Advanced Supercritical Carbon Dioxide Power Cycle Configurations for Use in Concentrating Solar Power Systems

by Zhiwen Ma and Craig S. Turchi
National Renewable Energy Laboratory

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Concentrating Solar Power (CSP) utilizes solar thermal energy to drive a thermal power cycle for the generation of electricity. CSP technologies include parabolic trough, linear Fresnel, central receiver or “power tower,” and dish/engine systems. The resurgent interest in CSP has been driven by renewable portfolio standards in southwestern states and renewable energy feed-in tariffs in Spain. CSP systems are deployed as large, centralized power plants to take advantage of economies of scale. A key advantage of certain CSP systems, in particular parabolic troughs and power towers, is the ability to incorporate thermal energy storage. Thermal energy storage is less expensive and more efficient than electric storage and allows CSP plants to increase capacity factor and dispatch power as needed – for example, to cover evening or other demand peaks.

Current CSP plants utilize oil or steam to transfer solar energy to the power block. These fluids 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 controls and has limited storage capacity. Higher operating temperatures generally translate into higher thermal cycle efficiency and often allow for more efficient thermal storage. To obviate these limitations, alternative fluids are under investigation by research teams worldwide.

Supercritical carbon dioxide (S-CO2) operated in a closed-loop recompression Brayton cycle offers the potential of equivalent or higher cycle efficiency versus supercritical or superheated steam cycles at temperatures relevant for CSP applications. The S-CO2 pressure is higher than superheated steam but lower than supercritical steam at temperatures of interest. The high pressure required for S-CO2 makes application to trough fields difficult [4], and preliminary analysis suggests the fluid may be better suited for use in Power Towers. Even circulating high pressure S-CO2 through a large Power Tower would be a challenge due to the volume and pressure of fluid being moved [9]. However, a modular power tower design introduced in this paper can take advantage of S-CO2’s potential without prohibitive piping costs.

In the proposed design, a single-phase process using S-CO2 as both heat transfer fluid (HTF) and thermal power cycle fluid simplifies the power system configuration. The design is compatible with sensible-heat thermal energy storage, if desired. The simpler machinery and compact size of the S-CO2 process may also reduce the installation, maintenance and operation cost of the system. Brayton-cycle systems using S-CO2 have smaller weight and volume, lower thermal mass, and less complex power blocks versus Rankine cycles due to the higher density of the fluid and simpler cycle design. The lower thermal mass makes startup and load change faster for frequent start up/shut down operations and load adaption than a HTF/steam based system. The research will characterize and evaluate advanced S-CO2 Brayton cycle power generation with a modular power tower CSP system.