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 η_ex improvement and LCOE reduction can be obtained by both the two combined cycles relative to the stand-alone S-CO₂ cycle. The maximal η_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 ȼ / (kW·h) and 0.60 ȼ / (kW·h), respectively.