CO2-based mixture working fluids used for the dry-cooling supercritical Brayton cycle: Thermodynamic evaluation

•The CO2-based mixture working fluids are used for the dry-cooling solar thermal power generation system.•The comprehensive thermodynamic and heat transfer analyses under typical design and operating conditions are investigated.•Mixture working fluids show excellent performance at higher rejection temperatures, especially CO2-H2S.•The CO2-propane has the highest heat transfer coefficient and relatively low pressure loss in the recuperator. Supercritical carbon dioxide Brayton cycle, distinguished by high thermal efficiency and compact structure, has great potential for application in solar thermal power generation systems. However, it faces the problem that the cycle thermal efficiency reduces with increasing ambient temperature in hot and water-deficient areas, where dry cooling is a necessary choice for power generation systems. In this paper, five CO2-based mixture working fluids, including CO2-cyclohexane, CO2-butane, CO2-isobutane, CO2-propane and CO2-H2S, are investigated in a 50 MW recompression Brayton cycles. Their impact on the cycle performance under typical operating conditions, such as main compressor inlet temperature, split ratio, turbine inlet temperature and maximum cycle pressure, are evaluated by energy and exergy analysis. The heat transfer and pressure drop in the recuperator are also discussed. The results show that the Brayton cycles using mixture working fluids have obvious performance advantages compared with sCO2 Brayton cycle under higher ambient temperature. According to the thermodynamic performance of working fluids in the cycle, CO2-H2S is the best choice. However, the CO2-propane has the highest heat transfer coefficient and relatively low pressure loss in the recuperator. This research can be used as a guide to the selection of mixture working fluids for dry-cooling supercritical Brayton cycle.