## 3 Results and Discussion
### Absorption Chiller Subcooling System Performance  
The use of the absorption chiller for dedicated subcooling is a choice that means the consumption of extra heat input in the system but not extra work, as is the case in the mechanical subcooling case. So, the reduction in the overall work consumption of the system can be higher than in the M-SC system. However, there is a need for significant amounts of heat input in the generator of the absorption chiller. The optimization of the ACH-SC system was performed using three optimization variables; the pressure ratio parameter (a), the evaporator temperature of the absorption chiller (T\({}_{\rm e,ach}\)), and the generator temperature level (T\({}_{\rm g}\)). Figure 12 illustrates the COP of the ACH-SC system and of the Ref system for all the optimized operating scenarios. It was obvious that the Ref system leads to higher COP compared to the ACH-SC system for all the cases. The reason for this result is based on the definition of COP according to Equation (9). This equation evaluates the work and the heat input in the same way by simply adding them in the denominator of the COP. The heat input in the generator is generally high and this fact leads to reduced COP. It is remarkable to state that the curves of the ACH-SC system were not very smooth in all cases because of restrictions in the optimization procedure. These restrictions are mainly associated with the minimum possible evaporator temperature (T\({}_{\rm e,ach}\)), as well the minimum possible generation temperature in every case, because of restrictions in the LiBr concentration in the streams of the absorption cycle. On the other hand, the exergy efficiency of the system was enhanced in the ACH-SC system compared to the Ref system, according to Figure 13. The enhancements were significant and so it can be said that the ACH-SC system is a system with important exegetic enhancement. The exergy efficiency was the proper criterion for evaluating the ACH-SC system because the temperature level of the generator heat input was not considered with the simplistic energetic index (COP). It is important to state that the overall maximum exergy efficiency was 37.23% and it was found for (T\({}_{\rm e}\) = \(-\)15 \({}^{\circ}\)C and T\({}_{\rm c}\) = 35 \({}^{\circ}\)C), whereas for this case the COP was 2.121. Moreover, it has to be said that the optimum evaporating temperatures were equal to \(-\)25 \({}^{\circ}\)C for heat rejection temperatures higher than 35 \({}^{\circ}\)C. It can be said that the optimum refrigeration temperatures were lower for the ACH-SC system compared to  
Figure 11: The exergy efficiency of the mechanical subcooling system for different values of the pressure ratio parameters and different evaporator temperatures (T\({}_{\rm e}\) = \(-\)15 \({}^{\circ}\)C and T\({}_{\rm c}\) = 40 \({}^{\circ}\)C). the Ref system, a result which was also found for the M-SC system. So, it can be said that, for both dedicated subcooling technologies, the optimum operating refrigeration temperature is lower in the subcooling systems, according to exegetic criteria. The next step in this analysis was the investigation of the optimization variables in the ACH-SC system for the scenario (T\({}_{\rm e}\) = \(-\)15 \({}^{\circ}\)C and T\({}_{\rm c}\) = 40 \({}^{\circ}\)C). Figure 14 shows that higher values of the pressure ratio parameter increased the COP, but the optimum exergy efficiency was found for (\(\alpha\) = 1.24). Moreover, in this case, the optimum generator temperature was 80 \({}^{\circ}\)C, while the absorption chiller evaporator temperature was kept at 10 \({}^{\circ}\)C. It has to be said that the subcooling was more beneficial in the cases of higher-pressure ratio, and the COP increased with a higher rate of higher-pressure ratio values. Practically, in high-pressure levels, there is greater gain with the subcooling because of the inclination in the liquid saturation curve of the CO\({}_{2}\).