The feasibility of solar thermal-air source heat pump water heaters in renewable energy shortage regions

Abstract

Abstracts Despite the extensive literature on solar-assisted heat pump systems, including optimizations of systems configurations and operation strategies, there are few works of literature on the potential of applying solar thermal-air source heat pump water heaters (SA-WH) in solar shortage regions and none that compares alternative system improvements for the same boundary conditions and with the same level of comfort to the end-user. This work examines the feasibility of the SA-WH in solar shortage regions from a performance perspective and compares its performance to the solar water heater (SWH) and the air-source heat pump water heater (ASHP-WH). Experimental studies were carried out from March 22, 2014 to April 21, 2014, which covered spring-summer transition seasons of these particular areas. The parameter η was proposed and a reference ηbase of 2.4 was defined to rate the performance of these three systems. The results showed nearly 80% of working conditions of the SA-WH had a η which was higher than 2.4, wherein 70% were above 3. Furthermore, this study presents a regression model verified by measured data to predict the potential performance of the SA-WH in renewable energy shortage regions. This prediction model provides an optimal method of fully applying solar energy for the domestic hot water (DHW) and even heating by targeting better system performance. Then, three solar energy shortage cites of China were used as cases to illustrate the use of the prediction model. To meet the DHW demand in transitional seasons of these regions, the air-source heat pump of the SA-WH is suggested to be switched on 1 h before using when the average ambient temperature is below 20 °C; and be switched on at the hour with the highest temperature of the day when the average ambient temperature is above 20 °C and operate 1 h.