Experimental investigation of a novel two-stage heat recovery heat pump system employing the vapor injection compressor at cold ambience and high water temperature conditions

Heat pumps (HPs) are energy-efficient space heating devices that are key to global carbon reduction and carbon neutrality. However, current commercial HPs have performance issues in cold climates where space heating is needed most, including low COP and high energy consumption of defrosting. Aiming to tackle these issues, a novel two-stage heat recovery heat pump (THRHP) is therefore developed to enable heat recovery from exhaust air for improving COP and preventing dramatic energy use during winter defrosting. The performance of THRHP was optimized in the laboratory by investigating its expansion valve opening and exhaust air fans situation. Finally, the experiment results showed the prototype provided a heating capacity of 32.3 kW, generating 4 m3/h hot water of 55 degrees C with COP of 2.57 at outdoor temperature of 0 degrees C, achieving 20.1% higher COP than the commercial HPs, while the efficient and quick defrosting process only consumed 0.46 kW and 4 mins under outdoor temperature on-6 degrees C. The results gave more insights into the characteristics of THRHP and obtained the optimal control strategies of THRHP for better performance, thus promoting the wide deployment of HPs and achieving the ambitious carbon-neutrality targets.