An equivalent temperature drop method for evaluating the operating performances of ASHP units jointly affected by ambient air temperature and relative humidity

Highlights

• An equivalent temperature drop (ETD) method was developed.

• It considered the joint effect of T_a and RH on the performances of ASHPs.

• The applicability of the ETD method was verified by a field test.

• The performances of ASHPs may be conveniently evaluated.

• Impacts from either T_a or RH may be individually assessed.

Abstract

It has been known that the actual operating performances of a space heating ASHP unit are significantly impacted by the joint effect of ambient air temperature, T_a, and relative humidity, RH. To enable a convenient evaluation of the operating performances of a space heating ASHP unit with a reasonable accuracy, an equivalent temperature drop (ETD) method has been developed. The ETD method transferred the impacts of frosting-defrosting on the operating performances of ASHP units into that of an equivalent ambient air temperature drop, so that the joint effects of T_a and RH on the actual operating performances of the ASHP units may be assessed using a single parameter of equivalent ambient air temperature. In this paper, the models for predicting the output heating capacity of an ASHP unit operated at both frosting and non-frosting conditions were firstly established through either regression analysis or referring to previous related studies. Secondly, with the availability of the models, the ETD method was developed for evaluating the actual operating performances of ASHP units at different ambient conditions. Thirdly, the developed ETD method was verified by a field study on the operating performances from an ASHP unit installed in Beijing. The field study results demonstrated that for any frosting operating conditions, using the ETD method, a corresponding non-frosting condition at which the field ASHP would output the same heating capacity may be identified. Furthermore, the use of ETD method can also distinguish the individual impacts from either T_a or RH on the actual operating performances of an ASHP unit.

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 (T_a) 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 T_a. As shown in Fig. 1(a) [14], the actual output heating capacity of an ASHP unit is decreased with a decrease in T_a. 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 T_a 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 T_a 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 T_a presented in ASHRAE Handbook [14] shown in Fig. 1(a). Zhang et al [20] studied the impact of T_a 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 T_a 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 T_a 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:

• Some methods considering only the single influencing factor, T_a, cannot be used to accurately evaluate the operating performances of ASHPs, which was actually impacted jointly by T_a and RH, in particular during frosting operation.

• Although the joint effects of T_a 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 T_a or RH alone, on the operating performances of ASHPs at a specific ambient condition.

• 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 T_a and RH, and further to assess the level of impact of either T_a 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.