Energy Harvesting Using Small Renewable Energy Sources: UAV ...

Proceedings of the ASME 2015 International Mechanical Engineering Congress and Exposition IMECE2015 November 13-19, 2015, Houston, Texas

IMECE2015-51650

Energy Harvesting using Small Renewable Energy Sources: UAV Application Sayem Zafar Department of Mechanical Engineering American University of Sharjah, Sharjah, UAE [email protected]

Mohamed Gadalla* Department of Mechanical Engineering American University of Sharjah, Sharjah, UAE [email protected] *Corresponding author

ABSTRACT

NOMENCLATURE A AC AR b c CL CP DC Ff h L MTOW P PWT RPM S UAV

A renewable energy harvesting system is designed and tested for micro power generation. Such systems have applications ranging from mobile use to off-grid remote applications. This study analyzed the use of micro power generation for small unmanned aerial vehicle (UAV) flight operations. The renewable energy harvesting system consisted of a small wind turbine, flexible type PV panels and a small fuel cell. Fuel cell is considered the stable source while PV and wind turbine produced varying power output. The load of around 250 W is simulated by a small motor. The micro wind turbine with the total length of 4.5 m and the disk diameter of 1.8 m is tested. The micro wind turbine dimensions make it big enough to be used to charge batteries yet small enough to be installed on rooftops or easily transportable. The wind turbine blades are installed at an angle of 22o, with respect to the disk plane, as it gives the highest rotation. The voltage and current output for the corresponding RPM and wind speeds are recorded for the wind turbine. Two 2 m and a single 1 m long WaveSol Light PV panels are tested. The PV tests are conducted to get the current and voltage output with respect to the solar flux. The variation in solar flux represented the time of day and seasons. A 250 W PEM fuel cell is tested to run the desired load. Fuel cell’s hydrogen pressure drop is recorded against the output electrical power and the run time is recorded. System performance is evaluated under different operating and environmental conditions. Data is collected for a wide range of conditions to analyze the usability of renewable energy harvesting system. This energy harvesting method significantly improves the usability and output of the renewable energy sources. It also shows that small renewable energy systems have existing applications.

𝑣 Volt

Rotor area, m2 Alternate Current Aspact ratio Span, m Chord, m 3-D lift coefficient Specific heat, J/kg.K Direct Current Shaft resistive force, N specific enthalpy, J/kg Lift, N Maximum Takeoff Weoght Pressure, Pa Personal use Wind turbine Revolutions per minute Planform area, m2 Unmanned aerial vehicle Velocity, m/s Voltage, V

Greek Letters ρ Density, kg/m3

INTRODUCTION Energy demand is on the rise globally and to cater the demand, natural resources are being depleted at an alarming rate. Destruction of natural resources can be reduced if there is a global switch from non-renewable sources of energy to renewable energy. Such move will ensure the stability of global

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climate and have minimal environmental side effects. A hybrid renewable energy system is an effective way to generate renewable energy. A standalone single renewable energy system usually does not provide enough output to be independently used. Hybrid systems on the other hand combine the different types of renewable energy systems and yield a sizable output.

Fuel cell system shows that such integration improves the efficiency of the system [12]. This paper presents the novel design, tests and analysis of a wind turbine-fuel cell-PV powered hybrid UAV. This paper studies the increase in endurance of the UAV by using the hybrid energy method. It also presents a system which enables the flight operations even in the remote off-grid locations through renewable energy sources.. Modeled systems are explained and bench test results of the systems are presented. Governing equations of the systems are presented and the experimental results are analyzed. A novel power sourcing system is proposed and analyzed which helps improve the UAV endurance.

A combined hybrid system shows to be a promising way to generate renewable energy with relatively decent exergy efficiencies [1]. Such combined hybrid systems have been used for off-grid applications in which the surplus energy, from wind and solar, is used for hydrogen generation which is stored for later use [2]. The results showed that such systems can be managed and their output is promising overall [3]. A combined proton exchange membrane fuel cell (PEMFC), wind turbine and photovoltaic panel hybrid energy system is believed to be an effective way to generate renewable energy. The PEM fuel cell uses hydrogen to produce DC electrical power, heat and exhaust water. Since the PEM fuel cell emission is water, the energy produced through fuel cell is environmentally clean, have extremely low emission of oxides of nitrogen and sulfur, and have very low noise [4]. The added advantage of using PEM fuel cell is continuous supply of power without having to deal with the fluctuation.

ANALYSIS The basic equations of the wind turbine, PEM fuel cell and PV are presented in this section. The usable energy available in air, for the small wind turbine, can be entirely associated to the kinetic part. This is due to the fact that small wind turbines have very little impact on the temperature change on the air. The total power available from wind can be represented by: 𝑊̇ ava = (½)ρ 𝑣 2 A

The other component is wind turbine in this hybrid renewable energy source. Thermodynamic performance of a large wind energy system is assessed for different months of the year for the city of Sharjah, UAE [5]. Exergy and energy assessments are carried out along with the irreversibility assessment. The paper also shows that the month of March has the highest potential to generate wind energy for the city of Sharjah. In spite their high power output, large wind turbines are expensive to make, install and operate. Without the involvement from the government, such projects cannot be undertaken. The bureaucratic involvement often delays the projects and adds unwanted cost. A more effective way to harness wind energy is to use personal use wind turbines mounted on rooftops or on towers. Such wind turbines have to be affordable for a household, small in size yet start producing power at average wind speeds found in UAE. Small personal use wind turbine present a viable solution to the before mentioned problems [6-8]. They can be used to harness wind energy while mounted on rooftops or on towers

(1)

Another important design parameter for wind turbines is the cut-in speed. It is the speed at which wind turbine rotor starts rotating and yield power output. The cut-in speed is defined as

𝑣cut-in = [2(L+Ff)/(ρSCL)]1/2

(2)

The above described equation is based on stall equation when lift, L, is equal to the weight of the blade. The total required force, to turn the rotor, is the lift, L, to overcome the weight plus the resistive force, Ff, in the shaft. In Equation 2, wind turbine blade’s planform area is represented by ‘S’ and ‘CL’ is the 3-D lift coefficient. This equation is derived from the assumption that all the lift converts to rotate the blade instead of some fraction contributing towards bending the blade. The electric power, which can be extracted from a PV panel, depends on three broad parameters. The PV area ‘A’ of the panels exposed to sunlight, solar flux ‘ST’ of the location and the efficiency ‘η’ of the designed panels. The extracted electric power ‘Pele’ is a product of the maximum voltage ‘Vm’, and the maximum current ‘Im’.

The third renewable energy source is solar. PV panels use solar energy to convert it into electricity. Solar energy is not only readily available outdoors but can also be conveniently collected by an aerial system such as a UAV [9]. PV panel can provide free clean energy for the UAV and since its energy source is solar, fuel weight is not a concern [10, 11]. Energy from PV panels can be stored in a battery which can be used when required. Studies conducted for the land based hybrid PV-

Pele = Vmax Imax = η St A

(3)

The power produced by the fuel cell stack is given by following equation.

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𝑊̇𝑠𝑡𝑎𝑐𝑘 = 𝑉𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 i 𝐴𝑐𝑒𝑙𝑙 𝑛𝑐𝑒𝑙𝑙 (4) where 𝑉𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 is the operating cell voltage, 𝑛𝑐𝑒𝑙𝑙 is the number of fuel cells inside the stack, 𝐴𝑐𝑒𝑙𝑙 is the area of the each cell and i is the current density.

A specific UAV operation is selected for this study however this hybrid system can be used for any mobile application. A wind turbine takes energy from the wind and charges the ground battery. Before the UAV takeoff, the ground battery charges the UAV battery on-board UAV which is used during take-off and climb. Once the initial battery voltage reaches the cut-off point, the controller switches power to the on-board PEM fuel cell. Meanwhile, the PV panels start charging the on-board battery. Once the battery is fully charged, the battery kicks-in to supply power to the motor. During the UAV flight, the wind turbine continues to charge the ground battery while the ground battery charges the reserve UAV batteries. The reserve UAV batteries are charged to make sure the continuity of the flight operations of the UAV. Figure 2 shows the schematic of the system.

DESCRIPTION An integrated hybrid renewable energy system is designed, tested and analyzed to provide energy for UAV operations.

Figure 1: Picture of the designed UAV. Figure 1 shows the picture of the designed UAV used for the study. A UAV is specifically designed to be powered by fuel cell-PV panel hybrid system. The primary reason for the UAV design is to demonstrate the feasibility of using fuel cell-PV panel hybrid power system as a small UAV power plant. Since UAV is to be small, a primary requirement is its weight. A maximum take-off weight, MTOW, of 130 N is chosen for the UAV. This MTOW limit made the UAV light enough to be carried over long distances yet enabled fuel cell and PV panels to be incorporated in the aircraft. The UAV had to be light enough to be hand or catapult launched, yet the added mass of the structure and the system must remain within the limit. The sub-systems will be housed in the fuselage while the PV panel on the wing and horizontal stabilizer. It is important to know the UAV dimensions and constraints so that a true power system could be modeled. Table 1 describes the features of the UAV for which the power system is studied.

Figure 2: Schematic of the integrated renewable energy system. Individual components are tested and system analysis is conducted to compare the overall system output. The energy outputs of individual components are compared with the integrated renewable energy system. The hybrid renewable energy system consists of three individual renewable energy systems, namely - Wind Turbine - Flexible PV panels - PEM fuel cell - Motor The wind turbine used in the study is a small wind turbine. A small wind turbine is used since it can be easily mounted on a roof-top or on a mobile base station. Such a system is easy to transport and erect. It has rectangular, non-tapered blades for manufacturing ease and to reduce manufacturing cost. The blades are aerodynamically designed to produce highest rotation yet sustain the experienced loads. Further details of the rotor design and evaluations can be acquired in references [8]. The wind turbine parameters are given in Table 2.

Table 1: Description of the UAV used for the power system Structural Mass 4.5 [kg] MTOW 130 [N] Wing Span 3.83 [m] Wing Chord 0.425 [m] Horizontal Stabilizer Span 1.4 [m] Horizontal Stabilizer Chord 0.35 [m] Total Lift 130 [N] (cruise condition) Thrust 37.7 [N] Drag 14 [N]

The other component of the hybrid system is PV panels are two WaveSol Light 2 m PV panels for each wing and a single 1m PV panel for horizontal stabilizer. WaveSol Light PV panels are considered for this project because of their relative light

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weight, high current and compatible voltage output. They also have a convenient width which can easily be mounted on the UAV wings and stabilizer. Table 3 lists some of the characteristics and modeled power output of the described PV panels.

performance, availability, dimensions and good power to mass ratio. It is desired to study the effectiveness of using the hybrid PV-fuel cell system to power the UAV using AXI 5325/24 motor. Table 5 shows the parameters related to AXi 5325/24 motor. Table 4: Characteristics of AEROPAK fuel cell with Type I cartridge PEM Fuel Cell Average Output Power 250 [W] Continuous Current 7.5 [A] Output Voltage Range 28 - 54 [V] Mass 4.70 [kg] Lifetime at Rated Power 500 [hours] Operating Temperature 0-65 [C] Fuel Cartridge Energy 900 [Wh] Hydrate Weight 1.55 [kg] Dry Weight 0.55 [kg]

Table 2: Wind turbine rotor parameters and their values Parameter Value Airfoil NACA 63-418 Aspect Ratio – AR 6 Span – b 1.52 [m] Planform Area – S 0.25 [m2] Chord - c 0.27 [m] Rotor Area – A 7.54 [m2] Material Glass-reinforced plastic Surface Area 1.71 [m2] Thickness 1[mm] Number of blades 3 Averaged Wind Speed 4 m/s Averaged Power Output 65 W Averaged Voltage 17 V Averaged Current 3.8 Amps

Table 5: Parameter for AXi 5325/24 motor AXi 5325/24 Mass 575 [g] Diameter 63 [mm] Length 59 [mm] Rm 0.045 [Ohm] KV 0.0043 [Vm/RPM] Maximum Current 75 [Amps]

Table 3: Specification of the PV panels used in the modeling 2 [m] Module Nominal Power (Pmax) 50[W] Voltage at Pmax 17.3 [V] Current at Pmax 2.9 [Amps] Length 1.88 [m] Width 0.33[m] Weight 1.3[kg] Thickness 1.2 E-3[m] 1 [m] Module Nominal Power (Pmax) 24 [W] Voltage at Pmax 16.7 [V] Current at Pmax 1.45 [Amps] Length 0.995 [m] Width 0.33[m] Weight 0.65 [kg] Thickness 1.2 E-3[m]

EXPERIMENTS Experiments are conducted on each componement of the integrated hybrid renewable energy system. Wind turbine is tested in uncontrolled ambient conditions in City of Sharjah. For the test, ambient temperature is 24.3 oC and pressure is 101.3 kPa. The wind turbine voltage and current output is recorded against its rotating speed. A 12 V 90 Ah GEL deep cycle battery is used to store the energy produced by the wind turbine. Tests results are recorded using EagleTree system that logged RPM, voltage, current and time. The readings are recorded after every 10 seconds. The experiment is conducted until the ground battery is completely charged. Figure 3 shows the wind turbine during testing. Wind turbine power depends on the rotation on of the rotor since the electric generator produces electric power based on the shaft rotational speed. Since the wind turbine rotor converts the kinetic energy of the wind into rotation of the rotor, the wind speed becomes a dominant factor in determining the power. Several battery charging tests are conducted on the wind turbine to charge the battery. The measurements during the tests showed that a 90 Ah 12 V ground base battery takes 23.68 hours to be completely charged by this wind turbine. The average power output from the wind turbine is estimated to be

For continuous power, Horizon Energy Systems’ PEM fuel cell is investigated for the UAV. The tested fuel cell is a proton exchange membrane fuel cell which consists of 250 W fuel cell and a 900Wh hydrogen cylinder. This fuel cell is chosen because of its light weight, proven performance, size and hydrogen capacity. Some of the prominent features of Horizon fuel cell are tabulated in table 4. AXi 5325/24 is the most appropriate motor for the desired UAV. The motor is chosen based on manufacturer status,

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The experiments are conducted on the PV panels and readings are recorded. The produced electricity is stored in a battery pack to test the storage limits. Battery pack consisted of 13 AA rechargeable energizer batteries with the battery pack voltage of 20.8 V and the current of 1.4 Amps. In addition, a solar charge controller (the rating is 12/24 V) is also installed to help to regulate current and voltage coming from the PV panels. The circuit consisted of connecting the PV panels and the battery pack to the controller so to charge the battery package to full potential and get the value of the average charging rate. Moreover, all 3 components in the circuit are connected in series and the current is measured in the battery. Running these tests gave the average current discharged from the PV panels. Figure 5 shows the connection of PV panel with the charge control circuit and the battery pack.

44 W. Figure 4 shows the power curve of the wind turbine tested.

Figure 5: PV panel connected with solar charge controller and battery pack in a series connection Figure 3: A view of the small wind turbine used in the integrated renewable energy system

The PV panels are tested for their output voltage and current. The tests lasted for, on average, 19 minutes and the collected average data calculations are shown in table 6.

Wind turbine power depends on the rotation on of the rotor since the electric generator produces electric power based on the shaft rotational speed. Since the wind turbine rotor converts the kinetic energy of the wind into rotation of the rotor, the wind speed becomes a dominant factor in determining the power. Several battery charging tests are conducted on the wind turbine to charge the battery. The measurements during the tests showed that a 90 Ah 12 V ground base battery takes 23.68 hours to be completely charged by this wind turbine. The average power output from the wind turbine is estimated to be 44 W. Figure 4 shows the power curve of the wind turbine tested.

Table 6: Averaged values of the collected data for 1 m and 2 m panels

PV panel

Electrical Power W

60 50 40 30 20 10 0 2500

4500

6500

Current (Amps)

Power (W)

Energy (J)

1m

17.35

1.156

20.05313

22860

2m (each)

17.01

2.29

38.93079

44381

Tests are conducted on a 1 m PV-panel and a 2 m PV panel which are to be mounted on horizontal stabilizer and the wing, respectively. The measurements are taken for voltage and current being stored in the battery. The test results show a fairly stable charging thought the studied period of 19 minutes. For the 1 m panel, current is averaged at 1.16 Amps and voltage at 17.35 V while for 2 m panel, current and voltage are 2.29 Amps and 17 V respectively. The results show that the 1 m panel yields 20 W while the 2 m panel yields 39 W of power. The amount of energy stored from 1 m panel and the 2 m panel is 22.8 kJ and 44.3 kJ, respectively. The calculation of total stored energy is important as it gets converted into power for the desired duration.

70

500

Voltage (V)

8500

Wind Turbine RPM

A bench test is done on the PEM fuel cell to record its endurance, current, voltage and hydrogen tank pressure. AXi motor is operated at 2800 RPM and 250 W. A motor controller

Figure 4: Power curve of the wind turbine tested.

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is used to controll the motoe rotation. Figure 6 shows the components used for the bench test set-up.

RESULT AND DISCUSSION A study is presented to find out the feasibility of using a hybrid, wind turbine-fuel cell-PV panel, renewable energy system for small UAV flight operations. Power systems are ground tested for their performance and output. Real flight is not conducted. A microcontroller will be used in the UAV to alternate the motor input between PV panel charged battery and fuel cell. It is assumed that the initial battery charge that is charged on the ground by the wind turbine is completely consumed during take-off and climb. Working with the cruise phase assumption, the motor draws power from the fuel cell until PV panels have charged the battery. The wing and tail PV panels, combined, generate the total current output of 5.73 Amps at a voltage of 17 V. A 5 Ah battery, rated for 21 V, takes 4021 seconds to fully charge under 1 kW/m2 solar flux through the PV panels. This means that the motor is powered by fuel cell for 4,021 seconds. Once the battery is charged, it takes 480 seconds for the battery to be drained out by the motor running at 29 V and 8 Amps. The cycle repeats until the fuel cell has run out of hydrogen and battery is fully drained out.

Figure 6: The measuring instruments (a), propeller (b), motor rig (c) and controller (d).

4500

The readings are recorded after every 10 minutes until all the hydrogen from the tank drained out. Hydrogen cylinder is filled to 128 bars. EagleTree system is used to measure the desired redings. Table 7 shows the fuel cell current and voltage output with respect to hydrogen pressure. The test results in Table 7 show that fuel cell can sustain flight for 130 minutes when used as a stand alone power system for UAV.

4000 Run time, seconds

3500

Table 7: PEM fuel cell bench test data for 250 W motor

3000 2500 2000 1500 1000 500

Voltage (Volt)

Power (W)

Current (Amps)

H2 pressure (bars)

Time (sec)

29.03

245.00

8.38

128.00

0

29.05

236.00

8.22

115.00

600.00

28.84

235.00

8.22

105.00

1,200.00

28.92

232.00

8.01

95.00

1,800.00

29.28

237.00

8.01

85.00

2,400.00

29.24

232.00

8.06

75.00

3,000.00

29.07

232.00

8.06

67.00

3,600.00

29.05

236.00

8.06

57.00

4,200.00

29.07

232.00

8.06

48.00

4,800.00

29.34

233.00

8.06

39.00

5,400.00

29.13

229.00

7.90

30.00

6,000.00

29.25

231.00

7.96

20.00

6,600.00

29.21

234.00

7.95

10.00

7,200.00

29.24

232.00

7.69

0

7,800.00

0 Battery

Fuel Cell

Battery

Fuel Cell

Battery

Figure 7: Run time of fuel cell and battery during flight Figure 7 shows the run time of each power supplier with their sequence. Each sequence comprises of fuel cell followed by battery powered flight. In case of cloudy conditions, representing low solar flux, the UAV would have lower added endurance. This means that it would partially run on the fuel cell plus whatever power PV panels can produce. Under no solar flux, it would only run on the fuel cell. The total battery time is the improvement in the UAV’s endurance because of hybrid power system. Total endurance increase, with the hybrid system, comes out to be 931 seconds. Because of this increase and sequential use of power systems, the total endurance of the UAV comes out to be 9,210 seconds as compared to 7,800. This shows an improvement of 18% over the baseline fuel cell lone system. Figure 8 shows the graphic depiction of the endurances.

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REFERENCES UAV Endurance, seconds

9000

[1] Calderón, M., Calderón, A.J., Ramiro, A., González, J.F., González, I., 2011, “Evaluation of a hybrid photovoltaic-wind system with hydrogen storage performance using exergy analysis”, International Journal of Hydrogen Energy, 36, pp. 5751–5762

8000 7000 6000 5000 4000 3000

[2] Eroglu, M., Dursun, E., Sevencan, S., Song, J., Yazici, S., Kilic, O., 2011, “A mobile renewable house using PV/wind/fuel cell hybrid power system”, International Journal of Hydrogen Energy, 36, pp. 7985–7992

2000 1000 0 Fuel Cell

Combined

[3] Alam, M.S., Gao, D.W., 2007, “Modeling and analysis of Wind-PV-Fuel cell hybrid power system in HOMER”, Second IEEE Conference on Industrial Electronics and Applications, Herbin, China, pp. 1594-1599

Figure 8: Endurances time comparison between fuel cell powered UAV endurance and hybrid system powered UAV endurance Each UAV battery takes 45 minutes to be charged through ground battery. With the total flight time of 9210 seconds or 2.558 hours, the ground battery can charge 3.4 UAV batteries before the UAV returns. The tested wind turbine charges a ground based 90 Ah battery in 23.68 hours. This allows the wind turbine to charge 9 UAV batteries before requiring a recharge from the wind turbine. The total energy available from the wind turbine charge is 5.4 kJ.

[4] Zafar, S., Dincer, I, 2014 “Energy, exergy and exergoeconomic analyses of a combined renewable energy system for residential applications”, Energy and Buildings, 71, pp 68-79 [5] El-Sharkh, M.Y., Rahman, A., Alam, M.S., Byrne, P.C., Sakla, A.A., Thomas, T., 2004, “A dynamic model for a standalone PEM fuel cell power plant for residential applications”, Journal of Power Sources, 138, pp.199-204.

CONCLUSIONS

[6] Redha. A.M, Dincer,I., Gadalla, M., 2011, “Thermodynamic performance assessment of wind energy systems: An application”. Energy. 36, pp 4002 – 4010

This paper presented the modeling conducted on an integrated hybrid, wind turbine-hydrogen fuel cell-PV, energy system as a small UAV propulsion system. A hybrid power system serves as an alternative to bigger and costly nonrenewable power systems that would be required to achieve the same flight performance and operations.    



[7] Mishnaevsky. L. Jr, Freere.P., Sinha. R., Acharya.P., Shrestha.R., Manandhar.P., 2011. “Small wind turbines with timber blades for developing countries: Materials choice, development, installation and experiences”, Renewable Energy. 36. pp. 2128-2138

The integrated hybrid renewable energy system shows that the entire flight operations can be carried out completely on renewable energy. Having a small wind turbine part of the system, the operation can be carried out even on remote off-grid locations completely on renewable sources. The endurance of the small UAV increased with the use of hydrogen fuel cell-PV hybrid power system. The endurance of modeled UAV increases 18% with the use hybrid hydrogen fuel cell-PV power system. The UAV endurance increased to 9210 seconds with a hybrid system, as compared to 7800 seconds with just the fuel cell. The resulting improvement in the endurance of a small UAV would result in loner range operations and fewer take-offs and landings. This improvement in performance, in turn, would result in lower operational cost and reducing the risk of damage.

[8] Singh. R.K, Ahmed. M.R., 2012. “Blade design and performance testing of a small wind turbine rotor for low wind speed applications”. Renewable Energy. 50, pp. 812-819 [9] Zafar.S., Gadalla.M., Ahmad, S.., 2013. “Energy Production Through Wind using Personal use Wind Turbine: A UAE Case Study”.ASME IMECE 2013-66323, San Diego, USA [10] Thomas, PJ., Qidwai, MA., Kellogg, JC., 2006, “Energy scavenging for small-scale unmanned systems”, Journal of Power Sources, 159, pp.1494–1509 [11] Khaligh, A., and Onar, OC., 2010, Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems, CRC Press, Boca Raton, 2010 Chap. 1

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[13] Hosseini, M., Dincer, I., Rosen, MA., 2013, “Hybrid solar–fuel cell combined heat and power systems for residential applications: Energy and exergy analyses”, Journal of Power Sources 221, pp. 372–380

[12] Chen, H. and Khaligh, A., 2010, “Hybrid Energy Storage System for Unmanned Aerial Vehicle (UAV)”, IECON, Phoenix, USA, pp. 2851 - 2856

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