Improving defrosting performance of cascade air source heat pump using thermal energy storage based reverse cycle defrosting method

To encourage a wider application of air source heat pumps (ASHPs) to colder areas due to the advantage of a higher energy efficiency, adopting cascade air source heat pumps (CASHPs) is a promising option. When CASHPs are operated in heating mode, frosting/defrosting has been problematic. However, the defrosting methods commonly used by conventional ASHPs cannot be directly applied to CASHPs. Therefore, a thermal energy storage (TES) based reverse cycle defrosting method for CASHPs has been proposed and an experimental study on its operating performances was carried out. Comparative tests when using both the standard hot gas by-pass defrosting method and the TES based reverse cycle defrosting method were carried out. The results suggested that when using the TES based reverse cycle defrosting method, defrosting duration was shortened by 71.4-80.5%, and defrosting energy consumption reduced by 65.1-85.2%, as compared to those when using the standard hot gas by-pass defrosting method. In addition, the thermal energy stored can also become a source for indoor space heating, and 37.7% of the normal heating capacity can be provided to a heated indoor environment when using the TES based reverse cycle defrosting method.