Multimode operation control strategy for improving part-load performance of supercritical CO2 Brayton cycle

Abstract

The recompression supercritical CO2 (S-CO2) Brayton cycle is regarded as one of the most promising energy conversion systems owing to its high efficiency and compactness. Because the recompressor impairs the performance of a system at low loads, the concept of multimode operation by switching the recompressor on or off is proposed to improve the part-load performance. In this study, a dynamic model of the recompression S-CO2 cycle with reheating is developed, and three control strategies, including turbine throttle valve control, turbo-alternator-compressor (TAC) speed control, and inventory control, are selected to follow the part-load. Subsequently, the action mechanism of the multimode operation and the effects of different control strategies on the part-load performance of the system under multimode operation is investigated and compared. Simulation results indicate that the improved thermal efficiency by switching operation modes is due to improvement in the thermal match inside the high-temperature recuperator and reduced degradation of the recompressor. Moreover, switching the system to Mode 2 by switching off the recompressor at 20% load under TAC speed control, turbine throttle valve control, and inventory control can improve thermal efficiency by 1%, 3.5%, and 5.4%, respectively. This result can serve as a guideline for the efficient operation of the recompression cycle under part-load.