Abstract
The most widespread defrosting technique adopted by Air-Source Heat Pumps (ASHPs) during the heating season is Reverse Cycle Defrosting (RCD). In this paper a dynamic model of RCD, based on performance data provided by the heat pump manufacturer, designed for TRNSYS and with a core-structure suitable for commercial units, is presented. A defrost cycle is divided in three phases. First, the unit heating capacity is reduced as a linear function of the ice layer thickness (Pre-Defrost phase). Subsequently, the reverse cycle operating mode is modelled on the basis of the performance data given by the manufacturer (Defrost phase) and, finally, the heat pump performances are altered taking into account the higher surface temperature of the external coil after the reverse mode (Post-Defrost phase). Then, the influence of defrosting energy losses on the heat pump seasonal performance factor in sites characterized by different climatic conditions has been assessed. Results point out that the ASHP seasonal efficiency decreases of about 5% taking into account defrost energy losses; in addition, the influence of defrost cycles on the internal air temperature is studied by assessing under which conditions the indoor thermal comfort can be guaranteed even in presence of frequent defrost cycles.
Highlights
Technologies based on fossil fuels have been the basis of the main energy services, such as electricity generation and buildings climatization, of the contemporary society with the evidence of negative environmental effects
Two series of simulations have been performed: the first one takes into account defrost cycle energy losses by means of the model presented in this paper, while in the second one, referred as the Ideal case, these losses have not been considered in order to assess the influence of defrost cycles on the overall energy performance of the heat pump
The model is developed within TRNSYS 17 environment and takes into account Reverse Cycle Defrost (RCD) technique, the most widespread solution adopted by commercial air-source heat pumps
Summary
Technologies based on fossil fuels have been the basis of the main energy services, such as electricity generation and buildings climatization, of the contemporary society with the evidence of negative environmental effects (e.g. increase of greenhouse gas emissions, air pollution and global warming). The heat pump performance is reduced due to a dual penalization effect: on the one hand, the accumulated frost acts as a thermal insulation layer, decreasing the evaporator heat transfer coefficient, and, on the other hand, the air flow area is reduced due to the shrinkage of the cross-flow area For both reasons, the evaporation temperature Teva lowers and the unit energy performance are significantly degraded
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