THERMO-ECONOMIC ANALYSES AND COMPARISONS OF TWO S-CO2-BRAYTON-CYCLE-BASED COMBINED POWER CYCLES FOR CONCENTRATED SOLAR POWER PLANTS

In order to pursue superior cycle efficiency and lower power generation cost for the CSP plants, two S-CO2 Brayton cycle based power cycles with different utilization methods of the residual heat recover of the top S-CO2 Brayton cycle (SCBC) are investigated to seek alternatives to the stand-alone S-CO2 cycle as the power block of concentrated solar power plants. The residual heat released by the top S-CO2 cycle are either utilized to drive a LiBr absorption chiller (AC) for further chilling of the CO2 fluids exiting the precooler before entering the main compressor inlet temperature or recovered by an organic rankine cycle (ORC) for generating electricity. Thermo economic analysis and optimization are performed for the SCBC AC and SCBC ORC, respectively. The results show that the thermal and exergetic efficiencies of the SCBC AC are comparable with those of the SCBC ORC in low pressure ratio conditions (PR<2.7) but are apparently lower than SCBC ORC when PR is over 2.7. The LCOE of the CSP plant integrated with SCBC AC is more sensitive to the change of PR. The optimal PR to maximum the cycle efficiency or minimize the plant LCOE for the SCBC ORC is higher than that for the SCBC AC, while the optimal recuperator effectiveness to minimize the LCOE of CSP plant integrated with SCBC ORC is lower than that of SCBC AC. The optimization results show that the thermo-economic performance of the SCBC AC is comparable to that of the SCBC ORC. Significant eta(ex) improvement and LCOE reduction can be obtained by both the two combined cycles relative to the stand-alone S-CO2 cycle. The maximal eta(ex) improvements obtained by the SCBC ORC and SCBC AC are 6.83% and 4.12%, respectively. The maximal LCOE reduction obtained by the SCBC-ORC and SCBC-AC are 0.70(sic)/(kW.h) and 0.60(sic)/(kW.h), respectively.