Solar Energy Perspectives - IEA

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Solar Energy Perspectives: Solar thermal electricity The high concentrating ratio (hundreds of suns) of the tower concept achieves high temperatures – 565° for molten salts, 550°C for the steam – thereby increasing the efficiency at which heat is converted into electricity. Improved efficiency also means a lower cooling load, thus reducing water consumption in plants in arid areas. It would also reduce the performance penalty of dry cooling. In addition, the concept is highly flexible; designers can choose from a wide variety of heliostats, receivers, transfer fluids and power blocks (some plants have several towers that form one power block). The potential for cost reduction in thermal storage is particularly impressive. Solar towers using molten salts as HTFs and storage media need three times less storage media than current trough plants, thanks to the larger temperature difference between the hot and cold molten salts. The possibilities of even higher temperatures should be explored using different receiver technologies. One option is supercritical steam cycles, such as those used in modern coalfired power plants, which reach overall efficiencies of 42 to 46% with supercritical and ultrasupercritical designs (thermal-to-electric efficiencies of 45% to 50%). Typically, modern coal-fired power plants use steam at up to 620 °C and 24 MPa to 30 MPa, but by 2020 could reach 700 °C and 35 MPa, using nickel-based alloys to achieve overall efficiencies approaching 50%. The application of this technology to solar towers, however, will require some adaptation. Direct steam generation (DSG) will pose particular challenges in synchronising solar fields with receivers and supercritical steam turbines. A continuous management of solar collectors will be needed to avoid problems during start-up and variations caused by clouds and at sunset. Solar towers with high-temperature HTFs and storage may prove more capable of fulfilling these requirements, as they disconnect solar heat collection and power generation. Superheating with some fuel, or full hybridisation in solar-gas or solarcoal plants (see below) could also help address these challenges. Solar towers would need to be paired to fuel each single supercritical turbine, whose minimum electric capacity today is 400 MW. High-temperature tower concepts also include atmospheric air as the HTF (tested in Germany with the Jülich solar tower project) with solid material storage. Solar-to-electricity efficiencies of up to about 25% can be delivered by such towers, but it is not yet clear if the gain in efficiency may compensate for the cost and complication of the cycle. Molten salts decompose at higher temperatures, while corrosion limits the temperatures of steam turbines. Higher temperatures and efficiencies could rest on the use of fluorideliquid salts as HTFs up to temperatures of 700°C to 850°C, with closed-loop multi-reheat Brayton cycles using helium or nitrogen, which have initially been developed for hightemperature nuclear reactors (Figure 8.3). On top of higher plant efficiency, such power systems operate at relatively high pressure and power densities that implies smaller equipment than for steam cycles with their large low-pressure low-power-density turbines, so they could cost less. The preferred heat-storage medium in this case would be graphite. Solar-based open Brayton cycles offer a completely different way of exploiting the higher working temperatures that towers can achieve. Pressurised air would be heated in the solar receivers, and then sent directly to a gas turbine, at a temperature exceeding 800°C. The pressurised air can be further heated in a gas fired combustion chamber to reach 1300°C, 146 © OECD/IEA, 2011
Chapter 8: Solar thermal electricity which makes the gas turbine more efficient. Excess heat after running the gas turbine could be sent to a steam cycle running a second generator. The solar-to-electricity efficiency could be higher than 30%. Heat storage, however, is still an unresolved issue for such plants, while fossil-fuel (or gasified biomass) back-up is more straightforward. Back-up fuel heating the air from the solar receiver could be used to manage solar energy variations, and if necessary continuously raise the temperature level. This concept was developed through the 100-kW Solgate project led by the German Aerospace Center (DLR), as illustrated on Figure 8.4. Pressurised air tube receivers were successfully tested in the middle 1980s in the German-Spanish GAST Technological project in Almeria reaching 850°C and 9 bars with metallic tubes and 1 100°C with ceramic ones. The 2-MW demonstration Pegase project set up by the French research centre CNRS will follow on the Themis solar tower in the Pyrenees. A more powerful 4.6-MW project along the same lines, Solugas, will run as a demonstration plant on a specially erected new tower at the Abengoa Solar test facility near Seville, Spain. Figure 8.3 Scheme of fluoride-liquid salt solar tower associated with a closed Brayton cycle Solar power tower Heat and salt storage Brayton power cycle Hot liquid salt Store heat Remove heat Generator Graphite heat storage Helium or nitrogen Recuperator Store heat Cold liquid salt Remove heat Cooling water Gas compressor Salt storage tank Source: Forsberg, Peterson and Zhao, 2007. Key point Solar towers may gain in efficiency with higher-temperature cycles. Parabolic dishes offer the highest solar-to-electric conversion performance of any CSP system. Most dishes have an independent engine/generator (such as a Stirling machine or a microturbine) at the focal point. Several features – the compact size, absence of water for steam generation, and low compatibility with thermal storage and hybridisation – put parabolic dishes in competition with PV modules, especially concentrating photovoltaics (CPV), as much as with other CSP technologies. Very large dishes, which have been proven compatible to thermal storage and fuel back-up, are the exception. Promoters claim that mass production 147 © OECD/IEA, 2011
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Renewable Energy Renewable TECHNOLO
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Renewable Energy Technologies Solar
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INTERNATIONAL ENERGY AGENCY The Int
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Foreword Foreword Solar energy tech
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Acknowledgements Acknowledgements T
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Table of contents Table of Contents
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Table of contents Chapter 7 Solar h
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Table of contents Chapter 3 Chapter
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Table of contents Chapter 4 Figure
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Table of contents Figure 10.4 • N
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Executive Summary Executive Summary
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Executive Summary A possible vision
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Chapter 1: Rationale for harnessing
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PART A MARKETS AND OUTLOOK Chapter
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Chapter 2: The solar resource and i
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Chapter 3: Solar electricity Chapte
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Chapter 3: Solar electricity Versat
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Chapter 3: Solar electricity Figure
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Chapter 3: Solar electricity Figure
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Chapter 3: Solar electricity to a t
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Chapter 3: Solar electricity New in
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Chapter 3: Solar electricity Variou
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Chapter 3: Solar electricity as fro
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Chapter 3: Solar electricity peak t
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Chapter 3: Solar electricity Kenya
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Chapter 3: Solar electricity • Ad
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Chapter 4: Buildings Chapter 4 Buil
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Chapter 4: Buildings South Africa,
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Chapter 4: Buildings Figure 4.3 Bui
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Chapter 4: Buildings Défense near
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Chapter 4: Buildings larger the col
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Chapter 4: Buildings Pumps variant,
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Chapter 4: Buildings radiators or h
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Chapter 4: Buildings Most widesprea
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Chapter 4: Buildings Photo 4.5 Mans
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Chapter 4: Buildings the overall po
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Chapter 4: Buildings area is limite
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Chapter 4: Buildings Figure 4.11 An
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Chapter 5: Industry and transport C
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Page 111 and 112: PART B TECHNOLOGIES Chapter 6 Solar
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Chapter 11: Testing the limits grow
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Chapter 11: Testing the limits betw
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Water availability is unlikely to b
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Chapter 11: Testing the limits 10 0
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Chapter 11: Testing the limits tran
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Chapter 12: Conclusions and recomme
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Chapter 12: Conclusions and recomme
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Annex A Annex A Definitions, abbrev
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Annex A REC RPS SACP sc-Si SEI SEII
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Annex B Annex B References A.T. Kea
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Annex B Greenpeace International, S
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Annex B Malbranche, Ph. et C. Phili
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