Comparative Study and Optimization of Thermal Economic Performance in Different Supercritical CO2 Recompression-Brayton Combined Cycle Systems

Abstract To effectively utilize the low-temperature waste heat from the supercritical CO2 recompression Brayton cycle (SCRBC), this study selected the organic Rankine cycle (ORC), transcritical CO2 cycle (tCO2), and organic flash cycle (OFC) to form three combined cycles, i.e., SCRBC/ORC, SCRBC/tCO2, and SCRBC/OFC. First, based on the principles of mass conservation and energy conservation, steady-state mathematical models for the three combined cycles were established. Then, the impact of key decision variables on system performance were comprehensively examined through parametric analysis. To accommodate the diverse requirements of industrial applications, three different top-cycle turbine inlet temperatures were selected, i.e., 500°C, 550°C, and 600°C, respectively. Finally, multi-objective optimization studies were conducted for each of the three combined-cycle systems. The results indicate that there are optimal values for the top cycle split ratio and the main compressor inlet pressure, which can be used to optimize the system's thermodynamic performance. Additionally, a smaller evaporator pinch point temperature difference and higher turbine inlet temperature can enhance the system's overall performance. Under the optimal compromised solution at T5=550°C, the SCRBC/tCO2 system achieves the highest cycle efficiency, increasing by 1.2% and 2.1% compared to SCRBC/ORC and SCRBC/OFC, respectively. It also exhibits the lowest total product unit cost, decreasing by 0.37% and 1.37% compared to SCRBC/ORC and SCRBC/OFC, respectively. The same trend is observed when T5 is at 500°C and 600°C.