Phase I (Part I): Review of CSP Technologies and Cost Drivers Overview

Consulting services for a Study of local capabilities to manufacture and

supply components for development of Concentrating Solar thermal Power plants (CSP) in India

2010

South Asia Energy Unit

Sustainable Development Department

The World Bank

Table of Contents

0. Introduction 5 0.1. THE SUN AS SOURCE OF ENERGY 6 0.2. DESCRIPTION OF MAIN TECHNOLOGIES 8 0.3. CSP PLANT VALUE CHAIN 9 0.3.1. CSP PROJECTS IN INDIA 10 0.3.2. CSP INDUSTRY IN INDIA 11 0.4. MANUFACTURING PROCESSES 14

1. LINEAR FRESNEL REFLECTORS TECHNOLOGY 15

1.1. GENERAL DESCRIPTION 15 1.2. MAIN PLANT CONFIGURATIONS 16 1.2.1. SATURATED STEAM 17 1.2.2. SUPERHEATED STEAM 17 1.2.3. OTHER HEAT TRANSFER FLUIDS 17 1.2.4. MIXED LINEAR FRESNEL/PARABOLIC TROUGH SYSTEM 17 1.2.5. HYBRID SYSTEMS 18 1.3. MAIN COMPONENTS 21 1.3.1. HEAT COLLECTION ELEMENTS 21 1.3.2. MIRROR ASSEMBLIES 22 1.3.3. SUPPORT STRUCTURE 22 1.3.4. REFLECTOR DRIVE MECHANISMS 22 1.4. DEMONSTRATION PROJECTS 23 1.4.1. JOHN MARCHEFF SOLAR PROJECT AT LIDDELL COAL POWER STATION 23 1.4.2. FRESDEMO 23 1.5. COMMERCIAL PROJECTS 23 1.5.1. KIMBERLINA 24 1.5.2. PUERTO ERRADO 1 24 1.6. PROJECT PIPELINE 24 1.7. PLANT VALUE CHAIN 25 1.8. COMPONENTS VALUE CHAIN 25 1.9. MANUFACTURING PROCESSES 25

2. PARABOLIC TROUGH TECHNOLOGY 26

2.1. GENERAL DESCRIPTION 26 2.2. MAIN PLANT CONFIGURATIONS 27 2.3. MAIN COMPONENTS 28 2.3.1. HEAT COLLECTION ELEMENTS 29 2.3.2. CURVED MIRROR ASSEMBLIES 32 2.3.3. HOSE CONNECTIONS BETWEEN TROUGH SECTIONS 32 2.3.4. TROUGH SUPPORT STRUCTURE 33 2.3.5. TROUGH DRIVE MECHANISMS 34 2.3.6. HEAT TRANSFER FLUID 35 2.3.7. DIRECT STEAM GENERATION (DSG) 36 2.4. DEMONSTRATION PROJECTS 37 2.4.1. PLATAFORMA SOLAR DE ALMERÍA (PSA) 37 2.4.2. ENEA‘S SOLAR COLLECTORS TESTING FACILITIES 37 2.4.3. SAGUARO DEMONSTRATION PLANT 38 2.5. COMMERCIAL PROJECTS 38 2.5.1. SEGS POWER PLANTS 38 2.5.2. NEVADA SOLAR ONE 38 2.5.3. ANDASOL PLANTS 39 2.5.4. PUERTOLLANO 40 2.5.5. ISCCS IN NORTHERN AFRICA 40 2.6. PROJECT PIPELINE 41 2.7. PLANT VALUE CHAIN 41 2.8. COMPONENTS VALUE CHAIN 43

2.8.1. HEAT TRANSFER FLUIDS 43 2.8.2. HEAT COLLECTION ELEMENT 43 2.8.3. MIRROR ASSEMBLY 44 2.8.4. SUPPORT STRUCTURE 44 2.9. MANUFACTURING PROCESSES 45 2.9.1. HEAT COLLECTION ELEMENTS 45 2.9.2. CURVED MIRROR ASSEMBLIES 47 2.9.3. TROUGH SUPPORT STRUCTURE 52

3. POWER TOWER TECHNOLOGY 56

3.1. GENERAL DESCRIPTION 56 3.2. MAIN PLANT CONFIGURATIONS 57 3.2.1. WATER/STEAM WORKING FLUID 57 3.2.2. MOLTEN SALTS AS WORKING FLUID 58 3.2.3. SODIUM AS WORKING FLUID 58 3.2.4. AIR AS WORKING FLUID 58 3.3. MAIN COMPONENTS 58 3.3.1. MIRRORS 60 3.3.2. HELIOSTAT LOCAL CONTROL 60 3.3.3. METALLIC STRUCTURE 61 3.3.4. PEDESTAL 62 3.3.5. DRIVE MECHANISM 63 3.3.6. FIELD CONTROL 64 3.3.7. FIELD WIRING 65 3.3.8. RECEIVER 65 3.3.9. TOWER CIVIL WORKS 68 3.3.10. HEAT TRANSFER FLUID 69 3.3.11. PIPING, VALVES AND SPARE PARTS 69 3.4. DEMONSTRATION PROJECTS 71 3.4.1. PLATAFORMA SOLAR DE ALMERÍA (PSA) 71 3.4.2. WEIZMANN 71 3.4.3. JÜLICH 72 3.4.4. CSIRO-NSEC 72 3.5. COMMERCIAL PROJECTS 72 3.5.1. ABENGOA SOLAR TOWERS 72 3.5.2. GEMASOLAR 72 3.5.3. ESOLAR 72 3.5.4. BRIGHTSOURCE 73 3.6. PROJECT PIPELINE 74 3.7. PLANT VALUE CHAIN 74 3.8. COMPONENTS VALUE CHAIN 75 3.8.1. HELIOSTAT MIRROR ASSEMBLY 75 3.8.2. HELIOSTAT METALLIC STRUCTURE 75 3.8.3. TOWER RECEIVER 76 3.8.4. TOWER CIVIL WORKS 76 3.9. MANUFACTURING PROCESSES 76 3.9.1. HELIOSTATS 76 3.9.2. TOWER CIVIL WORKS AND MATERIALS 78

4. DISH-ENGINE TECHNOLOGY 81

4.1. GENERAL DESCRIPTION 81 4.2. MAIN PLANT CONFIGURATIONS 82 4.3. MAIN COMPONENTS 82 4.3.1. DISH 83 4.3.2. RECEIVER 84 4.3.3. CURVED MIRROR ASSEMBLIES 85 4.3.4. DISH SUPPORT STRUCTURE 87 4.3.5. HEAT TRANSFER FLUID 88 4.3.6. DISH DRIVE MECHANISMS 88 4.3.7. STIRLING ENGINE 88 4.4. DEMONSTRATION PROJECTS 90

4.5. COMMERCIAL PROJECTS 90 4.5.1. VILLAROBLEDO 91 4.5.2. MARICOPA SOLAR (SES/TESSERA) 91 4.6. PROJECT PIPELINE 91 4.7. PLANT VALUE CHAIN 91 4.8. COMPONENTS VALUE CHAIN 92 4.8.1. STIRLING ENGINE 92 4.8.2. DISH MIRROR ASSEMBLY 92 4.8.3. DISH STRUCTURE 92 4.9. MANUFACTURING PROCESSES 93 4.9.1. CURVED MIRROR ASSEMBLIES 93 4.9.2. DISH SUPPORT STRUCTURE 94

5. POWER ISLAND 95

5.1. MAIN COMPONENTS 95 5.1.1. POWER BLOCK 95 5.1.2. BALANCE OF PLANT (BOP) 96 5.2. COMPONENTS VALUE CHAIN 96

6. THERMAL ENERGY STORAGE SYSTEM 104

6.1. GENERAL DESCRIPTION 104 6.2. MAIN COMPONENTS 104 6.2.1. STORAGE MEDIUM 104 6.2.2. TANKS 105 6.2.3. STORAGE FLUID / HTF HEAT EXCHANGERS 106 6.2.4. STORAGE FLUID HEATERS 106 6.3. COMPONENTS VALUE CHAIN 106 6.3.1. HEAT TRANSFER AND STORAGE FLUIDS 107

7. REFERENCES 108

Preface

The study for ‗assessment of manufacturing capacities for concentrating solar thermal technologies in India‘ is part of The

World Bank support on Concentrated Solar Power to the Ministry of New and Renewable Energy (MNRE). This report from

Phase 1 of the study was prepared by a consortium led by YES (Ynfiniti Engineering Services, S.L.) comprising of NIXUS,

and the Spanish National Centre for Renewable Energies, CENER under the contract for Contract Number P119536. The

report is in draft form, undergoing review by The World Bank and would be finalized after completion of Phase 2 on

‗‘assessment of competitive positioning of India‘s industries and preparation of an action plan to stimulate local production of

CST technologies in India‘‘ of the assignment (details are attached in the Annexure I on Terms of Reference).

The World Bank would like to thank counterparts in the MNRE who provided guidance and inputs through the assignment.

It should be noted that the analysis results does not represent The World Bank‘s view.

This work received Energy Sector Management Assistance Program (ESMAP) funding, for which we are grateful.

The task members from the World Bank team include Ashish Khanna, Gevorg Sargsyan, Nataliya Kulichenko,

Chandrasekeren Subramaniam, Anjali Garg and Ruchi Soni. For further clarifications and suggestions till the report is

finalized, please write to Ashish Khanna at akhanna2@worldbank.org and Nataliya Kulichenko at

nkulichenko@worldbank.org .

0. Introduction In this section we will provide some information on the Sun as a source of energy and after this we will provide a brief introduction to the main subjects of study: the different CSP technologies, their value chain and manufacturing processes.

0.1. The Sun as Source of Energy The sun‘s spectral class label indicates that its surface temperature reaches approximately 5,778 K, what means that solar energy has a very high exergy. The solar radiation at the sun is about 63 MW/m2, but the big distance to be covered lead to a high dilution of the flux, making it possible only around 1 kW/m2 to get to Earth. This entails that the sun radiation by itself would supply low temperatures to a thermal fluid, making it necessary to incorporate a concentration ratio of the radiation received in order to get higher solar fluxes. Figure 1 shows the high potential of the solar energy in comparison with other energy sources. Solar radiation received in the whole terrestrial surface is represented in this figure, together with all the known fossil fuel reserves including uranium, for one year time period. Annual energy consumption is also depicted.

Figure 1. Comparison of different energy sources in the World. Source: Cener. This huge potential is distributed along the Earth, having different incidence in specific locations depending on the latitude. These values can vary from 675 kWh/m2 year in Arctic Islands to 2,400 kWh/m2 year in some locations like Sahara. Global mean radiation is estimated around 1490 kWh/m2 year. Solar energy has two main characteristics:

it is highly diluted all over the Earth, and it is highly variable.

Figure 2. Solar Resources for CSP technologies (DNI in kWh/m2/y). Source: unknown.

Figure 3 shows a graphic of the yearly distribution at La Risca, Spain, location where the solar thermal power plant Alvarado 1, has been recently built.

Figure 3. Yearly distribution of DNI at La Risca, Spain. Day hours by DNI level (left) and DNI distribution by day of year and hour of day (right). Source: Cener.

Figure 4 shows a depiction of transient moments that take place in a typical meteorological year in Caceres (Spain). The high number of short periods with low solar radiation shows the interest of buffering the energy to avoid strong variations of power production.

Figure 4. Amount of time intervals with DNI values lower than 10 W/m2. The benefits of solar power are compelling: environmental protection, economic growth, job creation, diversity of fuel supply and rapid deployment, as well as the global potential for technology transfer and innovation. The underlying advantage of solar energy is that the fuel is free, abundant and inexhaustible. These technologies can be applied properly in regions with annual mean radiation values higher than 1,750 kWh/m2 year. Moreover, solar thermal power has an added advantage compared to other renewable energies: dispatchabity. Indeed it has the possibility of including a storage system or hybridization that could avoid stops during transient conditions. In addition, the scheme of a current solar thermal power plant allows the design and operation of hybrid systems which presents the following advantages:

Reducing polluting and greenhouse gases emissions compared to fossil plants, Offsetting variations of the solar radiation by fossil fuel to avoid partial load operation of the turbine, Improving the integration to the grid, Increasing the capacity factor without increasing investment costs as with storage systems

0.2. Description of Main Technologies Solar thermal technology makes advantage of incident solar radiation, concentrating and collecting it in a specific system that heats a thermal fluid. This heat is then used to run a turbine and produce electricity. This process can be carried out directly or indirectly, trough an intermediate heat transfer fluid and the use of a heat exchanger. There are four CSP technologies being currently developed and operated: linear Fresnel reflector, parabolic trough, power tower system and parabolic dish engine (Stirling). Every CSP technology shows different characteristics and efficiencies as a consequence of its configuration and operation conditions. In Figure 5 efficiencies for every subsystem are shown, depending on the CSP technology analyzed. Efficiencies are estimated within a range of values, showing the variability of these figures for every functional subsystem.

Figure 5. Comparative efficiencies between different technologies for every subsystem: solar Collection system, thermal generation system and electrical generation system. Source: Cener.

Linear concentration systems Point focus concentration systems

LINEAR FRESNEL REFLECTOR

PARABOLIC TROUGH POWER TOWER SYSTEM PARABOLIC DISH / ENGINE (STIRLING)

Linear Fresnel reflector concentrating systems use flat or slightly curved mirrors to focus solar radiation onto a linear receiver.

Parabolic trough systems concentrate the solar radiation with a parabolic-shaped mirror onto a linear receiver, located at its focal length. The parabolic trough system is the predominant linear system and is the most developed and commercially tested CSP technology.

A power tower system uses mirrors called heliostats with two-axis sun-tracking to focus concentrated solar radiation on a receiver at the top of a tower. Two main technical designs can be distinguished depending on the working fluid used: water/steam and molten salts.

The dish / engine is unique among CSP systems in using mechanical energy rather than a working fluid in order to produce electricity. Dish engine systems consist of a mirrored dish that collects and concentrates sunlight onto a receiver mounted at the focal point of the dish. The receiver is integrated into a high-efficiency engine (the Stirling engine is the most common type of heat engine used).

36

3. Overview of Concentrating Solar Thermal (CST) technologies

Parabolic Trough

100%

58%

48%

16%

Linear Fresnel

100%

42%

34%

11%

Tower System

100%

62%

53%

Parabolic Dish

100%

82%

62%

18%

22%

Optic

al

Therm

al

Annual D

irect S

ola

r E

nerg

y a

t th

e I

nput A

pert

ure

0.3. CSP Plant Value Chain Defined by Porter [1] [Porter, 1985], the value chain analysis helps to identify the sequence of activities required to make a product or provide a service. It allows companies to answer the following question: Are we taking advantage of our distinctive capacities and resources, focusing on the activities that bring the greatest benefits for our client? The value chain analysis points out potential competitive advantages within the activities and processes of the firm. Indeed from the detection of a demand, successive phases of design, logistics, production, marketing, sales and services constitutes a chain of activities that represents value on the client side, and costs on the supplier side. This strategy tool was first designed to improve the competitive advantage of companies, but then it was extended to industries and turned out to be useful for example to policy makers, local government agencies, or business associations. Figure 6 shows a generic scheme of a value chain analysis, which can be applied not only to companies but also to business segments.

Figure 6. Value chain analysis, generic scheme In this document this analysis will be applied to the Concentrated Solar Power (CSP) sector. CSP is a new industry, and the roles and actors in the value chain vary significantly by technology and project. In addition, the value chain structure is still evolving. At the basic level, [2] [Gereffi, 2008] proposes five stages in the value chain:

materials; components; the finished product including solar technology and plant development; distribution via ownership and operation of the CSP plant; and end use of power by utility companies.

In this document we will use the value chain defined in Figure 7, similar to the structure used in [Emerging, Energy].

Figure 7. CSP Value chain analysis Research and development is an integral part of the component and system design, production, construction and operation stages of the value chain. Much of the R&D, plant development, manufacturing, design, installation, and operation are conducted by a single company or by closely related companies. Therefore, there is significant vertical integration across the stages of the value chain. Ivanpah Solar Power Complex is a good example of this vertical

Firm Infrastructure

Human Resource Management

Technology Development

Procurement

Inbound

LogisticsOperations Outbound

Logistics

Marketing &

SalesService

Materials Components Finished

products

Distribution End Use

margin

margin

Firm Infrastructure

Human Resource Management

Technology Development

Procurement

Inbound

LogisticsOperations Outbound

Logistics

Marketing &

SalesService

Firm Infrastructure

Human Resource Management

Technology Development

Procurement

Inbound

LogisticsOperations Outbound

Logistics

Marketing &

SalesService

Materials Components Finished

products

Distribution End Use

margin

margin

Project Owner

Research and Development Centers

Material

SupplierTechnology

Provider Technology

Integrator

EPC

ContractorOperator

Materials CSP

Components

CSP Plant Plant

Construction

Plant

Operation

Project

Developer

margin

margin

Project Owner

Research and Development Centers

Material

SupplierTechnology

Provider Technology

Integrator

EPC

ContractorOperator

Materials CSP

Components

CSP Plant Plant

Construction

Plant

Operation

Project

Developer

margin

margin

integration: BrightSource Energy owns Luz II, one of the early CSP technology design and manufacturing companies, and Luz II will manufacture the CSP technology while BrightSource leads the development, operation, and management of the plant [2] [Gereffi, 2008]. Raw material needed for CSP component manufacturing are mainly steel, aluminium, copper, brass silica, and concrete, but also synthetic oil and / or molten salts as heat transfer and storage fluids. The CSP sector is unlikely to be impaired by a scarcity of raw materials. Large mirror areas will be required, which may exceed current global production by a factor of two to four, so timely investment in production capacity of mirrors will be necessary. This production would only account for a few percentage points of the global production of flat glasses, however. Similarly, accelerated deployment of trough plants would require investment in production of heat collector elements. Receivers for towers are a variety of high-temperature heat exchanger, which industry has largely deployed throughout the world. Only molten salts for thermal storage may raise some production problems. They are used in large quantities as fertilisers for agriculture, but their use as a storage medium requires a high degree of purity [CSP Technology RoadMap IEA, 2010] A CSP plant can be divided into specific and conventional components:

Specific components are components from the collector and thermal generation systems, and vary depending on the type of CSP plant. These are for example Heat Collection Elements, Solar Collector Assemblies, heliostats, or tower receivers. Many project developers conduct their own R&D to create unique, patented concentrating solar technologies.

Conventional components are mainly the components constituting the two other subsystems of a CSP plant: the power island and the Thermal Energy Storage. Control systems and civil works of the collector subsystem can also be considered as non specific components. CSP plant construction requires commodity type materials (steel and concrete), and many companies contract out the manufacturing of non-patented components.

The components are manufactured by the technology provider, while the developer or the project integrators integrate these components into a complete functioning plant. The owners are either the developer or the customers for whom the technology integrators integrate the plant. The end use consists of the sale of power to utility companies, and finally to the consumer. Some technology promoters are adding technology licensing to facilitate their technologies demonstration in the field. For example, because of their inability to develop and finance projects themselves, particularly in today‗s economic climate, eSolar and Ausra have begun to license their technologies as a means to raise capital and demonstrate their systems [Emerging Energy].

0.3.1. CSP Projects in India

We will be presenting in this Section just a brief summary of the current situation in the country. This information will be further developed in Part II of this project. In 2008 India launched its Jawaharlal Nehru National Solar Mission (JNNSM) to promote ecologically sustainable growth while addressing India‘s energy security challenge. The mission envisages a capacity of 20,000 MW by the year 2022. In March 2009, India-based ACME Group signed a 1 GW licensing agreement with eSolar for US$30 million to develop projects in India and thus received an equity stake in the US Company. Then ACME signed a 50 MW power purchase agreement (PPA) with BSES Delhi in January 2010, with commissioning planned for 2011. Other technology promoters, including Power Cube Pvt. Ltd. and Electrotherm India Ltd., are seeking CSP solutions that circumvent grid instability, primarily for industrial supply. The Italian consortium Solare XXI has announced a technology supply agreement with Entegra Ltd. of India for a 10 MW commercial plant in Rajasthan [Emerging Energy], and another 10 MW CSP power plants has been proposed in the Indian state of Rajasthan. Suryachakra MSM Solar India Pvt Ltd is a joint venture company formed between Suryachakra Power and MAN Solar Millennium for transfer of CSP Technology to project developers in India. This joint venture aims to indigenize the technology components and minimize import of critical components of the solar field and achieve cost reductions. This heralds the realization of a dream which goes back to early 1990s. Given India‘s solar power potential of 5000 trillion KWh per year, a favorable regulatory atmosphere and the supply demand gap it is only natural that solar power will be one of the thrust areas of future Indian governments.

0.3.2. CSP Industry in India

The Indian CSP regulatory framework given below depicts the players involved from the design stage till the end users. Currently the major chunk of activity in India is geared towards CSP plant operators who are collaborating with international CSP component manufacturers and Indian power generation equipment manufacturers to set up CSP plants in Rajasthan and Guajarat.

Indian CSP policy and regulatory framework.

In the following Tables we will describe for each value chain component:

Major Players Indian Outlook Potential Indian Players

Description of current and potential CSP industry in India

CSP Collector System Manufacturer These are the manufacturers of systems such as: Mirrors/Reflector Films; Receivers; Structures; Drives

Major Players Indian Outlook Potential Indian Players

CSP collector system Manufacturer

Technical Design & Construction

Partners

CSP Plant OperatorsTransmission and

Distribution Companies

End Users

Policy and Regulatory framework by JNNSM

Policy and Regulatory framework by CERC AND SERCs

Power generation and control equipment

manufacturer

Illustrative CSP Plant Value Chain

Research & Development

No major Indian players are present in this step of the value chain. Main worldwide players are identified in the next sections of this document.

Currently there is no major Indian company which specifically caters to CSP technology. However there are companies within the sectors mentioned below which have the potential of manufacturing CSP collector systems:

Glass Manufacturing

Auto and Auto Ancillary

Polymer

Steel

Players in Solar PV technology

Glass Manufacturing

ASAHI India Glass Ltd

Gujarat Borosil ltd.

Saint Gobain India

Auto and Auto Ancillary

Mahindra & Mahindra

TATA Motors

Bharat Forge

Polymer Manufacturer

3M India

Signet Solar

Moser Baer

Steel

SAIL

TATA Steel

Vizag Steel

Solar PV Technology

TATA-BP Solar India Ltd.

Small players like Photon Energy systems Ltd.

Power Generation and Control Equipment Manufacturer These are the manufacturers of systems such as: Steam Turbines; Boilers; Switchgear and Control gear

Major Players Indian Outlook Potential Indian Players

Bharat Heavy Electrical Ltd.

Larsen and Toubro

Siemens India

Crompton Greaves

ABB India

Adhunik Global Energy, Rajkot

Thermax Ltd.

The growth of the electrical machinery industry is directly related to the development of power generation and distribution. Excluding the non-utilities, India‘s power generation capacity of 2,300 MW in 1950 has expanded to over 147,806 MW at present. The Indian electrical machinery industry manufactures a wide range of turbines with a total capacity of more than 7000 MW per annum. While BHEL is the largest player, there are significant numbers of smaller units in the private sector. Thermax is already involved in CSP industry: indeed it has supplied two supplementary fired Heat Recovery Steam Generators (HRSGs) for the Hassi R‘Mel ISCC power plant, in Algeria.

This is a mature industry sector in India and the current major players are expected to cater to the demands of CSP Plants as the products used would be mostly the same as in a conventional coal based power plant.

Technical Design and Construction Partners These include: Project Design; Plant Assembly and Installation; Technology Integration

Major Players Indian Outlook Potential Indian Players

Larsen and Toubro

Bharat Forge

Maharishi Solar Technology

Gadhia Solar Energy Systems

Pvt Ltd.,Valsad

Clique Developments Pvt. Ltd.,

Mumbai

M/s Ultimate Technologies,

Nashik

Square Engineering Pvt. Ltd.,

Pune

Pradyna Consultants and

Engineers Pvt. Limited

M/s Purolator India Limited,

Gurgaon

Though there exists a technology gap for designing

and set up of CSP plants. India has a significant

amount of technological expertise in developing

solutions for power and infrastructure projects.

Local players are developing their CSP related

capabilities by entering into tie ups with more

experienced and established foreign players. For

instance:

Larsen & Toubro and Bharat Forge have a tie

up with Areva

Square Engineering Pvt. Ltd., Pune has a tie

up with the Wizard power Pty. Ltd., Australia.

The company has plans to install solar dish

concentrators for power generation at many

locations in India.

Singapore based Delta Power tied up with

local engineering and construction group Punj

Lloyd.

Germany‘s Solar Millennium struck an

agreement with Indian energy group

Suryachakra Power.

California-based group e-Solar signed an

agreement with local construction group

ACME for the development of 1GW of solar

thermal capacity over the next 10 years

Constructors

Larsen and Toubro

Bharat Forge

Gammon India

Punj Lloyd

Other Regional players

Engineering Consultants

SMEC

Engineers India Ltd

Independent individual

consultants

R&D Capabilities These include: Technology Development; Process and Plant Design; Operations optimization

Major Players Indian Outlook Potential Indian Players

BARC

BHEL

IIT Bombay

IIT Delhi

TERI

MNRE

MEDA

Solar Energy Centre (SEC)

Significant discussions have taken place on merits and demerits of the different technologies. A variety of opinions have been presented on the merits of centralized and decentralized power generation schemes. It has been decided that the work should be pursued in all the routes of solar power generation at National level. Focus is on immediately starting the installation a solar thermal power plant in the country for obtaining necessary experience on its design, installation, operation and maintenance. The Department of Energy Science and Engineering at IIT Bombay has expressed the desire to take the lead role in the solar thermal power generation program through medium temperature route. A few institutes and organizations are actively pursuing research activities in various aspects of solar thermal power generation. The Bhabha Atomic Research Centre, Mumbai and the Central Salt and Marine Chemicals Research Institute, Bhavnagar are involved in the development of solar power generation system using heliostats. IIT Bombay is involved in the development of indigenous Stirling Engines and the initial capacity target is 1.5 KWe. The Solar Energy Centre (SEC), established in 1982, is a dedicated unit of the Ministry of New and Renewable Energy (MNRE), Government of India for development of solar energy technologies and to promote its applications through product development.

3M India

Signet Solar

Moser Baer

TATA-BP Solar India Ltd.

NTPC

TIFAC

0.4. Manufacturing Processes Most of the components arranged in the solar field of a solar thermal power plant have been specifically designed. Even if some subsystems, like power block and thermal energy storage, are common to various solar thermal technologies and use classical manufacturing processes from other industries, some components related directly to the solar field (solar collection system and thermal generation system) have specific characteristics which can only be achieved through ad hoc manufacturing processes. Besides, advanced manufacturing processes could have a great impact on the capital cost reduction of CSP plants. Indeed mass production processes from conventional industries such as automotive or fossil fuel power industry could be adapted to CSP technologies and help them to reach competitiveness. In this document CSP specific manufacturing processes will be described for the considered four main technologies. They have to be implemented not only by technology providers during the fabrication and mounting of components (e.g. receiver tubes for parabolic trough plants, heliostats and central receivers for power tower plant), but also by EPC contractors during the plant construction (trough solar field mounting, tower building).

1. LINEAR FRESNEL REFLECTORS Technology In this section we will analyze linear Fresnel technology into detail. After a first general description, we will describe plant configurations and main components. Then we will name the projects (demonstration, commercial and in the pipe-line). Last, we will include as well an analysis of the value chain (plant and components) and manufacturing processes.

1.1. General Description Linear Fresnel (LF) collectors have a ground-mounted array of long, narrow, flat or slightly curved tracking mirrors to focus the sun‘s radiation on a linear receiver pipe located above the array. Collectors are made up of flat mirrors that can be slightly curved to the metal substrate during bonding, avoiding the expensive mirror ―sagging‖ process required for trough collector manufacture. According to their proponents, the flat mirror tracking subsystem is crucial to ensure the low cost of the concept.

Figure 8. Views of Linear Fresnel reflector arrays. [Morrison, 2006][3]. Linear Fresnel systems are usually used to produce low temperature water/steam that is directly coupled to a steam turbine. LF systems are asserted by its developers and promoters to be lower-cost and less accident-sensitive than other alternatives by virtue of the off-the-shelf components chosen for its construction and the use of low curvature mirrors. Besides they make a better land use than other technologies like parabolic-trough or power tower for the same power output [Emerging Energy, 2010] [4]. However, LF systems work at low operating temperatures and low field efficiencies tending to be less efficient than other technologies like molten salt power tower. Furthermore, there is no technically developed storage system available. Current LF systems are reported to achieve temperatures between 250˚C to 300˚C (Fresdemo reached up to 450ºC during performance tests. [Bernhard,2009]). They lack, however, the operational record proving they can achieve these temperatures consistently over long periods of time. There are currently two different Linear Fresnel solar field designs for electricity generation:

Classical linear Fresnel reflector (LF) and compact linear Fresnel reflector (CLFR).

The main difference between both configurations is that the second one has the possibility to focus the solar radiation onto either of two receiver arrays, depending on the sun position throughout the day. This feature decreases shadowing and blockage effects, making it possible to design more compact solar collector fields.

1.1 m

31 m77.5 m

10 m

3 m

LF and CLFR systems have similarities regarding geometry and size of the collector field. They have significant differences in the design of their receiver or solar to thermal energy conversion system. While LF systems use a single receiver tube with a secondary reflector, CLFR systems use multiple receiver tubes without secondary reflectors.

Figure 9. CLFR scheme with reflector rows oriented in order to minimize shadows and blockings. Source [15]. Comparisons of both technologies, classical LF and CLFR, show a higher technical and commercial maturity of CLFR but a 10% higher potential efficiency for technologies with only-tube receivers and secondary reflector [Morin, G., W. Platzer, 2006] [5]. Main possible plant configurations in LF technology are similar to parabolic trough configurations, taking into account the impact caused by the lower operation temperatures of this technology in its efficiencies. However, low temperatures systems also offer better opportunities to be implemented in innovative cycles like Organic Ranking Cycles, solar preheating and Integrated Solar Combined Cycle Systems and other low temperature applications as Solar Air Conditioning Systems.

1.2. Main Plant Configurations Although the operation of CSP plants is very similar to fossil fuel thermal plants operation, many options can be considered to design a Linear Fresnel solar plant, depending on the choice of thermal cycle, working fluid, solar fraction determined by the hybridization (if any),… and on the strategic objective of the installation. The most common configuration of Linear Fresnel plants is Direct Steam Generation (DSG). This innovative concept consists in using water/steam not only as working fluid in the power block but also as heat transfer fluid in the solar field. It can be implemented in almost all kind of CSP plants, in particular in strongly hybridized systems. Investment cost reductions and efficiency increases are expected, because these systems allow to:

Avoid the use of water-oil heat exchangers, simplifying the layout of the plant, Avoid the use of synthetic oil, reducing environmental risks (no more fire-protection and anti-freezing systems

needed) and removing temperature restrictions, Improve pressure and temperature conditions of the thermal cycle, resulting in higher cycle efficiencies, Improve heat exchanges in solar collectors, resulting in higher solar field efficiencies, Decrease parasitic consumptions for pumping.

However, to operate at such high temperatures and pressures, DSG systems absorber and insulated pipes should be more resistant and thus more expensive. This system presents some difficulties arisen from its operation conditions. High pressures and temperatures in water can be corrosive for materials, so its conductivity and pH level must be limited and controlled. However, the most important problem is the one which emerges as a consequence of the coexistence of both phases: liquid and vapor. Instabilities appear with the phase change, and this process also causes the fluid stratification inside the horizontal tubes leading to non-homogeneous temperature distributions that generate thermal stress.

Up to now, specific storage system for DSG facilities has not been yet developed. Nevertheless this is a matter that has already attracted the attention of several researchers of the solar thermal industry [Pincemin, 2008] [Bernhard, 2009] [6] [7].

1.2.1. Saturated Steam

Current commercial plants (Kimberlina and Puerto Errado I) use saturated steam turbines. Although the power block efficiency is lower than for superheated cycles, this conservative approach benefits from a reliable operation and stable performances.

Figure 10. Saturated-steam solar only plant configuration (source: Novatec Biosol)

1.2.2. Superheated Steam

Superheated steam production in linear Fresnel collectors has been demonstrated at Fresdemo test loop at the PSA, with temperatures up to 450 ºC [Bernhard, 2009][7]. In this case a superheated Rankine cycle could be used, with optional reheat and extractions to increase its efficiency. This design requires higher precision of the concentrator, which usually results in higher investment costs.

1.2.3. Other Heat Transfer Fluids

Although water / steam is the privileged heat transfer fluid for Linear Fresnel plants, some research centers and CSP companies considered the use of other fluids such as synthetic oil [Hoyer, 2009][8], or molten salts. For example SkyFuel is willing to develop a linear Fresnel collector optimized for direct molten salt, and compare its performance to a parabolic trough. After having completed a feasibility study of the concept, they are developing a prototype to be tested in 2011 [SkyFuel, 2010][9].

1.2.4. Mixed Linear Fresnel/Parabolic Trough System

A study carried out by Novatec Biosol shows the possibility of combining a 50 MW facility of parabolic trough with linear Fresnel reflector. This option considers the use of LF for preheating and evaporation, making it possible to substitute 180,000 m2 of parabolic trough field by 210,000 m2 linear Fresnel field, maintaining 75,000 m2 of superheating and reheating parabolic trough field [Novatec Biosol, 2009] [10].

Figure 11. Mixed system scheme proposed by Novatec Biosol [Novatec Biosol, 2009] [10].

1.2.5. Hybrid Systems

From an environmental point of view, solar-only configurations like the ones mentioned above are the best configurations as only heat from the solar field is used to generate steam. However, as no mature TES solutions are commercially available for DSG, hybridization with a fossil fuel boiler placed in parallel to the solar field could be interesting to increase the capacity factor of the plant. In Spain the range of hybridization is limited 12 to 15% (fraction of fossil fuel energy in the total thermal energy of the plant) by the legal framework, in the USA it can reach up to 25%. This design allows three operation modes (solar, fossil, or hybrid) providing great levels of versatility and dispatchability.

Figure 12. Saturated-steam hybrid plant configuration (source: Novatec Biosol) Aside from the configuration shown in Figure 12, other hybrid design can be considered for Linear Fresnel CSP plants: Conventional Rankine cycle with solar preheating This concept aims at adding a solar preheater to big fossil power plants in order to reduce their fuel consumption and gases emissions. It has been demonstrated at Liddell coal power plant in New South Wales, Australia. The annual solar fraction (amount of solar energy in the total thermal energy of the plant) is usually lower than 5%. However, solar energy is converted to power with high efficiencies [Lerchenmüller, 2004][11] and the investment cost is low, so it can be a relevant option to retrofit existing fossil fuel plant already in operation and introduce CSP technologies to the market. No solar energy is lost during start-up and shut-down periods. Integrated Solar Combined Cycle Systems (ISCCS) These systems consist in integrating solar energy into a combined cycle power plant, as shown in Figure 13. They have been primarily considered for parabolic trough collectors, but the characteristics of Linear Fresnel collectors (low cost, low temperature, DSG) made them very relevant for ISCC systems. They can result very effective, in particular if stable and continuous power production is needed. Solar thermal energy is delivered to the Heat Recovery Steam Generator (HRSG) of the combined cycle, thus the steam turbine receives higher heat input than in classical combined cycles, resulting in higher efficiencies.

Figure 13. Basic scheme of an ISCCS.There are two options for solar heat integration, from low pressure or high pressure solar steam. Source: ECOSTAR

These systems benefit from the high efficiencies of combined cycles compared to Rankine cycles: some studies assess annual fuel-to-power efficiencies of about 60% [Dersch, 2004], [Asssessment of the World Bank, 2005], [Status Report, 1996], [12] [13] [14]. Besides, as the investment cost for gas turbines is lower than for steam turbines, ISCCS are more cost-effective than hybrid solar Rankine cycles. As in conventional Rankine cycle with solar preheating, no solar energy is lost during start-up and shut-down periods. The design solar fraction is limited (lower than 15-20% [Dersch, 2004] [12], resulting in very low annual fraction (about 6% in favorable irradiation conditions, up to 12% a TES is included).

1.3. Main Components In the following table, the importance of the main components of a LF solar plant is qualitatively evaluated considering economic issues (according to breakdown in Activity 1.2) and technological issues (design / manufacturing / assembling complexity, criticality and improvement potential).

Subsystem Component Economic Importance Technological Issues

Solar collection Support structures High Medium

Mirrors Low Medium

Drive mechanisms Low Low

Thermal conversion Receiver tubes High Medium

Natural Gas Boilers Low Medium

Piping, valves and spare parts Low Low

Thermal storage Buffer tank Low Low

Electrical conversion Power block High High

Balance of plant (BOP) High Medium

Although the BOP does not represent any particular technical difficulty, it is assessed to have medium technological importance because of the issue of water resources, which are often scarce in the regions suitable for CSP plants. Besides these costs related to material suppliers or technology providers, some activities of the EPC contractor such as solar collector assembly and civil works have a medium economic importance. In the next subsections we present the main components of Linear Fresnel Reflectors components.

1.3.1. Heat Collection Elements

From 1995 to 2000 several Linear Fresnel system concepts and configurations were explored, mainly by Australian researchers. Aspects such as the orientation and inclination of the LF reflector (north-south and east-west solar collector fields, polar mounted mirrors, etc.), or the use of vacuum and secondary concentrators in the receiver tubes were analyzed. In a vacuum tube receiver rack, the water enters and exits from the same side of the vacuum tube rack, which allows the replacement of the racks without having to open the high pressure circuit [Burbidge, 2000] [15]. Computer simulation predicted best results with horizontal receiver tubes with a secondary reflector. The option to tilt the primary mirrors along the polar axis (Figure 15) minimizes de cosine effect [Mills and Morrison, 2000] [16] and allows the collection of larger amounts of solar energy throughout the year with a more homogeneous yearly distribution, although decreases the land use efficiency and increases the size and cost of the collector structures. The option to use flat receivers selectively coated with black chrome (up to 300ºC) to reduce infrared losses is also being explored [Burbidge, 2000] [15]. Current systems designs seem to favor north-south collector fields with horizontal receiver tubes parallel to the reflectors. Furthermore, most LF technology promoters use cavity receivers without vacuum tubes, which at least on paper are considered to have better optical and net efficiencies [Burbidge, 2000] [15], although the durability of the selective coatings in direct contact with atmospheric air is not well known.

Figure 14. Alternative receivers designs, vertical tubes (left) and horizontal tubes (right.). Source [16]

Figure 15. CLFR with inverted receiver and Fresnel mirrors tilted along the polar axis. Source [16] Various studies of the above mentioned designs showed that the best design consisted of a single or multiple horizontal absorber tubes oriented on the longitudinal axis of the concentrator, as shown in Figure 8. Current designs available in the market are:

Open air multiple-pipe with no secondary reflector, adopted by Ausra (Areva) with a carbon steel tube diameter of 50 mm, placed at 18 m high.

Open air single-pipe with secondary reflector, adopted by Novatec Biosol and Solar Power Group, with a diameter of 70 mm and 114 mm of the receiver tube, respectively.

1.3.2. Mirror Assemblies

Commercial LF systems use low cost glass-metal reflector (3 mm thickness and 88% reflectivity). These mirrors are slightly curved along their longitudinal axis by elastic deformation. The dimension of the mirror varies from one technology supplier/promoter to another. For instance the width of the primary Fresnel reflectors for the Solarmundo-SPG design is 0.5 m, for the Fresdemo design is 0.6 m, and Ausra has considered several designs: 1.84 m and 2.25 m. Ausra steel-backed mirrors rotate downward for protection.

1.3.3. Support Structure

There are two main designs of metallic support structure for Linear Fresnel reflectors. Both of them can be easily assembled and without specialized work:

The bench bar design used by SPG and Novatec Biosol, in which the reflector is placed over a parallel bench bar structure. This light structure is made of standard steel profiles or cold rolled steel and allows the use of space below the structure.

The ring design developed by Ausra, in which the reflector is supported by a metallic structure centric to the rings. Mirrors are supported by only two contact points, which requires less raw materials (steel) but results in lower optical precision.

1.3.4. Reflector Drive Mechanisms

There are several studies suggesting the mechanical coupling of the primary Fresnel mirrors as a strategy to reduce the cost of the primary mirror field and simplify the tracking system. Once the primary Fresnel mirrors are properly positioned, the rotational movement along their longitudinal axis can be coupled, enabling one rotating motor to rotate

several Fresnel mirrors at once. This approach is currently being used by part of the industry [Burbidge, 2000] [15], while others favor a tracking system per mirror due its improved tracking accuracy [Morin, 2006] [5].

1.4. Demonstration Projects We present here John Marcheff Solar Project at Liddell Coal Power Station and Fredesmo Spain 1.

1.4.1. John Marcheff Solar Project at Liddell Coal Power Station

Liddell Power Station is located in New South Wales, Australia. This power plant is coal powered, with four 500 MW GEC (UK) steam driven turbo alternators for a combined capacity of 2,000 MW. In 2004, AUSRA developed the world‘s first solar thermal power collector system for coal fired power augmentation. In a first phase, this solar module generated one megawatt equivalent (MW) of solar generated steam. This facility was expanded in 2008 with the construction of a second phase, which a power capacity of 3 MWe [31].

Figure 16. John Marcheff Solar Project at Lidell Coal Power Station, located in Australia.

1.4.2. Fresdemo

Fresdemo is the first LF demonstration power plant built in Spain. It is located in the PSA, Almería. The demonstration power plant, which has a 100 m long collector, generates 1 MWth (peak) and is designed as a modular system. In large-scale power plants, several of these modules will be connected up in series. The pilot plant was built by Ferrostaal in collaboration with Solar Power Group and the aim of the plant is to produce evidence that electricity can be generated more competitively proving that Fresnel technology is commercially viable for large scale projects. It was put into operation in July 2007 and the trial period lasted 2 years. The results of the operation and testing that took place at the PSA identified several key areas were substantial improvements must be achieved before the technology can be considered ready for commercial deployment. It is unclear that, at this stage of development, the cost reduction that this technology can claim in relation to conventional parabolic trough technology can compensate its lower solar to electricity yearly conversion efficiencies. [Bernhard, Eck, SolarPaces 2009]

1.5. Commercial Projects Fresnel technology is still at a first development level compared to other CSP technologies like parabolic trough. That is why there is only a few Fresnel experiences in the world. Up to now every Fresnel plant currently in operation can be considered as a demonstration plant since the technology is still not wholly developed; that explains their limited power capacities. However, as they deliver their electricity generated to the grid and sell it, they are also considered commercial plants.

1.5.1. Kimberlina

The 5 MW Kimberlina Solar Thermal Power Plant in Bakersfield, California, is the first commercial solar thermal power plant built by Ausra. Kimberlina uses Ausra's LF own technology. Figure 18 shows this power plant, which was started up in 2008.

Figure 17. Kimberlina LF power plant, located in Bakersville (California).

1.5.2. Puerto Errado 1

Puerto Errado 1, promoted by Novatec Biosol, is the most recent LF plant put into operation. It has an installed power capacity of 1.4 MWe taking up 18,000 m2 of mirrored area. This plant will generate an estimated annual electric energy of 2 GWh, by using the DSG technology. Novatec has developed its own collector technology (they have also patented it) which has been implemented for the first time in this power plant: the collector Fresnel NOVA-1, which was connected to grid in 2009. The Puerto Errado 1 plant is, to our knowledge, the only commercial grid connected plant using dry cooling in Spain.

1.6. Project Pipeline Novatec Biosol has a project pipeline including an additional linear Fresnel project included in the register of the Spanish Ministry of Industry. This project, Puerto Errado 2, which is the second phase of the already in operation Puerto Errado 1, will have a total installed power of 30 MW and will also be built in Murcia. The market activity in the rest the world is slim. Only Areva (Ausra) has announced a project pipeline which sums a total power capacity of 337 MW, in several projects located in Australia, Chile, Jordan and Portugal [Emerging Energy 2010] [4].

1.7. Plant Value Chain The Table below shows the main actors of the linear Fresnel plants value chain, and the countries in which they are active. Having added Ausra´s technology to its package of thermal system offerings to provide turnkey solutions, Areva can be now considered as a leader in this technology. However, the company does not have any new plant under construction, whereas Novatec Biosol is now building its 30 MWe Puerto Errado II solar plant.

Main actors of the linear Fresnel solar plants value chain [Emerging Energy, 2010]

Country Technology

provider Technology integrator

Project development EPC Operation

Project Ownership

Australia Areva Solar Heat &

Power Macquarie Generation

Chile Areva Areva, Chile Ambiente

Jordan Areva MENA Cleantech AG

Spain Novatec Biosol Prointec S.A. Novatec Biosol

USA Areva

1.8. Components Value Chain Major Linear Fresnel technology promoters, such as Ausra (bought by Areva in 2010) and Novatec Biosol develop and manufacture their own specific components with proprietary designs:

Ausra (Mountain View, CA, USA) has its own reflector production line at its first North American manufacturing/distribution center in Las Vegas, Nevada,

Novatec Biosol (Karlsruhe, Germany) is a relatively new entrant into the CSP sector. The company is focused on the manufacturing and development of linear Fresnel technology systems and is looking to serve the Spanish market, with the installation of a manufacturing facility in Spain to produce reflectors and receiver elements. Planned production capacity at the plant will total 220000 m2 (17 MW) of reflector surface per year.

Regarding mirror assemblies, Linear Fresnel technologies have fewer customized requirements, allowing technology integrators to rely on the more than 65 flat glass suppliers worldwide. Solar Power Group (Essen, Germany) relies on its partner Ferrostaal for EPC, but does not specify the specific component manufacturer for its plants.

1.9. Manufacturing Processes Manufacturing processes implied in linear Fresnel reflector technology components have not been considered, due to their simplicity. They do not entail specific processes to take into account.

2. PARABOLIC TROUGH Technology In this section we will analyze parabolic trough technology into detail. After a first general description, we will describe plant configurations and main components. Then we will name the projects (demonstration, commercial and in the pipe-line). Last, we will include as well an analysis of the value chain (plant and components) and manufacturing processes.

2.1. General Description Parabolic Trough technology is the most advanced CSP technology nowadays. Indeed commercial parabolic trough plants have been operating satisfactorily for more than 20 years, giving valuable information, improving operation and maintenance, so increasing their maturity, and thus reaching the leadership in power generation from CSP plants. The design of a parabolic trough solar thermal plant is similar to a conventional steam engine thermal power plant, but in this case, the boiler is replaced by a solar field formed by a large quantity of parabolic trough solar collectors, all oriented by a sun tracking system in a single axis, which concentrates the solar radiation onto a linear receiver located at its focal line (see Figure 18). The orientation of this field can be either North-South, with an East-West sun tracking, or East-West, with a North-South tracking. Currently the most common configuration is the North-South orientation.

Figure 18. Parabolic Mirror and Receiver. Solar radiation heats up the synthetic-oil that flows through the receiver to a temperature up to 400 ºC, which enables the steam generation in a downstream heat exchanger. As in a conventional power plant, the steam is pressurized inside the turbine that drives the generator. The basic scheme of a parabolic trough power plant can be observed in Figure 19. The system can be divided into the following three parts:

the solar field (in yellow), the power block (in blue, with optional reheater), and the piping and heat exchangers (in red).

Figure 19. Basic scheme of a Parabolic Trough power plant. Source: ECOSTAR.

In this scheme two optional elements of a CSP plant are also represented: the Thermal Energy Storage (TES) and the Back-Up Boiler (BUB), usually working with natural gas. Both of them increase the capacity factor of the system, that is to say that they allow the plant to operate even when there is not enough direct solar radiation, and sometimes to fit to a demand curve. Introducing one of these systems made solar thermal power plants able to generate reliable, dispatchable, and stable electrical energy to the grid. Moreover it improves the use and amortization of the power block. The most mature TES is the two-tank molten salts storage, already used in commercial plants. This type of TES consists in two-tanks of molten salts, a hot one and a cold one, connected to the Heat Transfer Fluid by a heat exchanger. When the thermal power delivered by the solar field is higher than the thermal power needed to operate the power block at full load, the excess power is used to charge the TES. Storage capacities of up to fifteen hours can be reached with such systems, but investments cost would be highly increased. Parabolic trough solar fields are modular; they can be implemented at any capacity, which provides a great versatility. Even so, the optimal capacity for current technology is estimated to be about 150-200 MWe.

Figure 20. Aerial view of SEGS III-VII power stations. Parabolic trough is the most commercially advanced of the CSP technologies. The installed power capacity in the U.S.A. exceeds 400 MWe, with the 354 MWe SEGS plants in the Mojave Desert, having operated for about 20 years, and the 64 MWe Nevada Solar One system near Boulder City, Nevada. Outside the U.S.A. there are 250 MWe of troughs operating in Spain, with more than 4000 MWe under construction. The parabolic trough technology is currently one of the cheapest ways to produce power from a renewable source (about 0.17 €/kWh in Spain and down to 0.10 €/kWh in sites with higher direct radiation), and its profitability is expected to compete with conventional the one of conventional thermal power plants at mid term.

2.2. Main Plant Configurations Although the operation of CSP plants is very similar to fossil fuel thermal plants operation, many options can be considered to design a parabolic trough solar plant, depending on the choice of the thermal cycle, working fluid, solar fraction determined by the hybridization (if any),… and on the strategic objective of the installation. Most of them have already been described in Section 1.2. of this document:

Rankine cycle without hybridization. These plants often include TES to make up for the strong variations of solar radiation and to increase the operation hours at full load.

Rankine cycle with hybridization. TES can also be included. Conventional Rankine cycle with solar preheating. Integrated Solar Combined Cycle Systems (ISCCS). Currently there is three ISCC solar projects in

advanced construction: Hassi R‘Mel (Algeria), Ain Beni Mathar (Morocco) and Kuraymat (Egypt). All of them are expected to be connected on the grid within the current year. They all include a 20 MW parabolic trough solar thermal field which generates electricity combined with a natural gas boiler.

Direct Steam Generation (DSG), which has been successfully tested in different experimental facilities around the world. Main studies have been carried out during more than 6000 hours of operation in the Plataforma Solar de Almería (PSA) by consecutive DISS projects.

Other thermal cycles that could be of interest for parabolic trough plants are Organic Rankine Cycle (ORC). These cycles have been used for years in low temperature applications, for example in geothermal energy or waste heat recovery. They are parabolic trough systems with Rankine cycles whose transfer fluid is an organic fluid adapted to its temperature range (see Figure 21).

Figure 21. Scheme of a parabolic trough plant with an Organic Rankine Cycle [Price, H., 2002] In spite of their low efficiency induced by low temperatures, these cycles have the following advantages:

Organic fluid condenses at pressure levels higher than atmospheric pressure, allowing simple dry cooling systems.

Low operating pressures allow to simplify the installation and to decrease parasitic consumptions. Low operating temperatures allow to use simplified collectors and to reduce the solar field aperture. Water consumption is reduced. Not much maintenance is needed for small installations, what make them adapted for distributed generation.

2.3. Main Components In the following table, the importance of the main components of a PT solar plant is qualitatively evaluated considering economic issues (according to breakdown in Activity 1.2) and technological issues (design / manufacturing / assembling complexity, criticality and improvement potential).

Subsystem Component Economic importance

Technological issues

Solar collection Support structures High High

Mirrors Medium High

Drive mechanisms Low Low

Thermal conversion Receiver tubes High High

Oil forwarding skid Medium Low

Ball joints Low Medium

Natural Gas Boilers Low Medium

Thermal oil Low Low

Oil purification system Low Low

Fire protection system Low Low

Inertization systen Low Low

Piping, valves and spare parts Low Low

Thermal storage Storage medium (molten salts) Medium Low

Molten salts forwarding skid Medium Low

Heat exchangers Low Medium

Initial filling system Low Low

Electrical conversion Power block High High

Oil/steam heat exchanger Medium Medium

Balance of plant (BOP) Medium Medium

It is worth mentioning that many activities of the EPC contractor such as solar collector assembly, solar field foundations, civil works or land leveling, correspond to a high share investment costs and thus have a high economic importance. In the following sections the main components of this technology are described.

2.3.1. Heat Collection Elements

The receiver is the component where solar energy is converted to thermal energy in the form of sensible or latent heat of the fluid which circulates through it. It is a critical component for the performance of the solar power plant because it is where thermal losses are produced. This makes it probably the most important component in the solar system. Currently, only one type of receiver is available for parabolic trough power plants: vacuum tube receiver. Main providers are Schott and Siemens (Solel Solar Systems), but also new manufacturers like Archimede Solar (from Angelatoni Group) are being lately emerging. Figure 22 shows a scheme of a receiver tube and its main components.

Figure 22. Scheme of a receiver tube. Source [34] A vacuum tube receiver is essentially a tube, usually stainless steel, through which a heat transfer fluid circulates, thermally insulated from the outside environment by a vacuum inside a surrounding glass tube. Both tubes have certain characteristics that optimize their functioning. The glass tube which surrounds the inner metal tube has the double mission of protecting the selective coating from weathering and reducing thermal convection loss in the receiver tube. The outer envelope has to be highly transparent and have low reflectivity in order to allow as much radiation as possible through, and therefore usually has an antireflective treatment on both sides to increase transmissivity and collector optical performance.

A selective coating is also applied to the steel tube to achieve efficient photo-thermal conversion. The selective surfaces of the receivers must be highly absorbent and have low thermal emissivity at the operating temperature. In fact, thermal emissivity is the chief source of thermal loss. Besides, they are spectrally characterized by a low reflectance (ρ ≈ 0) at wavelengths of λ ≤ 3 μm and a high reflectance (ρ ≈ 1) at wavelengths of λ ≥ 3 μm. For CSP applications it has been proved that the ideal selective surface must fulfill some requirements, shown below:

Good spectral response. α ≥ 0,98 and ε ≤ 0,05 at temperatures between 400 and 500ºC. Chemical and thermal stability in air at high temperatures (T ≥ 400-500ºC) and at long term. Low cost. Ease of manufacturing.

The thermal stability of the coatings over time is also very important. At high temperatures, thermal emissivity is the chief source of thermal losses and the requirement for low emissivity often leads to complicated designs susceptible to degradation at working temperatures. Nevertheless, for CSP applications, it is enough for the spectrally selective surface to have a solar absorptance over 0.95, a thermal emissivity of less than 0.15 at 400ºC, and ideally, be thermally stable in air over 400ºC. Among the most relevant causes of degradation of the absorber surfaces are oxidation, high humidity and air pollution. [Allen N and Edge, 1992], [Moens L. Blake D., 2008]. As mentioned above, there is a vacuum between the glass and steel tubes which is essential to keep the collector‘s thermal performance from falling. This makes the glass/metal seal which maintains the vacuum without losses the critical point in the receiver tube design. When the vacuum is formed, the ends of the glass tube are joined to a metal bellow or dilator by a glass/metal seal, which in turn is sealed on the other end to the inner metal tube. This bellow, in addition to keeping the space between the tubes airtight, compensates for the difference in their thermal expansion at the working temperature. The glass/metal seal is subjected to enormous thermal stress, and so to avoid this stress as much as possible, metals or alloys with low thermal expansion coefficients near to those of borosilicate glass are used. Between the glass tube and the inner steel tube, getters are installed to absorb the few molecules of substances which, over time, may penetrate in the annular space between the metal tube and the glass and that would degrade the original vacuum if they were not eliminated. It should be kept in mind that the vacuum that originally exists between the metal tube and the glass is 10-4Pa (deep vacuum). Barium getters have a silvery appearance when the absorber tube vacuum is good, but turn white from exposure to air if the absorber tube loses the vacuum. Figure 23 shows a list of properties of conventional getters.

Figure 23. Properties of conventional getters Design of new receiver tubes increases the getter‘s capability of capturing hydrogen. Schott Glass is changing the steel tube composition and using additional hydrogen barrier coatings for reducing permeability ratios in vacuum conditions, thus increasing the life span of the receiver tubes.

Figure 24. Schott getters strategy, produced since 2005. Source: SCHOTT.

Figure 25. Siemens (Solel) getters strategy. Source: SOLEL. Both main receiver tubes providers, SCHOTT and SIEMENS, use their own selective coatings. Main properties of the SCHOTT PTR 70 receiver tube are listed below [SCHOTT PTR 70 receiver. The Next Generation. www.schottsolar.com]:

Solar absorptance of ≤95% and emittance ≤10% at 400ºC Borosilicate glass of a higher transmisivity and higher coefficient of thermal expansion Durable glass-metal seal Durable AR coating; transmittance >96% New bellow patented design (active aperture area of 96%) Getters integrated in the bellow

Figure 26. Receiver tube Schott PTR 70. Source: SCHOTT. Main specifications of the SIEMENS (Solel) UVAC 2008 receiver tube are the following:

Absorptance ≤96% and emittance ≤10% at 400ºC Absorber coating with better properties without Mo Durable AR coating with transmittance >96.5% Protected glass to metal seal Patented ―Getter Bridge‖ New bellow design Long durability of more than 20 years, including H2 elimination

Figure 27. Receiver tube UVAC2008. Source: SOLEL.

2.3.2. Curved Mirror Assemblies

The purpose of the concentrator mirror is to concentrate the solar radiation incident on the receiver located in the line of focus. Its parabolic geometry and optical reflectivity are extremely important because they are the basic properties that make it possible to concentrate the solar energy efficiently. So the mirrors usually have a support structure to give them the rigidity they require and on which a film of a highly reflective material is deposited. In general, the support structure that provides the rigidity to the parabolic-trough mirror is a metal, glass or plastic plate, while the reflective material is usually silver or aluminum. Metal plate supports are usually made of polished aluminum and have no added reflective material. However, these mirrors have very poor outdoor durability due to degradation of the optical characteristics of metal. Their main advantage is their low cost, but as they are not durable, they are not usually used industrially. On the other hand, plastic supports are usually used to in the form of thin sheets on which the reflective film is deposited and must be attached to another rigid support. To date, this type of mirrors are not very durable to weather and get dirty faster than in other cases because this type of plastic is electrostatically charged by wind and attracts dust. The low-iron glass support option on which a reflective film is deposited is the one most widely used to date because it has none of the above problems. The reflective material is usually deposited on glass and is protected by a layer of copper and another of paint to protect it from outside agents. The glass can be either thick glass hot bent to give its parabolic shape or thin glass which is bent cold into the parabolic shape by attaching it to the collector structure. There are thus two different possibilities to shape the mirror, either the support itself is rigid or the mirror is flexible and takes on the shape given it by the structure it is attached to. The material most commonly used to date for collector reflector mirrors is the glass substrate mirror with silver deposition which reaches maximum reflectivities of around 93.5%. The facets comprising the reflective part of the concentrator can be different shapes and sizes. Normally they have three or four anchor points by which they are fastened to the corresponding structural supports to make up the complete collector.

2.3.3. Hose Connections between Trough Sections

In a typical parabolic trough field, several collectors are connected to each other in a row, to headers, and also a number of rows are connected in parallel according to the solar field design. In every row of collectors, receiver tubes have to be connected in a way that enables independent rotation of both collectors while tracking the sun during the day. These connections need to be flexible and they also have to allow linear thermal expansion of the receiver tubes when their temperature increases from ambient to nominal temperature during system start-up.

There are two main types of flexible connections available:

Flexible hoses are composed of an inner hose that withstands temperatures below 300 º C, protected by a thermally insulated outer metal braid shield. For higher operation temperatures, stainless steel bellows are usually used. This type of hose is not as flexible, and causes a significant pressure drop in the circuit because of its high friction coefficient.

Ball joints are the second option for flexible connection between adjacent receiver tubes and tubes to headers. The main benefit of this option is a significantly lower pressure drop, but it also has reduced heat losses and lower cost [Cohen, Final report 1999]. Another advantage of ball joints is the connected pipes can move with two degrees of freedom, because the connected pipes can freely rotate (360º) [Romero, M., Zarza, E. ―Handbook of energy efficiency and renewable energy].

Flexible hoses have experienced high failure rates at the early SEGS plants. Later plants used an improved design with a substantially increased life that significantly reduced failures. In addition, a new design that replaces the flexible hoses with a hard piped assembly with ball joints is being used at the SEGS III-VII plants located at Kramer Junction [Cohen, Final report 1999] Furthermore, flexible hoses are likely to suffer from fatigue failures resulting in a leak, whereas ball joints only require the graphite sealing to be refilled after many thousands of hours of operation. Today‘s parabolic trough power plants working at temperatures above 300ºC are connected by ball joints instead of flexible hoses. Figure 28 shows two images of both types of flexible connections

Figure 28. Image of a collector module with flexible hoses (left) and a collector module with ball joints (right). A new kind of flexible pipe connection for parabolic trough collectors, called ROTATIONFLEX, has been tested at the PSA. It demonstrated a maintenance-free operation in a EuroTrough collector. Its developers claim that it presents the following advantages compared with a graphite ball sealing: elimination of torque forces, shared compensation of the two movements of longitudinal expansion and rotation, higher lifetime, reduced pressure losses [Ortiz Vives, 2009].

2.3.4. Trough Support Structure

The concentrator mirrors are installed on a rigid metal structure which gives them the parabolic shape necessary to be able to concentrate the radiation in their linear focus. There are many varied metal structure concepts. One of the most common is that the concentrator structure is shaped by a bridge-truss system supporting the facets, which are screwed on. These trusses are fastened to an axis (cylindrical or quadrangular) which is joined to the drive system that moves the concentrator by a reduction mechanism. To date, the material used in the structures has always been metal, although other materials such as fiberglass and some plastics still not used commercially have been studied.

To configure the structure that gives the concentrator mirror its parabolic shape, there have historically been two main techniques:

Torque tube structure: this kind of structure was used in LS-1 and LS-2 collectors, developed by Luz International Ltd for SEGS I and SEGS II power plants. Its structural principle consists of an axial tube which supports the profiles where mirrors are connected to. Tolerances for components used in this kind of structures were wide, so optical accuracy needed was achieved by making mechanical adjustments during collector assembly.

Space-frame structure: this structure was first used with LS-3 collector, also developed by Luz International Ltd for SEGS III-IX power plants. This structure consists of a central frame adjusted with accuracy before installation. This frame is used as a guide pattern for the rest of the structure and it guarantees a more precise operation, mainly in hard wind conditions.

Figure 29 shows schemes of LS-2 and LS-3 structures, of torque tube structure and space-frame structure respectively.

Figure 29. Schemes of LS-2 and LS-3 structures, of torque tube structure and space-frame structure One of the most relevant collectors recently developed was designed by Solargenix Energy, which uses an aluminum space structure. This collector, which has been evaluated by NREL, has better structural properties than earlier designs such as the LS-2 and LS-3, such as its weight, manufacturing simplicity, resistance to corrosion, cost and easy installation. The commercial collector, the SGX-1 used in Nevada Solar One, the first commercial plant in operation since SEGS IX was built in 1991, is the first collector installed in a modern commercial plant. Another of the collectors recently developed is the Eurotrough, in its different versions, developed by a consortium of European companies and laboratories. Although it is based on the central torque-tube structure, the helicoidal tube in the LS-2 and LS-3 has been replaced in the ET by a longitudinal torque box that has mirror support ―arms‖. This type of structure drastically reduces the torsion the mirrors have to support, avoiding their breakage to a large extent even in high wind loads. Furthermore, structural deformation from wind loads and its own weight during operation are lower, so optical performance is considerably increased. The structure‘s stiffness makes it possible to work with collector lengths of up to 150 m and 5.76 m aperture. Besides these collectors, there are other designs like Senertrough, developed by Sener, Albiasa Solar, developed by Albiasa, and SkyTrough, developed by SkyFuel. Most of these structures are galvanized steel and have a 5.76 m aperture except for the SGX-1 collector which is aluminum and has a 5 m aperture.

2.3.5. Trough Drive Mechanisms

Collectors are placed along parallel rows and track the sun by rotating around its own longitudinal axe. Collectors can be either oriented in a North-South direction, tracking the sun from East to West, or in an East-West direction, tracking the sun according to its elevation (height of the sun relating to the horizon) at every instant.

Solar tracking can be carried out with two different types of mechanisms, which enable the collector rotation around its axe. These are electric (LS-2 type) and hydraulic (LS-3, Eurotrough and Solargenix type). Figure 30 shows an example of both trough drive mechanisms currently available. Tracking accuracy achieved by both of them is very high.

Figure 30. Different trough drive mechanisms At present, hydraulic mechanisms are the ones with a major application in existing solar thermal power plants.

2.3.6. Heat Transfer Fluid

The heat transfer fluid that circulates through the solar field is one of the most important components in the whole plant. Its mission is to absorb the energy provided by the absorber tube in the form of enthalpic gain, by increasing in temperature as it goes through the loops in the solar field. The hot HTF goes to a heat exchanger to heat water and generate steam at a certain pressure and temperature. The solar field outlet temperature is restricted by the HTF properties, and this means that the fluids that can perform these functions are also limited. Experience over the years has shown that by increasing the solar field outlet temperature, the performance of the power block and thereby the whole plant also increases significantly. The commercially proven technology is limited to a temperature of around 400ºC, after which, in addition to degrading the fluid, thermal losses increase and the selective coatings also are degraded. Therefore, there are several lines of R&D today directed at studying both working fluids and the rest of the components. Some parabolic-trough plant applications with maximum desirable temperatures that do not surpass 175ºC can use demineralized water as the heat transfer fluid, which can be maintained in liquid phase with working pressures that are not very high. But for electricity generation, which is the case at hand, these temperatures have to be much higher to increase plant performance. Synthetic oil The fluid currently in use in commercial plants has a wide advantage over water and other fluids, although it is relatively expensive. Synthetic oil has a much lower vapor pressure than water at the same given temperature, so pressures required in the system are much lower. This simplifies the facility and safety measures required. Furthermore, current oils have responded very well up to now to the needs of commercial plants, as their maximum temperature coincides with the optimum collector operating temperature. Nevertheless, in addition to the above mentioned drawback of price, it has a limited maximum working temperature at below 400ºC, which limits the power cycle temperature and therefore its electrical conversion efficiency. Molten salt

The search for higher electricity production efficiency in solar thermal power plants based on parabolic-trough technology has led to research in new fluids with higher working temperatures, mainly molten salt and water/steam. Even though there are no commercial plants in operation with these innovative working fluids, there is wide experience in tested prototypes and experimental plants. The salt most commonly used in solar applications is nitrate salt, since its advantages are well known. The specific advantages are low corrosion by salt of materials used for solar field piping, high thermal stability at high temperatures, low steam pressure making it possible to operate at relatively low pressures in its liquid state and also, its availability and low cost. In opposition to these advantages are mainly the high freezing point of salt, which although it may oscillate from 120º to 200ºC depending on the type used, must be near 220ºC to mix the salt currently used in pre-commercial projects. This is a major problem for installation and investment costs. The freeze-protection strategy is very important in this case, and several different techniques are necessary to maintain the fluid above a certain temperature: constant circulation of salt, auxiliary heating and heat tracing throughout the piping [Kearney, D., et al., 2004]. Several institutions are studying the use of molten salt, the most common of which are Solar Salt, Hitec and Hitec XL. Their basic properties are shown. The use of molten salt as the HTF has several advantages over synthetic oil, although it should be recalled that its commercial use has not yet been proven.

Main characteristics of molten salts used in solar applications

PROPERTY Solar Salt Hitec Hitec XL

Composition (%)

NaNO3 60 7 7

KNO3 40 53 45

NaNO2 - 40 -

Ca(NO3)2 - - 48

Freezing point (ºC) 220 142 120

Maximum operating temperature (ºC) 600 535 500

Density at 300 ºC (kg/m3) 1899 1640 1992

Viscosity at 300 ºC (mPa*s) 3.26 3.16 6.37

Specific heat at 300 ºC (kJ/kg-K) 1.495 1.56 1.447

2.3.7. Direct Steam Generation (DSG)

In addition to the study of the alternatives described above, another very attractive option has been under study for several years, the Direct Steam Generation technology (DSG). This technology involves the use of water directly in the absorber tubes, allowing its evaporation and superheating, thereby generating the steam used to move the turbine in the power cycle. DSG has a series of obvious advantages over the HTF technology.

In the first place, the heat exchanger is eliminated, avoiding thermal loss due to its performance and simplifying the installation.

In the second place, the low cost of the fluid used (water) is evident, and it is not degraded like the rest of the high-temperature fluids.

Furthermore, this fluid is not polluting or dangerous, which allows some devices essential to solar fields with salt and oil to be eliminated, such as fire extinguishing system and sump tank, purging system for non-condensable, antifreeze protection, and so forth.

In a commercial DSG application, these advantages could be reflected in very significant electricity production cost reductions (30%). However, the disadvantages of using this fluid should also be taken into consideration (see section 2.3.)

2.4. Demonstration Projects Besides commercial projects mentioned in next section, there are numerous experimental projects to develop and test off-the-shelf and new components and systems. Some of them are described below.

2.4.1. Plataforma Solar de Almería (PSA)

This platform operated by the CIEMAT (Centro de Investigaciones Energética, Medioambientales y Tecnológicas) is the world major CSP experimental and demonstration complex, located in the desert of Tabernas (Almería, Spain). It began its operation in 1977, and from then on all kinds of CSP technologies have been designed and tested, as observed in Figure 31.

Figure 31. Scheme of the PSA test installations. Source: PSA. Among all PSA facilities, the following ones are related to parabolic trough technology:

The DISS test loop was designed to test Direct Steam Generation (DSG) in parabolic trough collectors, under high pressure and temperature conditions (100 bar/400 ºC). It is formed by two Eurotrough and eleven LS3 solar collectors. It can be operated under once-through, recirculation, or injection mode and it is very well instrumented so as to monitor many operation parameters. Research on DSG was later completed in the frame of INDITEP and Almeria GDV projects.

The LS-3 test loop is used to test solar collector assemblies and components (absorber tubes, mirrors, tracking systems, etc.) under realistic operation conditions. It is now formed by LS-3, Eurotrough, and Albiasa collectors, and uses Syltherm 800 thermal oil working up to 400 ºC and 16 bar.

The Small Solar Power System SSPS-DCS plant was the first parabolic trough system to be installed at the PSA. It is formed by a 1,2 MWth solar field of 40 ACUREX solar collectors working with Santotherm fluid at 290 ºC, a 5 MWhth thermocline oil TES with solar desalination system, a 500 kWe Rankine cycle, and a desalination plant able to produce 3 m3/h distillated water.

2.4.2. ENEA’s Solar Collectors Testing Facilities

This 500 kW experimental installation called MOSE (Molten Salts Experiments) is aimed at developing parabolic trough technology with molten salts as working fluid [Fabrizi, F., 2007] [19]. This implies modifications of most of the components (pumps, valves, heat exchangers, sensors…) and operation procedures (TES charges and discharges, failure management) [Macari, 2008] [20]. Molten salts used are 40% KNO3 and 60% NaNO3, working between 270ºC and 550ºC The test loop started operation in April 2004 and is composed by a 540 m2 solar loop. Its final goal is to introduce this molten salts technology to the market through the Archimede commercial project consisting in integrating a solar field into a combined cycle in Sicilia.

2.4.3. Saguaro Demonstration Plant

This 1 MWe demonstration facility built by Acciona-Solargenix in Saguaro (Arizona, USA) has been selling electricity to the grid since 2006. It has demonstrated the viability of Organic Rankine Cycles (ORC) powered by parabolic trough collectors. The solar plant has 10,340 m2 of parabolic trough solar field and uses Solargenix collectors, as well as Xceltherm 60 as heat transfer fluid. This organic fluid is amyl hydride (n-pentane), a refrigerant whose boiling point ranges around 36ºC and its vapor pressure gets to 0.568 bar (T = 20ºC). Its vapor density is 3.2325 kg/m3.

2.5. Commercial Projects Experiments using concentrated solar radiation to produce heat or power raised during the nineteenth century, but modern parabolic trough plants are a relatively recent technology starting with experimental tests at the Plataforma Solar de Almería (500 kW) and the first commercial plants for power generation called SEGS (Solar Electric Generating Systems) in California in 1981. In the following paragraphs, commercial parabolic trough plants connected to the grid before December 2009 are described.

2.5.1. SEGS Power Plants

It consists of 9 solar thermal parabolic plants built by Luz International Ltd. between 1981 and 1991 in the Mojave Desert in southern California, USA, with a total installed capacity of 354 MWe. They are still in operation today and provide peak load electricity with a capacity factor of about 30%. Main characteristics of these plants are gathered in the following Table.

Main characteristics of SEGS plants

SEGS I II III IV V VI VII VIII IX

First year of operation 1985 1986 1987 1987 1988 1989 1989 1990 1991

Net Power (MWe) 13.8 30 30 30 30 30 30 80 80

Gross Power (MWe) 14.7 33 33 33 33 33 33 88 88

Solar field aperture (m2) 82960 190338 230300 230300 250560 188000 194280 464340 483960

TES capacity (MWh) 110 - - - - - - - -

Hybridization Super heat.

Boiler Boiler Boiler Boiler Boiler Boiler HTF

heater HTF

heater

Oil type (HTF) Mineral Synth. Synth. Synth. Synth. Synth. Synth. Synth. Synth.

2.5.2. Nevada Solar One

This plant was the first commercial parabolic trough plant built after the SEGS plants, more than 15 years later. Built by Acciona Energy in 2007 in Nevada, USA, this 72 MWe plant has a 357000 m2 solar field occupying 121 ha (see Figure 32). In 2008 it produced more than the expected 130 GWh.

Figure 32. View of the solar field of Nevada Solar One. The solar field is composed by SGX-2 collectors from Solargenix with Flabeg mirrors. 61% of the field is equipped with Schott tubes and 39% with Solel tubes. Heat transfer Fluid is Dowtherm A, working between 293ºC and 391 ºC. The power block is a superheated regenerative Rankine cycle provided by Siemens. This plant has an auxiliary gas boiler on the oil loop to improve its operation preventing oil freezing and decreasing the number of start-ups and shut-downs during cloud transients. Legal framework limits the auxiliary power from fossil fuel burning to 2% of design power.

2.5.3. Andasol Plants

Located in Guadix (Granada, Spain), Andasol 1 is the first CSP commercial plant to be built in Europe, part of an extended project of up to 7 plants promoted by ACS-Cobra and Milenio Solar (Spanish subsidiary of Solar Millenium AG) groups. The three first Andasol plants have the same characteristics: Andasol 1 was connected to the grid in June 2009, Andasol 2 in 2010, and Andasol 3 is under development.

Figure 33. View of Andasol 1 plant. Andasol 1 occupies 195 ha, its design net power is 50 MW for a solar field aperture of 510,000 m2. A 30 MWth back-up boiler allows increasing production and improving power dispatchability, but its use must stay within the limits of 12% of the annual power production, as stated in Real Decreto 661/2007. Solar collectors are north-south oriented SKALET-150 from Flagsol (subsidiary of Solar Millenium) (Figure 33), composed by Schott and Solel tubes, and Flabeg mirrors. As in Nevada Solar One, the Heat transfer Fluid is Dowtherm A and the power block is a superheated regenerative Rankine cycle provided by Siemens.

Figure 34. Basic scheme of Andasol I solar plant. Source: ECOSTAR. A two-tanks molten salts TES (shown in Figure 34) is implemented for the first time in a commercial CSP plant. It corresponds to 7.5 hours of full load operation.

2.5.4. Puertollano

In 2007, Iberdrola Renovables began works on the first 50 MW field in Puertollano (Castilla-La Mancha, Spain), which was connected to the grid in 2009. Its 287760 m² solar field is composed by Iberdrola Collector with Flabeg / Rioglass mirror and Schott tubes. The Heat transfer Fluid from Dow Chemical‘s is operated between 304°C and 391°C. The power block is a Siemens Rankine cycle, powered by the solar field and a gas-fired backup HTF heater.

2.5.5. ISCCS in Northern Africa

At the moment, there are three ISCC power plants in advanced construction.

Abengoa is the promoter of the first ISCC power plant being built with parabolic trough technology. This power plant is located in Hassi R’Mel, Algeria. The installed power reaches 20 MW with 120,000 m2 of solar field. The global power capacity of the plant is 150 MW, and the hybridization is carried out with natural gas burning.

The ISCC power plant located in Ain Beni Mathar, Morocco, is also promoted by Abengoa. This power plant is the biggest ISCC power plant in the world with parabolic trough technology. The power installed comes to 470 MW, with 20 MW coming from a 120,000 m2 solar field and the rest, from natural gas burning.

The ISCC Kuraymat is located about 87 km South of Cairo, Egypt, on the eastern side of the river Nile. The plant is owned by the New and Renewable Energy Authority (NREA) of the Ministry of Energy of Egypt. The global power installed comes to 125 MW with 20 MW coming from the 130,000 m2 solar field. The hybridization is also implemented with natural gas.

2.6. Project Pipeline

In Spain, the feed-in tariffs guaranteed by the Real Decreto 661/2007 caused a major boost for CSP technologies, with 2,300 MW to be connected to the grid before 2013. Some of these plants are already built and now under commissioning process and guarantee tests (before the starting up, every solar power plant has to pass three test phases: 1) support the commissioning of the whole plant, 2) perform the corresponding guarantee tests, 3) demonstrate the dispatchability of the plant): Acciona‘s Alvarado 1, ACS Cobra‘s Extresol 1 and Andasol 2, SAMCA‘s La Florida, Abengoa‘s Solnova 1&3…. Because of the legal framework, most of the Spanish projected plants are quite similar, whereas in other countries other incentives lead to different plant configurations and technologies, such as in the USA or in the MENA region (Morocco, Algeria, Iran, UAE…). The USA project pipeline includes several parabolic trough power plants concentrated mainly in the state of California. Some of these projects are the following:

Martin Next Generation Solar Energy Center, 75 MW, Florida, under construction. Ridgecrest Solar Power Project, 250 MW, California, still under development. Palen Solar Power Project, 250 MW, California, still under development. Genesis Solar Energy Project, 250 MW, California, still under development. Blythe Solar Power Project, 250 MW, California, still under development. Abengoa Mojave Solar Project, 250 MW, California, still under development. Solana, 280 MW, Arizona, still under development.

2.7. Plant Value Chain The Table below shows the main actors of the parabolic trough plants value chain, and the countries in which they are active. After its investment in Solel (100%) and Archimede Solar Energy (28%), the turbine specialist Siemens has a unique position in the parabolic trough plants sector, being vertically integrated over as much as 70% of a delivered CSP plant from the component supply (receiver tubes, mirrors, control systems, steam turbines) to the plant operation. It should get a leading position as a turnkey provider for independent power producers (IPPs) and utilities lacking their own technology (e.g., E.ON, NextEra Energy Resources, Cogentrix, NextLight, and Starwood) [Emerging Energy, 2010]. Abengoa Solar and Acciona are also vertically integrated, offering EPC through their respective subsidiaries Abener and Acciona Infraestructuras, but have to rely on turbines from other companies for the power island of its plants. It is worth mentioning that Abengoa Solar is the only CSP player to be active in three of the four main CSP technologies (parabolic trough, power tower, and dish engine). In many cases, various companies are forming consortiums to share the investment and risks related to the ownership of a parabolic trough plant, as for example ACS Cobra, with the German firms Solanda Gmbh, Stadtwerke München, RWE Innogy, and RheinEnergie, or Dioxipe Solar with Elecnor and ABN Amro. Many project developers such as NextLight, Mohave Sun Power, Starwood Group, Bethel Energy, Inland Energy (USA) and Albiasa Solar (Spain) are not included in this table because they have not found yet partners to complete their value chain. Italy occupies a special position in the CSP sector: a group of Italian companies formed a consortium called Solare XXI to benefit from one another‗s core competencies to provide a full turnkey trough package with TES using molten salts as heat transfer fluid, with the help of the national research center (ENEA): Rondo Reflex provides the mirrors, Techint and Xeliox design the collector structure, Archimede Solar Energy develops the receiver tubes, Duplomatic provides the control system and Techint is the EPC contractor. A key part of the group‗s strategy is to develop technology for export into Middle East/North Africa and Asia Pacific regions [Emerging Energy, 2010]. Solare XXI has signed a 10 MW technology supply agreement with Indian project developer Entegra.

Country Technology

provider Technology integrator

Project development EPC Operation

Project Ownership

Australia Flagsol (Solar Millenium) Solar Millenium

Leighton Contractors Pty

Siemens CSP Ltd. ERM Power ERM Power

China Flagsol (Solar Millenium) Inner Mongolia Luneng New Energy Inner Mongolia Luneng New Energy

India Flagsol (Solar Millenium) Suryachakra

Power Suryachakra Power

Solare XXI Entegra Ltd. Entegra Ltd.

Italy ENEA Enel Enel

Solare XXI Techint

Abengoa Solar Italgest SpA

Xeliox srl FERA srl

Sorgenia

Algeria Abengoa Solar NEAL, World Bank Abener Sonatrach. Cofides, ONE

Egypt Solar Millenium NREA Orascom, Ferrostaal

Egyptian Electric Authority (EEA)

UAE Abengoa Solar Abengoa, Total Abener, Teyma Abengoa, Total Abengoa, Total,

Masdar

Spain Abengoa Solar

Abengoa Solar / Hyperion

Abener Abengoa Solar Abengoa, EON,

Hyperion

Aries Solar Termoeléctrica Dioxipe Solar Aries / Elecnor Dioxipe Solar Dioxipe

Ingemetal / SAMCA

SAMCA GEA21, TSK

Energía Grupo SAMCA

Flagsol (Solar Millenium)

Solar Millenium Ferrostaal, ACS

Cobra, Duro Felguera S.A

Solar Millenium ACS Cobra

Flagsol , Sener Flagsol, Sener,

ACS Cobra ACS Cobra

SolarMillenium, Sener

ACS Cobra

Acciona Energía Acciona

Infraestructuras Acciona Energía

Solel (Siemens) Fotowatio OHL Fotowatio Fotowatio, OHL

NextEra Energy NextEra Energy

Iberdrola Iberinco Iberdrola

Flagsol (Solar Millenium) Ibereólica Inveravante Ibereólica Ibereólica,

Inveravante

FCC

Abantia, Enerstar Técnicas Reunidas

Abantia, Enerstar

Magtel

Epuron, Conergy Epuron

USA Abengoa Solar

Siemens

Flagsol Solar Millenium MAN Ferrostaal Solar Trust America, Chevron Energy

Solutions

SkyFuel

Cogentrix Energy

Iberdrola Renovables

NextEra Worley Parsons Worley Parsons,

NextEra NextEra

FPL Lauren Lauren, FPL FPL

Acciona Solar Power Lauren Acciona Energía

2.8. Components Value Chain In this section we present the main components value chain of this technology.

2.8.1. Heat Transfer Fluids

The main providers of thermal oil for parabolic trough plants are the 3 US companies Dow Chemical (Dowtherm, Syltherm), Solutia (Therminol VP-1), and Radco (Xceltherm). With more than 80% of market share, Dowtherm is the most used HTF. However, most of developers are willing to try various products and also test Solutia‘s Therminol in some of their projects. Both synthetic oils have very similar thermal properties and freeze at 12ºC. Xceltherm from Radco has been used in Saguaro ORC parabolic trough solar plant, which is operating at lower temperatures (300 ºC).

2.8.2. Heat Collection Element

The absorber tube of the Heat Collection element is made of stainless steel. Steel top producing countries are the USA, Brazil, China, France, Germany, Italy, and Japan. Glass envelope of the HCE is made of borosilicate glass. Silica top producing countries are the USA, Slovenia, Austria, and Spain. The major manufacturer of borosilicate glass in India is Borosil. This is the single largest manufacturer of borosilicate components in the country, whose main customers are pharmaceutical sector and high school and colleges. Other providers are Glassco, Vensil and Glaseq. Up to now there are only two absorber tube manufacturers at commercial scale: SOLEL (bought by Siemens in 2009) and Schott. Their tube have very similar designs (same dimensions, comparable thermal properties), but differs for example in bellows, as it can be seen in Figure 35.

Figure 35. Differences between Siemens’ (Solel’s) (left) and Schott’s (right) tubes. Source: Solel. Supplier of the SEGS plants, Solel (Israel) is the historic tube manufacturer for CSP industry for the past 20 years. It will now benefit from Siemens investments and project pipeline (703 MW of projects in the US and Spain) to increase its manufacturing capacity (up to 600 MW by 2012) and decrease its production costs. Schott AG (Germany) main activities are glass, household appliances, optics, electronics, automotive, and since 2003, concentrated solar energy, taking on the market‗s leadership position in recent years. The company is very active in the R&D field, in particular on:

New selective coating to decrease emissivity at high temperatures (up to 550 ºC). Increased tube‘s dimensions to augment solar absorbance to 95%. Direct Steam Generation receiver tubes, with a steam HCE derived from its PTR 70 receiver as part of the

DIVA Project. Molten salts receiver tubes, in collaboration with Italy‗s ENEA, expected to be commercialized within three to

five years.

Aside these two major technology promoters, two other European companies are trying to get market shares in evacuated solar tube receivers:

Enertol Santana (Spain) is a component-development group formed by the association of the CSP developer Enertol and the auto manufacturer Santana. Its strategy is to lead R&D and manufacture a parabolic trough receiver tube based on experience gained from Luz.

Archimede Solar (subsidiary of Angelantoni Industrie, Italy, is a multisector technology company focusing on solar energy (PV and CSP), environmental testing, life sciences, and refrigeration. Its strategy is to develop and manufacture an advanced molten salt tube receiver able to reach higher temperatures than oil tubes (above 550 ºC). Archimede has been working with Italian research agency ENEA and is part of the consortium Solare XXI to deliver tubes for the 5 MW Priolo Gargallo project in Sicily.

2.8.3. Mirror Assembly

The most common option for the parabolic trough mirror assembly is thick glass-silver mirrors. The glass is curved to adopt the desired rigid parabolic shape, following a mirror curving technique specific to each company. Other options are thin glass mirrors (also called flexible mirrors), which adopt the shape given by its back structure, and polymer films that can be stuck to smooth curved surfaces. Technical glass specialist Flabeg (Nürnberg, Germany) is the main parabolic trough mirror supplier, having provided mirrors for Andasol and Nevada Solar One plants, with manufacturing facilities in Europe and in the US, and projects totaling 900 MW signed. Now it has to compete with newcomers in solar industry such as Rioglass, Saint-Gobain, Guardian, which has dedicated strong R&D efforts on minimizing glass weight and thickness without decreasing reflectivity. Like Flabeg, they are all coming from the automotive sector and all uses thick-glass mirrors. Saint-Gobain (France) and RioGlass (Spain) have built facilities to manufacture parabolic mirror, in Portugal for Saint-Gobain and in Spain and Arizona (USA) for Rioglass. Guardian Industries is supplying Sener in Spain and working on R&D projects with NREL. Benefiting from a partnership with Abengoa, Rioglass has started the manufacturing of tempered glass that is stronger, thinner, and capable of a competitive 95% reflectance. Abengoa is also working with NREL on advanced front surface polymeric reflectors. Ronda Reflex (Treviso, Italy) is a mirror composite specialist designed the parabolic mirrors for ENEA‗s and Enel‗s 5 MW CSP plant at Priolo Gargallo. It utilizes a proprietary sheet molding compound with adhesive polymers to fasten its thin glass mirrors to the trough‗s metal structure. Along with their dedicated receiver supply, Siemens has a joint venture agreement with Glaston (Finland) for 50 MW per year of parabolic mirrors. Aside from these companies, others developers have invested in research on mirror assemblies for CSP systems, such as the Spanish groups Alucoil and Zytech which included CSP components in its R&D activities, the German group Alanod is working with the DLR on high performance anodized aluminum mirrors, but also AGC Flat Glass, PPG Industries, Alcoa, Cardinal Glass, and 3M which are all working with NREL to test products in the laboratory and in the field. In the last years, the option of plastic reflectors, which are cost-competitive, has been considered. Each plastic or polymer has its own properties, depending on manufacturing process of the provider (DuPont, Hoechst…). ReflecTech‘s polymer multilayer film with a layer of pure silver has been specially developed to provide a good durability of the reflective surface. It is commercialized as ultra light weight low cost self adhesive rolls and can be applied on an aluminum substrate. The company has been testing its product at the SEGS plants, and its first commercial application will likely be with partner CSP developer SkyFuel, which has no announced projects to date. The industrial and consumer goods company 3M (based in St. Paul, Minnesota, USA) developed a protective front surface for polymeric mirrors and is working with Gossamer to deliver an integrated solution. Like 3M, Alcoa and Alcan are capable of potentially undercutting the current market leaders by developing polymer and aluminum solutions to reduce mirror costs by as much as 30% to 40% [EE].

2.8.4. Support Structure

Support structures of the parabolic trough collectors are made essentially of aluminum and galvanized steel, and their manufacturing requires not only material providers but also component assemblers. Aluminum top producing countries are China, Russia, Canada, and the USA.

One important metal provider having a long history of producing trough frames for the CSP industry is Hydro Aluminum (36000 employees, Oslo, Norway and Phoenix, USA). This extruded aluminum specialist, operating in transportation, industry, and energy, is the supplier of Acciona‘s CSP plants in Spain and in the USA (for example its partnership with Gossamer to provide aluminum tubing and components for Nevada Solar One). For each project, the frames will be sent to Hydro‘s plant in Mexico for fabrication and then to Spain for final assembly. Each frame is about 26 feet long and 12 feet high. Hydro is manufacturing the frame components, support beams, clips, connectors, and other parts at its Phoenix, Arizona, manufacturing facility [EE]. Alcoa (107000 employees, Pittsburgh, Pennsylvania, USA) has won the contract to supply aluminum fasteners (C50L30 Huck) to the Kuraymat ISCC plant in Egypt. This aluminum provider specialized in engineered space frames for aerospace, automotive, packaging, building and construction, commercial transportation, and industrial markets is now investing on R&D to develop its own aluminum-based parabolic trough collector with a grant from US DOE. Regarding structure assemblies, the main early technology promoters have their own design, developed thanks to the experience gained with previous models.

SGX-series collectors were developed by Solargenix, now Acciona Power (Sarriguren, Spain). It has a light spatial aluminum structure, resistant to oxidation and easy to mount, what results in lower installation costs. The Gossamer Space Frame connectors design used in Nevada Solar One‘s SGX-2 collectors reduced fasteners by as much as 80%.

Eurotrough collector has been developed by a European consortium formed by private companies (Inabensa, Fichtner Solar, Flabeg Solar, SBP, Iberdrola, Solel) and national laboratories (Ciemat, DLR, CRES). Its structure is a pre-galvanized steel frame work, based on a central torque box, which have low weight and reduced torsion and bending of the structure during operation resulting in increased optical performance and wind resistance. Solar Millennium‘s subsidiary Flagsol (Cologne, Germany) designed a commercial version of the Eurotrough collector, called Skal-ET, extensively tested at SEGS V and used in Andasol projects. Flagsol is now testing its Heliotrough, which is an upgrade to the current Skal-ET design. Solel also has its own parabolic trough design for its vertically integrated CSP plants concept.

Abengoa Solar (Seville, Spain) designed a proprietary parabolic trough called Astro for its own pipeline, in particular for its Solnova plants.

The engineering and aerospace group Sener (Bilbao, Spain) developed a parabolic trough collector called Senertrough, used in Extresol and Manchasol power plants.

Aside these CSP historic promoters, relative newcomers are designing their own support structure design:

The Spanish company Samca, in collaboration with Ingemetal, has designed a proprietary parabolic trough (SAMCA-trough), which will be deployed in SAMCA‘s pipeline. There is currently no indication that SAMCA and Ingemetal will provide their trough design to other CSP developers.

The turnkey solar developer Albiasa Solar (Bilbao, Spain) also designed a parabolic trough collector tested at the PSA. It claims to reduce mounting time by about 30%.

The Italian national research centre ENEA is now developing and testing a new collector named –ITE 01 (100 m long, 5.9 m wide) at its experimental facilities for the Italian companies Solare XXI y Heliox.

SkyFuel (Albuquerque, NM, USA) is developing an innovative collector called SkyTrough, using ReflechTech mirrors, Schott receiver tubes, and lightweight structures (30% less than other utility-scale troughs) with 40% reduction in structural components to reduce mounting costs [32]. The company has been demonstrating its technology in two test-loops at its research facility in Arvada, Colorado and at one of Cogentrix‘s SEGS plants.

Sopogy (Honolulu, Hawaii, USA) entered the CSP market as a micro-CSP technology provider, and is now directing the marketing of its technology to larger-scale systems. Its SopoNova system is a parabolic trough technology that uses aluminum backing instead of mirrors in the collector.

Sapa (Stockholm, Sweden) used the properties of extruded aluminum to offer a space frame for parabolic trough collectors that can be assembled quickly without welding.

2.9. Manufacturing Processes In this section we describe the manufacturing processes of the main relevant elements of this technology.

2.9.1. Heat Collection Elements

Heat Collection elements are evacuated tubes formed by a stainless steel absorber with a selective coating and a glass envelope with an anti-reflexive coating. In this chapter we will discuss which types of absorptive and anti-reflexive coatings can be used, how they are applied, and which process is used to ensure vacuum conditions under the glass envelope. Selective coating Schott and Solel are currently developing new selective coatings to increase the yield of their receiver tubes. Figure 36 shows the influence of the properties of the selective coating on the cost of the produced electricity.

Figure 36. Influence of receiver technology on cost of electricity [54]. (DOE parabolic trough 2005 reference power plant configuration, 100 MWe solar-only SEGS, 6-hours TES)

Amongst the numerous coating techniques, the ones that have been most successfully implemented to the CSP industry are PVD (Physical Vapor Deposition, or sputtering), CVD (Chemical Vapor Deposition), PECVD (Plasma Enhanced Chemical Vapor Deposition), and dip-coating processes [Carreras, 2003]. All these techniques can be used to obtain anti-reflexive, selective, and reflexive materials. Currently, the main HCE manufacturers, Schott and Siemens (Solel), use the sputtering technique (PVD) for their absorber tubes, whereas a sol-gel technique (dip-coating, allowing to coat simultaneously the inside and the outside of the tube) is usually employed for the anti-reflexive coating of the glass envelope [Bautista, 2003] [22]. Vacuum methods The main problem with selective coatings is they degrade in contact with air when they are hot, so they require a deep vacuum in the annulus between the inner metal tube and the glass envelope. The entire vacuum or evacuated tube used in solar thermal collectors must ensure the vacuum during the useful lifetime of the collector to keep the thermal performance of the collector from falling. The critical point, then, in its design and/or manufacture is how the glass/metal or glass/glass joint is solved to avoid loss of vacuum. So the selective surface is not degraded when the vacuum between the metal tube and the glass tube is formed, the ends of the glass tube are joined by a glass-to-metal seal to a bellow which is sealed at the other end to the inner metal tube. This way, an airtight annular space is achieved between the inner metal tube and the outer glass, at the same time the metal bellow compensates the difference in thermal expansion of the glass and metal tubes when they reach the working temperature. The glass metal seal is one of the most sensitive elements in the system, since it is subjected to strong thermal stress, and so to avoid this stress as much as possible, metals or alloys with low thermal expansion coefficients (CTE) near to those of borosilicate glass are used (Figure 37).

Figure 37. New design of Schott for glass-metal welding. Source: SCHOTT.

2.9.2. Curved Mirror Assemblies

The curving processes take an important part in the manufacturing cost of a parabolic trough collector. The reflexive element is usually silver or aluminum, which have reflectivities close to 95% and 90% respectively. The most common option for the parabolic trough mirror assembly is thick glass-silver mirrors, which does not require self-supporting substrate, and high benefits from a long on-field experience. The glass is curved to adopt the desired rigid parabolic shape, following a mirror curving technique specific to each company. Other options are thin glass mirrors (also called flexible mirrors), which adopt the shape given by its back self-supporting structure, front surface aluminized reflectors, and polymer films that can be stuck to smooth curved surfaces. Thick glass mirrors (3-5mm) The most widely used configuration to date consists of thick low-iron glass, hot bent to the rigid parabolic shape that gives the mirror its shape. This process is carried out by companies such as Flabeg, Riolass, and Saint Gobain, each of which has its own bending techniques. These mirrors are manufactured by applying a reflective layer of silver and a top coating to promote adhesion (generally copper) on a thick glass (>1 mm) by wet chemistry processes followed by a layer of protective paint on the back. Low-iron glass is normally used to minimize absorption that impacts directly on decreased transmittance and reflectance of the mirrors. It has excellent durability to corrosion of the protective layer, is available on the market and has the confidence of the solar industry, although it is heavy, fragile and its bending is difficult. This type of reflector can be degraded in three main ways, the silver/glass interface can become unattached, halides can form in the metal layers and the silver layer can become lumpy.

Figure 38. Thick glass mirror structure. [Cheryl Kennedy, 2003] [23] AGC Flat Glass and Flabeg have developed and patented lead and copper-free processes for thick mirror production, based on 4 mm soda-lime glass. Saint Gobain valued 3 mm copper-free glass and lead-free painting on a steel substrate for Gemasolar heliostats.

The most common method to give the desired curving to the parabolic trough mirrors is using annealed bent glass. This is the production system used by Flabeg for its reflectors: heating the glass, bending it, and annealing by slow cooling. Flabeg‗s approach has been to supply a more robust mirror for a plant‗s outer rows, while the inner rows are equipped with 3 mm mirrors. Using a semi-conservative approach, Rioglass Solar has introduced the solar reflectors manufactured out of tempered glass mirrors; this is to say with fast-cooling. This product is the mixture for two proven technologies, three-layer copper-based mirror performance and durability and glass increased mechanical resistance of tempered glass widely used in automotive and architecture. By tempering, glass mechanical resistance is largely increased up to five fold: mirrors made of tempered glass can withstand stresses in excess of 120 MPa, the same reflector manufactured out of annealed glass will fail when stresses rise above 35 MPa. It allows a dramatical glass breakage reduction and safer handling [Ubach, 2009] [24]. The difference between Flabeg and Rioglass methods can be seen in Figure 39.

Figure 39. Bending methods for parabolic trough collector facets: annealed glass (Flabeg – left) and tempered glass (Rioglass Solar- right)

Guardian has developed two types of mirrors for parabolic trough applications:

A monolithic parabolic mirror whose durability is ensured by a multi-layer backing paint system specifically designed for the challenges of the solar industry.

A laminated parabolic mirror (Figure 40) whose durability is ensured by a laminated glass assembly similar to an automobile windshield, where the mirror is encapsulated between two layers of glass and held together by a polyvinyl butyral interlayer.

Guardian has a fully integrated process to manufacture the entire product from the float glass to the finished parabolic assembly. Raw glass, forming, mirroring, lamination, manufacture of mounts and assembly are done within Guardian facilities.

Figure 40. Guardian’s laminated mirror (source: Guardian) The back coatings on the mirrors are done by wet chemistry processes where the clean glass is sensitivized with SnCl2, silver-coated by chemical reduction and painted on the back by different techniques finished off with a heat treatment. A copper-free process has been developed that eliminates the copper layer used to inhibit corrosion of the silver in manufacture of the mirrors. Thus manufacture of the mirrors does not generate polluting copper effluents. FLABEG converted its 4-5-mm mirror manufacturing line to a new low-lead coating process, which achieves equivalent durability in outdoor exposure. The back-coating manufacturers have developed new lead-free coatings that resist durability tests well, but are usually designed for indoor conditions. Acceptable durability under outdoor conditions of these new formulas is still pending demonstration by testing. Additional recommendations may be: add a protective coating of dense impermeable paint, cover the paint with some highly impermeable metal or inorganic coating, and edge-sealing strategies to keep humidity from getting in.

Under development processes for reflective surface After the early developments by Acurex in the 80s, several glass manufacturers, including Naugatuck (USA, acquired by FLABEG), AGC, and Ronda Relfex have been experimenting with thinner mirror prototypes (closer to 1 mm, see Figure 41), which are curved mechanically without heating (flexible mirrors). They are supposed to have a great durability, but the challenge facing these lighter-weight mirrors is their ability to withstand high desert winds and sandstorms [Global Concentrated Solar Power, 2010]. Ronda Reflex (Treviso, Italy) designed the parabolic mirrors for ENEA‗s 5 MW CSP plant at Priolo Gargallo, using a lime soda composite as the foundation for its float glass parabolic mirrors, which have a nominal thickness of 0.85 mm. Ronda Reflex utilizes a proprietary sheet molding compound with adhesive polymers to fasten the mirrors to the trough‗s metal structure [Carmichael] [26]

Figure 41. Thin glass mirror structure. [Cheryl Kennedy, 2003] [23] Transparent plastics are cheap, light, easily mirrored with either silver or aluminum, and therefore their use may be of interest for solar thermal power plants. Finally, emphasize that glass materials require an antireflective coating to reduce reflection losses in solar applications, while polymers may be antireflective in themselves due to their composition. However, although the properties can vary quite a lot from one type of plastic to another, in general, they degrade over time, lose transmissivity, yellow, and lose their optical and mechanical properties more quickly than glass, which is inorganic (and therefore in principle more stable). The aluminized or silvered film must be placed on a support or substrate to provide it with sufficient rigidity to support wind loads without significant deformation. Usually, this film-support is attached using polymer adhesives. The durability of these materials has been their main obstacle. Even though reflective polymers, usually derived from acrylate, are highly reflective, they also have very low resistance to abrasion, and are degraded by UV rays. To avoid these effects, manufacturers put additives or stabilizers in their composition [Allen, 2009] [17], which absorb part of the UV radiation and avoid or delay polymer degradation. On the other hand, the cost of the material for support and durability must be added to the cost of the reflective material, while glass/metal mirrors today are reasonably priced and the glass itself acts as the mirror support structure. The potential of reflective polymers is very high, but their properties and current costs compared to glass/metal mirrors make their future difficult to predict. Polymethyl methacrylate (PMMA) is a good candidate for solar applications due to its excellent optical and physical properties and economics. On the other hand, it degrades considerably when exposed to UV light or abrasion. Several laboratories and manufacturers have tried to mitigate this effect through the use of additives of different types. Furthermore, in recent years, there has been a boom in solar energy applications for photocatalytic processes, which involves solar rectors having to make better use of the UV band of the solar spectrum. This makes reflective materials other than aluminum useless for such applications. In 1998 a new polymer solar reflector was developed based on cooperation between NREL who joined their experience with ReflecTech This product has uniquely desirable properties for solar reflectors and incorporates UV protection that provides strong outdoor durability (Figure 42). In 1999 the first samples were tested in outdoor exposure tests and accelerated aging in weather chambers (WOM) at the NREL with less durability than expected.

Figure 42. Structure of ReflecTech polymeric film. [Cheryl Kennedy, 2003] [23]. ReflecTech built in 2001 a small manufacturing pilot plant for its polymeric film with an improved UV protection (Figure 42), and tested it at the ACUVEX test loop (Arizona, USA) and at the SEGS VI solar field (California, USA). Meanwhile 3M is investigating on all-polymeric reflectors, which could be available after 2015. Another low-cost option for the reflector material is the Advanced Solar Reflective Mirrors (ASRM). They are formed by a PET or steel substrate covered by a silver layer and protected by a microscopic alumina layer (Al2O3), applied under high-vacuum conditions by Ion beam Assisted Deposition (IBAD). The copper layer is needed to block UV radiation that could degrade the substrate (Figure 43).

Figure 43. Structure of an Advanced Solar Reflective Mirrors (ASRM). [Cheryl Kennedy, 2003] [23].

Solel developed an ASRM material with a polymeric substrate (PET), a metal or dielectric adhesion layer, a silver reflective layer and a patented protective overcoat (Figure 44) that showed low degradation rates compared to previous experiments with PET substrates.

Figure 44. Structure of the Advanced Solar Reflective Mirrors (ASRM) from Solel. [Cheryl Kennedy, 2003] [23].

2.9.3. Trough Support Structure

Trough support structures are mounted on-site by using specific jigs. Only Solargenix collectors have a different on-site mounting technique, according to the aluminum structure design which makes it possible to mount mirrors directly to the structure requiring no alignment in the field. The structure has to be designed to be able to support the intrinsic load of the components attached to it, as well as wind loads, including the solar tracking system on its longitudinal axis. There are two types of tracking system, electric and hydraulic, which allow the collector to track the Sun by rotating on its axis. The collector structure is built on a reinforced concrete foundation which anchors the collector structure to the ground. The material used up to now for its construction is metal, which provides the rigidity necessary for the set of components that make up the complete collector. Even though there is still no commercial application, other materials such as fiberglass, concrete and some plastics that could replace these metals in the future have been studied. Plate metal supports are usually made of polished aluminum and do not include reflective material. A big advantage is their low cost, although their outdoor durability is very poor because of degradation of the optical characteristics of the metal. This is why they are not usually used industrially. Among the functions of the structure, the most important is to maintain the parabolic shape of the concentrator mirror when it is mounted on its support frame. There are two main arrangements for this as described below. Torque tube structure The first two generations of collectors, LS-1 and LS-2, consisted of similar assemblies, mounted on a structure of similar length, but the aperture width of the LS-2 collector was twice that of the LS-1 collector. The structure is based on a rigid structural support tube, called the torque tube, which supports the steel profiles to which the parabolic mirrors are attached [60] (Figure 45). In this kind of solar collector assembly, the components are designed with high tolerances and the required precision is reached thanks to a precise canting process during the collector mounting.

Figure 45. Scheme of the LS-2 collector structure SenerTrough-I collector, developed by Sener, uses the torque-tube concept from LS-2 support structure. This high-torsional-stiffness cylindrical tube is made of steel sheet, and varies in thickness and quality depending on wind load requirements. Cantilever arms, made using metal-sheet stamping techniques, connect the mirrors to the central torque tube, thus reducing both manufacturing and erecting costs and total mass (about 30%) [65]. The next generation, SenerTrough-2, being designed in collaboration with key component suppliers (Flabeg Solar International and Schott), has a wider aperture and absorber tube diameter [68] and is made by direct stamping using a technique from the automotive industry [69].

Receiver Tube

Mirrors

Helicoidal pipe

Support

Mirrors support

Figure 46. SenerTrough stamped structure. [Castañeda, 2006] [65] Albiasa Solar (Spain) has also developed a new collector, called Albiasa Trough, tested at the PSA and now commercially available. The design concept is similar to the SenerTrough, but in this case the cylindrical torque tube is made of four 908-arc pieces (made of cold rolled galvanized-steel profiles) with half-T flaps assembled with screws. The arms are hot-formed galvanized steel [66]. This special torque tube concept provides a very robust closed section with improved torsional and flexural stiffness, and lowers manufacturing time and costs [70].

Figure 47. Albiasa Solar collector structure. Source: Albiasa The improved assembly procedure of the HelioTrough implements a new patent-registered method to connect the parabolic mirrors to the steel structure, with a three-dimensional clearance compensation. Steel structure tolerances can be loosened, and at the same time a high optical accuracy is achieved. Collector aperture width and overall length were increased (one HelioTrough collector is more than 50% larger than a SKALET collector) to reduce the aperture area specific number of expensive single components like drive units and swivel joints [Riffelmann, 2009]. Siemens CSP (formerly Solel Solar Systems) is also working on an advanced design, the Solel-6, based on the LS-3 dimensions, but with the torque tube structural approach [66].

Space frame structure In the LS-3, the torque tube is replaced by a metal lattice framework, the aperture width is 14% wider than the LS-2 and collector length is doubled. Changes were made in the pedestal and reflector supports, and the collectors are positioned by a hydraulic control system instead of the mechanical gear and cable system used in the LS-2. LS-3 collector design makes use not only of previous Luz power plant experience (SEGS-I to SEGS-VI), but also mass production, cost and performance requirements [65]. Eurotrough collector has been developed by a European consortium formed by private companies (Inabensa, Fichtner Solar, Flabeg Solar, SBP, Iberdrola, Solel) and national laboratories (Ciemat, DLR, CRES). Its structure is a pre-galvanized steel frame work, based on a central torque box, which have low weight and reduced torsion and bending of the structure during operation resulting in increased optical performance and wind resistance. Its design combines the LS-2 torque tube design benefits of torsional stiffness and alignment with the reduced cost of an LS-3-like truss design [66].

Figure 48. Scheme of the LS-3 collector structure

Figure 49. Eurotrough collector structure. [Lüpfert, 2000.][67] Other parabolic trough collectors with space frame structures are the SGX collectors from Solargenix (Acciona).

Figure 50. Solargenix’s SGX collector structure. Source: NREL.

Receiver tube

MirrorsSupport structure (tube)

Pedestal

Reductor wire Main support structure

The SGX1 and SGX2 collector are derived from the LS-2 collector, except they are twice as long. The main effort was invested in the lightweight space frame structure, which is made entirely of aluminum (whereas structures from other manufacturers are generally made of steel with galvanized layer) and is superior in terms of shipping, handling during manufacturing, field installation and corrosion resistance [65]. Gossamer Space Frames designed a space frame for Nevada Solar One that led to several major improvements over the previous design used in the 1 MW Saguaro facilities. The innovative new aluminum hubbing design uses 50% fewer pieces and 80% fewer fasteners and is 30% lighter than the previous standard. The system requires one-third less time to construct, and mirrors do not require alignment during assembly, thereby reducing installation times considerably [Gretchen Menand, ―NREL research helps delivery clean, solar electricity to thousands of homes in the southwest‖, 2006]. The aluminum structure provides better corrosion resistance and has been designed so that the mirrors are mounted directly to the structure and do not require any alignment in the field. SkyTrough is a new 6 m-wide collector manufactured by SkyFuel (USA) with dimensions similar to the LS-3, which differs mainly in the reflector, the ReflecTech silver-metalized polymer film. The reflective film is laminated onto curved aluminum panels which are assembled on site on an aluminum space frame with low manpower requirements (no welding is needed) [65]. The lightweight SkyTrough mirrors do not come from the factory with a pre-defined contour. Instead, they are flat reflector sheets which are assembled onto shape-forming ribs attached to the underlying space frame. The primary structure is a space frame, an efficient truss structure made from aluminum tubing with joints enabling rapid assembly. This design approach speeds field installation and enables compact transportation from the factory to the site. Indeed three complete SCA‘s (over 2000 m2 aperture area) including mirrors, space frames, receivers, pylons, drive units, etc, may be transported to the site using only two standard truck trailers. Installing the mirror sheets is rapid: A single panel providing 9 m2 aperture areas may be inserted and stiffeners attached in a few minutes [Brost, 2009].

3. POWER TOWER Technology In this section we will analyze power tower technology into detail. After a first general description, we will describe plant configurations and main components. Then we will name the projects (demonstration, commercial and in the pipe-line). Last, we will include as well an analysis of the value chain (plant and components) and manufacturing processes.

3.1. General Description A power tower system or central receiver system (CRS) uses mirrors called heliostats with two-axis sun-tracking to focus concentrated solar radiation on a receiver at the top of a tower. The receiver absorbs the concentrated radiation and transforms it into thermal energy of a working fluid. Three main technical designs can be distinguished depending on the working fluid used:

water/steam, molten salts, or air.

In the receiver, either steam is generated directly (commonly referred to as a water/steam or direct steam generation receiver), or a heat transfer medium (typically molten salt) is heated. CRS plants can have different solar field layouts, but the most common are: the “North field” systems (for a power plant located in the north hemisphere), where heliostats are located in the north of the tower, and the “surround field”, where heliostats surround the tower. (See Figure 51).

Figure 51. Layout of a North field (left) and surround field (right) [Mills D.R, Schramek, 2002] An advantage of central power towers over most other concentrating solar power (CSP) technologies is that the solar collection occurs at one receiver atop a central tower, so piping is not required throughout the solar field.

Figure 52. Scheme of a Molten Salt Power Tower

Due to high radiation fluxes reached in the receiver, it is possible to work at very high temperatures without significant thermal losses, what makes it possible to integrate this module in more efficient thermodynamic cycles. Parabolic trough technology enables to aspire to get high efficiencies in electricity generation, even higher than 25% from solar radiation to electricity. Thank to these high operation temperatures, it is easy to integrate hybrid operation in these power plants, as well as thermal storage, at a lower cost. Requirements to install power tower plants are very similar to those necessary for a parabolic trough solar thermal plant. However, in this case, the field slope admits angles up to around 3-4º, not being necessary to level it in cases with little inclination terrains.

3.2. Main Plant Configurations Many heat transfer and storage fluids have been investigated for more than 30 years. A very promising option is to use air as heat transfer fluid to power a gas turbine, but it is still at laboratory scale. In the following paragraphs we will detail the two main technologies which are currently used in commercial projects:

DSG and molten salts power towers.

3.2.1. Water/Steam Working Fluid

The most used CRS system throughout the years is the configuration that uses water/steam as heat transfer fluid (see Figure 53). The steam is produced (and sometimes superheated, getting higher efficiencies) at the receiver and directly sent to the turbine. Some complications can be experienced, inherent to the receiver and the superheated steam, whose deficient thermal properties lead to regulation problems. In order to reduce the impact on cloudy moments, a storage system can be included.

Figure 53. Process flow diagram of water/steam SCR plant with oil/rocks tank storage system. Figure 53 shows a process flow diagram of this scheme, including an oil/rock tank as a storage system, which was one of the first configurations tested along the years (in Solar One solar thermal power plant).

3.2.2. Molten Salts as Working Fluid

Power tower systems would use molten salt primarily because of its superior heat-transfer and energy-storage capabilities. A molten salt system schematic is illustrated in Figure 52. Sunlight is redirected onto a receiver atop a tower by two-axis tracking mirrors called heliostats. Cold salt is pumped from the cold salt storage tank through the receiver. In the receiver, the salt is heated and then pumped to the hot salt storage tank. From the hot salt storage tank, the hot salt is pumped to the steam generator to produce superheated steam which is directed to a turbine. The molten salt, cooled again, is routed back to the cold tank for storage until it is heated again in the receiver. A further advantage of the molten salt systems is that the working fluid (molten salt) is an excellent storage medium, allowing thermal storage to be built integrally into the system.

3.2.3. Sodium as Working Fluid

The design of a power tower solar plant with sodium as a working fluid is similar to the molten salts. Only demonstration facilities have been built with this plant configuration (SSPS in the PSA).

3.2.4. Air as Working Fluid

New designs bet for schemes including air as a working fluid using a volumetric receiver. These designs can apply either atmospheric air or pressurized air, reaching very high temperatures. However, these configurations have been only implemented in demonstration power plants and have not been yet commercially tested.

3.3. Main Components In the following table, the importance of the main components of a CR solar plant is qualitatively evaluated considering economic issues (according to breakdown in Activity 1.2) and technological issues (design / manufacturing / assembling complexity, criticality and improvement potential).

Subsystem Component Economic importance

Technological issues

Solar collection Support structures High Medium

Drive mechanisms High Medium

Mirrors Low Low

Thermal conversion Solar receiver High High

Natural Gas Boilers Low Medium

Mechanical system ( piping, salts pumps) Low Medium

Heat exchange fluid (molten salts) Low Low

Fire protection system Low Low

Inertization system Low Low

Thermal storage Molten salts forwarding skid Low Low

Initial filling system Low Low

Electrical conversion Power block High High

Salt/steam heat exchanger Medium Medium

Balance of plant (BOP) Medium Medium

Civil works activities of the EPC contractor (solar field foundations, land leveling, and tower) correspond to a high share investment costs and thus have a high economic importance. Solar field assembly activities have a medium economic importance. The CRS collector system reflects concentrated low-density solar energy from the heliostat field onto the receiver. The size of the heliostat area depends on the facility power requirements and is usually several square kilometers.

Figure 54 shows some of the most important heliostat designs and their sizes, which have been developed throughout the years.

A

B

C

D

E

F

Figure 54. Heliostat different designs throughout the years. a). SIREC (EU), 16 m2; b). Martín-Marietta (USA), 40 m2;

c). SENER (EU), 115.6 m2; d). Sanlúcar-140 (EU), 140 m2; e). ATS (USA) 146.9 m2; f). ASM-150 (EU)

The performance of a heliostat field is given by its optical efficiency, that is, the quotient of the power intercepted by the solar receiver surface and the direct solar energy incident on the mirror surface. This efficiency will depend largely on the quality of the solar field components. A present-generation heliostat consists basically of a reflective surface, a support structure, an azimuth and elevation drive mechanism, a pedestal, the foundation and a control system. There is two main types of heliostats:

the traditional heliostat made of facets the stretched-membrane heliostat (heliostats with a very thin metal surface stretched over a ring which is in

turn the support for the membrane itself). The front of the membrane is covered by a reflective film, usually based on deposited silver coating. The stretched-membrane technology, however, has been abandoned in recent years due to its high cost. Therefore, this description concentrates on traditional heliostats.

3.3.1. Mirrors

The reflective surface is the main component of the heliostat, the purpose of which is to reflect and concentrate the incident solar radiation onto the receiver. It is classically composed of a set of small-area facets (a few m2), held by a metal frame to which they are fastened and directed by the heliostat support structure. Facets are usually made of glass on which a reflective silver or aluminum coating is deposited with paint on the back to protect it. They usually have three or four anchor points by which they are fastened to the corresponding supports. Each facet is in itself able to reflect and concentrate the solar radiation, so the set of facets that make up the heliostat must be geometrically organized for their combined reflection and the heliostat behaves as a single optical system. This procedure is called heliostat alignment (or canting) and can be done according to different optical criteria depending on the distribution of irradiance desired. The alignment provides the heliostat‘s final focus. The heliostat optics thus admits a wide combination of parameters that combine to form its reflective surface: size, shape and facet focus; size, shape and focus of the heliostat derived from the arrangement and alignment of its facets.

3.3.2. Heliostat Local Control

Heliostat operability is entrusted to the control system, which is in charge of every basic task that guarantees the correct daily operation of the heliostat.

Figure 55. Heliostat local control. Source: CIEMAT. This control can be done in two different ways:

With a centralized system, based in a central computer and a communications‘ topology which enables the management of every task.

With a distributed control system, where local controller arrangement is emphasized, downloading then to the central computer most of the tasks, now managed by each heliostat (Figure 55).

In both cases there is a procedure to follow:

Sun position calculation Calculation of heliostat position Measurement of heliostat current position Heliostat position correction Communication management with each heliostat Heliostat operation modes Detection of errors and breakdowns Emergency actions and signaling

The position of all of the heliostats has to be corrected from time to time depending on the distance of each heliostat from the target it is aiming at to maintain a precision on the order of 1-2 mrad. For example, in the case of CESA-1, in the PSA, this time fluctuates between 5 and 15 seconds. To find the position of the heliostat, it has encoders, one per axis, that send a signal to the corresponding control, where it is compared to the signal it should have according to calculations. Depending on the result of the comparison, the drive motors receive the corresponding order to adjust the position. One possible configuration in a field control system is to use independent heliostats that calculate their own positions, known as local control. The other option is to use a central computer that controls the position of all of the heliostats in the field.

Figure 56. Main elements of en heliostat local control. Source: CIEMAT. The local control has to solve all of the heliostat actions described above and communications with the central computer by the corresponding protocols. A current trend is to attempt to increase heliostat intelligence more and more to increase its autonomy from the central control and eliminate wiring. This, at its highest level, is the stand-alone heliostat, design currently under testing at the PSA [García G, Egea A.]. This heliostat includes a photovoltaic system for the electrical supply, a radio modem to communicate with the control room and an anemometer, wind switch, light and temperature sensors, and self-protection routines for adverse external conditions.

3.3.3. Metallic Structure

The heliostat structure is the mechanical support for the reflective surface. This structure is usually made up of a latticework to hold the facets, which are screwed onto it. The lattices are fastened in turn to a horizontal cylindrical axis which, by a reduction mechanism, is connected to the heliostat drive system. Tracking movement is on two axes, azimuth and elevation, and this is moved by two drive motors which are later described. Up to know, two different support structures have been used:

T type structure, which has been the most used design since the 80s decade, and the spatial structure, developed fundamentally for big sized heliostat with stretched membrane design.

Figure 57 and Figure 58 show these designs.

Figure 57-58. T type support structure (GM-100) (left) and Spatial support structure (ASM-150) (right). [Kolb, 2007].

T type structure allows two variations: configurations that make it possible to turn down the heliostat completely, and configurations without this option (Figure 59 and Figure 60). This first case protects the heliostat better against soiling, but the mirror surface is penalized with a reduction of mirror area.

Figure 59-60. Opened T structure with complete turn down position (left) and Closed T structure without complete turn down position (right). [Kolb, 2007].

These T-shaped structures are made up of galvanized steel trusses and/or profiles, usually screwed to a large center tube. The spatial structure does not have a pedestal and rests on a circular concrete foundation on which the heliostat rotates on the horizon plane. These foundations, unlike T-shaped heliostats, have very demanding manufacturing tolerances, and heliostat leveling if out of line is complicated, so the correction must be made by algorithms implemented in the aiming control system

3.3.4. Pedestal

The pedestals also admit different configurations that go from different materials used (concrete, steel), that usually condition the fastening procedure, usually fitting the concrete pile into the foundation (Figure 61), and bolting it down (Figure 62). The fitted pedestal does not allow the heliostat to be leveled from the base of the pedestal if out of line, which is a drawback.

Figure 61-62. Concrete pile fitted into foundations (left) and Metallic pedestal with crews (right)

3.3.5. Drive Mechanism

The purpose of the drive mechanisms is to keep the sun‘s image reflected by the heliostat stationary in a certain place, classically the aperture of a solar receiver. There is usually two drive motors for this which rotate the surface on two axes, one zenith and the other azimuth, so the direction of the reflected beam verifies a certain vector equation [Winter C-J, Sizmann, 1991]. Technical differences between drive mechanisms developed, are basically the following:

Type of drive mechanism: mechanic, hydraulic. Horizontal mount condition: centralized and decentralized. Sensor type used for register orientation of mount axes: encoders (incremental, absolute), Hall Effect

sensors. Motors (AC, DC, steps) Sizes

Figure 63 and Figure 64 show two different types of drive mechanism (mechanic, hydraulic) for a same Sanlucar heliostat prototype. Figure 65 shows a system based in linear hydraulic drive mechanisms, implemented in the HELLAS heliostat.

63

64

65

Figure 63-64-65. Mechanic drive mechanisms (Sanlúcar heliostat) (left); Hydraulic drive mechanism (Sanlúcar heliostat) (centre);

and Hydraulic drive mechanism (HELLAS heliostat) (right). [Kolb, 2007]. Heliostat evolution has lead to bigger heliostat size designs. This fact has forced manufacturers to also evolve in the same direction. This has caused the re-adaptation and constant redesign of these mechanisms, some of which can be seen in Figure 66.

A B

C

D

Figure 66. Drive mechanisms evolution. A) CASA. Ralpe,1980 (40 m2); B) GM-100. Pujol, 1997 (105 m2);

C) SENER. PUJOL-SENER, 2005 (115 m2); D) GM-140. Winsmith, 2005 (140 m2) This mechanism is the most expensive heliostat component. That is why there has been a trend in heliostat technology development to larger-area designs that lower the specific price per heliostat. Nevertheless, a new trend has opened to the contrary; using very small size heliostats so a much simpler and thereby cheaper tracking drive mechanism can be used. This line of action is being adopted by companies like eSolar and Brightsource, with 1 m2 and 7 m2 heliostat mirrors, respectively.

3.3.6. Field Control

As it was already explained, heliostats have to correct their position continuously. The calculated error between the set point and the real position in each heliostat axis is called the offset or stationary error. At present in solar plants, the offset correction of heliostats is a task that is done periodically by a specialized operator aiming at an auxiliary target. Depending on the image, the operator changes the azimuth and elevation angles manually (in collector bits) to make the heliostat reflection coincide with the center of the target or auxiliary target

Figure 67. Image capture system for offsets correction.

3.3.7. Field Wiring

The installation has to have a wiring system for communications between the central control and each of the local heliostat control systems, and the electrical supply for the motors to work. This means laying many kilometers of cable all over the field. Stand-alone heliostats have the advantage that, as they are supplied by photovoltaic cells and communicated by radio modem, installation wiring is a priori unnecessary.

3.3.8. Receiver

The receiver is the component that receives the concentrated solar radiation and transforms it into thermal energy to be used in later processes (usually in the power block), which makes it an essential plant component. Most receivers have a solid black surface (high absorptivity) or some other mechanism that strengthens absorptivity in the solar range (for example, the cavity effect) as the absorber. The process of transferring the concentrated solar flux incident on the working fluid or material usually involves working at high temperatures (300 to 1200 ºC) and high incident fluxes (200 to 1200 kW/m2), depending on the heat transfer fluid selected. These operating conditions usually generate high material stress, which added to the cyclic nature of the solar energy and keeping in mind that solar thermal plants must be designed for a lifetime of 20-30 years, makes the receiver design for fatigue a crucial subject. The receiver is usually located at the top of a tower the purpose of which is to support its weight and keep it in the focus of the optical system it forms with the heliostat field. Receivers‘ technology has various challenges that condition the global design of a power tower plant:

Selection of the heat transfer fluid inside the receiver (it also conditions the selection of the receiver typology and absorber materials, as well as some limitations in operation nominal temperature and pressure)

Selection of receiver materials, which have to withstand high level of thermal stress and fatigue in a durable way, in order to achieve a life span estimated around 20-30 years.

Selection of nominal working temperature and pressure in the receiver (it conditions the receiver performance, the heat exchanger design and the thermodynamic cycle exergy).

Selection of the receiver’s configuration (cavity, exposed panel, flat receiver) that conditions the power plant optical configuration, as a consequence of its influence in the layout of the solar field and the tower height.

The selection criterion and pre-design of the receiver requires an overall analysis of the resulting plant to optimize the involvement of other associated components for each receiver selection and therefore must obey the same decision criteria as the overall plant (cost of electricity produced, perception of current technological risk, identification of development niche of economically feasible equipment, total electricity production, etc.).

Receiver type Receivers may be classified by different criteria. There are different construction configurations, with directional receivers that only accept radiation from a certain direction and circular receivers that accept radiation from any angle of origin around it. Thermal Energy can be absorbed by the heat transfer fluid through different basic processes, according to the receiver heat transfer design:

Tube receivers. The exchange is produced in tubes that receive the radiation on the outside, carry the energy through their walls and transmit it to the thermal fluid that circulates inside them. Within this type there are two different receivers, the cavity receiver and the external receiver. The tube receiver works like an indirect heat recovery exchanger.

Volumetric receiver. The thermal energy is transferred by convection to an air stream that goes through the free volume of a metal or ceramic absorber matrix. The volumetric receiver operates like a convective heat exchanger.

Particle receiver. The thermal exchange is performed by the use of fluids or flows of particles that receive the direct and immediately in their volume or surface. This type of receiver operates like a direct heat exchanger.

Main basic configurations of solar receivers can be seen in Figure 68.

A

B

C

D

Figure 68. Basic concepts of solar receivers used in power tower power plants. A) Cavity receive; B) Particle receiver;

C) External cylindrical receiver; D) Volumetric receiver. [Winter, 1991] Figure 69 shows estimated ranges of operation for different type of receivers. In general, tubular receivers enable high pressure operations (up to 125 bar) with temperatures up to 500 ºC. If working pressure is reduced below 30 bar, temperatures up to 650 ºC can be reached in external receivers and up to 1,000 ºC in cavity receivers. In the case of volumetric receivers, higher temperatures can be reached, above 1,300 ºC, but with a limited pressure of 30 bar.

Optional window

Solid particles

Input air

Optional window

Volumetric matrix

Figure 69. Pressure and temperature operation ranges for different solar receiver configurations. In addition to the type of receiver, the influence of the type of plant and the thermodynamic cycle on the type of receiver used and receiver operating conditions have to be considered. The tube receivers have been used with all the common working fluids: water-steam, sodium, molten salt and air. On the other hand, the volumetric receivers have always used air as the working fluid. The maximum working flux of the receivers is marked by the properties of the materials to be used and by the cooling capacity (mainly the liquid film coefficient) of the heat transfer fluid.

Future developments

External tubular

Cavity tubular

Volumetric

Tem

per

atu

re (º

C)

Pressure (bar)

In the following table peak fluxes achieved in each receiver type are indicated, in relationship with the heat transfer fluid used.

Solar receiver characteristics

Heat transfer fluid

Saturated steam

Overheated steam

Liquid sodium

Molten salts

Air (tubes)

Air (Volumetric)

Peak flux (kW/m2)

650 500 2.500 1.200 <200 1.000

Figure 70 shows some pictures of different receiver prototypes used and tested throughout the years.

A

B

C

D

Figure 70. a) External cylindrical tubular receiver; b) Cavity tubular receiver; c) Flat tubular receiver; d) Volumetric receiver.

3.3.9. Tower Civil Works

The power tower is a characteristic component of a central receiver plant. Its main function is to house the receiver and keep it at the height required by the plant characteristics. The tower transfers to the foundations the loads from the weight of the system, the wind and earthquakes. The power tower is the structure that supports the receiver loads, and its design, along with the receiver and the heliostat field, affects the optical and thermal losses of the solar thermal energy conversion. Moreover, it houses mechanical equipment, pumps, exchangers, piping and all of the associated electrical system inside it. The height of the tower mainly depends on the plant power rating and also the configuration of the receiver and the heat transfer fluid [Falcone P.K., 1986]. Towers used in circular fields are lower than those in north (or south) fields. However, a larger number of heliostats and larger area of land are necessary for circular fields than for the north

configuration. This is because of the greater optical efficiency of the north field, mainly because of the better cosine effect. Today, towers may be built of two different materials:

steel or reinforced concrete.

Metal towers used for power towers are very similar to communications towers, usually called self-supported towers and are made of steel profiles and angles screwed, bolted, riveted or welded together. Commercial power towers made of reinforced concrete (PS 10, PS 20 and Gemasolar) have used sliding formwork. Although both materials are suitable for use in the construction of the power tower, the final decision depends only on price, which grows exponentially with its height.

3.3.10. Heat Transfer Fluid

The working fluid that circulates through the central receiver, in the various demonstration projects carried out to date have mainly used four systems:

water-steam (superheated or saturated), liquid sodium, molten salt and air.

The use of water-steam as a heat transfer fluid is the option that has acquired the most experience as direct steam generation in the receiver (CESA-1, Solar One…). Even though the thermodynamic cycle with superheated steam is more efficient, the greater control problems derived from its use have led to use of saturated steam in the plants PS10 and PS20. The high pressures that the receiver tubes undergo due to the change of phase of the fluid make designs complicated. Molten salt used in power tower applications is nontoxic and very stable. Its low steam pressure at high temperatures and inert behavior with water facilitate the design of its components. Its high thermal conductivity, and at the same time, its high heat capacity, makes it a good compromise as receiver working fluid and storage fluid. The high thermal conductivity of sodium, in turn allows receivers to operate at solar fluxes of up to 2.5 MW/m², much higher than other fluids (0.5-0.65 MW/m² for water/steam and 1.2 MW/m² for molten salt). However, it is a highly reactive material and the costs associated with the safety system, mainly in the sodium/steam generator, are very high [Grasse W., Hertlein H.P., Winter C.J., (1991)."Thermal Solar Power Plants Experience". Berlin, pp 215-282. ISBN 3-540-18897-5]. The use of air is an option that offers good expectations given the high temperatures and fluxes that can be reached, as well as its acquisition for free directly from the environment. However, its properties as a heat transfer fluid (thermal conductivity, liquid film coefficient, etc.) make it a poor choice compared to the other fluids.

3.3.11. Piping, Valves and Spare Parts

All of the piping, valves, boiler, pumps (pumping the fluid to the top of the tower, and if not natural circulation for steam) and other additional facility components are housed in the tower. This is a big advantage compared to the PTC and Fresnel technologies, as they avoid construction of many km of piping, and as these components are inside the tower, their thermal losses are lower. The characteristics of many of these elements depend on the fluid. For example, piping is not designed the same for a water-steam tower as for a molten salt tower, since the operating pressures and temperatures are very different, and therefore, their thickness will vary. Similarly, the use of a pump in a water-steam facility is not the same as with salt, since the density of each of these fluids is different and the force necessary to pump it to the top of the tower is different. In the case of water-steam, it is even possible to eliminate the pumps by making use of natural convection, depending on the design of the installation. All these decisions affect the tower design, since the weight that will have to be supported by it will be different in each case.

3.4. Demonstration Projects Although power towers are a recent technology at commercial level, they benefit from 30 years of experimental and demonstration projects. The main ones are gathered in the next table.

Experimental and demonstration power tower projects.

Project Sponsoring Country

Power Output (MWe)

Heat transfer fluid Start of Operation

EURELIOS Italy 1.0 Steam 1981

SUNSHINE Japan 1.0 Steam 1981

Solar One United States 10.0 Steam 1982

CESA-1 Spain 1.0 Steam 1982

MSEE/Cat B United States 1.0 Molten salts 1983

THEMIS France 2.5 Molten salts 1984

SPP-5 Russia 5.0 Steam 1986

TSA Europe 1.0 Air 1993

Solar Two United States 10.0 Molten salts 1996

Weizmann Israel 0,5 Air 2001

Jülich Germany 1.5 Air 2009

CSIRO NSEC Australia 0.5 MWth - 2010

Dahan China 1.5 Water-steam 2011

Most of these experimental power tower projects were completed in the last decade and some of the demonstration plants were completely dismantled. Today only some demonstration plants (PSA and Themis) that began their operation in the eighties keep on developing experimental projects.

3.4.1. Plataforma Solar de Almería (PSA)

The CESA-I project, was promoted by the Spanish Ministry of Industry and Energy to demonstrate the feasibility of central receiver solar plants and enable the development of the necessary technology. The power tower concept used was implemented with 300 heliostat, a water/steam receiver working at 520ºC and 100 bar and a molten salts thermal storage system. Today, CESA-1 is used for development and testing of subsystems and components such as heliostats, solar receivers, thermal storage, etc. It is also used for other applications that require high photon concentrations on relatively large surfaces, such as in chemical or high-temperature processes, surface treatment of materials or astrophysics experiments. The tower measures 80 m high and is made up in reinforced concrete. It has three different levels:

At 45 m height, a cavity has been adapted for use as a solar furnace for materials testing. At 60 m, a cavity with a calorimetry test bed for pressurized volumetric receivers. At this moment, this cavity

houses the SOLGATE project infrastructure. At 80 m, the top of the tower, is located the 2.5-MW TSA volumetric receiver test facility.

3.4.2. Weizmann

Weizmann Institute has been recently used as test platform for saturated steam receivers and pressure air receivers for gas turbine. One of the most remarkable projects developed in this facility is the test of the DIAPR (Directly Irradiated Annular Pressurized Receiver) volumetric receiver. Recently, a 75 m2 secondary reflector has been incorporated to the installation. This concept, known as ―beam down‖ will be here evaluated for the first time.

3.4.3. Jülich

The main objective of this experimental power plant is to prove the high temperature air receiver technology (HiTRec) within a complete facility, at a small scale (1,5 MW). This plant operates according to the scheme followed in the PHOEBUS project, carried out in the PSA.

3.4.4. CSIRO-NSEC

The National Solar Energy Centre (NSEC), located in South Australia, is a part of the CSIRO Energy Centre. This facility aims to research and develop innovative applications in concentrating solar energy, in collaboration with Australian and international research centres.

3.5. Commercial Projects We present here Abengoa Solar Towers, Gemasolar, eSolar and Brightsource.

3.5.1. Abengoa Solar Towers

PS-10 (11 MWe, connected to the grid in 2007) and PS-20 (20 MWe, connected to the grid in 2009), promoted and built by Abengoa, are the first commercial CRS plants in operation the world. Before them, only demonstration projects had been executed. The technology used is saturated steam technology, which simplifies the receiver design and operation and allows the use of a 30 minute steam accumulator for thermal storage.

3.5.2. Gemasolar

Gemasolar, currently under construction, will be the first CRS commercial plant with molten salts technology. It is being promoted by Torresol in Spain, and Sener, a Spanish company, is the EPC contractor and the provider of the receiver, molten salt thermal storage, and heliostat technology. This plant will have a power of 17 MWe and the heliostats used will have an area of 115 m2 each. The plant will have 15 hours of thermal storage. The consortium has accurate knowledge of this project, having participated in some stages of it.

3.5.3. eSolar

An innovative design is the rectangular field distributed CR system, as proposed by eSolar, with 46 MWe modules consisting of 16 towers and subfields for each module. They make use of very small (1.4 m2 mirror area) heliostats, with the concept that mass production will overcome the economy of scale advantages of larger heliostats. The eSolar concept is being tested at their 5 MWe facility called Sierra Sun Power near Lancaster, California. No public information is available about the successes of that demonstration. This concept departs radically from conventional wisdom in CRS design. The cost effectiveness of the small heliostats, ability to control a large number of steam sources which may be several thousand feet away from the turbine, and the achievement of satisfactory system efficiency are key challenges. Furthermore, these systems would likely require phase-change thermal storage, an undeveloped technology.

Figure 71. Sierra Sun Power solar thermal power plant in Lancaster, California. Source: eSolar.

3.5.4. Brightsource

BrightSource Energy, Inc., developer of utility-scale solar thermal power plants has bet on overheated steam CRS technology. The Energy Development Center (SEDC), a pre-commercial demonstration facility located in the Negev desert (Israel), has been used since 2008 by Brightsource in order to test components, equipment, materials and operation strategies.

Figure 72. Satellite image of the SEDC demonstration solar power plant. [Elon Silberstein et al]. This demonstration power plant reproduces a small scale commercial CRS with water-steam technology, capable of generating a thermal power between 4 and 6 MW. The solar field consists of 1,640 small heliostats, making up a portion from a real surround field of a bigger size. (See Figure 72) The tower is 60 m height and the receiver is capable of producing overheated steam at 530 ºC and 130 bar [Elon Silberstein et al].

3.6. Project Pipeline The current project pipeline of Spain does not include new power tower solar plants in the register of the Spanish Ministry of Industry, apart from Gemasolar, still under construction. However, new power tower plants projects are considered in USA. The most remarkable are the following stated below:

Alpine Sun Tower, 92 MW, California, still under development. Crescent Dunes Solar Energy Project, 100 MW, Nevada, still under development. Ivanpah, 440 MW, California, still under development. New Mexico Sun Power, 92 MW, New Mexico, still under development. Rice Solar Energy Project, 150 MW, California, still under development.

3.7. Plant Value Chain The following Table shows the main actors of the power tower plants value chain, and the countries in which they are active. Abengoa is leader in Spain with its commercial plants PS10 and PS20, but eSolar is the most active worldwide. eSolar wants to become cost-competitive by using off-the-shelf components and outsourcing their fabrication to a variety of experienced thermal power players (like ABB, Babcock & Wilcox, Victory Energy) and local supplier in emerging markets rather than to vertically integrate. Internally, the company is focused on its computer algorithms used to control and operate the heliostats, and patents on prefabricated structural design for heliostats and receiver. eSolar still has to demonstrate its technology over an extended period, but has already reach agreements as an Original Equipment Manufacturer (OEM) through technology licensing with many companies worldwide such as NRG in the US, Penglai Electric in China and ACME in India [Emerging Energy, 2010]. BrightSource and SolarReserve have so far used a vertically integrated strategy (design, build, and operate) to demonstrate their technologies, but for further commercial projects outside investors and owners may step in [Emerging Energy, 2010]. For example, in May 2010, Alstom, a specialist in equipment and services for power generation, invested $55 million in BrightSource Energy Inc., with an equity stake that positions Alstom as one of the main shareholders in the company. The Spanish company Sener has made an alliance with Masdar from Abu Dhabi to form Torresol for the design, construction and commissioning of CSP plants will be in Southern Europe, Northern Africa, the Middle East, and the south east of the USA, where Torresol Energy will promote plants with central receiver tower system based on the experience from the projects in Spain and Abu Dhabi.

Country Technology

provider Technology integrator

Project development EPC Operation

Project Ownership

Australia Lloyd Energy Storage SMEC

Worley Parsons

China Chinese Academy of Sciences / National High-Tech research

eSolar Penglai Electric China Huadian Engineering

China Shaanxi Yulin Huayang New Energy

India eSolar Acme Energy Solutions Acme Energy Solutions

Jordan eSolar Millenium Energy Industries

Israel Aora

South Africa eSolar Clean Energy Systems

SolarReserve

Eskom Eskom

Spain Abengoa Solar Abener Abengoa Solar Abengoa Solar, EON

Sener Torresol Energy Sener, Amsa Torresol Energy

Sener / Masdar

USA BrightSource Industries Israel BrightSource Bechtel Bechtel, BrightSource

BrightSource Energy

eSolar eSolar, NRG Fluor NRG

United Technologies

SolarReserve US Renewables

Lockheed Martin

3.8. Components Value Chain In this section we describe the value chain for the most relevant components of this technology.

3.8.1. Heliostat Mirror Assembly

Amongst all structural options, the more common is the metallic structure with glass-silver mirrors, used in all CSP technologies. Central receiver technology promoters can rely on prefabricated flat glass producers, which are abundant and have excess capacity. The more involved in heliostat mirror production are Guardian, which is supplying the 15 MW Gemasolar project in Spain, and FLABEG, supplier of the 1-square-meter flat mirrors for eSolar‗s 5 MW Lancaster plant. Sanlúcar 120 heliostats of PS10 and PS20 plants are Abengoa‘s proprietary technology with mirrors from Cristalería Española (Saint Gobain).

3.8.2. Heliostat Metallic Structure

To minimize capital costs eSolar is using prefabricated stamped frames supplied by Art Precision (Hong Kong, China) and prefabricated pedestals molds manufactured by China Mfg (China). Sanlúcar 120 heliostats of PS10 and PS20 plants are made of hot galvanized steel structure from assembled by Abengoa‘s subsidiary Inabensa.

3.8.3. Tower Receiver

Most technology promoters design their receiver, like Abengoa Solar NT for PS10 and PS20 (in partnership with Aalborg Industries and Técnicas Reunidas), Grupo Sener for Gemasolar, Brigthsource Energy for Ivanpah. However, and SolarReserve for its own projects, as CSP central receiver technology overlaps well with boiler technology design, promoters use to outsource the manufacturing of the receiver to traditional power players with boiler manufacturing experience:

Pratt & Whitney will supply the SolarReserve systems in California and Nevada. Babcock Power (Danvers, MA, USA) is supplying BrightSource‘s 392 MW Ivanpah project. Victory Energy (Collinsville, OK, USA) and Babcock & Wilcox (Lynchburg, VA, USA) are supplying eSolar

with receivers for its California project. Babcock & Wilcox is a thermal coal boiler technologies specialist, re-engaged with the CSP sector through a supply agreement with eSolar after having initially participated in the Barstow, California, Solar One demonstration project during the early 1980s [EE].

3.8.4. Tower Civil Works

The design and construction of PS10 tower was subcontracted to ALTAC (Madrid, Spain), which is a company specialized in design and construction of high concrete structure.

Figure 73. Some of the preliminary proposal from ALTAC for PS10 tower design. [García Sobrinos, “Tower of Power”]

3.9. Manufacturing Processes In this section we describe the manufacturing processes for the most relevant components of this technology.

3.9.1. Heliostats

Current heliostat designs use precision molds and mandrels over which the mirrors are formed, to provide focusing of the beam. Current state-of-the-art heliostats are composed of large components that are assembled in the field. The system is bolted together from shippable components on-site and a large crane is needed for installation of both the pedestal and the heliostat structure. Handling and shipping of large mirror structures is difficult and inefficient. Handling and installation are limited by wind speeds and require large equipment such as cranes and large forklifts. [Davenport, 2009]. Conventional heliostats require a large foundation, approximately as deep as the pedestal is high above the ground. These foundations contain tons of concrete and reinforcing steel. [Davenport, 2009]. Once the foundation is placed, the pedestal is set in position using a crane, and after the heliostat structure is assembled it is lifted onto the pedestal using another crane [Davenport, 2009]. The development of heliostats shows clear evolution from the first prototypes, with a heavy rigid structure, glass second surface and reflective areas of around 40 m2 [Mavis, 1989], to the more recent, much lighter-weight, larger-area and lower-cost heliostats with a wide variety of reflective materials based on low-iron mirrors or polymers with front-silvered

surface [Romero, 1991]. Historically, two perfectly defined lines in the development of heliostats have been defined, always directed at lowering the cost without reducing performance.

The first line is directed at heliostat configurations based on facets and large areas of over 100 m2, with the corresponding reduction in price per sq. meter of structure, mechanisms, wiring and foundations. On the other hand, the larger the heliostat, the greater the optical problems are and difficulty in washing.

The second are heliostats made of new reflective materials such as stretched membranes (heliostats with a very thin metal surface stretched over a ring which in turn supports the membrane itself). The front of the membrane is covered by reflective films, usually based on deposited silver coatings.

Experience accumulated to date is limited to what may be called conventional heliostats, with glass mirrors which are described below. A detailed description of the first generation of heliostats is made in [75]. Inabensa assembled its Sanlucar 90 heliostat by fixing the mirror to a steel frame with steel nails on a facets jig table. The structure is made of hot dipped galvanized steel. Single-facet and multi-faceted stretched-membrane heliostats have been developed by different companies and consortia. ASM150 single-facet heliostat of SBP and the 170m2 multi-faceted heliostat of SAIC could be mentioned as representative examples. To avoid the high costs of construction and assembly associated with the facet designs and also achieve better optical quality, the stretched-membrane heliostat was conceived, similar to the stretched-membrane dish, in which thin metal membranes are stretched on drums to achieve the desired reflectivity. Using a small fan or pump, a slight vacuum is maintained in the concentrator plenum. Other facet and component structures and designs have applied fiberglass technology, as Ghersa for its the Heplas heliostat (38.5m2).

Figure 74. HEPLAS heliostat from Ghersa The most common method to curve the facets of a heliostat was developed by CIEMAT during the 90s. A flat mirror is placed on a curving table whose curvature can be adjusted to the specifications of the heliostat, and then the required bending is given by a vacuum system applied on the front side of the mirrors and finally the back sides of the facets are stuck to the metallic structure with silicon.

Figure 75. Curving table for heliostat facets. In the SAIC design, the Glass-Reinforced Concrete (GRC) casting is made on low-cost, low-precision tooling using automated equipment. After the GRC casting is cured and any shrinkage or curing deformation has occurred, the mirrors are attached using a simple flat tool that provides accurate mirror placement. No focusing of the mirrors is

needed since the heliostats themselves are small relative to the receiver. The concrete azimuth track for our heliostat is also cast in a factory, in a mold that causes it to be round and flat and which incorporates components of the azimuth drive mechanism (e.g., molded-in ―gear teeth‖). [Davenport, 2009].

3.9.2. Tower Civil Works and Materials

The tower of a central receiver system can be made either of steel or of reinforced concrete. The election between both materials is mainly due to the tower height: steel is usually employed for tower smaller than 120 m, whereas concrete is used for taller structures [76]. Metallic tower The metal towers used for power towers plants are very similar to those used for communications. They are called self-supported towers and are usually made of steel profiles and trusses screwed, bolted, riveted or welded together. The models may be optimized for proper functioning of the structure, where the profiles and braces vary in size and thickness depending on the height of the tower and where it is going to be built. The main design factor is the weight of the receiver system and its configuration, although other factors such as wind speed must also be taken into consideration. Concrete tower Concrete top producing countries are Germany, Italy, Japan, the USA, and Canada. Concrete is a material with excellent compressive strength, however, it is too weak to support traction force. Although in the case of a power tower CSP plant, it is mainly going to support compression generated by the weight of the components it has to house, the effect of wind or earthquakes can be drastic. Therefore, reinforced concrete is used for construction. The steel bars that go through the concrete give it enough resistance to traction to become a very suitable material for large civil construction work. Although the properties of steel and of reinforced concrete are both perfectly suitable for use in the construction of the central receiver tower, there is one property in concrete that can make it more suitable depending on the location of the plant: its resistance to corrosion. This property makes concrete very important in remote areas near coastal environments [77]. The methods used for construction of a concrete tower lead to different options. One of the methods used to build these towers, used for example in some air turbine towers, is to divide the tower in levels of precast concrete, which are delivered to the plant and assembled by crane [78]. However, the most commonly used method in recent years for construction of concrete towers is sliding formwork, which has been used in PSº10 (120 m), PSº20 (160 m) and Gemasolar (140 m) towers. This technique is frequently used for construction of vertical or horizontal structures with constant or very similar section, allowing the same formwork to be reused as the building grows in height or length. The formwork is made in a certain shape on top of the foundations, thus preparing the shape desired for the structure to be built. A scaffold structure ensures that this profile is maintained at all times as the formwork is moved [79]. The formwork forms a mold where steel reinforcement bars are placed and then the concrete is poured in. The peculiarity of this method is that the formwork is moved without having to disassemble it as the tower is built upward from the base. Traditionally, the rebars are moved using hoists, cranes or jacks. Latest designs are usually assisted by a hydraulic jacking system [80]. There has been long experience with this technique. One of the examples is the Toronto communications tower (CN Tower), which is 553 m high and was erected at a speed of 6 m a day [81] and the Barclays Bank building in the large Canary Wharf Business Center in London [82]. The PS10 tower has a constant thickness, which made it relatively easy to erect using sliding formwork techniques. Design issues (receiver, boiler) and meteorological conditions (temperatures higher than 35 ‖C) made that 9 months were necessary to build the tower, but the lessons learned allowed the PS 20 tower (with a design very similar to PS 10) to be built in only 45 days.

Figure 76. View of the towers of PS 10 and PS 20 solar plants. Sliding formwork was also used to build the Gemasolar tower, as it can be seen in Figure 77.

Figure 77. Left, evolution of the erection of the Gemasolar tower with the sliding casing technique. Right, same tower, 2 days after. [Burgaleta, 2009]

Brightsouce and eSolar (47 m) went for for steel structures (Figure 78). eSolar towers are very similar to wind turbines masts.

Figure 78. Construction of the towers of BrightSource Energy (left) and eSolar (right) demonstration plants..

4. DISH-ENGINE Technology In this section we will analyze dish-engine technology into detail. After a first general description, we will describe plant configurations and main components. Then we will name the projects (demonstration, commercial and in the pipe-line). Last, we will include as well an analysis of the value chain (plant and components) and manufacturing processes.

4.1. General Description The dish/engine is unique among CSP systems in using mechanical energy rather than a working fluid in order to produce electricity. Dish engine systems consist of a mirrored dish that collects and concentrates sunlight onto a receiver mounted at the focal point of the dish. The receiver is integrated into a high-efficiency engine (the Stirling engine is the most common type of heat engine used due to its high efficiency). Solar parabolic dish engine systems comprise two main parts:

a large parabolic dish and a power conversion unit (PCU).

The PCU is held at the focal point of the concentrator dish and includes a receiver as well as a heat engine and generator assembly for converting the collected thermal energy to electricity. Typically, a high efficiency Stirling engine is used. Individual units range in size from 10 to 25 kW and are self-contained, with no cooling water requirement. An inherent issue with dish Stirling systems is that they shut down operation immediately upon loss of sun. In that respect, they are similar to solar photovoltaic plants. Up to now, no concept for thermal storage has been implemented for dish engine systems.

Figure 79. SES SunCatcher dish Stirling design. Compared to the other CSP technology, dish-engine systems suffers from higher investment costs, lack of existing storage and hybridization solutions. However, they benefit from the next features:

Central or decentralized operation Operation range between 5 kW and several 100 MW High energy density, low land use Short construction time

The Stirling engine technology offers three major advantages over other thermal steam technologies:

water usage is limited to operation and maintenance activities (e.g., mirror washing); it has attained efficiencies as high as 30% at Sandia Laboratories; and its modularity allows for a range of system sizes, from several megawatts to hundreds of megawatts.

Contrary to the other CSP technologies, dish–engines do not need water availability for cooling and proximity of grid connection.

However, dish Stirling technology still faces the overarching challenge of delivering a commercially viable installation for financial and utility verification.

Main pre-commercial dish-Stirling systems [Nepveu, F, 2008]

Sundish SunCatcher WGA Eurodish AZ-TH PowerDish Sunmachine

Type Stretched membrane

Parabola with facet Parabola Parabola

Parabola with facet Parabola Parabola

Aperture 113 m2 88 m2 41 m2 57 m2 56 m2 14,7 m2 15-17 m2

Diameter 15 m 10,5 m 8,8 m 8,5 m - 4,2 m 4-5 m

Reflectivity 95% 91% 94% 94% 94% - -

Focal length 12 m 7,45 m 5,45 m 4,5 m 5 m - -

Concentration ratio Max

2500 7500 11000 12700 - - -

Receiver diameter

0,38 m 0,2 m 0,19 m 0,19 m 0,19 m - -

Engine STM 4-120 4-95

Kockums V161 Solo

V161 Solo

V161 Solo Infinia Sunmachine

Working fluid H2 H2 H2/He H2/He H2/He - Nitrogen

Working temperature

720 ºC 720 ºC 650 ºC 650 ºC 650 ºC - -

Promoter SAIC SES WGA SBP Abengoa Infinia Sunmachine

Year 1999 1998 1999 2001 2007 2007 2007

Design power 22 kW 25 kW 11/8 kW 11 kW 11 kW 3 kW 3 kW

Max. efficiency 23% 29% 24% 23% 23% 24% 20-25%

4.2. Main Plant Configurations Dish Stirling Systems can be installed either individually or in little/big clusters. This technology features a modular arrangement: up to now Stirling motors currently used for solar applications do not exceed 25 kW power in best cases. This characteristic makes it possible to implement the technology in decentralized or off-grid power generation, for particular use, as well as in big centralized power farms. Stirling is an external combustion engine that makes it possible to incorporate different external heat sources like solar energy, biomass or fuel burn. This makes it possible to configure its operation with hybridization too.

4.3. Main Components Dish Stirling technology consists of a parabolic shaped concentrator and a Stirling engine located at the concentrator‘s focal point. This concentrator dish has a two axis tracking system and concentrates the solar radiation in a heat exchanger (receiver). The receiver acts as a connection between concentrator and power unit. Thermal energy is absorbed by the receiver, converted into mechanical energy by the Stirling engine, and finally into electricity through a generator.

In the following table, the importance of the main components of a PD solar plant is qualitatively evaluated considering economic issues (according to breakdown in Activity 1.2) and technological issues (design / manufacturing / assembling complexity, criticality and improvement potential).

Subsystem Component Economic importance Technological issues

Solar collection Support structures High High

Drive mechanisms Medium Low

Mirrors Low Medium

Plant infrastructure Electric infrastructure Medium Low

Lighting, safety, and lightning conductor Low Low

Thermal and electrical conversion

Receiver-engine High High

Balance of plant (BOP) High Low

In addition to component manufacturing, some assembly activities (mechanical and electrical equipment) that are not mentioned in the able above have a medium economic importance. In the following sections the main components of this technology are described.

4.3.1. Dish

Many different dish Stirling designs have been developed in the latest 20 years. Figure 80 shows some of these examples: some dishes were designed with the only purpose of being used in dish Stirling systems, but other designs with lower performance were used in farms as a centralized power unit. The evolution experienced by dish engine systems is remarkable.

Figure 80. Historic evolution of parabolic dish prototypes. [Winter, 1991].

4.3.2. Receiver

The receiver is the element that absorbs concentrated solar energy and converts it to thermal energy that heats the working fluid (gas) inside the engine. These receivers usually adopt the cavity geometric configuration, with a small aperture and its own isolation system. In order to carry out this energy transformation, it is necessary to reach a high temperature and high levels of incident radiation fluxes [Gener, A. ―Tesis doctoral‖]. It is then really important to minimize every possible loss. Many different configurations of receivers have been proposed, adapted to different heat transfer fluids. These configurations can be gathered in two main groups:

Direct Interchange Receiver (DIR): fluid absorbs the radiation being directly exposed to it. Indirect interchange receivers: there is an additional element which transforms solar radiation into heat and

then delivers it to the heat transfer fluid through convection. This thermal interchange can be carried out through any of the basic processes explained next:

Directly illuminated tubes: radiation reaches the outside of the tube panels and goes through its walls transferring the heat through conduction. Then it is once more transferred to the heat transfer fluid through convection (Figure 82). This configuration can be implemented either with cavity or external geometry.

Volumetric: radiation is converted into heat and transferred to the air trough convection, while crossing the volume of a metallic absorber or a porous ceramic.

Reflux: these kinds of fluids receive the radiation directly on their volume or surface and use an intermediate heat transfer fluid. This group includes a new technology called ―heat pipe‖ which has offered new expectations in the solar receivers.

Figure 81 shows a directly illuminated tubes receiver, which is the most current design used for dish Stirling today.

Figure 81. Directly iluminated tubes receiver. The receiver is the most critical element of the system. Working temperatures and solar radiation fluxes are so high that materials suffer from high thermal gradients and stress. This fact can be dangerous for the reliability and durability of these systems. It is necessary to design the system receiver-engine together with the system concentrator-receiver, to take in mind possible repercussions of the power block control and the optical/geometrical design. Apart from the current options, there are some emerging concepts which can be highlighted. One of these concepts is the heat pipe. In a heat pipe a liquid metal is vaporized (normally sodium) by the heat taken from the absorber and later it condensates on the surface of the tubes where the working fluid is flowing through, flowing once again to the absorber.

4.3.3. Curved Mirror Assemblies

The concentrator dish is made up of a parabolic shape reflector, which concentrates the incident solar irradiation into a receiver located at the dish focal point. The ideal shape of the concentrator is a paraboloid of revolution, even if some designs approach this shape by using multiple spherical mirrors. The dish can be made up either of individual reflector surfaces or of a continuous surface. Because of the parabolic shape and the low ratio focal length-diameter (f/D=0.6), what is needed for keeping the optimal concentrator within a limit angle of 45º, it can reach mean concentrating ratios higher than 2000. This avoids operation temperatures up to 800ºC, leading to efficiencies up to 40% of the Stirling engine. Reflectors used in concentrators consist of a glass or plastic substrate with a thin aluminum or silver layer deposited over it. The most durable material known up to know is the current silver/glass thick mirror, which reaches reflectivity values around 91-94% [Solar Dish Engine]. However silvered polymer solar reflectors (thin mirror) are finding increasing use in dish concentrator applications [John Harrison, 2001]. An innovative trend towards a new concept that would avoid better optical efficiencies was introduced in the 90s: the stretched membrane mirror, implemented in the SBP design. The size of the parabolic dish is mainly determined by two factors:

Thermal power demand of the power block (Stirling engine) in nominal conditions. Wind loads: they restrict the economical viability of big size installations.

Even if most of current designs have a mirrored surface smaller than 100 m2 (See table in Section 4.1), there are also some prototypes with much bigger sizes. An example of this is the new design of the Big Dish, recently built by the Australian National University (ANU), which covers a mirror surface of 500 m2 [K. Lovegroove, 2010].

Figure 82. ANU dish Stirling prototype, with a mirror surface of 500m2. [K. Lovegroove, 2010]. Historically, two perfectly defined lines of development of dish/Stirling concentrators have been developed, always directed at achieving the lowest cost without reducing performance.

The first line, the most common, is directed at configurations based on facets and large surfaces of over 100 m2, with the corresponding reductions in price per sq. meter of structure, mechanisms, wiring and foundations. On the other hand, the larger the size of the dishes, the greater is the optical problems and washing problems.

The second is an innovation in the design of the concentrator introduced in the 90s which consists of the use of a stretched-membrane over a supporting ring.

In the first case, the concave surface is covered by facets that form the reflective part of the concentrator, and which can be of different shapes, such as triangular wedge or spherical, and sizes, commonly made of glass on which a reflective layer of silver or aluminum is deposited and a layer of paint on the back to protect it. They normally have three or four anchor points where they are fastened to the corresponding supports. These facets have two construction variations: On one hand, in designs such as the MDAC/SES (Figure 84) and WGA ADD in which the mirrors are flat and acquire their curvature by being glued to a mold which is part of the support (Figure 84), and on the other hand designs like Abengoa Solar which use curved mirrors (Figure 84).

Different dish/Stirling system designs have been studied and tested, the Stirling Energy Systems design being the best known. This design is based on the McDonnell Douglas Aerospace Corp. (MDAC) design patented in the eighties [Lopez, C., and Stone, 1993] [Gener, A. ―Tesis doctoral], [Lopez, C., and Stone, 1993].

Figure 83. SES dish Stirling design, based on MDAC design [Morrison, 2006]. Other designs have been manufactured with metallic structures and glass-metal reflector. Among them, WGA ADD and Abengoa Solar designs can be remarked (Figure 84).

Figure 84. WGA ADD design (left) and Abengoa Solar design( right). Another model developed for dish Stirling systems is the stretched membrane design. In this system, a very thin metallic surface is stretched from a ring that withstands at the same time the own membrane. The front side of the membrane is covered by reflector films, normally deposited silver. The space in the back side is closed by another membrane, and a partial vacuum is made inside this space, that makes the reflector membrane adopt a parabolic shape.

Figure 85. SBP Stretched membrane dish Stirling design.

4.3.4. Dish Support Structure

There are many varied structure concepts, for which metal is the most used, although some dish/Stirling prototypes have a fiberglass structure (Figure 86). One of the most common is that the concentrator structure is made of different lattices that serve as a support for the facets, which are screwed onto it. These lattices are fastened to an axis (cylindrical or quadrangular) which is attached by a reduction mechanism to the system that moves the concentrator. Of all the structural options, the most common is the metal structure with glass/silver mirrors.

Figure 86. Eurodish design, at the Plataforma Solar de Almería (PSA). [T. Keck, et al., 2001]. It should be kept in mind that solar concentrators are optical components and that the higher the optical quality, the higher the concentration is. This quality has to be maintained during operating conditions, regardless of wind and the position the collector is in at the moment. (Maximum wind conditions for operation depend on the design but they are in the 36-65 km/h range). Furthermore, the structure has to have a survival position where it is able to protect itself and keep the collector qualities in tact at high wind speeds (120-160 km/h), so the collector can be used normally again after having survived adverse weather conditions. It is important to remark the influence of the structure on the final optical quality of the concentrator, since the structure itself ―deforms‖ the optical concentrator in such a way that it ―copies‖ structural errors.

4.3.5. Heat Transfer Fluid

The working fluid that this system uses is a compressible fluid, usually helium or hydrogen which works in a closed loop, and therefore does not emit any pollution at all.

Hydrogen has better cycle efficiency. This is because hydrogen has a higher specific heat and its pressure losses from friction are minimized by its lower viscosity. However it is rather more dangerous to handle.

On the other hand, helium has the advantages of being an inert gas and having fewer leaks due to its higher viscosity [Gener, A. ―Tesis doctoral‖].

4.3.6. Dish Drive Mechanisms

Since the concentrator always needs to be perfectly oriented towards the sun, it is mounted on a two-axial tracking system. Therefore a movable construction standing on several wheels is developed making it possible to carry out the sun tracking. Both the horizontal and the vertical orientation of the concentrator are done by a small servomotor. The orientation towards the sun is either determined by a sun tracking sensor, or by a special computer program which predicts the position of the sun.

Figure 87. Drive mechanism of the Eurodish design. design [ S.B.u Partner., 2002] [55]

4.3.7. Stirling Engine

The Stirling cycle is the most efficient thermodynamic cycle to transform heat into mechanical or electrical energy. It was invented by the Scottish Rev. Robert and in the 19th century already thousands of engines of this type had already been in use. The working principle of this engine is easy to understand. In the Stirling engine a constant amount of working gas (helium or hydrogen) is constantly heated and cooled. Due to expansion when heated and contraction when cooled, the working gas sets two pistons in motion, which both are connected to a crankshaft, and thus delivers energy. Since the efficiency of the Stirling engine increases with increasing upper process temperature, this engine is the ideal combination to produce energy with a solar collector. Stirling engine efficiency can be over 40%, and annual efficiency of parabolic dish – Stirling engine has been proved to be around 24-25%. Due to the flexibility of the heat source, a Stirling engine can also be operated with a hybrid receiver. This means that with an additionally installed burner, the required heat can also be generated with fossil fuels (Bio-gas etc.).Thus the system is also available during cloudy periods and during night-time [Schlaig Bergermann und Partner, 2001]. Nowadays there are only some Stirling engine designs applied to parabolic dish in the market. In the World, only the following models have been included in a parabolic dish system:

Kinematic engine SOLO 161 of 10 kW. Kinematic engine Kockums 4-95 of 25 kW (formerly United Stirling). Kinematic engine Stirling Termal Motors STM 4-120 of 25 kW. Free piston engine Infinia of 3 kW.

These engines were already being manufactured for different purposes before entering the market of solar applications. Examples of their previous commercial applications are the Swedish submarines that use Stirling engines for propulsion [Brett, C. ―The 4 - 95 Stirling Engine For Underwater Application‖. Energy Conversion Engineering Conference, 1990. IECEC-90. Proceedings of the 25th Intersociety. Volume 5, Issue , 12-17 Aug 1990, Page(s):530 – 533], and cogeneration machines [Wei Dong; Lucentini, M.; Nasp, V. ―The potential market analysis of a small cogeneration system based on Stirling cycle‖. Energy Conversion Engineering Conference and Exhibit, (IECEC) 35th Intersociety. Volume 1, Issue , 2000 Page(s):719 - 722 vol.1. 2000.]. Recently, in June 2008, SOLO STIRLING GmbH went bankrupt due to lack of liquidity. The Swiss company, Stirling Systems AG, bought it along with all of its rights and patents, so this company now has two models, one 1-kW engine of its own and the one it acquired that has already disappeared, the SOLO 9-kW engine. STM Power Inc. also closed at the beginning of 2007, but in July 2007 reappeared under the name Stirling Biopower. Unfortunately, this company is no longer developing one of its models, the STM 4-120, an alpha-type Stirling engine able to vary the outlet power. In spite of the technology‘s many years of development, the Stirling motor industry is unstable and still fighting to become commercially feasible. Another company, Sunpower, Inc., also manufactures engines from 42 W to 1000 W. It recently presented an ingenious configuration combining a four-cylinder free-piston alpha engine with a gas turbine in the final stage. This company has long experience in free-piston technology and in cryocoolers. Another company, Infinia, has developed an engine which, unlike the rest of the Stirling engines, whose power is extracted kinetically by a crankshaft, the pistons are not connected by crankshafts or any similar mechanism. In this case, they are bounced forward and backward by means of compress gas springs (the same as the working fluid) and its power is extracted by means of a linear alternator or a pump. Figure 88 shows the inner structure of a SOLO 161 Stirling motor with each of its components, as well as a picture of the whole Stirling power block

Figure 88. Stirling SOLO 161 engine scheme (left) SES Stirling power block (right). Conventional Stirling engines present some problems:

Hydrogen losses due to high pressures and to difficulty of getting a perfectly sealed system inside the machine.

Limited lubrication: it is impossible to lubricate the piston, because the oil would be mixed with the gas, getting to problems inside absorber tubes. There is an oil pump that injects oil directly to the crankshaft.

In order to be capable of controlling the engine power, there is a need of a complex valve system which regulates it in function of the injected gas. The control of these valves has been problematic, usual fails makes them stay opened or closed and they ware out.

Gas storage

Compressor cylinder

Gas cooling

Regenerator

Crankcase

Receiver

Expansion cylinder

4.4. Demonstration Projects Up to now every dish Stirling plant currently in operation could be considered as a demonstration plant since the technology is still not wholly developed; that explains their limited power capacities. However, as they deliver their electricity generated to the grid and sell it, they are considered commercial plants. Many different dish engine configurations have been developed and tested throughout the years (see Figure 89). However, every experience was carried out in individual or small clusters prototype units. The Plataforma Solar de Almería has the most complete facility for testing and evaluation of dish/Stirling systems. From 1991 to 1995 durability of three SBP dishes was tested under the DISTAL I Project. These dishes accumulated over 30,000 hours (world record), with solar-only operation. With the experience acquired in the DISTAL I Project, in 1997, the DISTAL II Project was begun, consisting of another three 10-kWe units manufactured by the German Steinmüller company, which were an improvement over the previous ones as their operation was completely automatic. These six units configure the most important dish/Stirling facility, both in the number of dishes and accumulated operating hours. Both the Distal I and Distal II facilities are based on the stretched membrane concept and have a round-table-type structure similar to the ASM-150 heliostat described before.

Figure 89. SBP Distall I and Distall II dish designs. The Spanish-german project EUROdish is the latest attempt of dish Stiling technology to be tested in the PSA. The EuroDish is an innovative 10 kW dish/Stirling system for decentralized power generation, developed between 1998 and 2001. Currently the EnviroDish project is under way with the objective to prepare for market introduction of the EuroDish system.

4.5. Commercial Projects Nowadays this technology is still in its early development phase, and no big capacity dish-engine plants are in operation or under construction. The two existing commercial plants have been erected recently, and their small size is a sign of the high technological risk associated to this technology. However plant projects with big capacity are announced.

Figure 90. SunCatcher design incorporated in Maricopa power plant(left)

and Infinia dish Stirling design (right). Source: SunCatcher & Infinia.

4.5.1. Villarobledo

The first commercial experience in Spain involving dish Stirling technology is located in Villarobledo (Albacete). Renovalia Energy and Infinia have promoted together this technology in this first 1 MWe solar thermal power plant. More than 300 dish units including a 3 kW Infinia free piston engine generate electricity in the first phase of a project that aims to reach 150 MW installed.

4.5.2. Maricopa Solar (SES/Tessera)

Maricopa Solar power plant has been recently put into operation. This solar thermal power plant, located in Peoria (Arizona), has an installed capacity of 1.5 MW implemented with dish Stirling technology. Tessera Solar has promoted this project constituted with 60 SunCatcher units, developed by Arizona Stirling Energy Systems (SES). Each unit includes a 25 kW Stirling engine, which uses a 38-foot mirrored parabolic dish combined with an automatic tracking system to collect and focus solar energy onto a Stirling engine to convert the solar thermal energy into grid-quality electricity. Tessera has planned the construction of 400 MW installed power within different projects during to be built in 2011.

Figure 91. Maricopa Solar power plant in Arizona (EEUU).

4.6. Project Pipeline The Project pipeline of dish Stirling power plants in Spain is also modest. Two more projects are considered in the register of the Spanish Ministry of Industry apart from the already in operation 1.5 MW Villarobledo power plant. A 70 MW power plant is planned in Ciudad Real, and a small 1 MW installation is also expected to be built in Albacete. The Project pipeline in USA expects also 2 projects to be developed. Calico-Solar One, of 850 MW and Imperial Solar Valley, of 750 MW are both located in California and still under development. They have a common promoter, Tessera Solar, and they will incorporate de dish Stirling unit designed by SES.

4.7. Plant Value Chain The Table below shows the three main players of the dish engine plants value chain. Up to now they are only active in Spain and the USA [Emerging Energy, 2010]:

Stirling Energy Systems (SES) is positioned to supply product and must soon deliver a proven technology, aside from project development issues. SES and its project development sister company Tessera Solar want to develop a long-term project pipeline (beginning with a 29 MW plant in Texas, USA, with CPS Energy) and turn on its own manufacturing supply chain.

Infinia Corp. (Kennewick, Washington, USA) is also positioned as a technology provider and integrator, which has been following a similar strategy to SES‗s in building out manufacturing capacity. Stirling intends to

enter the utility-scale power sector with its 3 kW free piston system design, which may be better suited for distributed systems, after originally targeting the smaller-scale distributed generation segment. Infinia opened an office in India in 2008 and plans to begin manufacturing its 3 kW machines in 2010.

Abengoa Solar developed and tested its own dish concentrator, and proved is ability to act over the whole value chain with other technology, but no commercial project is announced so far, maybe due to the shortage of Stirling engine manufacturer. Actually Abengoa is now developing its own 25 kW Stirling engine.

Main actors of the dish engine solar plants value chain [Emerging Energy, 2010]

Country Technology

provider Technology integrator

Project development EPC Operation

Project Ownership

Spain Abengoa Solar Abener (Abengoa)

Abengoa Solar

USA Stirling Energy Systems Tessera Solar RMT. Morteneson

Stirling Energy Systems

Stirling Energy Systems, NTR

Infinia

4.8. Components Value Chain In this section we detail the value chain of the main components in this technology.

4.8.1. Stirling Engine

Currently there are a very few Stirling Engine manufacturers in the world. This could be a bottleneck for the technology. Stirling Energy Systems (SES) has chosen Linamar to assembly their Stirling engine, and its subsidiary McLaren to perform engine design testing.

Name Sundish SunCatcher WGA Eurodish AZ-TH PowerDish Sunmachine

Promoter SAIC SES WGA SBP Abengoa Infinia Sunmachine

Design power 22 kW 25 kW 11/8 kW 11 kW 11 kW 3 kW 3 kW

Engine STM 4-120 4-95

Kockums V161 Solo V161 Solo V161 Solo Infinia Sunmachine

Infinia developed its own 3 kW free-piston Stirling engine. Although Abengoa Solar used Solo 10 kW V161 engines for its demonstration plant AZ-TH, the company is now developing its own 25 kW engine for new advanced dish-Stirling systems, with the help of various national technology centers.

4.8.2. Dish Mirror Assembly

After having subcontracted Paneltec for its demonstration systems at Sandia, Stirling Energy Systems (SES) chose Tower Automotive as its facet provider for its new commercial projects.

4.8.3. Dish Structure

Stirling Energy Systems (SES) has chosen Schuff Steel as its frame supplier.

4.9. Manufacturing Processes In this section we describe the manufacturing processes for the most relevant components of this technology.

4.9.1. Curved Mirror Assemblies

Concerning dish engine systems, thin mirrors were applied on compound materials by the German company SBP and on stretched membranes by Steinmüller and SAIC. Composite Materials / Mirror Structures One of the preferred options was the use of compound materials made of fiberglass and polymers resins. The Eurodish concentrator is a parabolic dish composed by 12 triangular facets made of 0.8 mm mirrors stuck on fiberglass (Figure 92).

A

B

C

D

Figure 92. Mounting of the Eurodish system. [Partner, S.B.u., 2002] [55] Stretched membrane Instead of using a polymer or film as the reflective material, a thin mirror (0.8 mm) is glued on and its curvature is formed by adapting it to the stainless steel stretched membrane, as for example in the concentrator DISTAL II constructed by welding strips of 1 m, or parables designed by the SAIC company (Figure 93). The front membrane is shaped by hydropneumatic molding, without the need of a negative mold to achieve the parabolic shape (Figure 93). In operation, the membrane is stabilized by a slight negative pressure of 20 to 50 mbar. Once shaped, 50 x 30-cm facets of 0.9-mm thin glass are glued to the surface.

Figure 93. Stretched membrane manufacturing

Figure 94. SAIC parabolic dish design. Source: Internet. At the current stage of development, stretched membrane systems require very precise mounting processes made by skilled workers, which made them very expensive.

4.9.2. Dish Support Structure

Stirling Energy Systems concentrator dish is based on a design from MDAC patented in the 80s, with the following structural details:

The Stirling engine support structure is part of the dish support structure. The dish structure is made of five triangulated vertical structures. Each mirror is attached to the structure at three points. Each section of the horizontal arm is an integral part of the vertical triangulated structures. The mirrors are glued directly to a mold which is what gives the curvature to the whole.

Other designs have been based on metal structures with glass metal concentrator mirrors. Among them, it can be selected the ADD WGA and Abengoa Solar designs. It is worth noting that WGA ADD and Eurodish designs employ identical triangular facets, whereas MDAC/SES and Abengoa Solar facets have different curvature depending on their place in the parabola. The later designs differ one from other in the curving method: Abengoa uses pre-curved mirrors while MDAC/SES mirrors get their curvature when being stuck to the support structure.

5. POWER ISLAND We present in this section the main components of a Power Island and their value chain.

5.1. Main Components The components that make up the power block in a solar thermal power plant are equivalent to the components of conventional thermal power plants. In this section the components are listed and those components that derive basically from the cyclical operation of solar power plants are described in detail. The power block is common to the first three solar thermal technologies explained in this document (linear Fresnel, parabolic-trough, and power tower), except the heat exchangers and fire protection system specific to thermal oil are not included in the DSG technology. Components in the power island have been divided into two main groups, according to their functional characteristics.

In a first group, the power block has been considered. This category includes every component which closes the thermodynamic cycle, what implies elements operating at high pressures and temperatures, demanded by the cycle.

In a second group, the balance of plant (BOP) has been considered. In this group, every auxiliary additional element necessary to the correct operation of the power block has been included.

5.1.1. Power Block

As explained, the power block is composed by the elements listed below:

Steam turbine, condenser and generator Low and high pressure preheaters (HP, LP) Steam generator (heat exchanger) Pumps Deaerator and supply tank

Other additional elements needed like pipes, valves and isolation to close this subsystem would also be included. Except for the steam generator, the primary fluid of which is oil instead of combustion smoke gases as in the conventional boilers, the systems included in the power block of solar thermal power plants are no different from those of a conventional thermal power plant. However, the incorporation of a Rankine cycle in a solar thermal power plant has a series of difficulties as a consequence of the cyclic nature of solar energy. An attempt is made to minimize transients through the use of thermal storage (see section 6) and the external support of a boiler, but even so, daily stoppage is the way they usually work due to the physical and legislative limitations on gas consumption. Therefore, it is important to keep in mind a series of additional considerations, both in the design of the equipment and during operation of the plant for its proper operation. That is the reason why a suitable design of a steam turbine adapted to solar applications should fulfill the following conditions:

Since the plant is not going to be operating 24 hours a day, it is necessary to reach high efficiencies for returns to be sufficient to make it economically feasible. This condition is essentially imposed on the steam turbine used in the process. Therefore, it is important for the turbine used to be high-speed and high-pressure to allow a certain power output and a reduction in the size of the solar field, which translates into a reduction in investment costs and therefore of the power generated.

The thermodynamic cycle could also include a reheat stage, especially, depending on the quality of steam with which it is going to operate. This could improve the efficiency and reduce problems of erosion, corrosion and humidity.

The annual plant production is also going to be affected by turbine startup time, due to the cyclical nature of the solar radiation. Both the daily cyclicity and the variations in temperature require special attention. One important characteristic of the turbine is the mass of its components. Optimizing the mass of machine rotors and cladding can shorten start-up time.

Another important factor, especially for plants that do not include storage, is the turbine technical minimum. This will also determine to a certain extent the number of plant operating hours. By optimizing the technical minimum as low as possible power generation hours can be gained, although it is penalized by reduced efficiency of the turbine at part loads.

With experience acquired throughout the years in installation and operation of solar thermal power plants, more and more manufacturers are adapting their equipments to the operational exigent conditions of these facilities, achieving better efficiencies and reducing O&M costs. Maintenance labors deserve a special attention as well. Some of these maintenance labors considered during operation of solar thermal power plants are critical, like the ones listed below:

Vibration in both steam turbine casing and bearings are also critical, since successive start-ups generate thermal expansion in the turbine components, with possibility of fissures that are detected by this instrumentation.

Checking the main components, such as turbine and main feed pumps, should be done keeping in mind the total number of operating hours, and equivalent operating hours each time it is stopped. This factor in turbine behavior is equally important to degradation.

The water-steam cycle valves will be continuously subjected to high differential pressures. Their maintenance is important, since it can lead to other phenomena such as malfunctioning of control.

Check heat exchangers, since they are also subjected to cyclical loads. One solution adapted to these technical inconveniences is the SST-700 DRH of Siemens, capable of generating up to 175 MW, or the 19 MW SST-600 model, also Siemens‘. In order to create these turbines, Siemens cooperated with some EPC companies specialized in solar thermal plants projects [Siemens, 2008]. The first model mentioned has been used in many solar thermal power plants of parabolic trough technology, and the second one is going to be soon implemented in the power block of Gemasolar, of power tower technology.

5.1.2. Balance of Plant (BoP)

The balance of plant includes every other additional element not integrated in the power block. This group involves each component not directly needed to close the cycle itself, but also necessary for the correct operation of the power island. These elements have no specific operation conditions compared to conventional thermal power plants, since they are not exposed to high pressure and temperature cycles. Elements included in BOP are the following:

Cooling towers Pumps of the cooling system Atmospheric and pressurized tanks Pressurized air Utility systems (water treatment, effluent treatment, nitrogen skid, chemical sampling equipment…) Fire protection system (Fire protection system is directly related to the use of synthetic oil as heat transfer

fluid, so this system could also be considered as a part of the fluid handling system instead of the balance of plant)

Instrumentation and control equipments And other additional elements needed like piping, valves and isolation would also be included. As told before, there are no specific requirements for this equipment included in the BOP. However, there are some aspects to remark related to the water treatment, demineralization and chemical sampling equipment. These components must be also designed considering daily stops. Osmosis processes are not designed, in principle, for operation with continuous stops. The main problem derived from this fact is that, during stops, air enters and contaminates the steam blocked in the circuit, raising its conductivity at values higher than permitted. This leads to two main consequences: it is necessary to control the water/steam quality and it is also important that the turbine can withstand higher conductivity values at precise moments (start-up).

5.2. Components Value Chain Power island suppliers are receiving much less attention for their contributions to the solar CSP industry, because much of their components are off-the-shelf technologies.

Multinational steam turbine and BOP players used to be cautious with solar concentrated technologies, considered of high technological risk. However, since the re-birth of CSP plants in Spain, many of them have been adding CSP to their business development strategies. As a result, competition for contracts has accelerated dramatically, with new steam turbine, cooling tower, and control systems suppliers stepping in to meet the growing demand. The first multinational power block player to enter the CSP market was Siemens (Munich, Germany), which has now a nearly monopolistic position on steam turbine supply for CSP plant. Indeed the company secured as many as 45 orders for steam turbines in Spain, the US, and North Africa [EE]. Since acquiring Alstom‗s industrial turbine portfolio (3 MW to 100 MW in size) in 2003, Siemens has been aggressive in capturing business. Its SST turbines are manufactured in Sweden:

SST-600 is used at Gemasolar 19 MW power tower. SST-700RH is used at Andasol 1, Nevada Solar One, and other small- to mid-sized plants. Siemens has fine-

tuned the ST-700RH for solar steam cycles, and it is capable of generating up to 175 MW. A SST-900 turbine has been ordered by BrightSource for its Ivanpah 123 MW central receiver plant, to be delivered in 2011.

Hassi R‘Mel ISCC power block is formed by Siemens turbines: two 40 MW SGT-800 gas turbines and one 80 MW SST-900 steam turbine. In the meantime, other suppliers such as MAN Turbo, Ormat, Alstom and GE are looking to get in CSP technologies:

Full-service compressor and turbine supplier MAN Turbo (Augsburg, Germany) was selected to supply steam turbines for Solar Millennium‗s Andasol 3 project. MAN Turbo has also reportedly been selected as the turbine supplier to the Shams 1 plant in United Arab Emirates, although developer selection is pending.

Geothermal and recovered heat specialist Ormat Technologies (Reno, Nevada, USA, and Israel) supplied the power island (steam turbine and generator, heat exchangers, cooling towers, instrumentation and control equipments) for 1 MW Saguaro plant, in 2007. Unlike its turbine competitors, Ormat has joined French company Orsol as a minority partner in bidding for the currently outstanding Israeli CSP tender for the Negev Desert. While this strategy matches its geothermal activities, this represents a first for a steam turbine supplier in the CSP sector [EE].

In October 2007, power generation player Alstom (Levallois-Perret, France) was awarded by Abengoa a US$234 million contract on behalf of Morocco‗s Office National de l‗Electricité (ONE) to supply two GT13E2 gas turbine generators, one steam turbine generator, and three air-cooled turbo generators to the 470 MW ISCC Aïn Beni Mathar power project.

General Electric Oil & Gas (Florence, Italy) has to date refrained from the CSP sector because its turbine models do not match up to CSP‗s demand. However, GE‗s affiliate Thermodyn (France) has delivered two steam turbines to Abengoa Solar for Abengoa‗s central receiver plants PS10 and PS 20. Moreover, gas turbine for Solar Millenium‘s Kuraymat ISCC plant is expected to be from GE, and further moves of GE are expected in the sector.

The steam turbine manufacturers mentioned above are gathered in the following table.

Power island steam turbine and generator components

Regarding the others components of the power island (pumps, filters, heat exchangers, tanks, pressurized equipments, utility systems, boilers, pressurized air, fire protection system, electrical equipments, instrumentation and control equipments, piping, valves, insulations and supports), a lot of traditional suppliers for conventional thermal power plants are involved in CSP industry, as it can be seen in the following tables. Amongst them, the Swiss company ABB signed

Power Island

component

Company

Name WEB Country City Primary Business

Alstom www.alstom.com France Levallois-Perret Power generation

General

Electric Co.

www.ge.com Italy Florence Power technology and solutions

Ormat

Technologies

www.ormat.com USA and IsraelReno, Nevada Geothermal and recovered heat

MAN Turbo www.mandieselturbo.com Germany Oberhausen Full-service compressor and turbine

technology

Steam turbine and

generator

corporate agreements with many CSP promoters, and for example was contracted to design, supply, transport, assembly and commission the Balance Of Plant (BOP) of Hassi R‘Mel ISCC facility. The main provider of components for the BOP of CSP plants are shown in the following tables.

Power island heat exchangers suppliers

Power Island

component

Company

Name WEB Country City Primary Business

Foster

Wheeler

www.fwc.com Spain Las Rozas, Madrid Engineering design and construction

services, energy equipment

GEA Group www.geagroup.com Germany Bochum Power technology and solutions

Stork www.fastech.com Spain Malaga Power technology and solutions

SPIG www.spig-int.com Italy Arona Air Cooled Condenser manufacturer

Atepisa www.atepisa.es Spain Madrid Consulting and Basic Engiennering

services

Talleres MAC www.talleresmac.com Spain Miranda de Ebro,

Burgos

Industrial boilers

SPX Cooling

Technologies

www.spxcooling.com USA Charlotte, North

Carolina

Power generation solutions

B.A.C.

(MOVISAF)

www.baltimoreaircoil.com Belgium Heist-op-den-

Berg

Heat exchangers

Soljet

(Thermax)

www.soljet.com Spain Madrid Power generation applications

(traditional, cogeneration, and

renewables)

Hamon

(Esindus)

www.esindus.es Spain Madrid Design, manufacture and installation

of thermal and mechanical systems

Alfa Laval www.alfalaval.com Sweden Lund Heat exchangers

Graver www.graver.es Spain Bilbao Design and supply of process plants ,

Industrial water treatment

Holtec

International

www.holtecinternational.com USA Marlton, New

Jersey

Power generation solutions

Mecet www.mecet.net Spain Bilbao Energy improvments in power plants

Sedical www.sedical.com Spain San Sebastián de

los Reyes, Madrid

Chillers and heat pumps

Viessmann www.viessmann.es Spain Pinto, Madrid Efficient systems for all fuel types

and applications

Heat exchangers

Power island boilers and electrical equipments suppliers

Power Island

component

Company

Name WEB Country City Primary Business

GTS Energy www.gtenergy.net USA Atlanta Thermal fluid heater systems

including gas and oil thermal fluid

heating

PolyComp www.polycomp.cz Czech RepublicPoděbrady VIII Products for operating steam-boiler

plants

Standard Sky N/A Canada Calgary, Alberta Boilers

Cerney www.cerney.es Spain Zaragoza Design and production of industrial

boilers

Teyvi www.teyvi.es Spain Paterna, Valencia Steam boilers, thermal fluid, hot

water and superheated, and auxiliary

equipment.

Aalborg

Industries

www.aalborg-industries.com Denmark Aalborg Steam boilers, Engineering

Caterpillar www.cat.com USA Peoria, Illinois Construction and mining equipment,

diesel and natural gas engines,

industrial gas turbines

Koncar www.koncar.hr Croatia Zagreb Energy and transportation

Circutor www.circutor.es Spain Viladecavalls,

Barcelona

Design and manufacture of electrical

energy efficiency equipment

Ansaldo www.ansaldo-sts.com Italy Milan Traffic management, planning, train

control and signalling systems and

services

Landis & Gyr www.landisgyr.com Spain Seville Integrated solutions for energy

management

Schneider

Electric

www.schneiderelectric.es Spain Madrid Energy management, with solutions

for power and control

Boilers

Electrical

equipments

Power island instrumentation and control equipments and mounting,

fire protection systems and pressurized air suppliers

Power Island

component

Company

Name WEB Country City Primary Business

Honeywell www.honeywell.com Spain Madrid Manufacturer of civil and military

avionics and other aerospace

products, integrator and also service

provider.

Campbell

Scientific

Spain

www.campbellsci.com Spain Barcelona Data acquisition and control products

WIKA www.wika.es Spain Alcalá de

Henares, Madrid

Pressure, Temperature level

measurement

Krohne www.krohne.com Germany Duisburg Process instrumentation

Emerson

Electric

www.emerson.com USA Chanhassen,

Minnesota

Control systems

Meisa www.meisa-e.com Spain Argamasilla de

Calatrava, Ciudadl

Real

Electrical and instrumentation

assembly and maintenance

ATC-Control www.atc-control.com Spain Madrid Systems and instruments for

measurement and control

Endress+Haus

er

www.endress.com Spain Sant Just Desvern Process control systems for industrial

measurement and automation

technology.

Crespo y

Blasco

N/A Spain Madrid Control systems

Sirsa www.sirsa.es USA Newport, Rhode

Island

Air Quality, Instrumentation and

Engineering

Instrumentation

and control

mounting

Oinse www.oinse.net Spain Huelva Electrical and instrumentation

mounting

Fire protection

system

Comin www.comin.es Spain Seville Fire protection facilities

Pressurized air Compair www.compair.com Spain Pinto, Madrid Compressors

Instrumentation

and control

equipments

Power island pumps and filters suppliers

Power Island

component

Company

Name WEB Country City Primary Business

Friatec www.friatec.com Germany Mannheim Pumps and valves

Emica www.emicabombas.es Spain Gallarta, Vizcaya Pumps and valves

Ruhrpumpen

GmbH

www.ruhrpumpen.com Germany Witten Process services in petrochemical

industrie and power plants

Imo Pump www.imo-pump.com USA Monroe Pumps and valves

Varisco www.varisco.it Italy Padova Pumps and valves

Sulzer www.sulzer.com Spain Madrid Industrial machinery and equipment

Grundfos www.grundfos.es Spain Algete, Madrid Pumps and pumping systems,

electric motors and electronics

Sterling www.sterlingsihi.com Spain Madrid Liquid, gas, and vapour handling

SPP Pumps www.spppumps.com France Château d´Eau Engineered Pumping Solutions

Weir www.weir.com France Vendin LeVieil Minerals, Oil & Gas and Power

KSB www.ksb.com Germany Frankenthal Pumps and valves

PFS Pumps www.pfspumps.com France Buchelay Pumps and valves

Hidrafilter www.hidrafilter.com Spain Las Rozas, Madrid Filters

Pumps and filters

Power island tank and pressurized equipments and utility systems suppliers

Power Island

component

Company

Name WEB Country City Primary Business

Calprisa www.calprisa.com Spain Almendralejo Roofing, cladding, metal structures,

paints and coatings, tanks, works

Koch Heat www.kochheattransfer.com Italy Bagnolo

Cremasco

Heat exchangers

Talleres Vaca www.talleresvaca.com Spain Badajoz Metallic structures and boilers

Talleres

Lombo

N/A Spain Heras, Cantabria Pressure Vessels, Storage Tanks,

Assembly

Sugimat www.sugimat.com Spain Qart de Poblet,

Valencia

Design and construction of

installations for energy saving

Wedeco www.wedeco.com Spain San Sebastián de

los Reyes, Madrid

Water purification, disinfection and

ozone oxidation systems

Ondeo www.ondeo-is.com Spain Bilbao Optimisation and global management

of the industrial water cycle.

Regasa N/A Spain Coslada, Madrid LNG regasification

Praxair www.praxair.com Spain Madrid Atmospheric, process and specialty

gases, high-performance coatings

Eurowater www.eurowater.com Denmark Skanderborg Treatment of water for waterworks

and the industry

Aquafrisch www.aquafrisch.com Spain Madrid Water treatment

Cryonorm www.cryonorm.nl Netherlands Alphen aan des

Rijn

Development and application of

cryogenic vaporizers

Chart Ferox www.chart-ferox.com Czech RepublicDěčín Supplier of a wide variety of

cryogenic products

Spirax Sarco www.spiraxsarco.com Spain Sant Feliu de

Llobregat,

Barcelona

Steam system Service & Product

Adiquímica www.adiquimica.com Spain Seville Services and products for water

treatment

ProMinent www.prominent.es Spain Argelaguer,

Girona

Experts in chem-feed and water

treatment

Pastech www.pastech.es Spain Perafort,

Tarragona

Analytical projects and supplies

Nalco www.nalco.com Spain Barcelona Water treatment and process

improvement company

Idagua www.idagua.com Spain Barcelona Water treatment

Deisa www.deisa.es Spain Barcelona Water treatment systems

KEU www.keugroup.com Germany Krefeld Industrial automation products

Tank and

pressurized

equipments

Utility systems

(water treatment,

effluent

treatment,

nitrogen skid…)

6. THERMAL ENERGY STORAGE SYSTEM In this section we provide first a general description of thermal energy storage systems. After this is done, the main components are analyzed and their value chain as well.

6.1. General Description The implementation of Thermal Energy Storage (TES) in CSP plants is intended to reduce the cost of electricity by increasing their capacity factor and their ability to meet peak demand and consequently to receive a higher rate in markets with varying prices along the day. Storage capabilities must be proven at large scale before widespread adoption takes place. More critically, it is the key factor that differentiates CSP from wind and photovoltaic (PV) energies and must be delivered in the medium term for CSP to grow. Molten salt storage is the more mature option, already included in operating commercial plants such as Andasol 1 in Spain. With increasing intermittent renewable added to the grid, TES technology is expected to play an even greater role. Up to know, many different thermal energy storage (TES) configurations have been studied. Some of them are listed below, classified mainly by terms of the storage medium:

Oil as storage medium - Two tanks - Single termocline tank - Single fix bed tank

Molten salts as storage medium in two tanks Water-steam as storage medium

- Saturated steam tank - Phase change material

Storage in solids for air receivers Storage in solids for water-steam receivers (graphite) [Emerging Energy 2010] Thermochemical storage

Even if many purposes have been done through the years, the most common storage configuration to be implemented nowadays is the two tanks molten salts thermal storage. In fact, this configuration is the only one with commercial application up to know. The storage system that uses two tanks of molten salts bases its operation in thermal storage by sensible heat and mass transfer. Salts located at the cold tank are heated (either directly by solar radiation in the solar field, in the case of the heat transfer fluid being the same as the storage medium, or through a heat exchanger) and then transported to the hot tanks, during the storage charge process. When storage is needed during the plant operation, it is used by discharging of the hot tank through the inverse process.

6.2. Main Components This section describes the main components of thermal energy storage systems.

6.2.1. Storage Medium

Most of TES use a material, which keeps the heat transferred by the heat transfer fluid and afterwards, gives it back again. There are many possible storage medium candidates, and an adequate selection is very important.

That is the reason why some important technical characteristics have to be taken into account: High heat capacity per mass unit and per volume unit, in order to decrease the storage volume for a same

value of stored energy. Good mechanical and chemical stability. Compatibility with materials used in piping, heat exchangers and recipients. Low vapor pressure, that decreases investment costs. Medium chosen, preferably neither inflammable nor toxic. High thermal conductivity, leading to a good heat transfer in heat exchangers.

Every characteristic just mentioned has to be fulfilled also at high temperatures. Even if molten salts are the currently storage medium used for the commercial power plants which include storage system, the following table shows the main properties of other storage mediums also studied.

Main properties and costs of different storage mediums studied

Storage medium Operational

fluid

Operating temperature (ºC)

Average heat

capacity (KJ/Kg K)

Average heat

conductivity (W/mK)

Media costs ($/kWhth)

Cold Hot

Solid

Reinforced concrete Air 20

0 400 0.85 1.5 1.0

Ceramics

Silica fire bricks Air 20

0 700 1.0 1.5 7.0

Magnesia fire bricks

Air 200

1.200 1.15 5.0 6.0

Sand rock mineral oil Oil 20

0 300 1.3 1.0 4.2

Pebbles Air 50

0 1.000 N.A. N.A. N.A.

Liquid

Molten salts

Nitrate salts Molten

salt 290

380 1.6 0.57 25.4 (28.6) (1)

Nitrate salts Molten

salt 290

565 1.6 0.57 8.5 (13.4)(1)

Carbonate salts Molten

salt 450

850 1.8 2.0 11.0

Oil

Synthetic Oil 25

0 390 2.3 0.11 43.0

Mineral Oil 20

0 300 2.6 0.12 4.2

6.2.2. Tanks

There are different options according to the material to be used, and each of them has its advantages and disadvantages compared to others. Furthermore, there are different ways of configurating the storage system, what makes it ask the same questions as when choosing the correct material. Tests and applications developed at an industrial scale make it possible to consider two main configurations:

Storage in one single tank. There are several possibilities, one of which uses the thermocline concept, in which the fluid is stratified by temperature due to the difference in density. Another variation uses a dual medium tank, for example, a bed of rocks that allows the amount of fluid necessary to be reduced and increases the thermal capacity of the whole, since, for example, the capacity of rocks is 5% higher than salt.

Storage in several tanks. This can be in two tanks or several. The most common is in two tanks, one with cold fluid and the other with hot fluid so the volume of each must have sufficient capacity to contain the total volume of working fluid. Although it has not been much studied, the multi-tank storage concept would allow the capacity of each tank to be decreased at expenses of complicating the mechanical and control systems.

6.2.3. Storage Fluid / HTF Heat Exchangers

The use of a storage system often implicitly involves the need to incorporate a heat exchanger between the working fluid in the solar field and the storage fluid. Only in cases when this fluid is the same in both circuits this can be avoided. This not only introduces efficiency losses, due to their performance, but also affects the working temperatures, which are lowered by exchanger approaches and pinch point. (See Figure 95).

Figure 95. Energy transferred by the storage system in charge and discharge processes for a facility using water as heat transfer fluid and oil as storage medium. [Baker, A. F., 1989] [87]

Solar power plants using the same fluid as heat transfer fluid and storage medium do not need this element, making this characteristic an important advantage in comparison with other storage systems.

6.2.4. Storage Fluid Heaters

The high melting point of salt (142ºC, for Hitec nitrate salt with three components and approximately 220ºC for binary salts) makes it necessary for the installation to include electrical tracing and proper insulation in piping and components. The electrical tracing consists of the installation of electrical heating elements that heat the ducts in emergency situations in which the salt could reach its melting point. In demonstration projects carried out, problems of obstructions due to solidification of salts appeared, especially in the intakes of process instruments and valve rods. So it is therefore important to try to minimize the use of valves and also avoid ball valves.

6.3. Components Value Chain The European companies Solar Millennium (Flagsol), ACS Cobra, and Sener have taken the lead on TES applications for parabolic trough projects that are operating in Spain and are planned for the US. Sener, through its joint-venture with Masdar called Torresol, has also pioneered TES applications for commercial power towers with its 17 MW Gemasolar plant under advanced construction. SolarReserve is developing a similar approach, based on the experience of Solar One and Solar two demonstration plants. Solare XXI (Italy) is the only company to promote the use of molten salts as heat transfer fluid in parabolic trough field and storage fluid in two tanks. It plans a demonstration plant in Italy and has programmed projects in India with Entegra. Abengoa did not include large TES in its first power towers (PS10 and PS20) and parabolic trough plants (Solnova 1 to 4), but is considering molten salts storage for its Solana plant in Arizona, in spite of the technology risk associated with molten salts coupled with high installed capacity. Abengoa‘s subsidiary Abener and the US engineering company MECS created Abencs (now subsidiary of Abener) in charge of the design of TES systems for Abengoa‘s plants. Abencs operates also in India (Mumbai).

Many components of the TES systems are very similar to power island components (tank, piping, heat exchangers, pumps…), that is why some power island component suppliers like Friatec or GEA are also involved in TES components supply. Heat transfer systems manufacturer Bertrams Heatec (Pratteln, Switzerland) is specialized in engineering systems that use fluids such as organic media (mineral/synthetic oil) or molten salts as conduits for high-temperature heat processes. It provides salts/HTF heat exchangers for TES to ENEA and other research institutions. Aside from molten salts, many other storage materials are investigated at laboratory scale, but none of them is considered in current commercial project, excepted graphite storage system from Lloyd Energy Storage (Sydney, Australia) and hydrogen storage system from Ibereólica. Lloyd Energy Storage focuses on power storage capabilities for renewable energy applications, using high-purity graphite as a high temperature storage medium positioned atop the plant‘s central tower and acting as both receiver and storage block. In 2007, the company received subsidies from Australian institutions develop central receiver CSP plants employing the company‘s design and graphite storage technology. In 2007 too, Ibereólica signed a 20-year license with Clean Hydrogen Producers (Geneva, Switzerland) for their solar hydrogen storage technology, claiming that it could cause a dramatic decrease in plant investment costs.

6.3.1. Heat Transfer and Storage Fluids

Molten salt Main salt suppliers for both parabolic trough and power tower storage systems are the fertilizer producers Haifa Chemicals (Israel) and Sociedad Quimica y Minera de Chile SA (SQM, Chile). They provide sodium and potassium nitrate, melted on-site in a 60% sodium nitrate and 40% potassium nitrate blend which can be used up to 565 ºC. SQM will have a premixed blended salt available for sale in 2012. Hitec and Hitec XL salts from Coastal Chemical (USA) were also considered and used in previous CSP experimental facilities up to 500 ºC. Molten salts for thermal storage may raise some production problems. They are used in large quantities as fertilizers for agriculture, but their use as a storage medium requires a high degree of purity [CSP Technology RoadMap IEA, 2010]. There are several R&D initiatives to increase the temperature range in molten salts by changing the composition. One of these initiatives consists in the addition of lithium to the molten salt mixture to increase the working range but this is still in a very early stage at R&D level.

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ANNEXURE I:

TERMS OF REFERENCE

ASSESSMENT OF MANUFACTURING CAPACITIES FOR CONCENTRATING SOLAR THERMAL TECHNOLOGIES IN INDIA

BACKGROUND

1. In November 2009, the Government of India (GoI) approved its National Solar Mission (the Jawaharlal Nehru National Solar Mission), which aims at bringing on line 20 GW of solar power by 2022. The first phase of the NSM envisions the installation of 1,000 MW of grid-connected solar power plants, 100 MW of roof top and small-scale solar power, and 200 MW of non-grid connected power. The GoI has approved funding for the first phase in the amount of Rs43.37 billion (USD 950 million). Beyond the obvious goal of deploying more carbon-free electricity throughout the economic sectors, which currently largely depend upon power supplies from coal-fired plants, the GoI hopes to join the top tier of world solar power developers. According to the Ministry of New and Renewable Energy (MNRE), GoI, three major initiatives are planned under the NSM including (i) the creation of volumes, which will allow large scale domestic manufacture, (ii) announcement of a long term policy to purchase power; and (ii) support for R&D to reduce material consumption and improve efficiency and develop new materials and storage methods. The implementation of the Mission will proceed on the basis of the technology advancements and cost reduction, which will be necessary for rapid scale-up and to achieve the target of 20,000 megawatts1. It is envisioned in the NSM that 50% of the targeted goal for electricity generation from solar power will come from concentrating solar thermal technologies (CST).

2. High initial capital costs are a significant issue for the adoption of CST technologies. The current industry experience unveils the installation cost between $4,000 and $6,000 per kW, as opposed to around $1,000 to $2,000 per kW for fossil fuel fired plants (or wind farms). Although CST technologies do not entail any fuel associated expenses, the levelized cost of electricity from CST generation facilities are therefore much more expensive than that of fossil fuel-based plants.

3. To make CST projects in India more cost effective in the short to medium term, a combination of factors is necessary, including but limited to local incentives and availability of concessional finance. In the longer term, to make concessional finance less critical, the generation cost from CST plants will need to be dramatically reduced. This implies that the major cost drivers related to manufacturing of solar field components and systems need to decline over time. It will be made possible by a combination of technical innovation, economies of scale, and the experience curve effect. The potential for such cost decrease is considerable, as CST is a young industry, with a limited number of large and/or experienced players. The structural elements of the CST technologies are characterized by high modularity of the main components (reflectors, receivers, tracking systems, etc.), so that manufactured volumes and experience should allow for extensive industrialization and sizeable productivity gains. CST technologies also exhibit moderate technological complexity2 and limited capital intensity.

4. The World Bank (WB) provides assistance to and work with the MNRE to support the establishment of an industrial base for manufacturing of CST technology components in India. With this respect, the WB intends to finance consulting services for a study of local capabilities to manufacture and supply components for development of concentrating solar thermal power plants.

1 http://www.pib.nic.in/release/release.asp?relid=56788

2 This means that innovation is expected to be more incremental than fundamental.

SCOPE OF WORK

5. The requested consulting services will help assess the potential for India‘s industries to set up a manufacturing base for production of CST technology components and equipment. The proposed scope of work for consulting services will include the following elements: 1) an assessment of a competitive position of industries in India to support the development of CST technologies; 2) an evaluation of short, medium and long-term economic benefits of the creation of a local manufacturing base, and 3) the preparation of an action plan to stimulate local manufacturing of CST technology components and equipment. The scope of work, defined below, will be implemented in two Phases.

Phase 1 – Assessment of CST Technology Elements and Cost Drivers

Overview CST technology and cost drivers:

A description of the main technologies, and associated components and systems (this should a brief overview since body of literature related to CST technology principles and components is available);

Overview of manufacturing processes for the main CST components and systems (including thermal storage); identification of key skills (e.g. in metallurgical production, mechanical engineering, electrical engineering, industrialization, etc.);

Cost analysis for the main CSP components and systems, and for CSP plants as a whole, including potential for cost reduction (materials, labor, automation, innovation, volume thresholds, etc.), based in particular on increasing local components.

Identification of changes and related costs that existing industries need to introduce in order to participate in supplying CSP components and systems (for example changes to production line or process for regular mirror production lines).

6. Milestones and Deliverables for Phase 1:

The implementation period for Phase 1 is from April 15 through June 15 2010;

The Phase 1 work will primarily involve desk top reviews of available literature and information on CSP technologies and associated component manufacturing processes;

The deliverable for Phase 1 will be a report on the CST technology and cost drivers overview covering the scope described above.

7. Phase 2 – Assessment of Competitive Positioning of India‘s Industries and Preparation of an Action Plan to Stimulate Local Production of CST Technologies in India

Assessment of Competitive Positioning:

Identification of present and potential international and local (if applicable) manufacturers, component suppliers and EPC contractors in the value chain of each CST system, including those that are active in India (present and potential), potential new entrants (automotive industry, glass industry, steel industry, etc.)

Country/state-specific level SWOT analyses;

Review of the main existing CST-related and potentially CST-related industrial sectors and companies in India;

An analysis of India‘s industries capabilities and potential for each of the main CST technologies and their components and auxiliary equipment including supply chains, cost structures, potential for economies of scale, potential export markets, competitive advantages/weaknesses (including labor skills/educational potential and R&D capabilities);

Definition of changes including related costs to be introduced in existing industries in India to enable them participate in the manufacturing and supply of CST components and systems;

An analysis of potential economic benefits resulting from the creation of a local industrial base for CST, including labor and skill impact, foreign trade impact, etc.

Preparation of Action Plan:

Outline of an action plan – country-wide and/or state specific - to attract investors, lower potential barriers, and address potential bottlenecks, including:

Financing, financial instruments;

Fiscal and trade measures;

Education and training, and R&D programs.

8. Milestones and Deliverables for Phase 2

The implementation period for Phase 1 is from July 15 through December 15 2010;

Consultants will gather information through questionnaires and one-on-one meetings with relevant industry representatives, both locally and internationally including existing CST project developers and suppliers; potential CST suppliers, local industrial companies in India; businesses, state specific institutions working on renewable energy development, etc.

9. Consultant‘s Qualifications: Consultants for this assignment should demonstrate international experience in the following areas: CST project preparation and engineering, knowledge of industrial processes and organizational structures, business competitive analyses, strategic planning, analysis of economic competitiveness, and skill assessments and design of capacity building programs.