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A novel solar power tower system that integrates with the cascade supercritical carbon dioxide Brayton-steam Rankine cycle is proposed to tackle the challenges of a simple supercritical carbon dioxide system in solar power systems. It provides a large storage capacity and can react to the fluctuation of solar radiation by adjusting the mass flow rate of molten salts in the receiver and heat exchanger. The fundamental is illustrated and comprehensive mathematical models are built. Energy and exergy analysis in the heat collection and power conversion processes is conducted. A comparison between the novel system and simple supercritical carbon dioxide system is made at a design plant output of 10 MW. Results indicated that: (1) the cascade system has a lower receiver inlet temperature, wider temperature difference across the receiver, higher specific work of the thermal energy storage system and lower mass flow rate of the working fluids. The solar-thermal conversion efficiency of the receiver is improved significantly. The heat gain of the tower receiver of the novel system is 53.4 MWh, which is about 7.1 MWh more than that of the simple system. The electricity production of the cascade system is improved by 9.5% at design point; (2) The novel system can generate constant electricity in a wide range of solar radiation and offer flexible control strategy for heat collection and storage. It is a promising option for central solar tower technology with a high efficiency, large storage capacity and short payback period.
supercritical carbon dioxide; brayton cycle; rankine cycle; solar tower; cascade system