Thermo-Economic Analysis of a Recompression Supercritical CO2 Cycle Combined With a Transcritical CO2 Cycle

Abstract

The transcritical CO2 cycle (TCO2 cycle) exhibits good performance in low-grade waste heat recovery area. In this study, a TCO2 cycle was employed as a bottoming cycle to recover the waste heat in the topping recompression supercritical CO2 Brayton cycle (SCO2 cycle). A detailed system analysis was performed of a recompression SCO2 cycle combined with a TCO2 cycle to improve the efficiency of energy conversion. Thermodynamic analysis, calculation of heat exchangers’ area and economic analysis were considered. The SCO2 turbine expansion ratio, TCO2 turbine inlet pressure, high temperature recuperator (HTR) effectiveness and condensation temperature were studied to investigate their effect on the system performance. For the basic analysis, SCO2 turbine inlet temperature was conservatively selected to be 550 °C and the compressor outlet pressure set at 20 MPa. For these operating conditions the proposed combined SCO2-TCO2 cycle yielded about 46% thermal efficiency at a SCO2 turbine expansion ratio of 2.7 and TCO2 turbine inlet pressure of 10 MPa. Similarly, the capital cost per net power output of the combined cycle was found as 6.6 k$/kW, which was ∼ 6% more expensive than that of the recompression SCO2 cycle without the bottoming cycle under the same operating condition. An optimum TCO2 turbine inlet pressure and an optimum SCO2 turbine expansion ratio existed where the system thermal efficiency reached the maximum value. Furthermore, the system thermal efficiency was very sensitive to the changes in the condensation temperature and the HTR effectiveness. The HTR effectiveness also had a strong effect on the ratio of heat exchangers’ cost to the plant capital cost. Additionally, increasing SCO2 turbine inlet temperature would significantly improve the cycle overall thermal efficiency and decrease the plant capital cost per net power output.