AMP_04_May_June_2021_Digital_Edition

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 1 1 6 W hile lithium-ion batteries have enabled rapid deployment of energy storage, most commer- cial applications havebeen limited to four hours of storage or less. Longer-duration storage can help alleviate the impact of extended periods of cloudy weather, for example, or even seasonal variations of solar energy production. Concentrating solar-thermal pow- er (CSP) plants use mirrors to capture the sun’s energy as heat, which can be stored in a thermal energy storage (TES) system and used to produce elec- tricity on demand, even when the sun is not shining (Fig. 1). There are nearly 100 CSP plants in commercial operation worldwide, representing almost 7 GW of capacity [1] . Like coal or natural gas plants, CSP systems use turbine-based heat engines to generate electricity. Existing commercial CSP plants with TES have demonstrated the viabil- ity of thermal storage to be responsive to grid needs, particularly for long du- rations of daily storage, up to 17 hours, which is not currently economically vi- able with lithium-ion batteries. Ther- mal systems can decouple the storage capacity component (e.g., molten salt stored in tanks) from the power-gen- erating component (heat engine). This enables system designers to increase marginal storage capacity or duration— without having to build additional gen- erating capacity—by increasing the size of the storage tank. The combination of readily scalable TES and convention- al turbine technology can allow CSP to provide reliable and flexible renewable electricity production. To achieve cost-competitive de- ployment of CSP in the United States, SETO has set a cost target of $0.05 per kilowatt-hour electric (kWh e ) for base- load plant configurations with 12 or more hours of storage, which could enable deployment of 25 to 160 GW of U.S. CSP capacity by 2050 [2] . To reach this cost target, SETO is supporting the development of next-generation CSP plants that are capable of deliver- ing heat to the power cycle at tempera- tures exceeding 700°C, increasing plant efficiency. Current technologies that use molten salt cannot exceed 565°C 2 megawatt (MW) thermal integrated test facility to validate the proposed system. Although SETO selected the sol- id particle pathway for development of a MW-scale test facility, continued research in other high-temperature pathways for heat transfer may be war- ranted. Systems that use molten chlo- ride salt for TES may be useful for some applications, given the well-understood dynamics of liquid heat transfer. How- ever, the relative complexity of the sys- tem, particularly regarding chemical purification and environmental con- trol systems to prevent corrosion, ulti- mately led SETO to deprioritize future investments in pathways based on mol- ten chlorides until several significant risks—especially in the receiver, TES de- sign, and chemical control—could be sufficiently retired for full integrated system testing. Beyond electricity generation, so- lar-thermal technologies may also be able to help meet the national goal of decarbonizing the entire U.S. energy sector by 2050 by providing industrial process heat. Even with more renew- able electricity available, many industri- al processes will be difficult to electrify owing to thermal decomposition of the salt, which is used as a heat-transfer fluid. If successful, these third-gener- ation (Gen3) CSP designs will enable CSP systems to utilize advanced pow- er cycles based on supercritical carbon dioxide(sCO 2 ),whicharemuchmoreeffi- cient than existing steam-based cycles. To achieve these high tempera- tures, projects in SETO’s Gen3 CSP funding program have been investigat- ing and designing fully integrated ther- mal transport systems based on three competing concepts for the heat-trans- fer media (HTM). The choice of HTM is foundational for CSP component de- sign, as it drives most of the materials and design considerations. The Gen3 CSP program was organized by the phase of matter for leading HTM candi- dates—gas, liquid, or solid [3] . Over the past two and a half years, three teams competed to derisk and generate de- tailed thermal transport system designs for each of those potential pathways. In March 2021, SETO announced the selection of Sandia National Labo- ratories to develop a design based on solid particles as the HTM. Sand- ia will receive $25 million to construct a Fig. 1 — The Crescent Dunes Solar Energy Project, in Tonopah, Nevada. Courtesy of SolarReserve.