Thermal Performance and Economic Analysis of Low-Temperature Waste Heat Heating Systems

Against the backdrop of the "dual carbon" goals， in this study we aimed to achieve efficient recovery and reuse of low-grade waste heat generated by high-voltage direct current （HVDC） converter valves. Using R515b as the system refrigerant， thermodynamic models and economic models were established. Through experimental testing， the thermodynamic performance variations and economic analysis of evaporative compression heat pumps versus ejector compression heat pumps were compared at different evaporation and condensation temperatures. The results indicate that the COP of the injection compression heat pump was， on average， 8.75% higher than that of the vapor compression heat pump， with the compressor power consumption reduced by 4.78% and the system heating capacity increased by 3.50%. The total friction losses in the injection compression heat pump were primarily concentrated in the condenser and injector， accounting for 32.6% and 27%， respectively. Economically， the net present value of the injection compression heat pump system increased by approximately 10.9% compared to traditional systems， with a dynamic payback period of 4.08 years. The results show that injection compression heat pumps effectively recover expansion losses， enhance the system energy efficiency and economic performance， and provide an efficient and economical solution for recovering waste heat from inverter valves.