## 5 Conclusions  
In terms of endurance, this work studies the advantages of powering an Autonomous Underwater Vehicle (AUV) with a power plant based on a Direct Methanol Fuel Cell (DMFC). Accordingly, a quick preliminary design modeling software that makes use of a genetic algorithm to solve a multi-objective optimization problem has been developed. This tool allows defining the payload of the vehicle using real data of the equipment to be loaded as well as the operation profile. The result is the initial design of an AUV powered by a DMFC. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Property**} & \multicolumn{2}{c}{**Model 1000**} & \multicolumn{2}{c}{**Model 3000**} & \multicolumn{2}{c}{**Model 4500**} \\ \cline{2-7}  & **Zeolite 13X** & **Mg-MOF-74** & **Zeolite 13X** & **Mg-MOF-74** & **Zeolite 13X** & **Mg-MOF-74** \\ \hline Mass of methanol (kg) & 18.0 & 35.3 & 46.0 & 89.1 & 49.7 & 97.5 \\ Specific energy of methanol (MJ/kg) & 22.7 & 22.7 & 22.7 & 22.7 & 22.7 & 22.7 \\ Net energy stored 1 (MJ) & 102.2 & 200.3 & 261.1 & 505.7 & 282.1 & 553.3 \\ Cph. net volume (L) & 625.8 & 626.5 & 1521.3 & 1521.8 & 1641.9 & 1657.0 \\ Power plant mass 2 (kg) & 305.7 & 361.4 & 711.1 & 844.3 & 764.7 & 919.2 \\ Cph. energy density (MJ/m3) & 163.3 & 319.8 & 171.6 & 332.3 & 171.8 & 333.9 \\ Cph. specific energy (kJ/kg) & 334.2 & 554.3 & 367.1 & 598.9 & 368.8 & 601.9 \\ Reference AUV. Energy density (MJ/m3) & - 3 & -3 & 625.0 & 625.0 & 625.0 & 625.0 \\ Reference AUV. Specific energy (kJ/kg) & - 3 & -3 & 360.0 & 360.0 & 360.0 & 360.0 \\ \hline \hline \end{tabular}  
* Energy used to power the AUV. Calculated multiplying the energy content of the stored methanol by a typical DMFC efficiency value, 0.25 [14]. 2
* Includes the mass of the equipment inside the central pressure hull excluding the mass of the pressure hull itself. 3 Information not available.  
\end{table} 
Table 6: Energy stored by the DMFC-AUV. LHV of methanol from [14]. Reference AUV energy data from [6, 58]. As the power plant proposed generates CO\({}_{2}\), a capture system is needed (CO\({}_{2}\)CS). For this application, the adsorption mechanism has been considered as the most appropriate method. Two different adsorbent materials have been evaluated: Mg-MOF-74 and Zeolite 13X. Mg-MOF-74 has the highest CO\({}_{2}\) capture capacity under the same ambient conditions, but it is heavier and more expensive than Zeolite 13X is. This fact makes the selection of the adsorbent material a relevant aspect that impacts the performance of the resulting AUV. To make the results comparable to existent AUVs, one reference AUV powered by batteries and two powered by Al/HP semi-fuel cells have been selected and used as models. The tool has been configured with the same payload, operational profile, and main dimensions of the model AUVs, and the corresponding equivalent designs have been obtained. A total of six designs have been produced, two for each reference AUV with different adsorbent materials. The analysis of the work results concludes that the application of power plants as the one described here to power AUVs can be beneficial in terms of longer endurance values when compared to Li-Polymer batterie or semi-fuel cell powered AUVs for large AUVs. The CO\({}_{2}\)CS has been revealed as the most critical component of the power plant because of its volume that can reach up to 52% of the total volume of the power plant. In this regard, the capture capacity of the adsorbent material is a crucial matter. It was observed that a design using Mg-MOF-74 (CO\({}_{2}\) capture capacity at 348 K and 100 kPa equal to 8 mol\({}_{\text{CO}_{2}}\)/kg\({}_{\text{Mg-MO-74}}\)) has an endurance value of approximately twice the same design using Zeolite 13X (CO\({}_{2}\) capture capacity at 348 K and 100 kPa equal to 3.25 mol\({}_{\text{CO}_{2}}\)/kg\({}_{\text{Zeolite 13X}}\)) regardless of size.