The life cycle climate performance evaluation of low-GWP refrigerants for domestic heat pumps

Highlights

• Evaluated LCCP of various mixtures as low-GWP refrigerants for R410A substitute.

• Binary blends (HFC32/HFO1234yf and HFC32/HFO1234ze(E) with GWP<300) were examined.

• Adopted experimentally-evaluated COPs and optimum charge values were adopted.

• HFC32/HFO1234ze(E) or HFC32/HFO1234yf in 42/78% offers the lowest LCCP.

Abstract

Domestic heat pumps constitute a significant part of the global heat pump industry. R401A offers excellent performance with no influence on the ozone depletion and has been a dominant refrigerant in most domestic heat pumps. However, R410A has a significant impact on the climate due to its high global warming potential (GWP). Thus, the heat pump industry has been focusing on the development of R410A substitutes. Pure refrigerant alternates, such as HFC32, and the blends mixed with low-GWP refrigerants, such as HFO1234yf and HFO1234ze(E), are widely reported. Besides the direct impact of refrigerants (which is estimated as GWP), domestic heat pumps indirectly affect the environment from the energy consumption and manufacturing processes. Hence the life cycle climate performance (LCCP) analysis accounts for all emissions through the lifetime of a heat pump. This article reports the LCCP evaluation of various low-GWP refrigerants for R410A replacement on domestic heat pumps. Six refrigerants, i.e., HFC32, binary blends of HFC32 and HFO1234yf (with GWP values of 300, 200 and 150), and HFC32 and HFO1234ze(E) with GWP values of 300 and 200, were compared against R410A. The performance data of these refrigerants from the experimental heat pump facility were utilized to evaluate the LCCP. Among the selected refrigerants, the binary blend of HFC32/HFO1234ze(E) with GWP 300 shows the lowest LCCP. Low-GWP refrigerants would become more competitive than R410A when CO₂ emission from energy generations can be reduced by the optimization of the system or the usage of renewable energy.

Introduction

Climate change has become a global concern. The heating, ventilation, air-conditioning, and refrigeration (HVAC&R) industry is regarded as one of the main contributors to global warming. Thus, the industry has been making a considerable effort to reduce the environmental impact of the HVAC&R system. The optimization of domestic heat pumps, which dominate the HVAC&R sector, has been the most active research area for long-term sustainability. On the one hand, the application of renewable energy on domestic heat pumps seems a palpable way to reduce greenhouse gas. For instance, the latent heat from solar energy can be utilized for space heating by heat pumps (Esen, 2001). On the other hand, the improvement of refrigerants is considered the most accessible way to achieve the environmental target. R410A dominated domestic heat pump systems due to the excellent performance despite the high value of GWP, i.e., 2088 (Pachauri et al., 2014), while studies on R410A substitute have become prevalent in the last decade. HFC32 has been widely applied due to the GWP value, i.e., 675, with superior performance. For instance, almost all residential heat pumps in Japan have been replaced with HFC32 (Okada, 2019). However, the GWP of HFC32 is still far higher than the targets of several regulations, such as Japan's F-Gas control policy (Kawagishi et al., 2019), the Kigali Amendment (Clark and Wagner, 2016), and European F-Gas directives (Maratou, 2014). Miyara et al. (2012) outlined four ways for next-generation refrigerants and heat pump systems: natural refrigerants, low GWP synthetic refrigerants, refrigerant management, and refrigerant mixtures. Pham and Rajendran (2012) reported low-GWP refrigerants for air conditioning and mentioned that the blends of HFC32 and HFO1234yf or HFO1234ze offer potential solutions for R410A replacement. Meanwhile, the Koyama laboratory had done numerous researches about the refrigerant mixtures, including but not limited to HFC32/HFO-1234ze(E) (Kondou et al., 2013), HFC32/HFO1234yf (Fukuda et al., 2016), and HFC32/HFC-1123 (Koyama et al., 2018). Creamaschi et al. (2013) studied the compressor characteristics and performance using HFC32, HFO1234yf and two new low-GWP developmental refrigerants. The characteristics of numerous low GWP refrigerants were investigated as replacements of R134a, R404A, R410A, and R22 by Devecioğlu and Oruc (2015). The assessment of the thermodynamic performance of HFC32 and HFO1234yf mixture was studied by Zheng et al. (2017) in China. Domanski (2019) reported an overview of the application of refrigerants and screened the next-generation refrigerants and pointed out that the availability of low GWP refrigerants varies between applications.

From the above literature review, the focus was often placed on the development of new low-GWP refrigerants comparing to the performance of the target refrigerants. Methods to assess the climate impact mostly focused on GWP, which is often limited, as it just evaluates the direct emission of refrigerants and ignores the indirect influence, such as emissions from the energy consumption over the lifetime of the system and manufacturing of materials and refrigerants. Thus, other indicators have been developed, for example, the total equivalent warming impact (TEWI) and the life cycle climate performance (LCCP).

In addition to the direct emission that is conveniently estimated by GWP, TEWI also accounts for indirect emissions from fossil fuels to generate energy to operate the units over their lifetime. TEWI was proposed at the end of the last century (Fischer and Sand, 1997). In 1999, Yin et al. (1999) used TEWI to compare R744 and R134a for mobile air conditioning systems using experimental results. Sand et al. (1999) presented an analysis of the utility, shortcomings, and results of TEWI. Later, TEWI was applied in the studies of the refrigeration system design (Davies and Caretta, 2004). Rahhal and Clodic (2006) developed a method to choose low TEWI blends between HFCs and Carbon Dioxide comparing with the performance of R407c. TEWI was also used for the investigations of the environmental impact and economic analysis in the Chinese region (Liu et al., 2013). For applications with large capacity, TEWI showed a significant contribution to evaluating the global warming impact of the refrigeration system. Islam et al. (2019) used the TEWI index to assess the performance and environmental impact of the supermarket refrigeration units with R410A and R404A in Japan using a simple ideal cycle analysis.

Currently, an advanced metric known as the life cycle climate performance (LCCP) is gaining attention in evaluating heat pump systems. In the report of Makhnatch and Khodabandeh (2014), the roles of GWP, TEWI and LCCP were introduced for the selection of low-GWP refrigerants. The LCCP analysis is more complicated than TEWI, and it accounts for all the relevant indirect emissions, including the manufacturing and disposal of materials and refrigerants. The LCCP concept was first reported by the technology and economic assessment panel (TEAP) of the united nations environmental program (UNEP) in 1999 (Andersen, 1999) to calculate the “cradle to grave” climate impact of the direct and indirect greenhouse gas emissions. Little (2002) first used this method to evaluate the performance of hydrofluorocarbons (HFCs) in various systems. GREEN-MAC-LCCP was the first tool to assess the mobile air conditioning system (MAC) using the LCCP method (Papasavva et al., 2010). Hafner et al. (2004) used the LCCP technique to evaluate a mobile air conditioning system with R134a and R744, comparing the seasonal energy consumption for various climate locations. The cycle simulation and LCCP evaluation with HFO1234yf on the heat pump cycle were presented by Horie et al. (2010). To encourage the usage of LCCP metrics, the international institute of refrigerant (IIR) published a guideline for performing LCCP calculations on air conditioning, heat pump, and refrigeration systems in 2016 (Hwang, 2015). Later, the LCCP analysis of heat pumps was carried out for both cooling and heating systems (Troch, 2016). Based on the LCCP method, five cycle options and seven low-GWP refrigerants were evaluated for air conditioning applications by Lee et al. (2016). Aprea et al. (2016) reported an experimental comparative analysis between R134a and HFO1234yf on a domestic refrigerator and the LCCP analysis to evaluate the environmental impact due to HFO1234yf used as a substitute for R134a. Another research about the LCCP analysis of HFO1234ze for R134a substitute was also carried out (Aprea et al., 2016b). LCCP is also employed to assess cooling and heating systems in South Korean weather conditions to reduce the CO₂ emission (Choi et al., 2017). Botticella et al. (2018) used a multi-criteria approach to analyze possible design options for split systems for residential heating with six types of low-GWP refrigerants. Furthermore, the environmental impact was measured by the LCCP index to choose the optimal design on the market. To replace the traditional boiler with the residential heat pump in China, LCCP was applied by Dai et al. (2020) as one of the critical metrics.

Although it has been decades since the proposal of LCCP, the application of LCCP is still limited because of its complicated calculation, especially analyzing low-GWP refrigerants. On the other hand, most reported works in the literature mainly focus on the performance analysis in terms of COP, heat transfer characteristics and thermophysical properties. Thus, the objective of the present article is to assess low-GWP refrigerants from another perspective accounting for the “cradle to grave” climate impact. In gauging the climate impact of low-GWP refrigerants comprehensively, this paper develops the LCCP evaluation of a domestic heat pump system using various next-generation refrigerants. The LCCP of six types of refrigerants, including HFC32, the binary blends of HFC32/HFO1234yf with different mass compositions (0.42/0.58, 0.28/0.72, 0.22/0.78) with the target GWP of 300, 200 and 150, and the blends of HFC32/HFO1234ze(E) with 0.42/0.58 and 0.28/0.72 for the respective GWP of 300 and 200, are discussed. In contrast to other studies, experimentally measured performance data, including the COP and the refrigerant charge amount, are employed in the analysis. Here, the performance and emission of R410A are considered a reference.