## 6 Energy Analysis
### Cascade Configuration
In addition, the fuel savings due to the operation of the cascade system can be calculated considering that electricity generation on-board is obtained through a generator connected to the main engine:  
\[FS=\frac{EES}{\eta_{gen}LHV_{fuel}}, \tag{17}\]  
where the efficiency of the generator considered is 50% and the low heating value (LHV) of the fuel is 0.004878 kWh/g [20]. The results are shown in Figure 14, where both energy savings and fuel saved for the various cases are reported. The energy savings were more consistent in the Indian Ocean, due to the higher seawater temperature and therefore the greater advantage in employing cascade configuration over the reference system. They varied between 6.8 MWh/y for vessels A and D and 13 MWh/y for vessel B, up to a maximum of 15.7 MWh/y for vessel C. For the North Sea, the energy savings were around 15% lower. For all cases, the energy saved amounted to 33 to 35% of the overall energy consumption of the reference system. Fuel saved varied correspondingly between 5000 kg/y up to 14,000 kg/y for vessel C in the North Sea and 8500 kg/y for vessel C in the Indian Ocean. It is worth stating that the lower energy required from the main generator, due to the reduction in the consumption for refrigeration, can be used for other auxiliaries, thus eliminating--or limiting--the need for secondary generators, without reducing the PLR of the engine. \begin{table}
\begin{tabular}{c c} \hline \hline MTpump rated power consumption & 20 W/kW\({}_{\text{f}}\) \\ \hline LT pump rated power consumption & 23 W/kW\({}_{\text{f}}\) \\ \hline HT pump rated power consumption & 27.3 W/kW\({}_{\text{f}}\) \\ \hline MIsorp pump rated power consumption & 20 W/kW\({}_{\text{f}}\) \\ \hline LTsorp pump rated power consumption & 10 W/kW\({}_{\text{f}}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Specific electricity consumption of auxiliaries of the cascade and reference systems. The energy savings, and therefore fuel savings, were normalised with respect to the energy (and fuel) requested for the reference system. The results of the calculations are reported in Table 8. As can be seen, an energy reduction of more than 70% for the refrigeration system was obtained by replacing the system with the combined sorption-compression one. Such savings correspond to a reduction of 4-6% of the overall energy consumption of the engine, thus representing an important contribution towards the overall efficiency of the ship. The volume needed for the installation of the overall system (sorption + compression) for the different cases was calculated as well and is shown in Table 9. For the smaller vessels (A and D), the volume needed for the installation was between 1.2 m\({}^{3}\) and 1.5 m\({}^{3}\), whereas for the bigger ones (B and C) a volume from 2.4 m\({}^{3}\) up to 3.4 m\({}^{3}\) was needed. For comparison, Table 9 reports also the volume needed for the installation in case of the use of a CO\({}_{2}\) compression chiller to cover the whole load. As can be seen, the penalisation, in terms of volume needed for the installation, is quite high, from 500% up to 700%, and therefore it represents an issue to be carefully addressed. It is, however, worth noticing that the system considered is still an unoptimized lab-scale prototype and scaling it up for real installation might significantly reduce the dead volume and therefore increase the volumetric cooling power. The volume needed for the sorption system was calculated, starting from the volumetric cooling power of the two systems, in kW/m\({}^{3}\), reported in the literature references [16] and [19], while the volume needed for the R744 chiller was taken from the volumetric cooling power of the compression chiller reported in reference [37]. Figure 14: Results of energy analysis for the cascade system for different fishing vessels from A to D (see Table 1).