Solar Power System for Experimental Unmanned Aerial Vehicle (UAV); Design and Fabrication

H. Bahrami Torabi, M. Sadi AmirKabir University of Technology,

Tehran, Iran, Bahrami@aut.ac.ir, Mohsen1365ir@aut.ac.ir

A. Yazdian Varjani Tarbiat Modares University,

Tehran, Iran Yazdian@modares.ac.ir

Abstract- A Solar Power System for experimental

unmanned aerial vehicle (UAV) is designed and summarized. For the aircraft represented in this paper, solar cells were used to increase the endurance of the aircraft. Obtaining this goal, an electrical circuit was developed to measure the output power of the batteries of the aircraft during the flight. Flight tests showed that in cruise phase flight without battery is achievable. A microcontroller based controller was developed to collect the output power data of power source to be send to the ground station. Since solar cells were decided to be installed on the surface of the wing, an airfoil, upper surface of which is smooth enough for putting solar cells without bending them, should be selected. This airfoil should also be a low-Reynolds-number airfoil. The selection of the airfoil was done by using Design Foil software. Among 150 airfoils which had the two desired conditions, EP178 was selected.

Keywords-Unmanned Aerial Vehicle; Solar cells; Power consumption

I. INTRODUCTION Nowadays, one of the most challenging problems of the

world is the limitation of the source of energies. One way to overcome this problem is to looking for another source of energy. Solar power is one of them. In these days, using from solar cells has become conventional. In one aspect they are used in Unmanned Aerial Vehicles (UAV).

Conventionally, Unmanned aerial vehicles are aircrafts either using remote control or automatic control for their guidance. These aircrafts are used for carrying devices as a payload such as a camera, sensors, and communication devices. From 1950 these aircrafts were used in both recognizance and gathering data missions.

In recent years the interest to development of such aircrafts with variety missions has been increased. The most challenging problem in an Unmanned Aerial Vehicle (UAV) refers to its source of power. Since battery and fuel are two common sources of power for UAVs, the endurance of such aircrafts is restricted. Using other sources of power can be useful to increase the endurance of the aircrafts. Unmanned solar aircrafts are considered as a group of UAVs in which, solar cells are used as a source of power. The solar cells are used as an unlimited source of power for motor and other electrical subsystems. Utilizing an electrical circuit, not only

power can be provided for motor and other electrical subsystems via solar cells but also can charge the battery. Batteries are used as a backup when solar cells cannot provide enough energy (flying under shadow or cloudy weather). So flying will be continued till solar cells can provide energy from sun.

On the 4th of November 1974, the first flight of a solar-powered aircraft took place on the dry lake at Camp Irwin, California. Sunrise I, designed by R.J. Boucher from Astro Flight Inc. under a contract with ARPA, flew 20 minutes at an altitude of around 100 m during its inaugural flight. It had a wingspan of 9.76 m, weighed 12.25 kg and the power output of the 4096 solar cells was 450 W [1].

Since using from solar cells as the only source of power, and if in one of the flying phases, the produced power by the solar cells is lower than the power consumption of the aircraft, fly will led to crash. So, in the design of a solar-powered aircraft knowing the quantity of power consumption of the aircraft and the produced power by the solar cells is a significant matter [2-5].

In this paper we focus on the design evaluation of a SPS for an experimental UAV application which has to handle the rapid voltage variations due to attitude changes during maneuvers. The objective of the paper is present a procedure to develop a solar powered aircraft which is enabled to flight with the power of only solar cells during the cruise phase.

II. SYSTEM OVERVIEW After designing the airplane with the data of weight and

placement of solar cells, power consumption of the airplane and produced power of the solar cells should be calculated. Solar cells totally produce a maximum nominal power of 17.7 watts. Therefore calculation over the power consumption of the airplane in different phases of the flight is inevitable. If the power consumption is less than the produced power of the solar cells, then it can be resulted that the airplane is capable of flying without batteries. But there is a critical fact about solar cell behavior; while under a shadow of a cloud or a different angle of radiation the produced power of the solar cell will significantly reduce. So, using a backup battery will help us preventing crashes while flying with only solar cells as our main power source. But, before deciding of using solar cells as our main power source, we need to know if it can produce the

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required power in all phases of flight or not. Doing so, we need to calculate as well as testing power consumption of our airplane in all phases.

A. Calculation of power consumption of the aircraft:

Cruise phase: in this phase, in which the airplane flies mostly, lift and gravity forces are equal as well as drag and thrust forces. So we have two equations: , (1)

And the required power of the airplane is: (2) Where V is Velocity

And (2) yields: (3) Where is air density, is area of the wing and is drag

coefficient. (4) is zero-lift drag coefficient and calculated with digital

datcom software[2] and is equal to 0.024 (dimensionless).

is lift coefficient of the airplane and is depend on angle of attack. (5)

is aspect ratio of the wing and is the Oswald efficiency factor and can be calculated by this formula:

1.78(1 0.045 . ) 0.64 (6) (7)

Putting (7) into (4): ( ) (8) Equation (8) and (3) result:

(9) Equation (9) shows that in cruise phase is a function of

velocity and for our airplane the diagram is indicated in Fig.1.

Stall speed is the minimum speed required for the airplane to fly. For this airplane the stall speed is about 7 m/s and we can see that the minimum power is around stall speed. So, as cruise speed of 10 m/s the required power is around 3.9 watts

But this is the required power or output power of the motor; input power of the motor, that is output power of the batteries or solar cells, can be calculated with this formula: (10)

Figure 1. diagram for the aircraft

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Where η is motor efficiency and η is propeller efficiency. For our airplane η 0.5 and η 0.8

So the produced power of the solar cells should not be less than .. . 9.75watts.

Other sources of power consumption exist in airplane such as servos, receiver and power dissipated in the ESC. The total power devoted to these devises is estimated to 0.8 watt. Therefore, the total power required for the airplane is equal to 10.55w.

So the total power of the airplane is calculated. But the team does not trust on the calculated power. Because so many other factors may affect the calculation such as error in drag coefficient calculation, error in speed calculation due to turbulence and so on. Therefore, in order to verify the calculated result and to be confident to use solar cells as the only power source of the plane, the team developed a power meter electrical board for the airplane (Fig.2). This board is able to calculate the output power of the batteries during flight test (before installing solar cells on the airplane). By doing so, it can be found whether solar cells have the capability of being used alone or not

.

Figure 2. The Board Used for Probing Output Power of the batteries during flight

III. Experimental Results The board consists of a microcontroller which can measure

the output voltage and amperage of the batteries and send the data to the ground station while flight test. Batteries used in the airplane are three series of Lithium-Polymer each of which has a nominal voltage of 3.7v, causing an overall voltage of 11.1v. This voltage will change during the flight and should be probed by analog to digital convertor (ADC) of the micro controller. But the maximum voltage that can be probed by the micro controller is 5 volts. So, two resistors in series are put in the board between two poles of the batteries. One is 56kΩ and the other is 39kΩ. The voltage between pins of the second resistor is probed by the ADC pin of the micro controller. The maximum probed voltage is 4.55 v.

For probing the current we should use a high power resistor with low resistance. As a result, a 0.1Ω resistor is used in the circuit. The amount of voltage between two pins of the resistor will show 0.1 of the current. The order of the current is about 4 amps at maximum so the probed voltage is 0.4 volts. Reading this amount directly by the ADC will cause inaccuracy in current probing. Therefore, we need to use an O-Amp to multiply the ADC reading data by approximately 10. So it can be read by the ADC as a scale of 0 to 4 volts.

The board is installed in the airplane and during the flight it sends the output power directly to the ground station through a HMTR module. Fig.4. shows the output power of the airplane during the flight.

This graph shows the power consumed with the entire electrical system of the airplane including the electrical board. The average power used in the cruise phase (after 5 seconds of taking off and climbing) is 17.6w. But, in experimental flight using solar cells (with specification shown in Table 1) the functionality of this board is no longer necessary. So the power used by this board should be considered in order to find the exact consumed power by the electronic system.

Table 1: The Solar Cells Specification

Type Length(mm)

Width (mm)

Output Power(m Watt) Voltage

(v) Weight

(g)

1 50 25 195 0.6 0.9

2 63 13 105 0.6 0.6

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B. Calculation of the power of the board: The resistor power loss is due to the current which comes

out of the batteries passes through this resistor. The power loss can be calculated as below: 0.1 1.5 0.225 (11)

The micro controller power consumption is about 0.025w the power loss in the regulator (LM7805 regulator) is calculated as below:

Power loss = (11.1-5) × 0.005=0.031w

As a result, the whole power consumption of the board is 0.225+0.025+0.031=0.281w

Figure 3. The Schematic View of the Power Board

Figure 4. Output power of the airplane during the flight

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Another factor that will affect the power consumption of the airplane is weight. Equation (9) shows the relation between weight and required power. During the flight test, batteries with higher capacity were used and also the electrical board was added. Instead the wing on which the solar cells were installed was not used. Therefore, there was a change in the weight of the plane and hence a change in the power required for the flight.

The difference between the weight of the wing with solar cells on it and the weight of the simple wing is 85 grams. The used batteries have the weight of 130 grams while the backup batteries have the weight of 35 grams. And at last, the board is 50 grams. So, a difference of 145 grams in the weight of the airplane occurs. Weight of the airplane during the flight test was 450 grams.

The effect of weight on has been indicated in Fig.5. The red one is the data with weight of 450 grams while in the blue one a weight of 305 grams has been taken into the calculations. So in a specified velocity, there is a difference of 5 watts between them. As a result, the average power consumption in the flight is 17.6 watts; 0.281 w is devoted to board and 5 w of it is because of extra weight. Therefore, the power output needed for the cruise flight of the airplane is: 17.6 0.281 5 12.319 (12)

C. The Comparison: Calculations showed that the power needed for the airplane

to flight in the cruise phase is 10.55w while the cruise power in

the flight test is about 12.319w. The reason of this difference probably is because of the quality of construction including coverage of the body and the wing.

Finally, the nominal output power of the solar cells is 17.7 watts for the surface used in this airplane and the power needed for the cruise flight is 12.32w. So, we can infer that in a good condition of weather- no cloud and shadow- the airplane can cruise without batteries. One backup battery is used for other phases of flight (takeoff and climb) which need more power.

Conclusion:

In this paper we focus on the design evaluation of a SPS for an experimental UAV application which has to handle the rapid voltage variations due to attitude changes during maneuvers.

A Solar Power System design for experimental unmanned aerial vehicle (UAV) has been summarized. For the aircraft represented in this paper, solar cells were used to increase the endurance of the aircraft. Obtaining this goal, an electrical circuit was developed to measure the output power of the batteries of the aircraft during the flight

In this work, the feasibility study of using solar cells as the only source of power for the airplane has been done. The power needed for the airplane has been calculated and the results showed that the airplane is able to fly without need of batteries and just with the power of the solar cells in cruise phase.

Figure 5: The effect of weight on

References: [1] R. J. Boucher, History Of Solar Flight, AIAA Paper 84-1429, June

1984 [2] Duryea, S., Islam, S., and Lawrance, W. “A battery management system

for stand alone photovoltaic energy systems”. IEEE Industry Application Magazine, 7, 3 (May—June 2001), 67—72.

[3] Bhuiyan, M. M. H., and Asgar, M. A. “Sizing of a stand-alone photovoltaic power system at Dhaka” .Renewable Energy, 28 (2003), 929—938.

[4] Glavin, M., and Hurley, W. G. “Battery management system for solar energy application” In Proceedings of the 41st International Universities Power Engineering Conference, Sept. 6—8, 2006, 79—83.

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[5] Hohm, D. P., and Ropp, M. E."Comparative stusy of maximum power point tracking algorithms" Progress in Photovoltics: Research and Applications, 11 (2003), 47—62.

Nomenclature Power requirement

Input power

Velocity

Drag force

Air density

Surface

Drag coefficient

Zero lift Drag coefficient

Lift coefficient

Oswald efficiency factor

Wing aspect ratio

Weight

Lift force

Trust force η Motor efficiency η Propeller efficiency

Dynamic pressure

Current

Resistance

Power

Figure 6: The Power circuit in Aircraft

Figure 7: The experimental unmanned aerial vehicle

(UAV)

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