## 2 The Quick Preliminary Design Model
### Power Plant  
Figure 5 shows a sketch of the power plant studied herein. The core of the power plant consists of a DMFC stack  
1  
connected in parallel to a set of Li-Ion batteries  
2  
, thereby forming a hybridized system. This hybrid system is connected to a power bus  
3  
that distributes the electrical power to the consumers. To refrigerate the power plant, the sea  
4  
is used as a heat sink. The stack cooling loop consists of an open heat exchanger located outside the AUV  
5  
, a cooling pump  
6  
, and a reservoir that is also used also as an expansion vessel  
7  
. The methanol is stored in its pure state in a specific tank  
8  
that is equipped with a pressure equalization valve to avoid undesired vacuum effects due to emptying the tank. In the case of an emergency or shut down, this tank is automatically isolated by a normally-closed solenoid valve  
9  
. The concentration of methanol in the anode circuit is maintained at the desired value, _cMoOH_, by a concentration sensor  
10  
that is installed inside a methanol reservoir  
11  
, which acts also as gas separator. This sensor controls the methanol feeding pump  
2  
. The anode flow is impelled by a pump  
13  
placed at the inlet of the anode to avoid cavitation due to the presence of CO\({}_{2}\) bubbles. The CO\({}_{2}\) that leaves the reservoir is impelled by a a b  
Figure 4: Power demand patterns for two different operational profiles. A  
, operational profile characterized by long high power demand periods and  
B  
, operational profile characterized by short power peaks. Capture System (CO\({}_{2}\)CO) 11 where it is captured by the adsorbent material. The excess stream is led to a heat exchanger 16 where the heat produced inside the CO\({}_{2}\)CS is evacuated. After cooling, this stream is blended with the CO\({}_{2}\) stream coming from the reservoir and recirculated through the CO\({}_{2}\)CS. This loop allows both refrigerating the CO\({}_{2}\)CS and reducing the temperature of the stream coming from the reservoir, thereby favouring the CO\({}_{2}\) capture. A cooling loop consisting of an open heat exchanger located outside the AUV 12, a cooling pump 18, and a reservoir 19 evacuates the heat of the CO\({}_{2}\)CS to the sea. Regarding the oxygen reduced at the cathode of the fuel cell, it is stored as compressed pure oxygen at 35 MPa in a cylinder 20 equipped with a pressure reduction valve that lowers the pressure to 2 MPa 21. The reason for using compressed oxygen is its simplicity. The use of pure oxygen rather than air leads to a more compact oxygen storage system for the same endurance. The oxygen is released by a low flow electronic pressure regulator 22 that reduces the O\({}_{2}\) pressure to 100 kPa. This O\({}_{2}\) enters into a loop connected to the cathode where a blower 23 keeps the stream circulating. The water produced in the cathode and the excess of oxygen are directed to a gas separator 24 similar to the CO\({}_{2}\) separator in the anode loop 11. The excess oxygen is led to the blower 23 and recirculated to the fuel cell. The water produced is distributed by means of a dedicated pump 25 and a set of four 3-way valves 26 to the anode loop or to the water storing tanks 27 that form the trimming system. These tanks are located at the ends of the central pressure hull, thereby maximizing the righting momentum. This configuration allows both managing the trim of the AUV and refilling the anode circuit with just a single conventional pump. The anode gas separator 11 also acts as a solution reservoir allowing an intermittent refilling of water from the cathode and, in this way, the operation of the trim control system. Finally, the installation is managed by the corresponding electronic control and management system 28. This system implements a control algorithm to manage the power plant. In this sense, works as those published by [PERSON] et al. [28] and [PERSON] et al. [29] can be used. Figure 5: Sketch of the power plant. 1 Fuel cell stack. 2 Batteries. 3 Power bus. 4 Sea water.