Field Test Results from Combined Solar Thermal and Air-source Heat Pump Systems with a Special Focus on Defrosting

Three different combined solar thermal and air-source heat pump systems for domestic hot water preparation and space heating in single- and multi-family houses in Germany have been monitored since the beginning of 2012. Through these field test investigations component as well as system performances could be characterized and it was possible to identify errors regarding installation and control of the multiple operation modes. A particular attention was paid to the energetic effort and drop in thermal efficiency due to frosting and defrosting of the heat pump’ evaporator. Furthermore, some of the descriptive parameters in the defrosting process (e.g. amount of energy required, work cycle between two defrosting modes) have been put in relation to the prevalent outdoor conditions (temperature, relative humidity). From this analysis the effectiveness of the applied defrosting strategy could be estimated, i.e. to which extent defrosting operation was initiated at the ideal point of time to be needed.