Research on defrost free air-source heat pump with surface stripe concavo-convex fins

Abstract

An ASHP extracts heat from the air with a low power consumption, to be used as thermal energy, which is several times larger than the power consumption. Thus, ASHPs are expected to replace stoves utilizing fossil fuels (such as, oil, coal, natural gas, and wood) which cause the global warming. However, during the winter operation of a heat pump (HP), frost forms and grows on fins and tubes of the ASHP’s outdoor heat exchanger, increasing the resistance to the air flow through fins, and causing a decrease in the HP output, leading to a decrease in Coefficient of Performance (COP).The purpose of this study is to utilize the effects of (a) overall heat transfer coefficient promotion of defrost-free heat exchangers (DFHEX) and (b) extension of heat exchanger blockage time to achieve a higher HP output and COP improvement during frost formation. First, in order to clarify the mechanism of extending the time to the blockage, we measured the frost thickness on fins and tubes and found the effect of suppressing frost formation on the tubes. In addition, we acquired the heat exchange performance when the fin pitch of DFHEX was changed (between 1.5 and 10 mm), and obtained the optimum fin pitch that can promote the overall heat transfer coefficient. Based on these results, a DFHEX heat pump (DF-HP) was manufactured, and a comparative test was conducted in a cold region with a commercial heat pump (Com-HP) using the same compressor. As a result, we achieved both a high output and high COP in a HP for cold region use.