This study aimed to reach a new solution for the defrost problem for air source heat pumps. In order to achieve this goal, the study was divided in 3 different parts. A literature survey about what the industry and the academy are utilizing now; a benchmark study with products that fit in a specific range of characteristics and then select and try one of the feasible solutions in the laboratory.
A literature survey was done to search for alternative solutions for the defrost for air source heat pumps. Multiple types of solutions were found and they can be divided on electric defrost, hot gas injection methods, advanced circuiting systems, pre-dry treatment of air, surface treatment of the heat exchanger, thermal energy storage and other type of solutions that cannot be placed in the previous tags.
The benchmark study was done simultaneously with the literature survey and two products of 9000 Btu/h of heating capacity were dismounted and tested in the psychrometric chamber of Electrolux AB in Stockholm. The results were different and some conclusions were achieved, but the main difference between the units was the condensation temperature during defrost, which is related to a higher heat exchange between the refrigerant and the frost, also was noticed that the control of the compressor and electronic expansion valve, a high optimization of these can improve substantially the defrost of an air source heat pump.
The solution selected to be tried in the lab was based on a thermal energy storage utilizing phase change materials (PCM). A PCM heat exchanger was designed, built and installed in the test rig. Two types of PCM were utilized, with melting points of 18ºC and 28ºC. They were compared with a reference without the PCM thermal energy storage. The results showed that 18ºC is not a viable election for a melting point of the PCM, and with this solution the defrost using the 28ºC PCM, the heat transfer can be improved in 57%, which means a shorter defrost period. Furthermore, the defrost cycle duration could be decreased up to a 67% and the energy consumption during the defrost was also decreased by a 70%.
Source: KTH
Authors: Azizi, Shoaib | Castelló Pérez, Luis