ANALYSIS OF SUPERCRITICAL CO2 CLOSED BRAYTON CYCLE APPLIED TO SMR

Abstract

Supercritical CO2 Brayton cycle has been studied to be used as power conversion system in many fields because of its competitive efficiency and compact structure. When applied to nuclear power systems, cycle parameters that have direct effects on cycle performances will be restricted differently under different reactor designs. At the same time, the small reactor systems based on existing technology and experience in PWR, especially the Small Modular Reactor (SMR), have been favored owing to their advantages on safety, feasibility, and economic benefit. The focus of the present work is on the performances of recuperated S-CO2 Brayton cycle applied to small PWR systems that have a relatively low range of core outlet temperature below 330 oC. The calculation model is based on the laws of thermodynamics and the cycle performance were analyzed. The results show that the inlet parameters of compressor have obvious effects on cycle efficiency due to the radical change of CO2’s thermal physical properties near the critical point. With given reactor outlet temperature and recuperator effectiveness, the cycle efficiency reduces as the compressor inlet temperature rises from 32 to 40 oC. And there exists an optimal compressor inlet pressure to achieve maximum cycle efficiency. Furthermore, this optimal inlet pressure depends on the compressor inlet temperature. It will slightly increase and the maximum cycle efficiency will decrease at elevated compressor inlet temperature. Once the compressor inlet pressure exceeds the optimal value, the influence of compressor inlet temperature on cycle efficiency decreases to negligible. Pinch point is a key consideration for S-CO2 Brayton cycle. Promotion of the recuperator effectiveness can improve the cycle efficiency, but temperature difference at the cold end of recuperator ( ΔTc ) decreases at the same time. Thus the recuperator effectiveness and cycle efficiency has upper limitations to avoid the pinch point. Cycle efficiency shows two extremums according to compressor inlet pressure for given ΔTc . However, the latter situation is not suitable for practical engineering on account of internal pinch point. The results are helpful for the application of S-CO2 Brayton cycle in nuclear power system and provide a basis for subsequent experimental research.