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New text comparison between CO2 and other supercritical working fluids (ethane, Xe, CH4 and N-2) in line-focusing solar power plants coupled to supercritical Brayton power cycles
This study is focused on comparing four supercritical fluids: Ethane, Xenon, Methane and Nitrogen, as possible alternative to supercritical Carbon Dioxide (s-CO2) in Brayton power cycles coupled to line-focusing solar power plants with Solar Salt (60% NaNO3; 40% KNO3) as heat transfer fluid. The Simple Brayton cycle with heat recuperation and reheating is the configuration selected in this paper, providing a balance of plant design with reduced number of equipment and cost. The gross plant efficiency is calculated fixing the recuperator conductance (UA) for different Turbine Inlet Temperatures (TIT), confirming the maximum plant gross efficiency is related with the minimum allowable recuperator pinch point temperature. The reheating pressure and compressor inlet temperature are optimized with the mathematical algorithms SUBPLEX, UOBYQA and NEWOUA. According to the REFPROP database ranges of applicability, the maximum TIT limits are established for the supercritical fluids (N-2 TIT = 550 degrees C, CO2 TIT = 550 degrees C, C2H6 TIT = 400 degrees C, Xe TIT = 450 degrees C and CH4 TIT = 350 degrees C). The reference scenario considered for calculating the thermosolar plant energy balances and simulations is the wet-cooling system with a Compressor Inlet Temperature (CIT = 32 degrees C). The gross efficiency results with the wet-cooling system are: N2 (45.8%), CO2 (44.37%), C2H6 (40.74%), Xe (39.88%), CH4 (32.15%). The plant efficiency is also translated into solar field effective aperture area and estimated cost, for a fixed power output. For optimizing the solar collector aperture area and cost, the Primary Heat Exchanger (PHX) and the ReHeating Heat Exchanger (RHX) capacity ratio (CR) are fixed (CR = 1). The dry-cooling system scenario (CIT = 47 degrees C) is alto estimated: N-2 (43.34%), CO2 (42.42%), C2H6 (37.34%), Xe (37.26%), CH4 (29.53%).