An equivalent temperature drop method for evaluating the operating performances of ASHP units jointly affected by ambient air temperature and relative humidity
Introduction
Air-source heat pump (ASHP) units have gained more and more attentions worldwide owing to their merits of higher energy efficiency and a wider range of ambient operating conditions [1], [2], [3], [4], [5]. ASHPs have been classified as one of renewable energy technologies in China [6], European Union [7] and Japan. In recent years, with the progress of the “Clean-heating Project” in China, ASHP has become a typical alternative to the traditional coal-burning based space heating technology [8]. According to the latest Developmental Yearbook of Chinese Clean Heating Industry [9], 819,000 numbers of ASHP units were sold in China in 2018 and it is expected that the number of ASHP installations would grow by 20% in 2019. This suggested that ASHPs have been widely applied to space heating [10] in China, and improving their operating performances would contribute greatly to sustainable development [11].
At the nominal condition of an ambient air dry-bulb temperature of 7 °C and a wet-bulb temperature of 6 °C, an ASHP unit usually operates efficiently [12], [13]. However, over the period of a heating season at different outdoor air temperature (Ta) and relative humidity (RH) other than those at the above nominal condition, the operating performances of an ASHP unit may significantly deviate from that at the nominal condition. Firstly, the operating performances are greatly affected by Ta. As shown in Fig. 1(a) [14], the actual output heating capacity of an ASHP unit is decreased with a decrease in Ta. Secondly, the operating performances can also be affected by the changes in outdoor air relative humidity or wet-bulb temperature [15]. When an ASHP unit operates in a humid environment, frosting takes place and accumulates on its outdoor coil surface, leading to a reduction in its output heating capacity [16], [17]. Hence, with the joint effect [18] of Ta and RH, the actual output heating capacity of an ASHP unit can remarkably deviate from that at the nominal condition [19]. On the other hand, as ASHPs which are rated at the nominal condition may be used in different climate regions, their actual operating performances will not be the same. Therefore, to improve the actual operating performances of ASHPs when operated at different climate regions, it becomes necessary to be able to evaluate accurately their operating performances of ASHPs under the joint effect of Ta and RH.
Different methods have been attempted in earlier studies to evaluate the actual operating performances of space heating ASHPs. Most of the existing methods were originated from the relationship between the output heating capacity and Ta presented in ASHRAE Handbook [14] shown in Fig. 1(a). Zhang et al [20] studied the impact of Ta on the performances of an ASHP unit, and developed a model for the ASHP unit to evaluate its actual operating performances. To evaluate the operating performances of ASHPs used in Southern China, Li et al [21] developed a generalized model for an ASHP unit and proposed an annual comprehensive performance coefficient (APF), which was the ratio of the total output heating and cooling capacity to the total power input during heating and cooling seasons. Unlike the model by Zhang et al [20], this generalized model further took the effects of frosting-defrosting on the operating performances of the ASHP unit into account. Ameen [22] and Jiang et al. [23] evaluated the operating performances of an ASHP unit during frosting and defrosting by using a loss coefficient of frosting-defrosting, which was defined as the ratio of COP during a frosting operation to that during a non-frosting operation, at the same ambient air temperature. Zhu et al [24] developed a frosting map on a temperature-humidity chart through simulation and field tests. As shown in Fig. 1(b), the frosting map indicates the levels and severities of frosting on the outdoor coil surface of an ASHP unit under the different combinations of Ta and RH. Based on field measured data of an ASHP unit operated at frosting conditions, Cui et al [25] proposed the loss coefficient in the nominal heating energy to evaluate the operating performances of the ASHP unit during frosting and defrosting operations. This loss coefficient was defined as the ratio of nominal energy loss to the total available nominal heating energy during a complete frosting-defrosting operation cycle. As seen in Fig. 2, the performance loss of the ASHP unit varied with the joint effect of Ta and RH. In addition, different indexes to evaluate the operating performances of ASHPs, such as Coefficient of Performance (COP) [26], Integrated Part Load Value (IPLV) [27], and Heating Seasonal Performance Factor (HSPF) [28], were proposed in various Technical Standards.
Although great efforts have been paid to establish methods for evaluating the actual operating performances of ASHPs, there are still inadequacies for the currently established methods, as follows:
- 1.
Some methods considering only the single influencing factor, Ta, cannot be used to accurately evaluate the operating performances of ASHPs, which was actually impacted jointly by Ta and RH, in particular during frosting operation.
- 2.
Although the joint effects of Ta and RH on the operating performances of ASHPs were considered in certain methods, these methods cannot be used to assess the level of impact of either Ta or RH alone, on the operating performances of ASHPs at a specific ambient condition.
- 3.
Some other methods only assessed the operating performances of ASHPs at some typical operating ambient conditions by field or laboratory tests. The operating performances at all ambient conditions were however difficult to be evaluated using field or laboratory tests due to high test costs and long test duration involved.
Therefore, an equivalent temperature drop method that can be used to evaluate the operating performances of ASHPs jointly impacted by Ta and RH, and further to assess the level of impact of either Ta or RH alone, on the operating performances of ASHPs, has been developed. In this paper, firstly, the establishment of models for predicting the output heating capacity of an ASHP unit, following a detailed analysis on the actual operating performances of the ASHP unit at various ambient conditions, is presented. Secondly, the development of the equivalent temperature drop method is reported. Thirdly, using the developed equivalent temperature drop method, a case study on evaluating the measured operating performances from a field ASHP unit installed in Beijing was carried out to verify its applicability and the case study results are presented. Finally, a conclusion is given.
Section snippets
Analysis of the joint effect of Ta and RH on the operating performances of a constant speed ASHP unit
Based on ASHRAE Handbook [14], the relationships between the changes in output heating capacity of an ASHP unit/building heating load and that in ambient air temperature at both non-frosting and frosting conditions are indicated in Fig. 3(a). As seen, with a decrease in Ta, building heating load increases, but the output heating capacity of the ASHP unit decreases. Point N is the operating point at the nominal condition for the ASHP unit, which is normally at a non-frosting or condensing
Development of the equivalent temperature drop method
Fig. 5(a) shows the schematics of equivalent temperature drop (ETD) method developed for an ASHP unit. At a non-frosting condition, when the ASHP unit was operated at point A, the reduction in output heating capacity, ΔQTa, was caused by a drop in ambient air temperature, ΔTa1, expressed as follows:
At a frosting condition, due to an increased relative humidity, the operating point for the ASHP unit moved further from A to A1, and its output heating capacity was reduced
Field test setup and instrumentation
To verify the applicability of the ETD method, the field test was carried out over a heating season in 2015, in a small - scaled office building in Beijing, China. Fig. 6 shows the schematics of the field test setup. There were 11 rooms with a total heating floor area of 185 m2. A commercial constant-speed ASHP unit served as the heating and cooling source for the building, with an output heating capacity of 14 kW at the nominal condition. It was on/off controlled according to a preset outlet
Limitations of the current study
From the results presented in 2 Establishment of models for predicting the output heating capacity of an ASHP unit, 3 Development of the equivalent temperature drop method, the ETD method was developed based on constant speed ASHP units and therefore may be applicable only to constant speed ASHP units. On the other hand, there were also a large number of variable speed ASHP units whose operational characteristics can be significantly different from those of constant speed ASHP units. Therefore,
Conclusions
In this paper, the development of the equivalent temperature drop (ETD) method to evaluate the actual operating performances of an ASHP unit operated at different ambient conditions with a reasonable accuracy is reported. The applicability of the ETD method was verified by a field test for an ASHP unit installed in Beijing. The following conclusions may be drawn:
- 1.
The models for predicting the output heating capacity of an ASHP unit at both frosting and non-frosting conditions were respectively
CRediT authorship contribution statement
Yong Wu: Conceptualization, Investigation, Validation, Data curation, Writing - original draft, Writing - review & editing. Wei Wang: Conceptualization, Writing - original draft, Supervision, Project administration. Yuying Sun: Methodology. Yiming Cui: Investigation, Data curation. Dexing Duan: Investigation, Data curation. Shiming Deng: Writing - review & editing, Supervision.
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.
Acknowledgements
This work was supported partially by the China National Key R&D Program (Grant No. 2016YFC0700400), the China National Key R&D Program (Grant No. 2016YFC0700100), the Special Fund of State Key Laboratory of Building Safety and China Academy of Building Research (BSBE-EE2019-01) and the Beijing natural fund & municipal education commission co-sponsored project (Grant No. KZ202010005003).
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