Integration of an auxiliary heat pump into a transcritical carbon dioxide heating system provides an effective solution for managing high return-water temperatures in district heating applications, where conventional carbon dioxide heat pumps experience significant performance degradation. However, system performance is strongly affected by thermodynamic interactions between the auxiliary and carbon dioxide subsystems, requiring simultaneous optimization of multiple operating and design parameters. Existing studies have mainly optimized individual variables, such as discharge pressure or intermediate water temperature, rather than the complete integrated configuration. This study addresses this limitation by conducting a comprehensive system-level optimization using a fast and elitist Single-Objective Genetic Algorithm to simultaneously optimize operating pressures, temperature levels, and flow distribution to maximize the coefficient of performance. Six refrigerants namely isobutane, 1,1,1,2-tetrafluoroethane, 2,3,3,3-tetrafluoropropene, ammonia, 1,1-difluoroethane, and propane were evaluated as working fluids for the auxiliary heat pump. Isobutane achieved the highest system performance, followed by ammonia and 1,1,1,2-tetrafluoroethane. System performance was strongly influenced by the selected design variables, which required careful balancing to maximize the coefficient of performance. For most refrigerants, the optimal temperature reduction across the auxiliary evaporator ranged from 11.0 to 12.3 °C, whereas isobutane and 1,1,1,2-tetrafluoroethane achieved optimal performance at approximately 15 °C. The optimum total and split water flow rates directed to the carbon dioxide loop were approximately 7 kg/s and 2.9–3.25 kg/s, respectively. The results confirm that integrating auxiliary heat recovery with comprehensive multi-variable optimization reduced the power consumption of the parallel compression system by approximately 39.8% and improved the coefficient of performance from 1.83 to approximately 3.04.
