Supercritical CO2mixtures for Brayton power cycles complex configurations with concentrating solar power

Abstract

An evaluation of the impact of using supercritical carbon dioxide mixtures (s-CO2/C2H6, s-CO2/CH4, s-CO2/Kr, and s-CO2/SF6) as a working fluid is made here for Brayton s-CO2 power cycles. The considered complex configurations include recompression with two reheating (RCC-2RH), recompression with three reheating (RCC-3RH), recompression with main compressor intercooling and two reheating (RCMCI-2RH), and recompression with main compressor intercooling and three reheating (RCMCI-3RH), which were coupled to a linear-focus solar system with Solar Salt (60% NaNO3/40% KNO3) as the heat transfer fluid (HTF). The design parameters evaluated the solar plant performance at the design point, the aperture area of the solar field, and variations in costs regarding the plant's total conductance (UAtotal). The methodology used to calculate the performance established the total conductance values of the heat recuperator (UAtotal) to between 5 and 25 MW/K. The main conclusion is that the cycle efficiency has a considerable improvement compared with that obtained using pure s-CO2. The s-CO2/Kr mixture with a molar fraction ratio of 30/70 increases the cycle efficiency between 7-11% relative to pure s-CO2 and as a function of the UAtotal. The s-CO2/CH4 mixture with a molar fraction of 45/55 increases between 3-7%, and the s-CO2/C2H6 and s-CO2/SF6 mixtures only increase between 1- 2%. For the solar field unitary costs, the s-CO2/Kr mixture has the lowest cost at $29-34 million USD, which depends on the solar field aperture area and the UAtotal for the RCC-2RH and RCMCI-2RH configurations. Finally, the results demonstrate that variations in the working fluid properties play a significant role due to the positive impact on the increased thermal efficiency of the s-CO2 Brayton cycle when using the RCC and RCMCI configurations.