## 4 The Refrigeration Systems  
Currently, the vast majority of on-board refrigeration systems are vapour compression systems employing high-Global Warming Potential (GWP) refrigerants, among which R-22 is still the dominating one [34]. However, such a refrigerant is going into phase out starting from 2020, with a consequent need for suitable replacements. The focus of the present work is to consider not only waste-heat driven sorption systems, as is common in the literature as a replacement for the refrigeration facilities on-board [10], instead, combined systems, which also include compression chillers, are evaluated. The integration of sorption and compression systems is specifically studied to maximise the efficiency of both systems. To achieve this, as will be detailed in the next section, different parallel and series connections were considered. Regarding sorption systems, different cases were identified, according to the data available in the literature and aiming at representing both commercial or higher technology readiness level (TRL) systems and lab-scale systems at lower TRL. In particular, two sorption systems with the following working pairs were analysed:  
* Activated carbon/ethanol;
* CaCl\({}_{2}\)/ammonia. The activated carbon (SRD 1352/3)/ethanol system is the first-ever reported prototype for continuous cooling production employing such a refrigerant. The design of the prototype is presented in [19], whereas its full experimental characterization is reported in [15]. The system was designed for cooling production, in a range of -10 \({}^{\circ}\)C to +7 \({}^{\circ}\)C, and can be driven by low-grade heat sources, i.e., engine coolant water. The main features of the prototype are reported in Table 3, whereas a picture of the system is shown in Figure 7. The main characteristic of the system is the use of a natural refrigerant (ethanol), with 0 ozone depleting potential (ODP) and very low global warming potential GWP = 6. The system was tested in a lab-scale environment and was then at low TRL, thus the weight and volume that will be used for the subsequent analysis were not optimized and can be subject to a great improvement. Nonetheless, the results reported in [15] indicated the promising potential as a replacement for other systems employing toxic refrigerants, such as methanol. This is especially important for the application in cold rooms, where leakages of the refrigerant can induce safety and health issues on the food preserved. Figure 6: Waste heat available calculated. The second sorption system evaluated was a prototype employing CaCl\({}_{2}\) as sorbent and ammonia as refrigerant, whose design and experimental characterization is presented in [16]. Much like water, ammonia is a natural refrigerant with 0 ODP and 0 GWP. The prototype was specifically designed for application in fishing vessels, by paying attention to corrosion issues and realising heat exchangers in such a way that no performance degradation occurs under shaking. In order to improve the Coefficient Of Performance (COP), the cycle includes a four-step heat recovery process. A picture of the system is shown in Figure 8. Despite being a lab-scale system, three different generations with continuous improvement were presented, giving the system a higher TRL than the previous one (TRL 5 versus TRL 4). For the fourth-generation prototype, the estimated volume of the sorption refrigerator is 0.42 m\({}^{3}\) and includes a four-step heat recovery system to improve the COP of the unit. This system is composed of a generator, heat exchanger for rectification and distillation, seawater heat exchanger, evaporator, solution heat exchanger, refrigerant heat exchanger, condenser, absorber, pumps, and valves. All the heat exchangers are micro-channel heat exchanger, to improve heat transfer and avoid issues in the flow of the heat transfer fluid due to the shaking of the vessel.