Comparative experimental study on performance of air-source heat pump water heaters with electronic expansion valves and capillary

In order to study the impact of different throttling methods on air-source heat pump water heaters, a prototype is developed by using capillary (CAP) and electronic expansion valves (EEV) as throttling components, respectively, and their operating characteristics are compared through experiments. The results show that the inspiratory superheat of CAP system can be stabilized at about 1 ℃. The exhaust temperature of the CAP system is relatively lower, about 15 ℃ lower than that of the EEV system. The heating speed of the CAP system for heating the same tank of water is faster. The COP and the annual performance factor (APF) of the EEV system are higher than those of the CAP system, but under the nominal operating conditions, the CAP system has a comparable heating COP to the EEV system. Considering the factors such as heating performance and initial investment, capillary can be preferred as throttling devices according to needs.