## 3 Results and Discussion
### Comparative Analysis  
The last step in this analysis was the comparison of the three examined configurations. Firstly, the three systems were compared for different refrigeration temperatures and for the typical heat rejection temperature of 40 \({}^{\circ}\)C. Figures 16 and 17 show the COP, the exergy efficiency, and the work consumption of the examined systems, respectively. Figure 16 shows that the M-SC system had the highest COP for all the cases, while the Ref system and the ACH-SC system followed, respectively. The low COP of the ACH-SC system is explained by the definition of its COP, which evaluates, with the same way, the work consumption and the heat input. Moreover, Figure 16 depicts the exergy efficiency of the examined systems. The highest exergy efficiency was found for the M-SC system, with the ACH-SC system as the second choice with a small difference, while the Ref system had significantly lower exergy efficiency compared to the systems with subcooling. The greatest difference was observed in the lower refrigeration temperatures, because in these cases the reference system faces important difficulties to produce the demanded refrigeration load, due to the high-pressure ratio in the compressor and the lack of any subcooling assistance. Figure 17 shows the work consumption for the three systems, as well as the decrease in the work consumption of the system with subcooling compared to the reference system. It was obvious that the highest work consumption was found for the Ref system, whereas the ACH-SC system had the lowest work consumption. The decrease in the work consumption for the ACH-SC system ranged from 40% to 59%, whereas for the M-SC system it ranged from 32% to 41%. So, it can be said that the ACH-SC system leads to higher work savings compared to the M-SC system, and this is an important advantage of this configuration. Another advantage of the ACH-SC system is the use of the environmentally friendly working pair, LiBr-H\({}_{2}\)O, while, in this case, the M-SC system utilizes R134a, which has a GWP of 1430. However, there is the possibility to use other working fluids in the M-SC system, such as propane or butane, which have extremely low GWP (around 3-4), but they are flammable working fluids and they belong in the ASHRAE safety group A3. Generally, there is no optimum choice for the secondary cycle, and this is something that has been also discussed in the recent review paper of Ci  
Figure 15: COP and exergy efficiency of the absorption chiller subcooling system for different generator temperature levels and evaporator temperatures with optimum pressure ratio values (T\({}_{\mathrm{e}}\) = \(-15\)\({}^{\circ}\)C and T\({}_{\mathrm{c}}\) = \(40\)\({}^{\circ}\)C). The results for all the cases are given in Figures 18-20 for the COP, the exergy efficiency, and the work consumption, respectively. Moreover, Table 2 includes the results of work and the heat input in the system for all the examined cases. It has to be said that the work input was significant and it ranged from 51.14 up to 137 kW, whereas the refrigeration production was constant at 100 kW in all the cases. The greater heat input in the system was found for the cases with high heat rejection temperature levels (T\({}_{\rm c}\) = 45 \({}^{\circ}\)C or 50 \({}^{\circ}\)C), thus, in these cases, the absorption chiller was not the most suitable case. The highest work consumption decrease was found in the cases with extremely low refrigeration  
Figure 16: COP and exergy efficiency comparison of the optimized cases of the three systems (T\({}_{\rm c}\) = 40 \({}^{\circ}\)C). Figure 17: Work consumption of the optimized cases (continuous lines) of the three systems, and work consumption decrease of the subcooling cases compared to the reference case (non-continuous lines), (T\({}_{\rm c}\) = 40 \({}^{\circ}\)C). for the decrease in the case with the absorption chiller is the definition of the COP in this case (see Equation (9)); the generator heat input is simply added to the work consumption.