Frosting map of variable-frequency air-source heat pump for building space heating

When the variable-frequency air-source heat pump (VFASHP) is used for building space heating, its compressor speed is regulated based on the heating load, which changes the temperature difference between the outdoor coil surface and the air, thus affecting the frosting, making its frosting characteristics significantly different from the constant-frequency air-source heat pump (CFASHP). To reveal the frosting difference of VFASHP caused by different load control methods, this paper establishes the frosting map of VFASHP in severe cold zone through experiments, further draws the frosting map of experimental units in cold zone and hot summer and cold winter zone through numerical models, and quantitatively studies the influence of heating load regulation on frosting characteristics. The results show that the change of frosting critical relative humidity of VFASHP is completely opposite to that of CFASHP, which decreases with the decrease of outdoor air temperature. When the same unit is applied to different areas for space heating, the lower the outdoor calculated temperature for heating is, the higher the frosting critical relative humidity is, and the lower the upper and lower limit temperature of the frosting area is. The temperature and relative humidity range of the frosting area for heating by the unit in Harbin, Beijing and Shanghai are -26.5-1.0 ℃/63%-81%, -18.3-3.2 ℃/59%-72% and -10.1-5.0 ℃/53%-63%, respectively. However, when VFASHP operates at the highest frequency, its frosting shows the same trend as CFASHP.