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Transformation of the German energy system – Technology Readiness Levels 2018 VGB PowerTech 8 l 2019

Transformation of the German energy system – Technology Readiness Levels 2018

AuthorsDipl.-Wi.-Ing. Christoph PieperWissenschaftlicher MitarbeiterProf. Dr. Michael BeckmannTechnische Universität DresdenInstitut für Verfahrenstechnik und Umwelttechnik Professur für Energieverfahrenstechnik Dresden, Germany

Transformation of the German energy system Technology Readiness Levels 2018Christoph Pieper and Michael Beckmann

Kurzfassung

Transformation des deutschen Energiesystems – Technology Readiness Levels 2018Der Artikel fasst die Ergebnisse der wissenschaftli-chen Studie „Transformation des deutschen Ener-giesystems – Auf dem Weg zur Photovoltaik und Windkraft – Technology Readiness Levels 2018“ zusammen. Die Forschungsarbeit wurde vom wis-senschaftlichen Beirat des VGB unterstützt[2]. Im Mittelpunkt der Studie steht der Entwicklungs-stand relevanter Technologien auf Basis von Tech-nology Readiness Levels. Weiterhin werden Ent-wicklungspotenziale und -grenzen sowie die not-wendigen Leistungskapazitäten für ein primär strombasiertes Energiesystem aufgezeigt. So wer-den erneuerbare Energiequellen, die für die Strom-versorgung in Deutschland geeignet sind, Techno-logien zur Umwandlung von Primärstrom für an-dere Energiesektoren (Wärme und Mobilität) und Speichertechnologien in den Anwendungsbereich einbezogen. Darüber hinaus werden nicht-konven-tionelle Technologien für die Stromversorgung und -netztechnologien betrachtet. Das zugrunde lie-gende Technology Readiness Assessment ist eine Methode zur Bestimmung der Reife dieser Systeme oder ihrer wesentlichen Komponenten. Die Haupt-kriterien für die Bewertung sind Skalierung, Sys-temtreue und Einsatzumgebung. Um die relevan-ten Größen für bestimmte Energietechnologien hinsichtlich Leistungs- und Speicherkapazitäten abzuschätzen, wird ein Simulationsmodell erstellt und implementiert. Es ermöglicht die Berechnung einer erneuer-baren, volatilen Stromversorgung auf der Grundlage historischer Daten und die Dar-stellung von Last- und Speicherverläufen. Im Er-gebnis der Untersuchung variiert der Technology Readiness Level der verschiedenen Systeme stark. Für jeden Schritt der direkten oder indirekten Nut-zung erneuerbarer intermittierender Energiequel-len stehen Technologien zur Verfügung, die im Megawattbereich kommerziell verfügbar sind. Der notwendige Maßstab für die Energiespeicherkapa-zität liegt in Terawattstunden. Von den untersuch-ten Speichertechnologien liegen nur chemische Speicher in dieser Größenordnung. Darüber hin-aus liegen die erforderlichen Gesamtleistungska-pazitäten für komplementäre Umwandlungstech-nologien im zweistelligen Gigawattbereich. l

The article summarizes the results of a sci-entific study “Transformation of the Ger-man energy system – Towards photovoltaic and wind power – Technology Readiness Levels 2018” [1]. The research was sup-ported by VGB Scientific Advisory Board [2]. The study focuses on the state of devel-opment of relevant technologies on the ba-sis of Technology Readiness Levels. Further, it emphasizes development potentials and limits as well as the necessary power capac-ities needed for a certain energy system de-sign that is mainly based on electricity. Thus, the scope is set to renewable energy sources suited to provide electricity in Ger-many, technologies that convert primary electricity for other energy sectors (heating and mobility) and storage technologies. Ad-ditionally, non-conventional technologies for electricity supply and grid technologies are examined. The underlying Technology Readiness Assessment is a method used to determine the maturity of these systems or their essential components. The major cri-teria for assessment are scale, system fidel-ity and environment. In order to estimate the relevant magnitudes for certain energy technologies regarding power and storage capacities, a comprehensible simulation model is drafted and implemented. It allows the calculation of a renewable, volatile power supply based on historic data and the display of load and storage characteristics. As a result, the Technology Readiness Level of the different systems examined varies widely. For every step in the direct or indi-rect usage of renewable intermittent en ergy sources technologies on megawatt scale are commercially available. The necessary scale for the energy storage capacity is in terawatt hours. Based on the examined storage technologies, only chemical storag-es potentially provide this magnitude. Fur-ther, the required total power capacities for complementary conversion technologies lies in the two-digit gigawatt range.

Introduction

The sufficient, reliable, flexible and afford-able supply of a society with resources and energy is a cornerstone for the develop-ment of prosperity. Energy supply, in par-ticular, took on an increasingly role in Eu-rope at the end of the 19th century, as

mechanization led to considerable increas-es in productivity and energy demand. This process is by no means finished today. On the contrary, several studies show (cf. [3], [4]) that economic growth cannot be ex-plained by the factors labor and capital alone. An abundant and cheap energy sup-ply enables increases in productivity while scarce energy severely constrains econom-ic growth. It accelerates the production and substitutes expensive labor costs. Therefore, the vital necessity of a rich and economical energy supply is undisputed for an ongoing growth of prosperity. Be-sides this economic feature, a nation’s way of energy supply has to cope with the given ecological, social and political boundary conditions. If there is a social and political desire to change the prevailing way of energy sup-ply, the society sets the objectives and the legal framework that force the develop-ment of technical innovations. This devel-opment takes time and it takes even longer to implement these innovative energy sys-tems nationwide. An example may be giv-en by the history of nuclear energy in Ger-many (based on [5] and [6]). From the first discovery of radioactivity in 1896 by Antoine Henri Becquerel it took until 1954 when the first grid feeding nuclear power plant began operation in Obninsk, Russia with a power of 5 MWel,net. The next com-mercial reactor started with 60 MWel,net in 1956 in Calder Hall, England. The first German nuclear power plant with 15 MWel,net (grid connected) started 1961 in Kahl, Germany and was still designated as an experimental plant. It took further three more decades to install a number of nuclear power plants with ever-increasing power capacities. In Germany the expan-sion was stopped in 1989 and the current phase out is set to 2022 due to ecological and safety concerns. This short outline shows the rise and fall of an energy source in Germany and the time it took for devel-opment, implementation and shutdown (deconstruction is not included).With the current path of the (next) Ger-man energy transition towards a renewa-ble energy supply with minimal carbon di-oxide emissions a series of questions arise. From an engineering point of view some of these questions are:

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VGB PowerTech 8 l 2019 Transformation of the German energy system – Technology Readiness Levels 2018

– How can it be done? (What are necessary technologies for an at least secure supply of energy? What are the options for the energy system and how can different technologies be implemented?) And

– How fast can this transition be done? (What is the current state of the art and what technologies need further develop-ment? What is the necessary expansion for the respective technologies? Are the given goals and timeline for the energy transition feasible?)1

Scope

Based on the mentioned questions, the ob-jective of this work is to contribute to find-ing answers and to objectify the discussion regarding the available technologies. This work describes the state of development of relevant technologies on the basis of Tech-nology Readiness Levels [7]. Further, it points out development potentials and lim-its as well as the necessary power capaci-ties needed for a certain energy system design that is mainly based on electricity. For the investigation the sample or rather the examination object had to be limited. The essentials of the German energy transi-tion are found in the “Energy Concept for an Environmentally Friendly, Reliable and Affordable Energy Supply” [8]. The feasi-bility has been confirmed by a statement of the German Advisory Council on the Envi-ronment [9]. The aim of the energy transi-tion is to gradually increase the share of renewable energies to more than 80 % of the gross electricity consumption. At the same time, primary energy consumption is to be reduced by 50 % compared to the base year 2008. In a rough timeline, the year 2050 was set as final milestone. The transition of energy supply for heat, elec-tricity and mobility is expected to reduce greenhouse gas emissions by 80 to 95 % compared to the base year of 1990 [8]. A spotlight is set on the electric power supply, which has to be delivered by renewable en-ergy sources (RES) and thus must be ex-panded accordingly [10]. However, the use of volatile RES requires large-scale storage solutions. The options for direct, large-scale storage of electricity are limited. Therefore, an alternative energy supply system is needed that can buffer temporary oversupply from RES by storing it for later use and bring it in line with energy demand [11]. A sample structure for such a system is shown in F i g u r e 1.In F i g u r e 1 two possible (extreme) situ-ations are shown. The filled components represent full supply by RES. A mean pow-er load of 60 GWel, which includes the final energy demand for heating, cooling, elec-

tricity and mobility, is completely covered by volatile und by a limited degree of con-tinuous RES. Temporary surpluses are stored by means of the parallel storage sys-tem. This consists of (buffer) batteries, which reduce rapid load changes for the subsequent, sometimes slower, conversion technologies and further serve to provide electrochemical storage. Energy storage af-ter initial conversion may be thermal, chemical or mechanical. The second situa-tion is represented by the gray / transpar-ent components. In the event of an under-supply of electricity (10 GWel from RES), stored energy has to be converted a second time to provide power (here 50 GWel). It may also be used without a second conver-sion e.g. for heating. This consideration leads to the expectation that the power ca-pacity and storage capacity of transforma-tion and storage technologies have to be in the order of magnitude of gigawatts (GW) and terawatt hours (TWh). In this approach, the supply of electricity is considered primarily in the context of the technology matrix depicted in F i g u r e 2 .In order to cope with the vast range of tech-nologies some simplifications in the analy-sis where necessary, which are shown in the method chapter. It should be pointed out that the focus is based on a) today’s electrical energy requirements and b) com-paratively recent technologies. For exam-ple, conventional, mature technologies for the second conversion stage (gas and steam turbines, electric pumps and motors, generators, electrical transformers, etc.) are not included in the technology matrix. The direct use of electrical energy is limited to today’s electricity needs (for decentral-ized supply of heat, cooling, light, informa-tion and communications technology (ICT), stationary mechanical drives, etc.). In contrast, the future direct use of electric-ity for mobility might not be limited to rail traffic, but could also include trucks using

overhead transmission lines (e.g. eHigh-way concepts [12]). This topic is not con-sidered in depth for reasons of delineation. Today’s electromobility in passenger cars uses electrical energy only indirectly via portable storage batteries. The technology readiness assessment fol-lows the matrix in F i g u r e 2 and starts with the primary energy supply from RES. Particular emphasis is placed on their tech-nical potential, on significant improve-ments and expansion potential in Germa-ny. In the following step, the first conver-sion stage – the transformation of electri- city into other energy forms that are better suited for storage – is analyzed. Further, the storage solutions and finally the second conversion to electricity is investigated. For the second conversion stage, the focus is on hydrogen technologies as the conversion of fossil (chemical) energy into thermal, me-chanical and electrical energy is widely known and mature (TRL-wise). The con-version of energy in batteries is an inherent part of the technology and thus not explic-itly designated as first/second conversion.

Method of Technology Readiness Assessment

In the following section, the fundamentals and drawbacks of the Technology Readi-ness Assessment (TRA) are described. TRA is a method used to determine the maturity of a technical system or its essential compo-nents. Its result is a classification of nine levels – the so-called Technology Readiness Levels (TRL). These levels describe the tech-nology progress from concepts and studies to actual operation as an integrated system in full scale over a prolonged period. The TRL are shown in F i g u r e 3 with a qualita-tive description of the classification criteria. The classification is already partially adapted to the requirements of energy

1 Other important aspects of the life cycle and depreciation of energy infrastructure with re-gard to ecological and economic questions in construction and dismantling are not in the focus of this paper.

Direct usage ( incl. sector coupling)

60 GW

10 GW

Sector coupling:- Heat pumps- Electrode boiler- Charging infrastructure mobility- Motors- Cooling- ICT

X GWBufferbattery 1. Conversion Storage

Transport/distribution

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2. Conversion

Conventionalpower plants

50 GW

Fig. 1. Power supply system dominated by RES with parallel storage system (bold/filled compo-nents: sufficient RES supply with surplus; transparent/gray components: RES undersupply with compensation by stored RES energy and conventional power plants; degree of effec-tiveness illustrative).

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Transformation of the German energy system – Technology Readiness Levels 2018 VGB PowerTech 8 l 2019

technologies, since US National Aeronaut-ics and Space Administration (NASA) de-veloped the original scale and accordingly tailored it to aerospace issues. The first considerations for an assessment of tech-nology maturity can be found in a NASA report from 1969 for space station tech-nologies [7]. The concept evolved continu-ously over the following years and the scale was expanded from the initially seven to nine levels and in a 1995 paper comprehen-sively described and defined for the first time [13]. Another step towards global ac-ceptance was the adoption of the TRL con-cept by the United States Department of Defense (DoD) in 1999, initiated by the US Government Accountability Office (GAO). The United States Department of Energy (DOE) was also encouraged by the GAO in 2007 to use the TRA [14].

The main reason for using the TRA is, from the perspective of the GAO, the avoidance of cost explosions and delays in large pro-jects. A high degree of maturity of the tech-nologies used is associated with a high probability of project success. From the experience of GAO, the use of technologies with a TRL < 6 leads to cost increases and delays [15]. The examples given in [15] re-late primarily to defense engineering pro-jects. In energy engineering, the GAO 10-675 report [16] noted with particular con-cern that the DOE was unable to provide an assessment of the maturity of carbon cap-ture and storage (CCS) technologies for coal-fired power plants. The DOE prepared an overview of the TRLs for the Clean Coal Research Program (CCRP) by 2015 [17].The TRA method is presented in detail in [15] and a best practice process is given for

conducting TRAs. Additionally, the DIN ISO standard 16290 is available with the latest update of 2016-09, in which the TRLs are presented descriptively [18]. The gen-eral process for determining the TRL is as follows:

– 1. Select Critical Technology Elements – CTE:In general, parts are considered critical if they are new or completely novel and in-dispensable for the specified operation of the target system. An element is also critical if it poses a particular risk to com-pliance with cost, time and performance restrictions [15].

– 2. Evaluate critical technologies and de-termine the TRL: The classification of a technology or CTE is based on the following list of criteria (see Ta b l e 1 ). The definition of the re-

Primary energy (electricity from renewables)

1. Conversion(charge)

Energy storagesolutions

2. Conversion(discharge)

Indirect usage

Finalconsumption

Direct usage

RES Wind power Horizontal-axis wind turbines Vertical-axis wind turbines Offshore wind turbines Solar pv (various cell types) Solar thermal Geothermal Direct usage Binary system (ORC, Kalina) Hydro (osmotic power, tidal power) Biomass Biogas plant Other conv. technologies

Electricity distribution solutions HVDC FACTS GIL

Power-to-HeatPower-to–ColdElectricity distribution

Power-to-Heat Heating resistor boiler Heat pumps Power-to–Cold Compression refrigeration Ad-/absorption cooling Power-to-Gas Electrolysis AEL PEMEC SOEC

Power-to-Chemicals Catalytic methanation Bio methanation Methanol synthesis FTS

Power-to-Mechanical Electric motors Compressors Pumps

Hydrogen combustion Hydrogen turbine Hydrogen gas motor

Chemicals-to-Power AFC PEMFC PAFC SOFC MCFC

(conv.) Heat-to-Power Rankine cycles Gas turbines

Heat-to-Heat Heat exchanger

Mechanical-to-Power Electric generators

Mechanical: CAES LAES Pumped hydro Fly wheel storage Gravity storage

Thermal: (incl. cold storages) Latent heat storages PCM storage Sensible heat storages Hot water storage High temperature storage Sorption heat storage Thermochemical storage

Chemical: Caverns Pore storages H2-Storages LOHC Metallic hydrides

Electro-chemical: battery storage Galvanic cells Lithium-Ion-Battery Redox-flow batteries Lead acid batteries High temperature batteries

Electricity

Mobility/drives

Heating

Cooling

Fig. 2. Technology matrix.

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VGB PowerTech 8 l 2019 Transformation of the German energy system – Technology Readiness Levels 2018

spective criterion is based on the TRL scale used by the US Department of En-ergy [19] (see Ta b l e 2 ).

The classification can be supported by the use of TRL calculators, which derive the evaluation from a questionnaire (e.g. [20]). Their application is not undisputed, because the assessment is formalized but sometimes the critical assessment of indi-vidual components is disregarded. There-fore, it may be possible to derogate from the formal approach according to [15] and to simplify the method, in particular if the

addressee group is limited to an expert au-dience and a frequent or regular repetition of the assessment is done. A large amount of information about test results, model scaling, environmental conditions or per-formance specifications is required for a detailed assessment of the maturity. This data can only be found to a limited extent in the context of secondary research, so that some uncertainty in the classification re-mains. Even within the framework of pri-mary research (e.g. through expert inter-views), a classification risk remains. The

quantification of that risk has been attempt-ed in [21] by applying statistical methods.The TRL classification is therefore only a first step in the assessment of a develop-ment project. Consequently, a “Technology Maturity Plan” (TMP) should be prepared after the evaluation. It includes the next steps to further maturing a CTE and gives a rough estimate of development costs and time to provide decision-makers with nec-essary information [22].In summary, the TRA can show the develop-ment gap from the current state of the art to large-scale application. It further identifies critical components of a system in a trans-parent way and the necessary research steps to eliminate development risks [22].

Drawbacks and Limits of TRAThe TRA focuses in its original context on a technology’s technical feasibility or opera-tional capability for a mission. Therefore, many other factors for a technology de-ployment are not taken into account or are considered of minor importance. Thus, im-portant goals in the energy supply, namely sustainability and cost-effectiveness, are not in scope of the examination. This exclu-sion of aims is acceptable, since in this scope the technical feasibility of the Ger-man energy transition with today’s state of the art shall be considered in depth. In addition, the Technology Readiness As-sessment has a number of other flaws, which are listed below and briefly ex-plained (based on [15] and [18]):

– Limitation of context and timeThe TRL rating is a snapshot and with a limited validity period. It applies to a specific element in a particular system with particular timing, purpose and con-ditions. If the narrow frame is changed, a revaluation is required.

– Missing life cycle effects:On the one hand, a mature technology (in the sense of TRL 9) can be consider-ably improved through continuous im-provement processes (CIP) over time and, on the other hand, it can become outdated through the advent of new technologies (e.g. the transition from floppy disks to CDs). These effects are not taken into account and are generally difficult to predict.

– Independence of the components:If the evaluated CTE element is consid-ered detached from the target system, systematically higher TRL valuations may occur. The successful application of the component in the final system also depends on the maturity of the interfac-es and the integration with further com-ponents, however that connection is not directly examined in a detached maturi-ty rating.

– Ordinal metric:The ordinal TRL metric itself departs not allow an estimation of time or effort

Basic principles observed and reported

Technology concept and/or application formulated

Analytical and experimental critical function and/or characteristic proof of concept

Component and/or breadboard validation in laboratory environment

Component and/or breadboard validation in relevant environment

System/subsystem model or prototype demonstration in relevant environment

System prototype demonstration in operational environment

Actual system completed and qualified through test and demonstration

Actual system proven through successful mission operations

TRL 1

TRL 2

TRL 3

TRL 4

TRL 5

TRL 6

TRL 7

TRL 8

TRL 9

Prot

otyp

eLa

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tory

Theo

ry

Fig. 3. Technology readiness levels and basic properties, based on [12]

Tab. 1. Classification evaluation criteria of TRLs.

TRL Scale of Testing Fidelity Environment Proof

1 Paper

2 Paper Analytical

3 Laboratory/Bench Partial Simulated Analytical Experimental

4 Laboratory/Bench Partial Simulated Analytical Experimental

5 Laboratory/Bench Similar Relevant Analytical Experimental

6 Engineering Similar Relevant Analytical Experimental

7 Full Similar Relevant Analytical Experimental

8 Full Identical Operational (limited range) Analytical Experimental

9 Full Identical Operational (full range) Analytical Experimental

Tab. 2. Definition of TRL criteria.

Scale of Testing:

– Full scale: system matches final application in performance, power and dimensions– Engineering: the scale regarding performance, power or dimensions is 1:10 < system < full scale– Laboratory/bench: the scale regarding performance, power or dimensions is < 1:10

Fidelity:

– Identical: matches final application in all respects– Similar: matches final application in almost all respects– Partial: system partially matches the final application– Paper: system exists on paper (i.e. no hardware system)

Environment:

– Operational (full range): system is tested in deployment environment– Operational (limited range): system is tested with limited range of real conditions– Relevant: controlled environment with limited influence/use of real conditions– Simulated: controlled environment, necessary to prove concept or function

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Transformation of the German energy system – Technology Readiness Levels 2018 VGB PowerTech 8 l 2019

needed to reach the next maturity level. Therefore, the TMP is necessary.

– Standardized approach:The method must be adapted to the spe-cific industry, case or purpose. General definitions are possible, but only condi-tionally suitable. The complexity and scope of various elements to be evaluat-ed require specific approaches.

– Quality of the tools and the evaluation team:The objectivity of the maturity rating de-pends on the available data, the exper-tise of the reviewers and is not least lim-ited by their subjective assessment (in-fluenced by goals, corporate culture, expectations of the technology, etc.).

– Major impact of scale on the TRL rat-ingThe necessary scale of CTE has substan-tial impact, as there are only three grades for classification (< 1:10; > 1:10; full scale). Thus a mathematical model was developed during the study to show the required scale for technologies of the first conversion step, storages and back-up plants.

This assessment does not question the cur-rent maturity of certain processes, but the TRL in regard to the mission of a renewable energy supply. The results of the more de-tailed study and the key findings are sum-marized below. The above aspects should be kept in mind while reading and evaluat-ing the content.

Summary of TRL assessment results

The maturity of the different technologies is highly heterogeneous across the different areas. A visualization of the TRLs in con-junction with a respective scale is presented in the following sections (F i g u r e 4 - 8 ). There are generally high TRLs in renewa-ble energy supply systems. This applies for biomass conversion in combined heat and power applications as well as for hy-dropower and electricity transport and dis-tribution. Despite their maturity, technolo-gies can always be further optimized. These optimization measures are aimed at expanding life cycles or reducing costs. In photovoltaics, certain solar cell types are simply mature, mass-produced and con-nect to pv power plants. The use of more efficient multilayer pv cells is reserved for special applications. A significant break-through on the use of large building sur-faces by DSC or organic cells is still pending (TRL 6). It could significantly increase the available surfaces and thus the PV expan-sion potential. In other areas, such as the geothermal energy and wind power, there is still the potential for technological devel-opment, which enables existing plants to be made larger or more efficient. Onshore wind turbines are also durable and techni-cally mature. Although further scaling is desirable, it is reaching limits in terms of

transportation, height regulations and weight for the nacelle. The shortcomings of offshore wind power relate to the need for foundations in deep water and grid con-nection over long distances. Floating foun-dations could solve many challenges, but so far they have only been built and tested as demonstration projects in full scaling (TRL 8). The network connection by sub-marine cable is technically mature. In geo-thermal power the main components of the power train are mature, have proven them-selves in continuous operation and are used worldwide, but other complementary fields lag behind. Here, the challenge lies in the exploration and drilling of suitable geothermal sources. However, this does not represent a central problem with the energy technology itself.The initial conversion stage also includes many mature power-to-heat and power-to-cold technologies. Most of them, like electric process heaters, electrode boilers and large-scale heat pumps for district heating have a capacity up to several MWel. The development of heat pumps has also

improved recently: they run more efficient-ly, on a larger scale and at higher tempera-tures (>130  °C TRL 4-6). A major draw-back is the ongoing low flow temperature for industrial applications. New refriger-ants are still in R&D while CO2 seems to be usable but necessitates larger compression units and requires pilot scale plants to gain experience. Using combined heat pumps for cooling and heating extends energy ef-ficiency even further but can rarely be im-plemented. Power-to-cold technologies are still dominated by compression cooling (rated cooling-capacity > 20 MWth per unit). The drawbacks of absorption/ad-sorption cooling technologies are low ther-mal efficiency and the large size of the sys-tems concerned. Power-to-gas and power-to-chemicals (c.f. F i g u r e 6 ) are closely linked by the hydrogen both produce from electrolysis. The maturity of chemical pathways to con-vert syngas (mixture of CO, H2, CO2) to fuels via methanol or Fischer-Tropsch-syn-thesis is undisputed when using steam re-forming and natural gas with a continuous

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Fig. 4. TRLs of primary energy supply from renewables and electric power transmission.

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Fig. 5. TRLs of power-to-heat and power-to-cold.

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supply. However, electrolysis represents a key technology in transforming electrical energy into chemical energy sources for the German energy transition. Large-scale (alkaline) water electrolysis has been used in the past to produce fertilizer. The well-known electrolysers at the Aswan Dam (200 MWel) and Glomfjord (135 MWel) are both powered by comparatively cheap and continuously operated hydropower plants. The challenge of today’s electrolysers is (cost-)efficiency and durability in the con-text of dynamic and intermittent opera-tion. PEMECs support rapid load changes, but the system efficiency at high pressure does not match the efficiency of SOEC sys-tems. However, SOECs should not be shut down completely to avoid thermal stress on the material. Further research topics are currently cata-lytic and biological methanation. Especial-ly biological reactors are neither optimized nor available on the MW scale (TRL 5). On the other side, the catalytic methanation has achieved higher TRL ratings, especially with regard to operational capability, sys-tem fidelity and scale of testing. However, there is still no proof of a dynamic system that can be interrupted on at least an hour-ly and up to a daily basis due to a non-con-tinuous energy supply. The operational environment for the methanation technol-ogies (hydrogen from electrolysis, flexible operation) is being tested in multiple Ger-man pilot projects but can at best be cate-gorized as “relevant” (simulated + (limit-ed) real input) for the TRL assessment. The TRL is therefore categorized at no more than 7 for the particular application of al-kaline electrolysis and the small pilot scales.Storage solutions have been the subject of numerous research projects and offer a wide range of options. Mechanical storage methods present a bottleneck with their low energy density, which results in large land usage. New approaches try to either “hide” the problem underground (under-

ground water storage) or remedy the stor-age density problem by lifting heavy rocks. The technology underlying water pumps and turbines is mature. The concepts them-selves only reach TRL 2 since none of them have been built (mostly for economic and regulatory reasons). Compressed air ener-gy storage solutions face similar problems. In addition, they are more complex and re-quire high temperature thermal storage for adiabatic operation, which are of limited availability in the required MW-scale. Molten salt storages on a MW scale (up to 350 °C) are available worldwide in com-mercial pilot projects (TRL 8) in conjunc-tion with CSP-plants. For thermal power, sensible storage solutions based on water and gravel dominate amongst the mature approaches. Higher storage densities (and temperatures) can be achieved with la-tent or thermo-chemical storage (TRL 3-6), but most of them are still in R&D (= max. TRL 4) and research continues to focus on new, cheap, durable and available materi-als. The comparative simplicity of water and gravel are hard to overcome. Chemical storage methods also have a strong focus on hydrogen due to the abundant experi-

ence in transportation and storage of (fos-sil) gases, fluids and solids (TRL 9). The hydrogen tolerance of existing storage components is still in question and there are only a few demonstration units for large-scale underground storage solutions (TRL 8). The same applies to transporta-tion technologies such as metal hydrides, LOHC, liquid hydrogen or adsorption ma-terials (TRL 4-6). Although there have been numerous research projects, particu-larly in the automotive sector, the main drawback is the thermal management of these technologies. Discharging requires temperatures at the range of 200 °C which are difficult to provide in the context of highly intermittent and flexible operation. In the field of electrochemical storage, there are also mature, large-scale, proven battery types (lead-acid, high temperature

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NaS, lithium-ion). Their weak point is their capacity-wise scalability. Even redox flow batteries that might overcome this hurdle are only available up to 1 MWel (TRL 9). If battery storage capacity in the magnitude of GW (or at least > 100 MW per module or system) is needed for the energy transition, no battery type reaches TRL > 6. However, the widespread use of battery electric vehi-cles and several projects around the world demonstrate a TRL 9 for scale-ups to sev-eral MWel.In the second conversion stage, both global research efforts and the present study are dominated by fuel cells for the reasons set out above. In this last stage, end use is more important than the maturity of fuel cell types. While PEMFCs are produced in large quantities up to 100 kWel to provide the power train for fuel cell vehicles, there are far fewer PAFCs, MCFCs and SOFCs for large-scale stationary operations in the MW-class (TRL < 7). However, over 200,000 PEMFCs and SOFCs are shipped in Japan under the ene-farm-project for small-scale home applications in the kW-class (TRL 9). The remaining drawbacks are the longevity of the cells in unfavorable

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conditions (gas purity) and the general high costs for large-scale stacks (based on materials and production).An alternative approach is the use of hy-drogen in conventional power technologies such as gas turbines and gas motors. Inter-nal combustion engines can be basically adapted to use hydrogen (TRL 8). Howev-er, recent research on this topic is scarce and interest has dropped over the last dec-ade with the technological progress made by fuel cells. With regard to large-scale gas turbines, the greatest progress has been made with non-premixed burners for syn-gas operation in conjunction with IGCC (TRL 8). For premixed burners the possible hydrogen fraction in hydrogen-enriched natural gas has been increased to about 30 %. The main problems remaining in this field are high flame velocity (complicating flame stability and causing flashbacks) and high temperatures when burning hydro-gen, causing NOX emissions without lean-burn technologies. The injection of water to lower temperatures during combustion and therefore lower NOX emissions is be-ing investigated.

Conclusion and Outlook

In summary, the TRL of the different sys-tems examined varies widely. For every step in the direct or indirect usage of re-newable intermittent energy sources, tech-nologies are available that are commercial-ly available in the MW-scale. However, RESs potentially provide power in the GW-scale. Thus, conversion technologies and energy storage systems also have to be available in the GW-scale. Since the scale of testing is an essential criterion in tech-nology readiness assessment, it reduces the TRL of essential technologies such as battery storage and power-to-gas conver-sion significantly. A further bottleneck is the intermittent power supply and thus op-

eration of the technologies. Comparatively slow and thermally inert processes (e.g. power-to-gas/chemicals) are more suitable for continuous operation. Therefore, the environment criterion under current con-ditions for most demonstration projects has to be assessed as “relevant” or “opera-tional with limited range” and thus limits the TRLs. Another bottleneck is the restricted availa-bility of RESs and their dependence on natural conditions. Germany has long peri-ods with high temperatures (reducing the efficiency of solar PV modules) or low ra-diation combined with poor wind condi-tions. In these cases, even large wind and PV capacity provide low power outputs. Those conditions pose a threat to the en-ergy supply system, which has to be coun-tered by large-scale (GW) conversion and storage capacity. Such capacity cannot be installed on a short-term basis and there-fore requires conventional power supply technologies to bridge the gap towards a RES-dominated energy supply system. There are significant bottlenecks in terms of security of supply. Although the conver-sion of solar and wind energy into electri-cal energy has a very high TRL and is also highly scalable (land use, environmental impact and costs are neglected here), the intermittent availability of wind and solar energy is a natural bottleneck and requires storage solutions and backup systems. Of course, sector coupling may contribute to security of supply, but storage solutions on a GW and/or TWh scale are essential. Their TRL and scalability are still considered to be low, so that at least a decade of further development will be required to reach TRL 9. At that point nationwide implementa-tion can begin, which in turn will take sev-eral decades to complete. A more accurate estimation on the time necessary to in-crease the TRL of certain technologies de-pends heavily on the economic circum-

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stances. Rapid improvement of critical technology elements is possible even with complex solutions, when (financial) re-sources are available and economic con-straints exist. As an outlook, the assessment of TRLs should be repeated on a regular basis, as the validity period is limited. This study is a snapshot of the current expectation on technologies and the future. It underlies permanent change. On the other side, cer-tain boundary conditions, e.g. the laws of thermodynamics, remain constant and force a critical view on ideas and concepts regarding the energy system.

AcknowledgementThe authors would like to thank VGB PowerTech e.V. and the members Scientific Advisory Board for supporting this study. The authors would further like to thank Ms. Viedt for the editing of the article. A more in-depth examination of the technol-ogies is given in [1].

References[1] C. Pieper: Transformation of the German

energy system – Towards photovoltaic and wind power – Technology Readiness Levels, dissertation, submitted at Faculty of Me-chanical Science and Engineering TU Dresden, 19.11.2018.

[2] VGB project “Identification of the bottle-necks of the German Energiewende – Study on Technology Readiness Level (TRL) of crucial techniques”, Research Project 19/16.

[3] R.U. Ayres, J.C.J.M. van den Bergh, D. Lin-denberger, B. Warr: The underestimated contribution of energy to economic growth, Structural Change and Economic Dynam-ics, Volume 27, 2013, Pages 79-88, ISSN 0954-349X, https://doi.org/10.1016/j.strueco.2013.07.004.

[4] D.I. Stern: The role of energy in economic growth. Annals of the New York Academy of Sciences, 1219: 26-51, 2011, DOI:10.1111/j.1749-6632.2010.05921.x

[5] Deutsches Atomforum e.V.: Geschichte der Kernenergie, Website, INFORUM Verlags- und Verwaltungsgesellschaft mbH (eds.), online: https://www.kernenergie.de/kernenergie/Politik-und-Gesellschaft/Ge-schichte-der-Kernenergie/, retrieved 30.10.2018.

[6] World Nuclear Association: Outline History of Nuclear Energy, Website, online: http://www.world-nuclear.org/information-li-brary/current-and-future-generation/outline-history-of-nuclear-energy.aspx, retrieved 30.10.2018.

[7] J.C. Mankins: Technology readiness assess-ments: A retrospective. Acta Astronautica 65, 2009, doi:10.1016/j.actaastro.2009. 03.058.

[8] Technologie (BMWi): Energiekonzept für eine umweltschonende, zuverlässige und bezahlbare Energieversorgung [Energy con-cept for an environmentally friendly, reli-able and affordable energy supply], 28.09.2010, online: https://www.bmwi.de/Redaktion/DE/Downloads/E/energie-konzept-2010.pdf?__blob=publication File&v=3, retrieved 15.08.2018.

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[9] Sachverständigenrat für Umweltfragen (Hrsg.): 100% erneuerbare Stromver-sorgung bis 2050: klimaverträglich, sicher, bezahlbar [100 % RES power supply until 2050 – climate-friendly, save, affordable], Stellungnahme Nr. 15, Mai 2010, ISSN 1612-2968.

[10] R. Scholz, M. Beckmann, C. Pieper, M.  Muster, R. Weber: Considerations on providing the energy needs using exclusively renewable sources: Energiewende in Ger-many. In: Renewable and Sustainable En-ergy Reviews 35 (2014), S. 109-125, DOI: http://dx.doi.org/10.1016/j.rser.2014.03.053.

[11] Hack, N.; Unz, S.; Beckmann, M.: Stand der Technik und innovative Verfahren-skonzepte zur Umwandlung und Spei-cherung elektrischer Energie. In: Beck-mann, M.; Hurtado, A.: Kraftwerkstechnik – Sichere und nachhaltige Energiever-sorgung, Band 5. Neuruppin: TK Verlag Karl Thomé-Kozmiensky, 2013, S. 822 – 857. ISBN 978-3-944310-04-6.

[12] Siemens Mobility GmbH (Hrsg.): eHigh-way – die Elektrifizierung des Straßengü-terverkehrs [eHighway – Electrification of road haulage], Website, online: https://www.siemens.com/global/de/home/produkte/mobilitaet/strassenverkehr/ele-ktromobilitaet/ehighway.html, retrieved 15.08.2018.

[13] J.C. Mankins: Technology Readiness Levels – A White Paper, Advanced Concepts Of-

fice, NASA, 1995, online: http://www.ar-temisinnovation.com/images/TRL_White_ Paper_2004-Edited.pdf, retrieved 15.08.2018.

[14] U.S. Government Accountability Office (Hrsg.): Major Construction Projects Need a Consistent Approach for Assessing Technol-ogy Readiness to Help Avoid Cost Increases and Delays, Report to the Subcommittee on Energy and Water Development, and Related Agencies, Committee on Appro-priations, House of Representatives, GAO-07-336, March 2007, online: http://aries.ucsd.edu/ARIES/WDOCS/ARIES07/GAO %20Report%20May%202007.pdf, re-trieved 15.08.2018.

[15] U.S. Government Accountability Office (Hrsg.): Technology Readiness Assessment Guide – Best Practices for Evaluating the Readiness of Technology for Use in Acquisi-tion Programs and Projects, GAO-16-410G, August 2016, online: http://www.gao.gov/assets/680/679006.pdf, retrieved 15.08.2018.

[16] GAO, Coal Power Plants: Opportunities Ex-ist for DOE to Provide Better Information on the Maturity of Key Technologies to Reduce Carbon Dioxide Emission, GAO-10-675 (Washington, D.C.: June 16, 2010).

[17] U.S. Department of Energy (Hrsg.): 2014 Technology Readiness Assessment – Over-view – A checkpoint along a challenging journey, DOE/NETL-2015/1711, January 2015, online: https://www.netl.doe.gov/

File%20Library/Research/Coal/Reference %20Shelf/DOE-NETL-20151711-2014- Technology-Readiness-Assessment-Over-view.pdf, retrieved 15.08.2018.

[18] DIN ISO Norm 16290: 2016-09, Raum-fahrtsysteme – Definition des Technologie-Reifegrades (TRL) und der Beurteilung-skriterien (ISO 16290:2013).

[19] U.S. Department of Energy (Hrsg.): Tech-nology Readiness Assessement Guide, DOE G 413.3-4, October 2009, online: https://www.directives.doe.gov/directives-docu ments/400-series/0413.3-EGuide-04/ @@images/file, retrieved: 15.08.2018.

[20] W.L. Nolte, “Technology Readiness Level Calculator”, Space System Engineering and Acquisition Excellence Forum, the Aerospace Corporation, April 2005.

[21] D.W. Engel, A.C. Dalton, K. Anderson et al.: Development of Technology Readiness Level (TRL) Metrics and Risk Measures, U.S. Department of Energy (Hrsg.), PNNL-21737, October 2012, online: http://www.pnnl.gov/main/publications/external/technical_reports/PNNL-21737.pdf, re-trieved 15.08.2018.

[22] D. Alexander, K. Gerdes, L. Holton et al.: Technology Readiness Assessment of Depart-ment of Energy Waste Processing Facilities: Lessons Learned, Next Steps, WM2008 Conference, February 24-28, 2008, Phoe-nix, AZ, online: http://www.wmsym.org/archives/2008/pdfs/8290.pdf, retrieved: 15.08.2018. l

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The electricity sector

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The role of renewables energy

in Europe

Power market, technologies and acceptance

Dynamic process simulation as an engineering tool

European Generation Mix

Flexibility and Storage

1/2

2012

International Journal for Electricity and Heat Generation

ISSN 1435–3199 · K 123456 l International Edition

Publication of VGB PowerTech e.V. l www.vgb.org

The electricity sector

at a crossroads

The role of

renewables energy

in Europe

Power market,

technologies and

acceptance

Dynamic process

simulation as an

engineering tool

European

Generation Mix

Flexibility and

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