# Transcritical R744 Supermarket Refrigeration System Integrated with a Heat-Driven Ejector Chiller
## 3 Results and Discussion
### Experimental Validation of the HEC and the R744 Refrigeration System  
The model of the HEC system was validated against available experimental data [34] with R600a as the working fluid in our previous study [16]. The percentage deviation in the COP and entrainment ratio were found to be within \(\pm\)4.8% and \(\pm\)0.1%, respectively. The model of the R744 refrigeration system equipped with parallel compression (PC) was also validated in our previous study [16] against published experimental data [7] with percentage deviation within \(\pm\)10%. Performance Comparison of the Proposed System with R717, R718, and R600a as the Refrigerant for the HEC Cycle  
The COP of the proposed system was contrasted with several baseline systems. These included an R744 refrigeration system coupled to R718- and R600a-based HECs having six to eight fixed-geometry ejectors in the multi-ejector rack (ME)). The performance was also compared with an R744 system equipped with parallel compression (PC) and a standard R744 booster system (CB), both without the heat-driven cycle. The results are depicted in Figure 7. Even though the difference in the operating GC pressure was small (Figure 8), the R717-based system having five ejectors was found to outperform the six-ejector-based R718 system by 3.8% at 27 \({}^{\circ}\)C to 12.0% at 45 \({}^{\circ}\)C ambient temperature, while the same accounted for 3.6% at 27 \({}^{\circ}\)C to 10.7% at 45 \({}^{\circ}\)C, compared to the eight-ejector-based R718 system. This was due to the fact that the cooling capacity of the R718-based HEC was significantly lower compared to the R717-based system (Figure 9). Switching from six to eight operating ejectors enhanced the capacity of the R718-based HEC from 14 kW to 18 kW at 45 \({}^{\circ}\)C ambient temperature, while the same for the R717-based system was around 51 kW. The lower the cooling capacity, the lower the degree of subcooling achieved by the HEC unit. This resulted in the higher accumulation of flash vapor in the liquid receiver of the R744 refrigeration system due to an increase in the quality of the refrigerant entering the receiver. This further increased the power consumed by the parallel compressors, which eventually lowered the system's efficiency. The lower cooling capacity of the R718-based HEC is attributed to its intrinsic thermo-physical properties. The higher latent heat of vaporization of R718 (Figure 10) signifies that it would require lower mass flow rates compared to R717 for extracting the same amount of heat. However, due to relatively lower vapor density (Figure 11), the volumetric refrigeration capacity is significantly lower than R717. Thus, with the given number of ejectors, the volume of refrigerant flow handled is lower. To accommodate larger volume flow rates, the number of R718 ejectors had to be increased to more than eight. Larger flow rates would lead to higher heat extraction from the R744 in the subcooler. For instance, the use of 14 R718 ejectors reduced the inferiority margin in COP to 5.8% compared to the R717 system; however, the use of more than eight ejectors may not be practically feasible due to escalation in the investment cost. It could also be observed that the R718-based system was able to outperform the PC system only at an outdoor air temperature above 30 \({}^{\circ}\)C (Figure 7), while the R717-based HEC was reported to outperform the PC system beyond 24 \({}^{\circ}\)C [16]. This significantly reduced the operational range of the R718-based HEC (Figure 12). As a result, the R718-based HEC could be utilized for a relatively smaller fraction of time in a year compared to the R717 system. Below 30 \({}^{\circ}\)C, the cooling capacity of the R718-based HEC reduced to less than 2.7 kW at the cost of relatively higher GC pressure compared to the PC system.