This paper investigates the effects of frost formation on performance for an air-to-air heat pump. A transient model is used for evaluating frost formation on a parallel-plate heat pump evaporator. The frost formation model is derived by applying the equations of conservation of mass, momentum and energy, as well as some empirical correlations, to calculate the growth and densification of the frost layer. The model has been validated in a previous work by comparison with experimental results.

In this paper, the frost formation model is incorporated into the evaporator subroutine of an existing heat pump model to calculate performance losses due to frosting as a function of weather conditions and time of operation since the last evaporator defrost. The heat pump model takes into account the performance losses due to added pressure drop and heat transfer resistance that result from the growth of the frost layer.

The results show frost formation parameters and heat pump COP as a function of time and ambient conditions. It is determined that there is a range of ambient temperatures and humidities in which frosting effects are most severe, and this range is explored to calculate heat pump operating conditions. The heat pump analysis results are expected to be useful in predicting optimum defrosting conditions, and to evaluate alternative methods for defrosting.

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