Heat Transfer Performance Comparisons of Supercritical Carbon Dioxide and NaCl–KCl–ZnCl2 Eutectic Salts for Solar s-CO2 Brayton Cycle

Abstract

Supercritical carbon dioxide (s-CO2) and the new generation molten salts can both be used as the heat transfer fluid in the solar supercritical carbon dioxide Brayton cycle. One of the main concerns in the design of solar s-CO2 Brayton cycle is that the working fluids need to meet the desired heat transfer coefficient to reach the high operating temperature, responding to the high concentrated heat flux. In this paper, to satisfy the SunShot Initiative’s goal of a 10 MWe solar s-CO2 Brayton cycle power plant, the receiver is designed to heat the s-CO2 and the NaCl–KCl–ZnCl2 eutectic salts from 500 ºC to 750 ºC to gain heat of 20 MWth. A tubular receiver made of pressureproof Haynes 230 is assumed. A one-dimensional computational heat transfer model is developed. The influences of receiver operating and structure parameters on the performance of the HTFs are evaluated. Finally, the outlet temperature, the heat gain, the solar field efficiency and the solar exergy efficiency for the s-CO2 and the new molten salts are compared. It is shown that both the fluids can reach the desired temperature and heat gain under special operating and structure parameters, but their relevant effectiveness to convert solar radiation into heat are different. The results obtained in this study are expected to provide references for the further development of the solar s-CO2 Brayton cycle technology.