Sizing and Dynamic modeling of a Power System for the MUN Explorer Autonomous Underwater Vehicle using a Fuel Cell and Batteries

The combination of a fuel cell and batteries has promising potential for powering autonomous vehicles. The MUN Explorer Autonomous Underwater Vehicle (AUV) is built to do mapping-type missions of seabeds as well as survey missions. These missions require a great deal of power to reach underwater depths (i.e. 3000 meters). The MUN Explorer uses 11 rechargeable Lithium-ion (Li-ion) batteries as the main power source with a total capacity of 14.6kWh to 17.952kWh, and the vehicle can run for 10 hours. The draw-backs of operating the existing power system of the MUN Explorer, which was done by the researcher at the Holyrood management facility, include mobilization costs, logistics and transport, and facility access, all of which should be taken into consideration. Recharging the batteries for at least 8 hours is also very challenging and time consuming. To overcome these challenges and run the MUN Explorer for a long time, it is essential to integrate a fuel cell into an existing power system (i.e. battery bank). The integration of the fuel cell not only will increase the system power, but it will also reduce the number of batteries needed as suggested by HOMER software. In this paper, an integrated fuel cell is designed to be added into the MUN Explorer AUV along with a battery bank system to increase its power system. The system sizing is performed using HOMER software. The results from HOMER software show that a 1kW fuel cell and 8 Li-ion batteries can increase the power system capacity to 68 kWh. The dynamic model is then built in MATLAB/Simulink environment to provide a better understanding of the system behavior.The 1kW fuel cell is connected to a DC/DC Boost Converter to increase the output voltage from 24V to 48V as required by the battery and DC motor.


I. Introduction
The MUN Explorer AUV is an autonomous underwater vehicle used for missions such as mapping, surveillance, oceanographic data gathering, environmental monitoring, mine detecting and coastal defenced [1]. One of the challenges facing the MUN Explorer is the power system's capacity to complete its missions. To Hydrogen production by Proton Exchange Membrane (PEM) water electrolysis is a promising method that has been successfully developed and integrated into renewable and hydrogen energy-based systems. Renewable energy sources, such as solar and wind, are desirable for hydrogen production due to random variations and significant current density capabilities [2]. PEM water electrolysis technology that generates hydrogen primarily emits water moisture, nitrogen and oxygen [3].
Energy storage or backup power systems are needed for photovoltaic and wind energy systems due to their discontinuous energy production.
Batteries can be a good solution for daily storage but not for seasonal storage due to self-discharge.
Storing energy in the form of hydrogen gas that is generated from renewable sources is a possible solution for both daily and seasonal storage [4].

For example, Sopian et al. (2009) integrated a
Photovoltaic-wind-hydrogen energy production / storage system. The components of the system were a photovoltaic array, wind turbine, PEM electrolyzer, battery bank, and hydrogen tank.  [5]. Lithium-ion (Li-ion) battery technology has improved in the past decade. Liion batteries have higher energy and power density, higher efficiency and lower selfdischarge when compared to other batteries (NiCd, NiMH, and Lead Acid). To ensure the Liion battery is operating at a proper temperature and state of charge (SOC), a battery management supervision system (BMSS) must be applied [6].

Hydrogen / Oxygen Tanks and PEM Fuel Cell
The MUN Explorer Autonomous Underwater Vehicle as shown in Fig. 1 has plenty of vacant space that could be used to install the hydrogen and oxygen tanks as well as the fuel cell.

Lithium-Ion Battery and Converter
The MUN explorer uses Li-ion batteries as its main source of energy to power loads, which include all electronics onboard and the emergency lights. That is because these batteries have high energy density and efficiency compared to other types of batteries. A Li-ion battery is more attractive in portable applications such as automotive and autonomous vehicles.

Permanent Magnetic DC Motor (PMDC)
In this case, the PMDC motor represents the load in HOMER software, and it is powered by the fuel cell and the battery. Permanent magnetic direct current (PMDC) motors are electrical machines that convert direct current electrical energy into mechanical energy. They are commonly used in many industrial, residential, and commercial applications [13]. The MUN Explorer AUV runs for ten (10) hours, so that the load has been specified based on the hours of operations (i.e. 10 hours) to be 600 W as illustrated in Fig. 7. The load is also divided into two sections: a DC load, which represents the electronics onboard, and the AC load, which is a variable speed motor. The MUN Explorer has only DC components, so the reason for selecting AC in HOMER is to represent the motor drive in our sizing.   can be modified for oxygen storage systems [14].
"Since high-pressure oxygen has a simple delivery mechanism, the desired oxygen tank wall thickness increases with pressure, which causes a reduction in the energy density advantages" [15]. Liquid oxygen storage can be a suitable solution for limited space applications.
Some drawbacks of this storage system are its complexity due to the safety concerns associated with the handling and refueling process [15]. A liquid oxygen storage system prototype has been designed by Sierra Lobo, Inc. with a diameter of 54 cm (21 inches) [14]. This prototype can store 50 kg of liquid oxygen at 452 k to run a 1-kW output PEM fuel cell. The system is 0.94-m long and 0.32 m in diameter. The weight is 13.6 kg when it is empty and 63.6 kg when it is full [16]. Table 2 shows the specific energy and the energy density for compressed and liquid oxygen storage systems, respectively [14]. In this paper, the model for the compressed oxygen / hydrogen tank corresponds to the one used by [17] and [18]. The dynamic model of oxygen / hydrogen tank was built based on equations (1) and (2) Table 3 shows the fuel cell parameters.
Overall: 2 2 + 2 ↔ 2 2 + + ℎ For any fuel cell, both the anode and cathode can be represented by the mole conservation equations as follows [19]: The fuel cell dynamic is built in a model in Simulink using a controlled voltage source in series with a constant resistance as illustrated in  where Td is the stack settling time. Equation (11) represents the total fuel cell voltage by taking the losses into account due to electrodes and electrolyte resistances (ohmic losses). This model is a simplified model that can simulate a fuel cell stack at a nominal condition of pressure and temperature operations. To eliminate the flow of negative current into the fuel cell, a diode is used [20]. Polarization curves (V-I and P-I) from the simulation and data sheet are presented in Fig. 11 and Fig. 12  Discharge Model when * is grater than Zero Charge Model when * is less than Zero  Table 4 also shows the battery model input parameters. The simulation discharge curves for the Li-ion battery system (i.e. 48 V and 34 Ah) are shown in The average mode boost converter is used in this simulation, and its parameters are illustrated in

3) Permanent Magnetic DC Motor (PMDC)
The dynamic model for any PMDC motor can be represented by the following equations [24]: Table 6 shows the parameters for the DC motor

IV. Results and Discussion
The simulation in HOMER software was done to get the sizing results for the integrated power  Table 7. The nominal discharge current is 14.78 A. Fig. 19 shows the fuel cell power profile in HOMER through the year. This constant speed is maintained by the boost converter to run the AUV at a constant speed.
After that, the DC motor runs at its highest efficiency. The armature current is 16, which is very close to the manufacturer data sheet value.    Table 8 shows a significant improvement in terms of specific energy and energy density, especially for liquid oxygen and hydrogen storage options.
The largest improvements are in the specific energy of the fuel cell total systems when compared with the lithium-ion batteries. To show the buoyancy effect on the system, the density can be defined as energy density divided by specific energy [14]:  Fig. 25, some ballast or float material must be added to meet the buoyancy requirement [14]. Fig. 25: Buoyancy in terms of SE and ED [14] The power system's capacity is increased by integrating the fuel cell power system into the MUN Explorer according to the following calculations.
Available Energy=Power*time (20) Watt-hour=Battery Volt*Ah The energy capacity is increased by integrating the fuel cell into the system and the number of batteries is reduced by applying equations 20 and 21. More details are attached in the appendix.

V. Conclusions
The sizing and modeling of the MUN Explorer's power system were studied and simulated in this paper. The oxygen and hydrogen tanks were successfully studied in terms of specific energy and energy density.
They were also implemented in MATLAB / Simulink as compressed gas storage. The results showed that a fuel cell with hydrogen and oxygen storage options has a higher energy density than batteries alone. The system sizing by HOMER was studied and implemented. The power profiles from HOMER software were illustrated for the fuel cell and DC motor. A 1-kW fuel cell and 8 Li-ion batteries can increase the power system capacity to 68 kWh. Installing these options will greatly increase the hours of operation and will help the buoyancy force.