## 4 Results and Discussion

Table 5: Results obtained with DMFC-AUV\(\backslash\)QpDM using the input parameter values listed in Table 4 and the operational profile showed in Table 3. Bcn, bow cone. Bph, bow pressure hull. Cph, central pressure hull. Sph, stern pressure hull. Scn, stern cone. In all cases, the diameter of the central pressure hull is the maximum possible, that is, the outer diameter minus two times the thickness of the external hull. This was expected as it maximizes the volume of the central pressure hull thereby allowing to load larger amounts of fuel, O\({}_{2}\) and CO\({}_{2}\) adsorbent. Regarding the structure, colored in dark green in Figure 8, thicknesses increase with design navigation depth as expected, but it is observed that the designs for the same depth are not equal, as might be expected. This is not surprising as MF does not penalize any design by the arrangement of its central pressure hull but by the maximum external pressure they can withstand. In other words, two different structural arrangements, suitable for the same design navigation depth, are equal for DMFC-AUVQpDM. Anyway, this behavior does not influence the conclusions of this work since it does not impact the endurance results. If some arrangements were preferable to others, MF could be modified to prioritize them. The power demand of the payload and auxiliary equipment, 274.9 W and 52.6 W, respectively, is equal in all the cases as the installed sensors and the operation profile are the same. BoP power is lower for Model 1000 than for Model 3000 and Model 4500 where this value is the same. Once again, these results were expected as the power plant of Model 1000 is made of a single 1000 W fuel cell stack while the power plant of Model 3000 and Model 4500 contains two 700 W stacks. Despite the fact that heat and mass flows are greater, a two-stack power plant (Figure 6) involves the use of two cooling pumps and two normally-closed control valves rather than one cooling pump and no control valves used in a single-stack power plant. When comparing the volume claimed by the fuel cell stacks and the corresponding BoP with the the volume claimed by methanol, O\({}_{2}\) storage system, CO\({}_{2}\)CS, and water tanks, the first is, on average, 3.5% of the second; that is, most of the volume of the central pressure hull is dedicated to fuel, O\({}_{2}\) and by-products storage. More in detail, it is observed that the design of the power plant is highly influenced by the CO\({}_{2}\)CS. If Zeolite 13X is used as adsorbent material, the volume dedicated to the CO\({}_{2}\)CS is 52% on average of the volume of the central pressure hull in all the cases. This aspect can be improved by using Mg-MOF-74 that occupies 32% of the net volume of the central pressure hull, leading to more compact CO\({}_{2}\)CS along with longer values of endurance. Regarding the total dry mass, the AUVs outlined are heavier than the reference AUVs for the same dimensions (see Table 1); specifically, when using Zeolite 13X as CO\({}_{2}\) capture agent, the Model 1000 is around 1.30 times heavier than its reference AUV, Model 3000 1.63 times and Model 4500 1.54 times. If Mg-MOF-74 is used, these figures are bigger: 1.37 for Model 1000, 1.72 for Model 3000, and 1.62 for Model 4500. The weight of the adsorbent is on average 15% of the total weight of the AUV when Zeolite 13X is used and this rate is reduced to 11% when using Mg-MOF-74. This reduction is explained by the higher CO\({}_{2}\) capture capacity of Mg-MOF-74 compared to that of Zeolite 13 X, 2.46 times higher under the same pressure and temperature conditions, see Table 4. Although it is difficult to explain why DMFC AUVs are heavier than the reference models without a detailed knowledge of the construction of the last ones, a short analysis is done.