Air-Source heat pumps in a cold climate - Criteria for defrosting

Frost formation will occur when air-source heat pumps operate in a cold and humid climate. Defrosting is necessary since the frost lowers the performance of the heat pump. There are several methods for defrosting and it is usually carried out by heating from the inside of the tubes. The results from previous tests of heat pumps have shown that defrosting takes place even though not required. Unnecessary defrosts should be avoided since this is an energy demanding process and may cause wear of the heat pump. A method for optimized initiation or defrosting has been experimentally evaluated. The optimization was made for one cycle including frosting and defrosting. In this method defrosting is initiated when the maximum average COP is reached. The criterion for defrosting was applied on data from previous tests and the results showed a potential for improvement of COP as well as energy savings when using this optimization. The optimization can also be done regarding the cooling capacity which is of interest when the heat pump does not cover the whole heating requirement. The optimized control was tested in a climate chamber with a heat pump using hot gas defrost as defrosting method. The heat pump was also tested with the regular defrosting control. The tests were carried out at specified conditions for inlet air, +2 °C with two different levels of humidity and -7 °C. An evaluation according to SS-EN 255-2 was carried out for each test. The results from the optimized control did not come out as expected. It gave the best COP at -7 °C but the regular control gave the better COP at +2 °C. The optimized control turned out to be a sub optimization. It gives the maximal COP given a particular duration of defrosting. However, this COP may not necessarily be the best achievable with given heat pump and defrosting method. The optimized control showed a longer duration of defrosting for all tests and that contributes to the lower COP average. The cycle time was longer as well but it is limited by the frosting and was not able to compensate the longer defrosting. This was not the case in the one test with -7 °C. When the optimized method was applied on data from the test with regular control, it shows room for improvement of COP in all tests, if the cycle time is prolonged. To reach the maximum COP in all conditions further optimization is necessary regarding defrosting duration and efficiency