Effect of defrosting on the seasonal air heat pump performance

The present work is aimed at investigating the influence of the frost formation on the energy consumption of an air source heat pump (ASHP), when the defrost is obtained by inverting the cycle. A simplified model for estimating the number of defrost cycles per year as a function of the outdoor air temperature and moist content has been developed. By using this model the defrosting cycles for many Italian sites, representative of the different climate zones, have been evaluated. Finally, the heat pump performance for the whole heating season (SCOP) has been estimated, accounting for the energy spent during the defrost cycles. The results point out that the frost formation is significantly influenced by the outdoor air conditions. Although the amount of defrost cycles depends both on the temperature and relative humidity of the air, the heat pump that operates in cold and dry weather needs less defrost cycles. The heat pump performance is significantly reduced by the defrost cycles; during a typical winter day the average COP can decrease of about 15-20% in Northern Italian sites. However, considering the whole heating period the decrease of SCOP ranges between 0.5% and 9.6% in Italy.