Thermodynamic assessment of combined supercritical CO2 (SCO2) and organic Rankine cycle (ORC) systems for concentrated solar power

Concentrated solar power (CSP) systems are acknowledged as a promising technology for solar energy utilisation. Supercritical CO2 (SCO2) cycle systems have emerged as an attractive option for power generation in CSP applications due to the favourable properties of CO2 as a working fluid. In order to further improve the overall performance of such systems, organic Rankine cycle (ORC) systems can be used in bottoming-cycle configuration to recover the residual heat. This paper presents a thermodynamic performance assessment of a combined SCO2/ORC system in a CSP application using parabolic-trough collectors. The parametric analysis indicates that the heat transfer fluid (HTF) temperature at the inlet of the cold tank, and the corresponding HTF mass flow rate, have a significant influence on the overall system performance. The results suggest that the combined system can offer significant thermodynamic advantages at progressively lower temperatures. Annual simulations for a case study in Seville (Spain) show that, based on an installation area of 10,000 m2, the proposed combined cycle system could deliver an annual net electricity output of 2,680 MWh when the HTF temperature at the cold tank inlet is set to 250 °C, which is 3% higher than that of a stand-alone CO2 cycle system under the same conditions. Taking the size of the thermal storage tanks into consideration, a lower HTF temperature at the cold tank inlet and a lower mass flow rate would be desirable, and the combined system offers up to 66% more power than the stand-alone version when the HTF inlet temperature is 100 °C