Climatic division based on frosting characteristics of air source heat pumps
Introduction
Air source heat pump (ASHP) units have found worldwide applications due to their advantages of high energy efficient and environment friendly [1], [2], [3], [4]. However, when ASHPs are used for heating in cold regions, frosting occurs when the surface temperature of outdoor coil is lower than both the dew point and freezing point. Frost leads to the increase of thermal resistance and air flow resistance, which results in a decrease in system performance [5], [6]. Therefore, it is of great significance to study the frosting mechanism [7], [8], defrosting and frost suppression technology [9], [10], [11], [12] and developing efficient defrosting control strategy [13], [14], [15] for the performance improvement of ASHPs [2], [3], [8], [13].
Researchers have proposed many defrosting techniques, such as hot gas bypass defrosting [16], [17], compressor shutdown defrosting [15], electric heating defrosting [18], [19], hot water spraying defrosting [20], and reverse cycle defrosting [21]. To ensure the effectiveness of those defrosting techniques, appropriate defrosting control strategies are very important, which mainly includes: time-based start defrosting control, demand start defrosting control and temperature-based termination defrosting control [3]. However, for a vast country, like China, with large differences in climate of different geometry regions [22], [23], [24], [25], if the same defrosting technology and control strategy are adopted, unnecessary or insufficient defrosting actions are likely to occur in some regions, resulting in energy waste [26]. In other words, different defrosting techniques and defrosting control strategies have climate adaptation issues. Therefore, it is necessary to consider the local climatic conditions when developing ASHPs defrosting technologies and corresponding control strategies.
Since ASHPs are normally mass produced, they are impossible to be tailored for each city. Identifying different climatic regions, which normally includes dozens cities, based on frosting characteristics of ASHPs would be more practically useful. Meanwhile, the degree of frosting of an ASHP outdoor unit is not only affected by climatic conditions [24], but also by system operating performance [27], [28], [29], [30], and user habits. During the process of identifying climate region, the influence of factors rather than climate conditions should be eliminated. Therefore, this paper proposes the concept of frosting degree hour (FDH) to specifically characterize the impact of climatic conditions on ASHP frosting characteristics. In order to make the calculation of FDH closer to the actual situation, actual operation data of ASHPs were on-site collected from 12 cites in China, used for calibrating the FDH calculated by meteorological data of 275 cities. ASHPs investigated in this study were multi-evaporator type, with different number of indoor units connected to one outdoor unit. There were many choices for its rated heating capacity or compressor rated power [31]. In practical applications, the larger the rated power, the greater the probability of a partial load and the lower the partial load rate, which may greatly affect the value of the evaporation temperature [32]. This factor was taken into consideration in this study.
All on-site investigated ASHPs were divided into several categories according to their rated powers, and the corresponding FDH was calculated for different categories, respectively. Based on this, the quantitative index of FDH is used to characterize the degree of frosting on ASHP outdoor units of different power types. Finally, different climatic regions were divided according to the frost characteristics. This may facilitate the climate-adaptive design of ASHPs and corresponding defrosting control strategies.
Section snippets
Data collation
By placing 24 air source heat pumps and sampling equipment in 12 cities across the country, the actual operation of the ASHPs was monitored.
Sensors installed on the ASHPs are shown in Fig. 1 below, of which 4 temperature sensors were installed on the indoor units, as shown in Fig. 1a, which measured wall temperatures on the inlet of capillary (TC2), the inlet (TCJ) and outlet (TC1) of heat exchanger, and the air temperature at the outlet of heat exchanger (TA). Other 6 temperature sensors were
Concept of frosting degree hour
In order to quantitatively study the frosting characteristics of ASHPs, we propose the concept of frosting degree hour (FDH). It is well known that the temperature and humidity of outdoor environment has a great effect on the frost formation of ASHPs. In essence, the frosting phenomenon is determined by both the surface temperature of outdoor coil and the dew point temperature of air blowing across the surface [33]. Frosting begins when the surface temperature is below both zero and the dew
Analysis of FDH under characteristic temperature difference
The characteristic temperature differences of different power types obtained in section 2 were used to analyze FDHs in this section. It was assumed that when an ASHP was operated in the heating season, its heat transfer temperature difference was remained as the characteristic temperature difference.
A total of 275 meteorological data were collected, covering 32 provinces, municipalities, autonomous regions and special administrative regions. The meteorological data were derived from Chinese
Conclusion
Nowadays, most researches are carried out in labs with carefully controlled experimental environment. In current study, on-site operating data of ASHPs were utilized with the effort to reflect their actual performance. Although results obtained could be affected by many random factors, laws with universal value still can be extracted from it. During winter operation, the outdoor units' evaporation temperature is mainly affected by the outdoor temperature and the compressor speed. Based on the
CRediT authorship contribution statement
Xiangguo Xu: Conceptualization, Methodology, Supervision. Zhiqiang Fang: Investigation, Writing - original draft. Zhiqiang Wang: Investigation, Data curation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors thank the National Natural Science Foundation of China (Project No: 51976181) and the Toshiba Carrier Corporation (Japan) for financially supporting the work reported in this paper.
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