## 3 Problem Configuration
### Payload, Auxiliary Equipment and BoP  
To define the three basic attributes of the on-board devices, namely, mass, volume, and power demand, needed by DMFC-AUV\(\backslash\)QpDM, data from commercially available equipment was used whenever possible. This method has the advantage of being simple while working with actual data, which reduces the uncertainty of the design. This is the case of pumps, valves, blowers, or sensors. For equipment that is not commercially available or in case the data of which was not accessible, reasonable assumptions were made to define their attributes. This is applicable mainly to gas separators and heat exchangers. The characteristics of all the elements on-board the AUVs are included in Appendix A. Payload is described in Table A1, and these data are the same for the three designs under study. The same is applicable to Table A2 which contains the characteristics of the auxiliary equipment. Finally, the BoP is included in Table A3 for Model 1000 and Table A4 for Model 3000 and Model 4500. The definition of payload and auxiliary equipment is based on the capabilities of the reference AUVs and does not depend on the AUV size or navigation speed. However, this is not the case of BoP components whose definition needs of an iterative process because the size of the power plant is not known in advance and depends on the operational profile of the vehicle. Starting from the power plant schema (Figures 5 and 6), the mass and heat flows and the pressure drops of the different circuits are estimated. Then, the components of the BoP are selected from commercial catalogs and incorporated to the design. As these components modify the operational profile, the design tool must be run again to check if the proposed design is coherent. For the size estimation of cooling components, the following common assumptions were made. The cooling fluid is pure water. Sea water temperature is taken equal to 333 K to consider navigation in shallow tropical waters. This value is considered constant in spite of sea water temperature drops with depth. This allows to consider the worse conditions from the point of view of heat balance and size of the heat exchangers. The size of heat exchangers is estimated using the effectiveness-NTU method [56]. External heat exchangers are modeled similarly to counter current shell and pipe heat exchangers and CO\({}_{2}\)CS heat exchanger is modeled similarly to a plate heat exchanger. Regarding the cooling of stacks, the following assumptions apply. The components of the cooling loop of the stacks must satisfy the needs of the fuel cell stacks running at their nominal power, although this operating regimen is never reached in accordance with the operational profile of the AUV. Temperature variation of cooling water is equal to 10 K to avoid thermal tensions inside the stack. The heat flow to evacuate is equal to the energy of the methanol flow calculated through its Lower Heating Value minus the nominal power of the fuel cell. The heat produced by the CO\({}_{2}\)CS is calculated from the values of the isosteric heat of adsorption reported by [PERSON] et al. [43]. The CO\({}_{2}\) flow that exits the CO\({}_{2}\)CS is equal to twice the production rate of CO\({}_{2}\). The power consumption of pumps and blowers is considered fixed and equal to their nominal power although the real demand is lower. In the case of O\({}_{2}\) and CO\({}_{2}\) blowers, 23 and 16, respectively, their minimum flow must be three times the O\({}_{2}\) consumption and CO\({}_{2}\) production rates, respectively. For liquid and low-pressure gas pipes, nitrile butadiene rubber flexible pipes with an outer diameter equal to 7 mm were considered. This material shows good resistance to hydrocarbons, is light and flexible and can be used with fluid temperatures up to 373 K [57]. The connection between the O\({}_{2}\) pressure reduction valve 2 and the O\({}_{2}\) low flow electronic pressure regulator 2 is made by a 1/16\" stainless steel pipe (see Figure 5).