Experimental and theoretical study on an air-source heat pump water heater for northern China in cold winter: Effects of environment temperature and switch of operating modes

Because of the urgent need of environment protection and energy saving, Chinese government has proposed ‘coal-to-electricity’ policy in north China to use air-source heat pump for space heating. Therefore, a cascade heat pump working at −20 °C and providing 75 °C hot water is studied in this work to match original cast iron radiator in cold climate in north China, which was seldom reported. It is also able to provide hot water for industrial and commercial use and work under single-stage mode in summertime. An experimental prototype of the cascade heat pump is designed and set up. Experimental results show COP of the cascade heat pump system can remain 1.69 at environment temperature as low as −21 °C and the COP is around 2 for most of the tested cases. In the tested temperature range, heating capacity varies from 11.32 to 18.93 kW, and discharge temperature varies from 93.95 to 91.42 °C. Thus, the cascade heat pump reveals good energy saving potential and can work stably demonstrating the feasibility to supply high-temperature water for space heating in coal-to-electricity retrofit. To study the switch between cascade and single-stage modes, computational models of the heat pump are established, validated with experimental data. The results imply cascade mode should be switched to single-stage mode when ambient temperature is higher than 20 °C, taking into account the COP, heating capacity, and discharge temperature. The studied system and results can be also used in other cold weather worldwide to help save energy.