Energy performance evaluation of two-phase injection heat pump employing low-GWP refrigerant R32 under various outdoor conditions

Highlights

• Performances of R32 and R410A two-phase injection (TPI) heat pumps are compared.

• TPI is effective in improving the performance and reliability of R32 heat pumps.

• R32 TPI heat pump shows different optimum conditions from R410A equivalent.

• Injection parameters are optimized to achieve best performance with safe operation.

Abstract

The refrigerant R32 has been receiving significant attention as an alternative to R410A used in heat pumps, owing to its lower global warming potential. The two-phase injection (TPI) technique is introduced to achieve improved energy performance along with high system reliability in R32 heat pumps, owing to their high discharge temperature. The objectives of this study are to compare the performances as well as reliabilities of the R32 and R410A TPI heat pumps and provide the design guidelines for achieving the best performance with safe operation. The performances of the R32 and R410A TPI heat pumps are measured by varying compressor frequency and outdoor temperature. The TPI technique enables the R32 heat pump to operate at a higher compressor frequency under severe weather conditions. At low outdoor temperatures of −10 °C and −15 °C, the R32 TPI heat pump demonstrates a superior heating capacity and coefficient of performance (COP) compared with the R410A TPI heat pump. In addition, the injection parameters in the R32 TPI heat pump are optimized under various operating conditions.

Introduction

As hydrofluorocarbon refrigerants are considered one of the primary reasons for global warming, the Kyoto protocol and EURO F-gas regulations have imposed restrictions on the use of refrigerants with a high global warming potential (GWP) [1]. Therefore, R410A, which is one of the most extensively used refrigerants in heat pumps, must also be replaced by low-GWP refrigerants [2]. R32 is considered the most appropriate candidate among the existing alternatives for R410A. R32 can be used in R410A heat pumps without complex modifications, owing to the similarities of its properties and operating conditions with those of R410A [3]. Several studies have been conducted to investigate the performance improvement of an R32 heat pump. Mota-Babiloni et al. [4] reported that the performance of an R32 residential heat pump was similar or marginally better than that of an R410A heat pump. However, since an R32 heat pump can experience a high discharge temperature, decreasing this discharge temperature is necessary to ensure its reliability [5].

The refrigerant injection technique has been extensively adopted in heat pumps to reduce the discharge temperature and improve the capacity [[6], [7], [8], [9]]. Among the existing injection types, vapor injection (VI) is the most popular injection type in heat pumps because of its high potential in improving the capacity and coefficient of performance (COP). Xu et al. [8] compared the performance of VI heat pumps using 410A and R32. They reported that the R32 heat pump demonstrated capacity and COP improvements of up to 10% and 9%, respectively, compared with the R410A heat pump. Cho et al. [10] also measured the cooling and heating performances of VI heat pumps using R410A and R32. They concluded that the VI technique elicited performance improvements of the heat pumps during their operations in the cooling and heating modes. In addition, Shuxue et al. [11] reported that a VI heat pump using R32 decreased the discharge temperatures in both cooling and heating modes. Overall, the VI technique is effective in increasing the capacity and COP of an R32 heat pump. However, when the R32 heat pump operates at a high compressor frequency under extreme weather conditions, the increased discharge temperature is a significant problem that must be resolved. Furthermore, since R32 is a mildly flammable refrigerant (A2L), the reduction in the discharge temperature is necessary to ensure system safety.

In a VI heat pump, the injection mass flow rate can be limited by the injection superheat. Therefore, the two-phase injection (TPI) technique has been introduced in heat pumps when they are operated under extreme weather conditions [12]. Based on a simple cycle analysis, Lee et al. [13] analyzed the potential benefits of the saturation cycle with the TPI technique. They established that the TPI heat pump using R410A exhibited a good potential in improving the system performance and realizing the saturation cycle. Using a cycle simulation model, Yang et al. [14] estimated the effects of two-phase suction, liquid injection, and TPI using R32, to decrease the discharge temperature of scroll compressor. They concluded that all the three methods showed excellent potential in decreasing the discharge temperature. Kim et al. [15] reported through an analytical model that a TPI heat pump using R410A was very effective in expanding the operating range and improving the COP based on the optimized injection parameters of a scroll compressor. Furthermore, using an artificial neural network model, it was confirmed that a TPI heat pump employing R410A could improve the annual performance factor compared to a non-injection (NI) heat pump [16]. In addition, control methods for the injection quality in a TPI heat pump employing R410A were proposed to achieve the optimum performance without the risk of wet compression [17]. Based on the previous studies, it is clear that the TPI heat pumps using R410A demonstrates the potential to increase the system reliability and COP under severe weather conditions. However, there have been few experimental studies on TPI heat pumps using low-GWP refrigerants such as R32 with a proper control of the quality of the injected refrigerant.

For practical applications of TPI into heat pumps, the experimental database and design guidelines for the optimum injection quality and optimized system parameters are to be established. However, the experimental investigations on the system reliability (the discharge temperature and wet compression) and COP of the TPI heat pump using low-GWP refrigerants are very limited. As shown in Table 1, most previous studies [15,16,[18], [19], [20]] focused on VI technique, and a few TPI studies were mainly conducted by theoretical modeling [[13], [14], [15], [16], [17]]. As mentioned previously, R32 is receiving strong attention as an alternative refrigerant to R410A, but has difficulty in its applications to heat pumps owing to high discharge temperature. However, there are no experimental studies on the TPI heat pump using R32 to provide design guidelines for its applications. Thus, it is important to experimentally investigate the system reliability and performance characteristics of TPI heat pumps employing R32, because the differences in the refrigerant properties can change the optimum design parameters. In addition, the optimization of the TPI heat pump using R32 are required to achieve high system COP and reliability.

The objectives of this study are to investigate the system reliability and performance characteristics of R32 and R410A TPI heat pumps and provide the design guidelines for achieving the best performance with safe operation, under various operating conditions. Experiments were performed on R32 and R410A TPI heat pumps by varying the compressor frequency and outdoor temperature. The performances of the R32 and R410A TPI heat pumps were analyzed and compared by varying the operating conditions. The main contributions of this study are as follows: (1) the performance characteristics of the R32 and R410A TPI heat pumps are analyzed; (2) the advantages of the R32 TPI heat pump are presented; and (3) the optimum injection parameters of the R32 TPI heat pump are suggested to achieve performance improvements under various operating conditions.