Thermo-economic analysis of a recompression supercritical CO₂ cycle combined with a transcritical CO₂ cycle

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.