Simulating an evaporator field for large-scale air-source heat pumps subjected to frosting and defrosting

Defrosting of air source heat pump evaporator fields is necessary for securing fault-free operation of the heat pump. However, it should be secured that the defrost procedure decreases the performance of the heat pump as little as possible. This paper presents a simulation model of an evaporator field consisting of several parallel evaporator units operating under frosting conditions. A simple defrost strategy was applied, where two evaporator units are taken out for defrost at a time. The results show that the spatial distribution of frost accumulation and subsequent refrigerant- and airflow maldistribution on the unit level had a minor effect on the overall performance of the evaporator field. On the other hand, the distribution of heat load and fan power between the individual units of the evaporator field was significant and the overall performance should be modeled based on the entire field and not based on a single unit.