Abstract
The frosting type and frosting degree of the outdoor heat exchanger of air source heat pump (ASHP) in the heating season are greatly affected by the local environmental temperature and humidity. China has a vast territory, and the climate varies greatly in different regions. Therefore, when the ASHP is running in different climate zones for heating in winter, there are significant differences in the frosting type and frosting degree. In order to achieve deeper understanding of frost formation and provide more accurate guidance for the suitability application of antifrosting and defrosting technologies, a study on the division of frosting type and frosting degree was performed in this work. Based on the three-phase diagram of water and combined with the theory of phase change dynamics, the outdoor heat exchanger of the ASHP in the heating season was divided into four states: neither frosting nor condensation, condensation, condensation frosting, and sublimation frosting, and the proportion of each state in a typical city in different climate zones was calculated. The results showed that more than 80% of the heating seasons in Nanjing, Shanghai, Wuhan, Changsha, Xi’an, and Harbin had the frosting phenomenon. Sublimation frosting was the main frosting type in Xi’an, Beijing, and Harbin, while Chongqing was all condensation frosting. Moreover, a frosting model was developed and the frosting degree was divided into mild frosting, moderate frosting, and heavy frosting, according to the performance attenuation of the ASHP under frosting conditions. The proportion of each frosting degree in a typical city was achieved. About 50% of the heating seasons in Nanjing and Changsha were distributed in the heavy frost area and 100% in Chongqing.
Highlights
The air source heat pump (ASHP) is widely used as cooling and heating source in China for building air conditioning due to its advantages of having cooling and heating functions, energy saving, environmental protection, flexible installation, etc
The results of dividing the frosting degree area of the ASHP in each typical cities are shown in Figure 8: 1 represented the nonfrosting area; 2 represented the mild frosting area; 3 represented the moderate frosting area; and 4 represented the heavy frosting area
The heating seasons of Beijing, Xi’an, and Harbin were dominated by mild frosting, reaching 89, 57, and 94%, respectively, and it was basically under the sublimation frosting, mainly because the climate in these cities was relatively dry
Summary
The air source heat pump (ASHP) is widely used as cooling and heating source in China for building air conditioning due to its advantages of having cooling and heating functions, energy saving, environmental protection, flexible installation, etc. Its application plays an important role in improving energy efficiency, promoting energy conservation and emission reduction (Xu et al, 2020; Zhang et al, 2018a; Song et al, 2018; Zhang et al, 2017; Amer and Wang, 2017). When an ASHP runs for heating in winter, there is a problem of frost formation on the surface of its outdoor fin-tube heat exchanger. The growth of frost layer leads to the deterioration of the working condition of the ASHP and the attenuation of its heating performance (Dong et al, 2012; Song et al, 2016; Wang et al, 2017; Zhang et al, 2018b). Understanding the process and mechanism of frost formation is the prerequisite
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