Modelling and performance analysis of supercritical CO₂ pre-cooling cycle solar thermal power tower system

With the increasing shortage of fossil fuels and stricter environmental requirements, the development of new energy is receiving increasing attention. Among them, solar thermal power generation technology has developed rapidly in China in recent years, which can better adapt to the intermittent and random changes of solar energy benefitting from its long-term thermal storage device. As for the tower solar S-CO₂ Brayton cycle, various layouts of S-CO₂ Brayton cycles have been proposed, including reheating cycle, recompression cycle, intermediate cooling cycle, and staged expansion cycle. Among them, the recompression cycle is considered to be one of the most promising cycles due to its simple structure and high cycle efficiency. Nevertheless, the S-CO₂ tower solar recompression cycle power generation system requires a large amount of molten salt in the storage tank, therefore based on the recompression cycle, this paper proposes a S-CO₂ pre-cooling cycle tower solar thermal power generation system, which uses low-temperature molten salt flowing out of the heater in the pre-cooler to cool the high-temperature S-CO₂ at the turbine outlet. On the one hand, it increases the temperature difference of S-CO₂ in the heater, on the other hand, under a certain heating amount, it also increases the temperature difference of molten salt in heat storage tank, reduces the amount of molten salt used, reduces equipment costs, and improves the economic operation of the power generation system. Compared with the recompression cycle, the pre-cooled cycle omits the reheating process and adds one compression process and one intermediate cooling process, forming a two-stage compression intermediate cooling, further reducing the power consumption of the compression process. This article first applies EBSILON software to establish tower solar thermal power generation system models for S-CO₂ recompression cycle and S-CO₂ pre-cooled cycle, respectively; Secondly, the reliability of the mirror field simulation model and the power cycle simulation model was verified using existing measured data from solar thermal power plants and literature data respectively; Finally, a comparative analysis was conducted on the system performance of two power generation systems under design conditions. The results show that compared with the recompression cycle solar thermal power tower system, the S-CO₂ heat absorption temperature difference of the pre-cooling cycle solar thermal power tower system is increased by 200.21 ℃, an increase of 185.36%, and the power generation efficiency is increased by 0.38%; and the amount of molten salt for the pre-cooling cycle solar thermal power tower system is 9513.28 t, which is reduced by 7046.07 t. The S-CO₂ pre-cooling cycle solar thermal power tower system ensures a higher power generation efficiency with a larger S-CO₂ heat absorption temperature difference and a smaller amount of molten salt, which can be used as a reference for the design of tower solar thermal power plants.