A method for sizing air source heat pump considering the joint effect of outdoor air temperature and relative humidity

When operated in a highly humid environment, frost may accumulate on the surface of the outdoor coil in air source heat pump units (ASHPs), leading to a reduction in its output heating capacity. Currently, the existing methods for sizing ASHPs only considered the effect of outdoor air temperature, Ta, without taking into account the effect of relative humidity, RH, on output heating capacity. This often resulted in an insufficient heating capacity from an ASHP selected. To resolve this issue, a model for predicting the output heating capacity of ASHPs, Qh, was established firstly. Then, considering the variation in the rated output heating capacity of ASHPs, a method for sizing ASHPs considering the joint effect of Ta and RH has been developed. Besides, for practical application, a capacity design correction factor, K, was proposed, evaluated, and plotted as a cloud picture. Evaluating results indicated that K can be used for sizing ASHPs accurately. When the effect of frosting on Qh was considered, the maximum rates of decrease in the output heating capacity correction factor, KQ, was between 43.8% and 50.0%, and the outdoor air design temperatures for space heating in six typical Chinese cities were decreased by 1.8–3.5 °C, with the average of 2.8 °C. For highly humid cities, such as Qiannan area China, even if its outdoor air design temperature was 1.4 °C higher than that in Chongqing City, the K value was 0.24 lower.