## 4 Results and Discussion
On the one hand, and considering the values of specific energy of Al/HP semi-fuel cells, see Table 6, the theoretical weight of an Al/HP semi-fuel cell, with an efficiency equal to 1, and an energy content equal to that of Model 3000 using Zeolite 13X and Model 3000 using Mg-MOF-74, would be equal to 725.3 kg and 1404.7 kg, respectively. In the case of Model 4500, those figures would be, 783.6 kg and 1536.9 kg, respectively. Therefore, an AUV powered with Al/HP semi-fuel cells would be heavier for the same amount of energy. On the other hand, the payload is similar and not big weight deviations are expected. An explanation for this divergence can be found taking into account the presence of the central pressure hull that represents from 34.4% of the total dry mass in the case of the Model 1000 with Mg-MOF-74 to 48.4% in the case of the model 4500 using Zeolite 13X. Reference AUVs use pressure tolerant batteries and semi-fuel cells allowing a mass saving. This weight increment makes necessary the use of syntactic foam filling void spaces to get the hydrostatic equilibrium, but this foam adds an extra weight to the final balance contributing to the weight increment observed. In all the cases, the results obtained demonstrate that the volume and weight margins defined are respected, sufficient space for the installation of the equipment and addressing the deviations due to the uncertainties of the design process. It must be noted that for all the designs, the value of the volume margin is the lowest possible while the mass margin remains far above the minimum value. This means that the power plant proposed is essentially bulky rather than heavy and this trend is stronger as the AUV is smaller. In terms of energy stored, Table 6 sets forth the energy density and specific energy of the different designs taking the net volume of the central pressure hull and the mass of the components of the power plant, i.e., fuel cell stacks, BoP, consumables, and products inside, as well as the defined volume margin. The figures of this table are in line with the aforementioned results, considering that energy content is equivalent to endurance. On the one hand, if Zeolite 13X is used, the energy density and specific energy of a DMFC power plant is much lower than the same power plant using Mg-MOF-74. On the other hand, the size of an AUV has a great impact on the applicability of DMFC-based power plants to power AUVs. Larger AUVs are more likely to favour the installation of this type of power plant. Finally, Mg-MOF-74 provides endurance values approximately 40% longer than those provided by batteries. Table 6 also shows the energy density and specific energy of the reference AUVs for Model 3000 and Model 4500, that is, the figures for the Al/HP semi-fuel cell. From a comparison analysis, it follows that a DMFC-based power plant as the one proposed here would be bulkier than an Al/HP semi-fuel cell no matter the CO\({}_{2}\) capture agent is used. However, considering the specific energy values, it is observed that the DMFC power plant is lighter than a Al/HP semi-fuel cell when Mg-MOF-74 is used as CO\({}_{2}\) capture agent, being almost equally heavy when Zeolite13X is used.