Thermodynamic Comparison and Optimization of Supercritical CO2 Brayton Cycles with a Bottoming Transcritical CO2 Cycle

This study investigated the feasibility of a combined cycle comprising a topping SCO2 cycle and a bottoming TCO2 cycle (SCO2-TCO2 cycle). A simple SCO2 cycle and a recompression SCO2 cycle were considered as the topping configurations. Thermodynamic analyses and comparison were performed to evaluate the effects of key thermodynamic parameters on the behavior of combined SCO2-TCO2 cycles. In addition, a parameter optimization was achieved by means of a genetic algorithm to reach the maximum overall thermal efficiency. The results show that the thermal efficiency of the simple SCO2-TCO2 cycle increased with an increase in SCO2 turbine expansion ratio and compressor inlet temperature. However, for the recompression SCO2-TCO2 cycle the thermal efficiency increased and then decreased as the SCO2 turbine expansion ratio increased. Both the modified SCO2 cycles with a bottoming TCO2 cycle had higher performance, with thermal efficiency increase of 10.12 and 19.34% for combined recompression and simple configurations, respectively, in comparison to their original values. Furthermore, the values of exergy efficiency were 60.72 and 64.79% for the simple SCO2-TCO2 cycle and recompression SCO2-TCO2 cycle, respectively. The simple SCO2-TCO2 cycle and recompression SCO2-TCO2 cycle had a power ratio of 16.21 and 11.26%, respectively.