New text comparison between CO2 and other supercritical working fluids (ethane, Xe, CH4 and N2) 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%). For predicting the recuperator heat exchanger dimensions for a fixed conductance (UA), the heat transfer coefficient (HTC) is calculated with the Dittus Boelter correlation and compared with the CO2 as reference. The C2H6, and CH4 have relative higher HTC in relation with CO2. Also is calculated the recuperator pressure drop. The C2H6, CH4 and N-2 pressure drop is lower in comparison with the CO2 for the same operating conditions. The energy efficiency in solar power station coupled to Brayton cycle is very constrained by the ambient temperature variation, impacting directly in the dry-cooling system performance. For this reason a Compressor Inlet Temperature (CIT) sensing analysis is carried out ranging from 32 degrees C to 57 degrees C, and also varying TIT from 400 degrees C to 550 degrees C. A sensing analysis is also developed varying the Turbine Inlet Pressure (TIP) from 200 bar to 375 bar. The CO2 improves the plant efficiency when increasing the TIP from 250 bar to 350 bar, however the rest of fluids (Ethane, Methane, Nitrogen and Xenon) nearly not suffered any impact in the plant efficiency when increasing the TIP. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.