## 1 Introduction
The previous ways for enhancing the performance of a CO\({}_{2}\) refrigeration system are effective and interesting. However, in a recent review paper, [PERSON] et al. [23] highlighted that the dedicated subcooling is a more effective method among the previously discussed. More specifically, they stated that the dedicated subcooling increases, by 30%, the COP, which is a higher value compared to the other reviewed techniques, under the same operating conditions. They stated that the dedicated subcooling can be performed using three different technologies: (a) mechanical compression cycle, (b) absorption chiller, and (c) thermoelectric system. At this point, it has to be said that the dedicated subcooling is performed with an external refrigeration cycle, which can be driven by renewable energy sources, such as solar energy. For example, there is the possibility to use photovoltaic panels in order to drive the dedicated mechanical subcooling or solar thermal collectors (e.g., flat plate collector or evacuated tubes) for driving the dedicated subcooling with an absorption chiller. The most common dedicated subcooling is the use of an external mechanical compression subcooling system (M-SC) which consumes extra electricity. The basic idea is that the increase of the cooling capacity, due to the subcooling, and the simultaneous decrease of the optimum high-pressure lead to a high reduction in the electricity consumption of the primary compressor, which overcomes the extra electricity consumption in the secondary cycle. [PERSON] et al. [24] studied this configuration with various refrigerants, included R134a in the secondary refrigeration cycle, and they found 20% higher COP and 29% higher cooling capacity. Furthermore, [PERSON] et al. [25] stated that the COP enhancement can reach up to 30% when the heat rejection temperatures are high. Moreover, [PERSON] et al. [6] studied the use of R134a in the secondary loop and they found the total equivalent warming impact to be 10% lower compared to the booster system in Athens (Greece) and 25% for Valencia (Spain). [PERSON] et al. [26] compared the use of a cascade system with a mechanical subcooling system with R1234 yf in the secondary cycle, and they concluded that both systems increase the COP but the mechanical subcooling system gives the best results. [PERSON] et al. [27] studied the use of zeotropic mixtures in the secondary cycle in order to reduce the temperature difference during the heat transfer from the one cycle to the other. According to their results, the COP is found to be 35% with the subcooling and the optimum subcooling temperature difference can be up to 38 \({}^{\circ}\)C.  
The next part of the literature includes methods of dedicated subcooling by the utilization of a heat source. In these cases, the electricity consumption of the system is reduced and extra heat input is needed. The use of renewable or alternative energy sources, such as solar energy, geothermal energy, and waste heat, can be effective choices for reducing the environmental impact of the refrigeration system. [PERSON] and [PERSON] [28] studied the use of an absorption chiller operating with LiBr-H\({}_{2}\)O for subcooling the CO\({}_{2}\) after the gas cooler (ACH-SC). The absorption chiller was powered by a solid oxide fuel cell and they studied the system with and without subcooling, as well as with and without an internal heat exchanger. They found that the use of an absorption chiller for subcooling is a more effective technique than the internal heat exchanger, while they stated that the global optimum system combines both an absorption chiller and internal heat exchanger for subcooling. [PERSON] [29] investigated various configurations of the CO\({}_{2}\) refrigeration system with single or two-stage cycles and with the use of different absorption machines for subcooling. They utilized the heat of the compressor outlet in order to feed the absorption machines.