A Review of Small Scale Distributed Power Generation Technologies

Using Solar Energy Driven Stirling Engine

Salem Ghozzi 1,

and Rabah Boukhanouf 2

1 Sustainability and Energy Research Group in the Faculty of Engineering at the University of Nottingham

2 BEng, MSc, PhD, CEng, MCIBSE, MIET, HVAC and Heat Transfer Research Group

Abstract. Traditionally, off-grid power generation and supply in remote and inaccessible areas is provided

by fossil fuelled technologies such as diesel gensets. Currently the emphasis however has shifted towards

deploying renewable energy technologies which are becoming increasingly an important part of many

countries power generation infrastructure. Small scale solar energy projects are mainly dominated by PV

technology whereas large schemes use Concentrated Solar Power (CSP). This paper gives an insight into

current solar energy technologies that can be deployed for power generation either as stand-alone or

connected to the main power grid. Then an alternative technology using Stirling Cycle will be described

giving its potential application and limitation. A conceived new design of Free Piston Stirling Engine is

under research will be discussed and introduced.

Keywords: Concentrating Solar Power (CSP), stirling cycle and stirling engine, free piston stirling engine,

schmidt analysis.

1. Introduction

Recent studies found that 90% of CO2 emissions is a result of burning fossil fuel for power generation

and transportations [1]. it is projected that oil prices could increase to $225/barrel by 2035 [1]. Furthermore,

the global demand for primary energy grows by approximately 1.2% per year on average, that would

represent about 36% of the world energy demand between 2008 and 2035 [2]. Meanwhile, demand for

electricity worldwide stands at about 20,000 TWh and increases at a rate of 2.3% per year , the largest rise

of any other final energy form, accounting for around 80% of today’s demand, by 2040 [3], [4]. Other

estimations also point to a continuous increase in demand for power by around 2.2% per year until 2035,

with about 80% of this growth is expected to be in developing countries [2]. In order to meet the demand

growth for electrical energy, additional generation capacity of about 5900 GW will be required by 2035 [5].

Currently, large scale centralized fossil fuel power plants operating on the conventional steam Rankine cycle

(both steam cycle and combined cycle gas turbine) generates about 80% of the electrical power in the world

[6], [7]. Recent advances in materials technologies have allowed more efficient steam cycle power plants to

be built using supercritical steam cycle in combination with gas turbine cycle to achieve efficiencies higher

than 50% [6].

Concerns over energy security because of the rapid increase in the fuel prices and the negative

environmental impact due to greenhouse gases emission have led many countries to adopt new energy

policies with a focus on investing heavily in renewable and low carbon energy technologies. These include

solar, wind, hydro, geothermal and biomass as well as clean coal technologies such as carbon capture and

storage. For instance, the contribution of power generated from non-hydro renewable energy is projected to

rise from 2.5% in 2007 to 8.6% by 2030, with electricity generated from wind is set to increase by 82% to

2035 [7], [8]. Furthermore, solar power particularly, photovoltaic (PV) technologies and to a lesser extent Corresponding author. Tel.: 00447450126282

E-mail address: salem.ghozzi@nottingham.ac.uk.

2014 5th International Conference on Environmental Science and Technology

IPCBEE vol.69 (2014) © (2014) IACSIT Press, Singapore

DOI: 10.7763/IPCBEE. 2014. V69. 25

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concentrated solar power plants (CSP) have seen huge growth with hundreds Gigawatts of installed new

capacity. Concentrated solar power using parabolic troughs and concentrating dish are also the most mature

technology for large scale power generation. The interest for developing CSP power generation has been

growing again, particularly in Spain and the United states. Fig. 1 shows the cumulative power generation

capacity from concentrated solar power plants. It is shown that although interest in the technology started

back in 1980s, it is only in the last few years that the technology is starting to take off again.

Fig. 1: The cumulative capacity in MW of installed thermal power plant over the world [1]

This paper will focus on the use of Stirling engine as an alternative for power generation as standalone

and grid-connected. Stirling engine is an attractive power generation technology because of its flexibility to

be able to operate on a wide range of energy sources either conventional or renewable, particularly, solar

energy and biomass fuel, has low requirements for maintenance and safe operating.

2. Concentrating Solar Power (CSP) Technology

Solar thermal concentrating (CSP) are usually categorized according the geometry of focusing the solar

radiation which includes line-focus concentrators (linear Fresnel and parabolic trough) and point-focus

concentrators (solar dish and power tower) [6]. The CSP operates can operate from a temperature of lower

than 120°C to very high temperatures in excess of 1000°C [7]. The low temperature systems are generally

used for producing hot water or low pressure steam, whereas the medium and high temperature systems are

mainly for electrical power generation. Table 1 shows the main types of solar thermal collectors’

technologies (concentrators/reflectors) and their operational temperature and tracking mechanism.

Table 1: Types of solar thermal collector [8]

Technology Temperature range (°C) Tracking mechanism

Flat plate collector 30-100 -

Advanced flat plate collector 80-150 -

Evacuated tube collector 90-200 - Parabolic trough 70-400 One-axis

Fresnel reflector 100-400 One-axis

Solar dish 500-1200 Two-axis solar tower 500-800 Two-axis

Basically, all of the CSP systems reflect direct solar radiation (Direct Normal Irradiation, DNI) and

concentrate it onto a receiver component to convert short wave radiation into thermal energy. Fig. 2

illustrates the most common types of solar concentrating technologies.

(a) Solar dish (b) Solar tower (c) Parabolic trough (d) Linear Fresnel

Fig. 2: CSP Technologies

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The CSP is a proven and reliable technology and is usually used as part of hybrid power generation plant

integrated thermal energy storage system or combined with conventional steam boiler fuelled by fossil fuel

for continuous operation. Fig. 3 shows a schematic layout of a large scale CSP hybrid power plant. The

thermal energy input to operate the Rankine cycle is supplied from the CSP collectors’ field and

supplemented by a conventional fossil fuelled combustion process, usually using natural gas.

Fig. 3: The general layout for electrical power plant utilizes CSP as a heat source

2.1. The Integration of CSP Technology into Conventional Power Plants

CSP solar thermal fields can be integrated into the power plants in two main approaches:

Indirect Steam Generation (IDSG): a heat transfer medium such as molten salt or mineral oil is used

in a closed circuit to carry thermal energy from the solar field, where it is heated to around 400°C, to

a heat recovery steam generator where steam is produced to drive the steam turbine [9]. In addition a

thermal store using synthetic oil (thermal oil) is usually used for heat storage at maximum

temperature reaching 600°C [10].

Direct Steam Generation (DSG): in this arrangements a superheated steam is produced directly in the

solar field and then transferred to drives the steam turbine [11]. The efficiency of the direct steam

generation (DSG) potentially is higher as a result of higher temperature can be achieved in the

collector receivers. Moreover, they are simpler in design and more economical since the heat

exchanger component is no longer required. The drawbacks of DSG are the storage difficulties and

the high volumetric flow rate of steam in the solar receiver. Current research is focused on using

novel working fluids such as high pressure gases and new liquids as the energy carriers. Currently

there as a number of CSP plants that are in operation or being constructed as shown in Table 2.

Table 2: Operational and in construction CSP power plants around the world [12]

Type of CSP plant Operating Under construction

Parabolic trough 20 27

Linear Fresnel 3 1

Solar tower 5 2

Solar dish 1 1

Total 29 31

2.2. Solar Dish Stirling Engine

A parabolic dish solar uses two axes to track the sun and concentrate incident solar radiation onto a

single focal point above its centre where a receiver is mounted. Using a Stirling engine as an active thermal

receiver to convert incident solar radiation into electrical power is very effective in that the entire system (the

parabolic dish and the engine) tracks the sun and there is no need for a heat transfer fluid and associated heat

exchanging equipment. currently the major solar dish plant with a capacity of 1.5 MW in located in USA

using dish-Stirling engine units of 25kW, as shown in Fig. 4.

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Fig. 4: Parabolic dish-Stirling engine plant

3. Stirling Engine Technology

Stirling engine is not a new technology, its invention dates back to 1816 [13]. However, there is

currently a renewed interest in its application for small scale power generation using both fossil-fuels and

renewable sources of energy such as solar and biomass. The market for Stirling engine however is still very

limited to specific applications. Theoretically, the ideal Stirling cycle achieves the highest efficiency which

correspondence to Carnot efficiency. However, in practice typical efficiency is limited to about 35%.

Furthermore, Stirling engine is an external combustion heat engine that operates on closed regenerative

thermodynamic cycle where a working fluid in a gas form undergoes a compression and expansion process

at two temperature levels converting thermal energy into mechanical energy. Hence, Stirling engine lends

itself well to using solar energy in its concentrated form, which gives it an advantages compared to internal

combustion engines.

In its basic form, a Stirling engine is simple in construction. It is made up of a hermitically sealed

housing cylinder, a displacer, a power piston and three heat exchangers through which heat is supplied

(heater), rejected (cooler) and stored (regenerator), as shown in Fig. 5 (a).

(a) (b)

Fig. 5: Stirling engine (a) schematic of the main components (b) Ideal thermodynamic cycle

4. Stirling Engine Mechanical Arrangements

Stirling engines can be classified into two main mechanical arrangements which are set by the way the

reciprocating pistons (the power piston and displacer) are coupled to each other and/or to the common

rotating shaft.

4.1. Kinematic Stirling Engines

In the kinematic Stirling engine type, the reciprocating elements are connected through a rotating crank

mechanism to a common shaft that allows the synchronization of the movement of the power piston and

displacer with the fixed phase shift. A further distinction between types of kinematic Stirling engines can be

made according to the arrangement of the piston and displacer inside the cylinder(s). This can be in the form

of the two pistons mounted at each end of a common cylinder with the heater, regenerator and cooler inserted

in between (known as the α type), the two pistons located in one cylinder with the displacer shaft extended

through the piston to connect to the crank (β type), the displacer and the power piston act in two separated

cylinders (γ type). Fig. 6 show the different types of single acting Stirling engines.

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(a) α type Stirling engine (b) β type Stirling engine (c) γ type Stirling engine

Fig. 6: Types of single acting Stirling engines [14]

Other mechanical arrangements of the kinematic Stirling engine include double acting in which usually a

four cylinder engine with a swash plate is used for transmitting the reciprocating motion of the pistons into a

common rotary shaft.

4.2. Free Piston Stirling Engines (FPSE)

The free piston Stirling engine configuration is simple in that the power piston and the displaced do not

have a direct physical connection that links them together; instead the two components interact through

dynamically through exerted mechanical or air springs and dampers. Fig. 7 shows a simple sketch of a free

piston Stirling.

Fig. 7: Schematic of FPSE components

The linear motion of the piston and displacer is not converted to a rotary motion and the power output of

the power piston is used to drive a linear load such as an electric generator or a hydraulic pump. Unlike

kinematic Stirling engine, the swept volume of the FPSE is not constant and it decreases as the acting load

force increases and vice versa whereas it oscillation frequency remains constant which is mainly determined

by the oscillating pistons masses and mounting mechanical or gas spring rate. The FPSE has the advantage

of eliminating mechanical friction and components wear which leads to low maintenance and long life [15].

There are various mechanical arrangements of the moving parts in the FPSE which have been the subject

of a lot of academic and industrial research and development work in the last decades. FPSE could be of

simple harmonic (resonant engine) or non-resonant (over-driven) engine. Most FPSE designs are of two

pistons housed in a single cylinder and differ by the way in which the reciprocating elements are linked

dynamically. Fig. 8 (a) shows the displacer is connected to the power piston through an air spring located in

the body of the power piston, whereas in Fig. 8 (b) the air spring is located inside the displace. In other

arrangement, the piston and displacer use the bounce space as a common spring. This is shown in Fig. 8 (c).

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(a) Common spring in the piston (b) Common spring in the displacer (c) Common spring in the bounce space

Fig. 8: Single cylinder FPSE

Furthermore, other research work replaced the power piston with a flexible diaphragm that is clamped at

its circumference to the engine cylinder. This type of engine eliminates problems of sealing the working fluid.

Fig. 9 shows an example of such arrangement [16].

Fig. 9: Diaphragm Stirling engine

4.3. The Ongoing Research

A novel conceived FPSE design, basically depending on using completely sealed flexible parts in order

to enhance the thermal efficiency by eliminate the gas leakage between the compression and expansion

spaces. The model has been simulated using Matlab and a prototype of the new design is still under

construction. The new prototype is tended to work at low temperature levels (about 250° C).

5. Conclusion

Solar radiation is an abundant and free form of energy that can be converted directly or concentrated for

power generation. A literature review of solar concentrated heat engine with particular focus on operation of

a Stirling engine was presented. Being externally heated, Stirling engine presents an effective technology for

integration into small scale solar concentrated power systems as grid connected or in standalone operation.

The FPSE arrangement is a very simple energy conversion heat engine that require low maintenance, has low

noise and long life. FPSE receives an interest of researchers and some an ongoing researchers demonstrate a

novel FPSE design aiming to achieve a sensible thermal efficiency.

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