Thermodynamic Analysis and Comparison of Supercritical Carbon Dioxide Cycles

by Martin Kulhánek & Václav Dostál
Czech Technical University in Prague

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The recompression Supercritical Carbon Dioxide (S-CO2) power conversion cycle has been proposed for future energy applications because of its high thermal efficiency. However, specific properties of the recompression cycle may not fit all applications. Therefore, a comparative analysis of the simple Brayton, precompression, recompression, and partial cooling cycles was performed. Besides the usual analysis of cycle thermal efficiency, aspects related to coupling of a S-CO2 cycle to a nuclear reactor have been investigated, together with effects of turbine inlet temperature and pressure ratio on the cycle performance. In terms of thermal efficiency, the recompression cycle shows, consistent with previous studies, to be the best option in the range of turbine inlet temperatures between 500°C to 600°C. At higher temperatures, however, the partial cooling cycle is superior. Additionally, increasing turbine inlet temperature improves the precompression cycle thermal efficiency considerably. Above 700°C, the precompression cycle achieved equivalent efficiency as the recompression cycle; and its efficiency surpasses the recompression cycle at temperatures above 850°C despite of being less complex (no flow split) than the recompression cycle. All of the most sophisticated power cycle options achieved higher efficiency than the simple Brayton cycle. In terms of power output and number of loops (for a specified nuclear reactor thermal power, turbine inlet temperature, and cycle flow rate), the primary competitors are the recompression and partial cooling cycles. Both produce similar power, but the recompression cycle needs one power conversion loop more than the partial cooling cycle (5 versus 4). However, the partial cooling cycle is more complex, and more investigation is necessary before a final endorsement is appropriate. A sensitivity analysis of cycle response to a pressure ratio deviation from its optimum (efficiency based) value shows the suitability of the precompression cycle to pressure ratio control, as well as the maneuverability of the pressure ratio for the partial cooling and precompression cycles without adversely impacting thermal efficiency. The recompression cycle efficiency, however, drops markedly when pressure ratio deviates from the optimum value.