Environmental Impact of High Altitudes on the Operation of PEM Fuel Cell Based UAS

Fuel cell is a device that converts the chemical energy in the reactants into the electrical energy after steps of sequential electrochemical processes with no significant impact on the environment. For high altitude long endurance (HALE) of unmanned aircraft system (UAS) where fuel cell operates as a prime source of power, the operation and performance of a PEM fuel cell at different level of altitudes is vitally important. In this paper, the impact of direct using extracted air from high altitudes atmosphere in order to feed the stack is investigated, and the governing equations of the supplied air and oxygen to the PEM fuel cell stack are developed. The impact of high altitudes upon the operation and the consumption of air are determined in order to maintain certain level of delivered power to the load. Also the implications associated with operating the PEM fuel cell stack at high altitudes and different technical solutions are proposed. Various modes of Integral, Proportional-Integral, and Proportional-Integral-Derivative controller are introduced and examined for different time setting responses in order to determine the most adequate trade-off choice between fast response and reactants consumption which provides the necessary optimization of the air consumption for the developed model of PEM fuel cell used for UAS operation.

Energy and Power Engineering sion-production for fuel cell is comparatively higher than internal combustion engines. The efficiency of a conventional gasoline combustion engine is around 25% -30%, while the efficiency of the fuel cell is two to three times higher than combustion engine depending on the size and type of the fuel cell and its application [1] [2]. The operational temperatures of PEM fuel cell range from 30˚C to 100˚C and dynamical response is faster than solid oxide fuel cell which operates at over 700˚C [3].
In vehicle applications, the time consumed in recharging the battery is longer than the time spent in refuelling the hydrogen storage tank. Therefore major automotive companies such as Ford, Nissan, Daimeler Chrysler are developing electric vehicles based on PEM fuel cells [4]. The response of PEM fuel cell towards the rapid changes in the load demand tends to be slow because the operation of PEM fuel cell depends on monitoring and controlling the flow rate of air-hydrogen, reactants pressure, temperature, management of the produced heat and water as a result of the electrochemical reaction [5].
In a PEM fuel cell, the fuel (H 2 ) and oxidant (O 2 ) are supplied by mechanical system of pumps and valves. The mechanical adjustment time for these equipments is comparatively longer than the reaction time. This mismatch between the two times causes a real shortage in fuel and oxidant (reactants starvation) which consequently leads to breakdown the chemical reaction and to a rapid drop in the output power of fuel cell. These problems have been addressed by researchers [6] [7] in the control models of fuel cell power systems for hybrid vehicles.
A PEM fuel cell for UAS applications needs a careful water management, particularly for low temperature high altitude environment operations. Also, controlling the fuel cell temperature is a vital issue particularly at high current demand, as it needs a significant volume of air to cool down the stack, which otherwise increases the parasitic losses of the system. However, if the heat transfer from the stack system to the environment and vice versa is managed and controlled effectively by using a proper thermal insulation to maintain the power system within the operational temperature boundaries, thereby reducing the amount of cooling air required, which improves the system efficiency [8].

Implications of High Altitudes on the Performance of the PEM Fuel Cell
In the last few years, small numbers of the successful flight tests for the light unmanned aircraft systems powered by fuel cells have been reported. However, most of these tests were restricted to short duration and low altitude flights [9].
At high cruise altitudes of UAS-based fuel cells, where the temperature and pressure are very low, air needs to be compressed in order to raise its pressure to an acceptable operational pressure level, also the fuel and air must be warmed up before being supplied to the stack in order to avoid excessive stack cooling and hence thermal stress caused by cold streams [8].
A water cooling system can be used to maintain stack temperature, but it is not appropriate for UAS applications, as it needs a coolant tank, pump and heat exchanger, which leads to an increase in the size, weight and complexity of the entire system. Therefore, an air cooling system could be considered as an adequate alternative option for cooling the fuel cell stack [10].
Taegyu Kim, et al. [11] proposed that in order to provide a sufficient flow of air to the fuel cell stack system of the UAS, the stack must be located in the front part of the fuselage to directly receive the flow of air through its intake, which is used to feed the cathodes and to cool down the entire system. Sylvie Bégot, et al. [12] reported that it is a complicated task to start-up and to operate the fuel cell stack with sub-zero ambient temperatures, as the water produced in the cathode can turn to ice, which can block the passage of the reactant from reaching the reaction interface. The frozen water can change the conductive properties of the electrodes, membrane, and in the worst cases can damage the membrane. It has been found that the formation of ice would reduce the active surface of the electrode-catalytic layer which reduces the rate of the electrochemical reaction, and hence causing a high drop in the fuel cell output power.
Jürgen Dollmayer, et al. [13] proposed a 700 kW solid oxide fuel cell system as an alternative source of power to the combined generator and auxiliary power units in the conventional aircraft. Different fuel cell operations are proposed using ram air from the ambient atmosphere with two air temperatures (−24˚C and −57˚C), and cabin exhaust air at 22˚C. Heat exchangers will work to warm up the stream of air before supply it to the cathode, an auto thermal fuel reformer that uses a warmed air to warm up the stream of hydrogen up to 25˚C before supply it to the anode, a gas turbine with a generator and condenser for excess water recovery, and an afterburner that works to burn the exhaust air and the hydrogen leaving the cathode and anode of the stack. showed a decline in net power by 24% compared to the sea level operation. Furthermore, it has been reported that between altitudes of 1000 ft. and 10,000 ft., the power output of the fuel cell stack drops by 1.5% per 1000 ft., for a fuel cell using fresh air extracted directly from the atmosphere. However, predicting performance of a fuel cell for altitudes higher than 10,000 ft. might be different to the predicted values for lower altitudes, because higher than 10,000 ft, the temperature and pressure of the atmosphere drop dramatically to become lower than −5˚C and 0.7 bar, which is not an appropriate operational environment for the PEM fuel cell. Therefore, further studies are needed to accurately determine the impact of high altitudes on the performance of the fuel cell stack and its power output, which is very important for the design requirements of fuel cell systems in aerospace applications [14].

PEM Fuel Cell as a Prime Source of Power
The stack of the PEM fuel cells needs to operate with other components in order to form an integrated fuel cell power system. These components are mainly divided into four systems [15]. • Hydrogen supply system.
• Air supply system.
• Cooling system. Horizon (H-1000) PEM fuel cell stack is used as a prototype device for physically conducting the tests in the Laboratory in order to validate and tune the developed mathematical model with the output results of the tests. The stack is designed to be self-humidified PEM fuel cell stack, four fans are installed at the exit outlet of the manifold of cathode, thus the air system is designed by the manufacture to be ducted outlet, which means the flow of air is controlled by the differential pressure caused by the extractor fans at the exit outlet of the manifold of cathode [16] [17].
In order to maintain flow rate and pressure of hydrogen and air within the desired level, hydrogen and air must be replenished via hydrogen-air supply systems. Hydrogen supply system consists of a tank of pressurised pure hydrogen, inlet flow control valve, outlet purge valve, while air supply system consists of air supply fans, and the connection of the main supply manifold [16]. Figure 1 shows lasts for a very short period of time (10 ms), the controller is unable to cope with such a fast transition. A special controller was built by Horizon to switch the fuel cell stack on/off at almost every 10 seconds to overcome the issue of frequent purging of hydrogen, in order to maintain the maximum rated power from the fuel cell stack. However, the proposed controller technique might cause further problems, because of the continuous cycle of turning on/off for the stack, in a cycle of 10 seconds, would lead to unstable the performance and in the worstcase scenario would damage the stack after a period of continuous operation. It also might lead to further delay in the response of the stack toward the change in the load demand [17].

Impact of High Altitudes on the Consumption of Air for a PEM Fuel Cell
The earth's lower atmosphere, known as the Homosphere, which extends from sea level to 80 -100 km, and consists of three layers: Troposphere is lowest layer, the Stratosphere layer extends above the Troposphere, and the Mesosphere layer is above the stratosphere. Within the Homosphere, the main highly concentrated gases are nitrogen 78%, oxygen 21%, and the remaining 1% is composed of different types of gases such as argon, water vapour, carbon dioxide, methane, ozone, in addition to the traces of inert gases such as neon, helium, and xenon. Within the Homosphere, nitrogen and oxygen are considered as fixed gases where their concentrations are consistent and well mixed at any given altitude [18]. In earth's atmosphere, air density, pressure, and temperature vary with altitude, latitude, longitude, and the season. Air density and pressure decrease exponentially with increasing altitude, the highest air density and pressure are at the sea level [18]. Within Troposphere and up to the height of the Tropopause at 11 km (36,089 ft.), temperature decreases with the increase in altitude at a constant rate of −6.5˚C/km (−1.98˚C/1000 ft.). In the model of international standard atmosphere (ISA), air is considered as a perfect gas within the Troposphere, therefore, temperature and pressure of air at a certain altitude is given by the following equations [18] [19]: where, h is the altitude in metres, T o is the temperature of the air at sea level in Kelvin which is equal to 288.15 K (15˚C).
where, P o is the pressure of air at sea level which is equal to (101,325 Pa). Using ideal gas law, the density of dry air ρ (kg/m 3 ) at any altitude can be determined as a function of pressure and temperature [20]: with fresh air, with molar proportion of oxygen to air is 0.21 [20]. Then, at sea level, the mass flow rate of contributed existing air at the reaction interface of the cathode can be determined as given in Equation (6) At any given altitude, the mass flow rate of the contributed existing air at the reaction interface of the cathode can be determined as given in Equation (7) below, air air,altitude Based on the mathematical model of the PEM fuel cell developed by [16], Horizon (H-1000) PEM fuel cell stack consisting of 72 cells, with a maximum current of 20 A, and by using Equations (5) & (6) to determine the mass flow rate of contributed existing air at the reaction interface of the cathode at sea level, is found to be equal to 0.569 g/s.
If at high altitudes the PEM fuel cell is going to be supplied by fresh air extracted from the atmosphere, Equations (1)-(4) can be used to determine the relative density of air at a certain altitude. Then, using Equation (7)   to raise the flow rate of the supplied air. Such a compressor requires a significant fraction of the generated power, and contributes to increasing the total aircraft weight, which is very difficult to afford in a small scale UAS [9].
In fact, at an altitude of 10 km, the temperature of the air is equal to −50˚C and a fuel cell stack does not operate at such low temperature levels, unless special measures are applied to warming up the air before supplying it to the stack. It is assumed that at high altitudes, the PEM fuel cell is going to be supplied by a fresh air from the atmosphere, after warming it up to 15˚C. Similarly, the mass flow rate of required air at the reaction interface of the cathode to produce the maximum current at various altitudes, (at 15˚C constant temperature of supply air to the stack) can be determined and depicted in Figure 2 above. Therefore, at an altitude of 10 km, and at 15˚C constant temperature of supply air to the fuel cell stack, the mass flow rate of the necessary consumed air at the reaction interface is 2.18 g/s, and the ratio of mass flow rate of consumed air at the reaction interface to the consumed air at sea level is equal to (2.18/0.569 = 3.83). Thus, the mass flow rate of contributed air in the reaction of the PEM fuel cell stack must be increased by 383% in order to produce the same level of output current at sea level.
It is clear that maintaining the temperature of the supplied air to the PEM fuel cell stack at 15˚C for high altitude operations has an impact of increasing the mass flow rate of the supplied air to the reaction interface, which leads to more power being consumed by the compressor to raise the flow of air. Jürgen Dollmayer, et al. [13] reported for the proposed 700 kW solid oxide fuel cell system for aircraft applications, that supply streams of air and hydrogen must be warmed up to 25˚C before supplying it to the fuel cell stack. Also, using cabin exhaust (22˚C) air instead of ram air to supply the fuel cell stack is more efficient in reducing the fuel consumption.

PID Controller for Air and Hydrogen Supply
A proportional-integral-derivative (PID) controller is widely used in the industrial and commercial applications [22]. A proportional controller adjusts the speed, the integral part adjusts the steady state error, while the derivative part control regulates the stability and the dynamic response of the system [23].
A PID controller was used to regulate the output voltage at a certain set point via controlling the flow rate of hydrogen and oxygen for a hybrid power-plant system of a 240 W PEM fuel cell stack used for a C-130 Hercules aircraft, as reported by [24].
Hao Chen, et al. [25] used a proportional-integral (PI) controller to control the flow of current between the load, fuel cell/electrolyser stack, solar photovoltaic panel, and the rechargeable batteries for a hybrid energy storage system, in order to extend the endurance of the UAS.
In this research, a PID controller is adopted to control the flow of the fuel and the supplied air to the PEM fuel cell stack, based on determining the current of the fuel cell stack with respect to the changes in the demand current of the connected load. In order to determine the performance of the PID controller due to the changes in the load demand, the following strategy was adopted: 1) The simulation of the PEM fuel cell model will be started from time zero up to the first second, based on the initial value of the current equals to 1 A, with the stack output voltage is equal to 58.2 V.
2) After the first second, the load demand will be increased up to 2 A and the required power up to 111 W, resulting in an output voltage from the PEM fuel cell stack to be equal to 55.5 V.
3) The simulation will be lasted for four seconds further, in order to achieve five seconds of model simulation.

4) A mechanical adjustment delay of 200 ms is adopted which corresponds to
an arbitrary mechanical delay of flow valve adjustment.
It is observed that the PID controller time response must be tuned between zero and one second, any settings for a time response higher than one second must be avoided as this will cause a delay in the response of the developed model of the PEM fuel cell to the power demand, as shown in Figure 3, for a PID con-    The output responses of the simulation of the developed PEM fuel cell model for various PID settings are presented in Figure 4 and Figure 5, respectively. Table 1 presents the amount of air and hydrogen that needs to be supplied to the developed model of the PEM fuel cell stack in order to deliver the required level of power for the current load varies from 1 A to 2 A, with different PID controller settings. However, these settings apply only to the developed PEM fuel cell model based on the Horizon (H-1000) stack; other PEM stacks may have different tuning parameters.   Based on the proposed developed model of a PEM fuel cell in [16] and PID controller, the amounts of hydrogen and air required to be supplied to the fuel   Table 2. While looking at the performance of the fuel cell system model, it became clear that due to drops in the pressure, temperature and density of ambient air at 300 m altitude, the system stalled. While warming air up to 15˚C or 20˚C before supplying it to the fuel cell system lead to reduce the density of the air, which extended the operation of the fuel cell system up to 400 m altitude, as presented in Table 3 and Table 4, respectively.
It has been reported by [5] [16] [26], that increasing the temperature of the supply fuel and oxidant to a certain level < 30˚C will lead to warming up the stack of the fuel cell and this will reduce the activation losses via improving the exchange current density of the fuel cell, and hence increasing the output power and improving the performance of the fuel cell stack, which would lead to reduce the rate of fuel and oxidant consumption.
The temperature of the fuel cell will affect the activity of the catalyst, the hydration level of the membrane, the saturation of the gas diffusion layers, and diffusion of gases through the membrane [27]. However, increasing the stack temperature to higher levels should not be the proper solution for improving its output power, because this would dehydrate the membrane and reduce its conductivity, thus increase the ohmic losses of the cell and decline the output power.

High Altitude Operational Hypothesis
It has been reported in the literature that at high altitudes, where the temperature, pressure and density of air are very low, compressed air and hydrogen can be used to supply the fuel cells with a sustainable source of air or oxygen. Also, fuel and air must be warmed up before being supplied to the stack, in order to     [14].
In the case of the supply air to the manifold of cathodes is not going to be supplied directly from the external ambient. Instead of that, the air is going to be supplied from a pressurised tank of dry air connected to the supply manifold of cathodes. In order to overcome system stalling issues as presented in Tables 2-4, respectively, due to the drop in atmospheric pressure and temperature, it is necessary to maintain the operational environment of the stack, such as pressure,  where the normal operational temperature for the PEM fuel cell stack is between 30˚C -90˚C. Therefore, the stack of fuel cells will be considered to be located inside a sealant thermally isolated chamber with an internal ambient pressure and temperature equal to 1atm and 15˚C, respectively.
However, a certain amount of unreacted air and hydrogen will leave the stack of the fuel cells associated with water in the form of vapour or liquid, as a result of electrochemical reactions. These residual fluids will accumulate inside the sealant chamber. This will present another operational problem; hence, the air inside the chamber needs to be refreshed in order to maintain a certain operational temperature, relative humidity and pressure approximately close to sea level conditions.
At high altitudes, the pressure inside the chamber containing the fuel cell stack will be higher than altitude pressures. Therefore, any attempt to drain the chamber via opening the drain valve will create a high flow of air at the exit outlet of the chamber as a result of a great pressure difference between the chamber and external atmosphere. This will create a sudden and rapid increment in the flow of air entering the stack which may lead to a spike in the output voltage of the fuel cells; hence, instability in the operation and performance of the stack, and further unnecessary consumption in the usage of air. Also, this may damage the electrolyte membrane due to pressure differences.
The difference between the internal and external pressure of the main stack chamber can be eliminated by either installing a series of sequential settling drain chambers at the exit outlet of the main stack chamber, which can drain the contents (i.e. accumulated gases) of the main stack chamber slowly and sequentially, or by recycling the internal accumulated gases inside the main chamber after a process of refining and gas separating ending with recompressing air and hydrogen again to their storage cylinders, however this option will add more complexity, weight and cost to the entire system. Figure 7 shows the mechanical components and flow variables associated  with the proposed model of a fuel cell stack system operating at high altitudes.
The hydrogen and air supply system consists of tanks of pressurised dry air/oxygen and pure hydrogen, inlet pressure regulator valves (PRV), a hydrogen purging valve, drain tank, and connection pipes between the components.

Conclusions
The impacts of high altitudes on the operation and consumption of the reacted air were determined in order to establish how to maintain a certain level of delivered power to the load. Also, the implications associated with operating a PEM fuel cell stack at high altitudes were considered and different technical solutions were proposed.
It has been proven that warming up the extracted air from the atmosphere at high altitudes (~10 km) in order to feed the fuel cell stack must be avoided, as this increases the flow of supplied air to the cathode by about 400% in comparison with the flow at sea level in order to overcome the drop in the density of air at high altitudes and to produce a stable power from the stack, which would add more complexity, weight and cost to the system.
It is clear that due to the drops in the pressure, temperature and density of the ambient air at 300 m altitude, the fuel cell stack system is stalled. While warming up air up to 15˚C or 20˚C before supplying it to the fuel cell system will lead to extend the operation of the fuel cell system up to 400 m altitude. Moreover, increasing the temperature of the supply fuel and oxidant to a certain level will lead to warming up the stack of the fuel cell and this will reduce the activation losses via improving the exchange current density of the fuel cell, and hence in-