## 8 Future Trends and Alternatives  
The results reported in the previous sections highlight the good potential of the solution described, which allows a significant reduction in the fuel consumption of fishing vessels, with a clear benefit in terms of the sustainability of the shipping sector. This is a crucial point, especially considering the stringent international laws in terms of energy efficiency in the shipping sector [44] and the legally-binding measures adopted within MARPOL Annex VI for the reduction of greenhouse gas (GHG) emissions from international shipping [3], which set a 10% reduction with progressive tightening of the target every five years. The introduction of alternative refrigeration systems can then represent an important step forward, considering that refrigeration demand represents more than 40% of the energy demand in long-distance non-touristic shipping [22; 45]. However, further optimization is possible, for example, combining the waste-heat powered system with other alternative systems for air-conditioning and domestic hot water production or the extension of such systems also to cruise ships. For instance, in [5], the energy and exergy analysis for a cruise ship in the Baltic Sea indicated that around 30% of the energy demand of the ship is due to space heating, with several inefficiencies in the boilers used for providing such a demand. The authors also suggested that an even higher share of energy demand could be encountered in cruise ships operating in warm climates, due to the electricity needed for the chillers that cover cooling demand. It is clear that the solutions proposed could be even more beneficial in these cases, thanks to the intrinsically higher efficiency of the sorption system at higher evaporation temperatures (+5 \({}^{\circ}\)C to +15 \({}^{\circ}\)C for air conditioning purposes). For example, for the system proposed in [21], a volumetric cooling power up to 275 kW/m\({}^{3}\) was measured, which would also significantly reduce the space needed for installation, without any limitation due to the volume of the technical rooms. For the case of a chemical tank, waste heat recovery for covering heating demand could lower the fuel consumption up to 18% [46]. In this case, the application of a sorption system is even more interesting, due to the efficiency higher than 1 when operating as a heat pump. Further improvements of the system, however, do not only rely on a sorption system for heating and cooling. As reported in the previous analysis, even using sorption or hybrid sorption-compression systems, there is a significant amount of waste heat still potentially unused that can be recovered from exhaust gases. A promising application could be the integration of an ORC system for the production of electricity needed for the ancillary systems or for the chillers and auxiliaries of the heating, cooling, and refrigeration system. An analysis of a cruise ship in the North Sea indicated that up to 22% of the electricity demand could be covered by waste heat [46]. Moreover, the comparative analysis of different power cycles in [46] indicated the ORC solution as a promising alternative due to the simplicity in the layout and the possibility of using non-toxic refrigerants. For example, for the cases examined, considering the waste heat available that is not used by the refrigeration system and an efficiency of the ORC of 5% (which can instead be significantly improved), up to 20 kW of electricity might be produced, even higher than the electricity demand needed for the refrigeration system itself, which could be then totally sustainable with waste heat. Another aspect to be considered is the time mismatch and oscillations in the heat source and cooling load. An efficient way to cover this issue is the development of thermal energy storageswith high density.