# Transcritical R744 Supermarket Refrigeration System Integrated with a Heat-Driven Ejector Chiller
## 1 Introduction  
Supermarkets are a highly energy-intensive industry, requiring about \(\sim\)3.5% of the total national electricity in some industrialized nations [1], mainly due to their refrigeration systems [2]. These systems make supermarkets a significant contributor to the indirect emissions of greenhouse gases. In addition, the majority of the commercial refrigeration systems around the globe still operate on high-global warming potential (GWP) synthetic refrigerants, like R404A and R22 [3]. Leakage of these refrigerants thus makes supermarkets a significant contributor to the direct emission of greenhouse gases as well. Meanwhile, enactment of several legislative acts (e.g., reference [4]) have enforced stricter regulations on the use of these high-GWP refrigerants, and they are stipulated to be phased out within a given timeframe. This has encouraged the scientific community to look for suitable alternatives to these high-GWP refrigerants. Several low-GWP refrigerants, including hydro-fluoro-olefins (HFOs) [5], are under consideration. However, HFO usage has given rise to different types of challenges, such as the proliferation of persistent, bio-accumulative, and toxic (PBT) chemicals with potential contamination of soil and water. Therefore, there is substantial investment risk, as in the near future these chemicals could come underrestricted usage. Natural refrigerants like R744 appear to be a more dependable solution and a long-term alternative. However, technology development around natural refrigerants has yet to mature. One of the most promising natural refrigerants, R744, suffers from the fact that the efficiency of the refrigeration system rapidly declines with a rise in ambient temperature due to transcritical operation. To overcome such challenges, several technological innovations have been proposed in recent times, such as parallel compression [6], evaporative cooling [7], various subcooling techniques [8; 9], waste heat recovery using an organic Rankine cycle [10], use of ejectors [11; 12; 13], pressure exchangers [14], etc. There is an enormous amount of waste heat available in supermarkets due to the transcritical operation of R744 systems, which is generally used to meet the district and space heating demands in cold weather locations. However, for supermarkets operating in warm and hot climates, the heating demand is either modest or absent. This necessitates the rejection of the excessive amount of heat to the outdoor air, which is simply wasted. Instead, this heat could be utilized in various ways to improve the efficiency of R744 refrigeration systems. These include powering an organic Rankine cycle to generate electricity, which in turn can be used to partially meet the electricity demand of the refrigeration unit itself [10]. The waste heat can also be utilized for powering either an absorption cycle [15] or a heat-driven ejector chiller [16; 17]. These waste heat-driven cycles can be deployed in supermarkets for subcooling the R744 at the gas-cooler outlet. Although substantial research is already available on the integration of absorption cycles with R744 systems [15; 18; 19], the literature lacks extensive studies on the usage of heat-driven ejector chillers (HEC). [PERSON] et al. [20] reported an investigation on a heat-driven ejector cycle integrated with a single-stage R744 refrigeration unit. Several refrigerants were evaluated, among which R32 was identified to be the most suitable for the heat-driven cycle. The integrated system was reported to improve the cooling capacity by 10% to 50% over a single-stage refrigeration unit. However, the utilization of the heat-driven ejector unit for subcooling the R744 cycle was not explored in that study. [PERSON] and [PERSON] [21] reported an investigation on an R1234 ze(E)-based heat-driven ejector cycle coupled to a single-stage R744 refrigeration system. The integration of an internal heat exchanger in the R744 system with an economizer in the heat-driven system was reported to provide the maximum coefficient of performance (COP).