Comparing line-focusing and central tower solar power plants with s-CO2 binary mixture Brayton power cycles

Abstract

State-of-the-art supercritical carbon dioxide (s-CO2) power cycles represent an important technical solution for optimizing thermal efficiency in concentrated solar power (CSP) plants, and simultaneously provide compact solar fields and power-cycle footprints. The main drawback with CSPs coupled to s-CO2 Brayton power cycles (CSP-sCO2) is the limited compressor inlet temperature (CIT). CSPs are usually located in deserts, where the ambient temperature is above the s-CO2 critical point (31 °C). This has a negative impact on plant performance. To overcome this, high-molecular- weight substances can be added to pure s-CO2, increasing the power cycle working fluid critical temperature and maintaining a moderate operating pressure range. In this paper, two solar field configurations are compared: line-focusing solar collectors and central towers with heliostats. Four Brayton power cycle configurations are considered: simple, recompression, recompression with partial cooling, and recompression with main compressor intercooling (RCMCI). The solar power plant reference power is 115 MW and the CIT is fixed to 45 °C. The overall conductance of the low- and high-temperature recuperators varies from 2.5–35 MW/K, and the supercritical Brayton power cycle performance parameters are optimized. The thermal efficiency is improved, and the solar field effective aperture area can be optimized by adding high-molecular-weight substances to pure s-CO2. The RCMCI configuration achieves the best performance, with a thermal efficiency of 49.5%. This study shows that power cycle working fluid composition is a key issue in new- generation CSP-sCO2 design. The working fluid selection is closely related with the power plant location and depends on the local annual ambient temperature profile.