Conceptual Design of Fuel Cell Based Hybrid Unmanned Air Vehicle

Unmanned air vehicles (UAVs) are used successfully in many different fields mainly various military and civilian applications. Recently electric powered UAVs have become more preferable due to their better accessibility, cost, transportation and silence. However, the batteries used in electric UAVs still do not have enough energy density for long endurance flights. Due to the difficulty of using piston and gas turbine engines in small-sized UAVs, hydrogen fuel cells are seen as one of the considerable options to increase endurance and range. In this study, conceptual design of hybrid UAV including fuel cell, solar unit and battery is presented. Small, light and high endurance UAV was designed using aerodynamic calculations and minimum power requirement for cruise flight was determined. 100 W fuel cell produced by Horizon Fuel Cell Technologies and 3S, 5 Ah of battery were used for main power supplies, moreover solar cells to charge the battery were designed to mount on the surface of the wings. Also, a new power management system is designed for switching between the energy sources. As a result of this work, it is determined that fuel cells and solar units are the significant options which can increase the endurance of UAV up to 8 times depending on the design, solar flux, hydrogen capacity etc.

sing the number of batteries is not a practical solution due to weight and space constraints. In order to offer a solution to this endurance and range problem of small UAVs, researchers have great interest in fuel cell-based hybrid propulsion systems for small UAVs since 2003 and several successful efforts have been practiced within this time period [4][5][6][7][8][9][10][11][12][13][14][15].
Fuel cells use 99.99% pure hydrogen to generate electrical energy via an electrochemical reaction between hydrogen from a source and oxygen in the air. At this point the need for a suitable hydrogen storage method emerges. Compressed hydrogen storage is the most common method for UAV applications however this method requires extremely high pressures. Lately, although the weight of compressed tanks has been significantly reduced with the new composite technology, the different methods such as liquid hydrogen and sodium borohydride continue to be explored [7,8,13,16,17]. In one of these studies, with the use of liquid hydrogen, exceeding 48 hours endurance was recorded [8]. In Table 1, some examples of fuel cell powered UAVs researches are presented.
In brief, nowadays the number of studies on long endurance UAVs by renewable and emission-free energy sources has increased significantly. One of the first options to achieve to increase endurance is fuel cell stacks that use hydrogen as fuel. Fuel cells (1000 Wh/kg) have about four or five times higher energy density than LiPo batteries (250 Wh/ kg) [15,16]energy researches on aviation have turned to renewable energy sources providing electric propulsion. Fuel cells are one of the most popular sources of power for electric vehicles terms of being greener, more reliability, and higher efficiency. Among fuel cells, the most attractive option for flight is polymer electrolyte membrane fuel cells (PEMFC. When the researches on fuel cell powered UAVs are examined, it is predicted that some issues will be handled more in the near future; these are improving hybrid management system, reducing the weight, improving reliability and increasing their power density. In this regard, a novel design of fuel cell-based hybrid propulsion system of small UAV is presented in this study. Firstly, design of concept UAV is explained in details and then, an overview of the fuel cell, solar and battery system is given. Secondly, performance of hybrid propulsion system is evaluated experimentally and finally the endurance of battery and hybrid system are compared for the same model UAV.

Conceptual Design
Fuel cell-powered UAV model ( Figure 1) is designed as fixed wing to perform surveillance mission. The main purpose of the novel UAV is to fly extended period of time and for this reason almost all flight phases is cruise stage. In this stage, the air vehicles consume minimum energy to equalize the drag force of the UAV. In connection with the limited power and relatively high weight of the fuel cell and solar system, model air vehicle was designed as a glider which can fly in cruise mode at a low power rating of about 50 W. To start the design of UAV, firstly empty weight has to be calculated. Since this air vehicle consists of three different power sources, this calculation should be done carefully. Weights fraction of the UAV system and the conceptual design parameters are given in Table 2 and Table 3 respectively. Figure 2 shows the weight distribution of the conceptual UAV.   Maximum take-off weight was assumed 3.3 kg according to first calculations. Wing span, aspect ratio and wing span values were calculated with the help of formulations in next chapter.

UAV Power Requirement
After estimating the approximate weight of UAV, the aerodynamic parameters and power requirement for cruise flight are to be calculated based on the related formulations. High lift/drag ratio and the appropriate surface for solar cells are the main concern to choose the optimum airfoil. As a result of research in literature and XFLR5 analysis, WE3.55 airfoil was determined.
In the horizontal (x) direction, thrust force (F T ) should be equal to the drag force (F D ) according to the conservation of momentum, therefore the equation could be described as [17]: and the drag force can be calculated as: are density of ambient air, the frontal area, the drag coefficient and induced drag coefficient of the wing, respectively. As V shows the velocity of the air vehicle, wind V expresses the wind velocity. It can be seen from eq. 3; , D lift C can be found with using lift coefficient of airfoil, the aspect ratio which describes the square of the span divided by the wing area and ( Os e the Oswald efficiency number). Examining the literature of same type of fixed wings UAV, it can be seen that AR is around 6 and 9. In this study, there is also solar panel area factor to determine the wing span. As a result, AR is decided as 7.06. ( ) Equation 5 and 6 show the pressure and temperature calculations at the operating altitude. Another equation from the Conservation of Momentum direction is the vertical (z) way as: The lift force formula can be calculated as: in this formula, p A describes the planform area and b is the wingspan of the air vehicle.
An air vehicle should produce enough lift equal to the gravitational force to keep itself at cruise flight. Therefore, the required minimum cruise speed was calculated according to eq.8. Model UAV parameters and values are given in Table   4 in detail.
Finally, required power of cruise flight of UAV can be calculated as: Where; , , p m c η η η propeller, motor and controller efficiencies, respectively.

Horizon H-100 Fuel Cell
PEM type fuel cells are more suitable for small UAVs due to its low operating temperature range, small size and relatively higher efficiency [18]. Therefore, a PEMFC stack (Horizon Fuel Cell Technologies, H-100, 100 W) was selected in this study as seen in Figure 3. H-100 fuel cell stack has 24 single cells with output power of 100 W at 12 V and 8.3 A. This model is self-humidified type. Hydrogen was provided by a compressed hydrogen tank with full of 99.999% of pure H 2 . Table 5 shows the general specifications of H-100 fuel cell system. A fuel cell controller is used to keep working the stack at nominal conditions and commands periodic short-circuiting and purging of the fuel cell stack. In general, fuel cell controller regulates the temperature management, air management (flow rate considerations) and purging (removing the water vapor from the cell). In addition, as a source of hydrogen, it was chosen to feed a total of 12 fuel cells from the hydrostik model which is the product of Horizon Company, each containing 10 liters.

Solar System and Battery
Fuel cells offer high energy density but they are inadequate in high power demands. Back-up system has to be used such as battery since take-off, climbing and maneuvering require high power (x3 or x4 more than cruise phase). Lithium ion and lithium polymer (LiPo) batteries are most commonly used in small UAVs to boost power. In this design, 3S (3 cells in series), 5000 mAh, 25C LiPo battery was chosen to feed the load. While the nominal voltage of the battery is 11.1 V, maximum voltage is 12.6 V at fully charged. There are mainly two reasons to be chosen 3S model. Firstly, it could be hard to provide enough power during the high-power demands with 1S or 2S. Secondly, using more than 3 series battery could need more voltage to charge and also it causes increasing wingspan and the weight of UAV.
In this hybrid design model, while fuel cell is active, solar panels on the wings start charging the battery. The Sunpower C-60 solar panel was chosen for its relatively high efficiency flexible photovoltaic model. C-60 has a length of 125x125 mm and 0.008 kg with rated voltage and current 0.57 V, 5.37 A respectively. 30 solar cells are planned to mount of the UAV wings in series to charge LiPo battery. Figure 5 illustrates the battery and solar unit system.

Hybrid Propulsion System
Battery, solar panel and fuel cell are the sources of the hybrid propulsion system of the model UAV. A novel hybrid management system was set up to control these three separate power units. A control card was integrated into the system with DC-DC relays that regulate the transitions. In this case, both fuel cell and solar panel are inactive while the battery voltage is full of charge (12.61 V). The test was started by setting DC load unit to 50 W which is the cruise power demand of UAV. When the battery voltage drops to nominal Figure 6. Hybrid propulsion ground test unit scheme value (11.1 V), the battery goes off and the fuel cell starts feeding the system with the help of the DC-DC relay. Solar panel starts to charge Li-Po battery while fuel cell is active until the fully charged status. After the battery voltage reaches the 12.61 V, the relay stops the fuel cell power supply and the battery restarts to feed the load. This transformation continues as long as solar energy production continues and the compressed hydrogen tank becomes empty.
Hybrid propulsion system including experimental set up is illustrated in Figure 6.
Hydrogen tank consists of 200 bars of H 2 at normal which is not appropriate for fuel cell stack. Thus, a pressure regulator was used to keep the pressure in the range of 0.45-0.55 bar.
In addition to this, ALICAT mass flow meter was mounted between fuel cell stack and pressure regulator to measure the hydrogen consumption.

RESULTS AND DISCUSSION
In this study, the conceptual design of a small unmanned aircraft powered by three separate power units (battery, fuel cell and solar cell) was made. At this stage, some assumptions were made for certain required parameters according to the related literature. According to related formulations, 50 W power is required for cruise flight of model UAV. Since the small aircraft is modelled as a hand-launchable and also surveillance mission, more than 90% of flight time is cruise phase. For this reason, ground tests were applied with the 50 W of DC load.
After mathematical calculations of model UAV's requirements, hybrid system was tested successfully. Current and voltage parameters of solar, fuel cell and battery system were recorded to computer with the help of Ardunio card and current-voltage sensors. Figure 9 and 10 show conversion of the voltage and current values at the load of 50 W. While the battery is on, the current of battery is increasing and the voltage is decreasing until the 10.5 V. There is no considerable fluctuation in voltage and current for fuel cell except purging situation. Tests was started at 10 am in a sunny day of November 2019, so when the solar flux decreased, the charging time of battery was extended as can be seen in Figure 7.   Changing in power demands are ignored in this study which can be occurred at normal flight such as maneuvering, climbing and wind effects. These effects would cause in lower endurance of UAV. In particular, more power will be required to keep the aircraft at the same altitude, since the maneuver will cause the total drag force to increase. For this reason, more detailed study including the energy consumption of sub-parts and different flight phases is required to precisely predict the endurance of the fuel cell powered UAV.

CONCLUSION
Renewable energy sources offer high power and energy densities, lower costs, non-carbon emissions, more efficient power, lower acoustic and heat emissions, thus preventing detections and making tasks easier. Fuel cell systems which use hydrogen (one of the most preferred renewable energy source) as a fuel have recently been regarded as the primary candidate in increasing the endurance of air vehicles. The new ultralight fuel cell power and energy densities exceed 600 W/kg, 2000 Wh/kg respectively, based on the commercial fuel cell producer's datasheets. Due to the difficulty of using piston and gas turbine engines in small-sized UAVs, hydrogen fuel cells could be seen as one of the considerable options to increase endurance and range however there are still many issues to be challenged such as hydrogen storage, initial investment cost (especially for very light models) and insufficiency in high power demands.
Before producing hybrid systems powered UAV, many factors and issues should be taken into account in the conceptual and preliminary design phases. In this study, a conceptual design of small UAV was presented and the ground tests of hybrid system with three components tests were performed. As a result, it has been determined that fuel cell powered UAVs with support of solar panels could reach more than eight times compared to battery powered UAVs for conceptual design phase.