Supercritical CO2 for Application in Concentrating Solar Power Systems

by Craig S. Turchi
National Renewable Energy Laboratory

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Current Concentrating Solar Power (CSP) plants utilize oil or steam to transfer solar thermal energy to a steam Rankine Cycle power block. These heat transfer fluids (HTFs) have properties that limit plant performance, for example, the synthetic oil has an upper temperature limit of 400°C while direct steam generation requires complex control and has limited storage capacity. Accordingly, alternative fluids are under investigation by research teams worldwide. Supercritical carbon dioxide (s-CO2) has been identified as a potential HTF for CSP plants.

Supercritical CO2 could be deployed as the HTF in the solar field or utilized as both HTF and power cycle fluid in a Brayton Cycle power block. If used simply as an HTF, s-CO2 provides advantages of higher operating temperatures, no threat of freezing, and single-phase operation. Applying s-CO2 in a Brayton Cycle offers additional potential for system efficiency and savings by eliminating heat exchangers and reducing the size of the power block equipment. Challenges include the high pressure necessary for s-CO2 operation, interfacing s-CO2 with thermal energy storage, and the lack of operating experience with s-CO2 Brayton Cycles.

The presentation will summarize ongoing research by the Department of Energy, via the National Renewable Energy Laboratory (NREL), Sandia National Laboratories, and private contractors to investigate s-CO2 use in parabolic trough and solar power tower systems. Specific attention is given to the match between s-CO2 properties and typical solar thermal operating conditions. Opportunities for further investigation and collaborative research will be discussed.