Performance analysis of different high-temperature heat pump systems for low-grade waste heat recovery

Different heat pump systems were used to recover the heat from waste water with mean temperature of 45 °C and produce hot water with the temperature up to 95 °C. Those systems include single-stage vapor compression heat pump (system 1), two-stage heat pump with external heat exchanger (system 2), two-stage heat pump with refrigerant injection (system 3), two-stage heat pump with refrigerant injection and internal heat exchanger (system 4), two-stage heat pump with flash tank (system 5) and two-stage heat pump with flash tank and intercooler (system 6). Thermodynamic and economic analyses were conducted to compare the performance of each system. Results showed that the COP and exergy efficiency for both system 5 and system 6 are quite close, and much higher than those of other systems. Besides, the payback period of both system 5 and system 6 are also shorter as compared to other systems. Considering both the thermodynamic performance and economic quality of the system, system 5 is finally preferred since less initial investment is required for system 5 as compared to system 6.