Solar Power UAV

Solar Power System for Experimental Unmanned Aerial
Vehicle (UAV); Design and Fabrication
H. Bahrami Torabi, M. Sadi
AmirKabir University of Technology,
Tehran, Iran,
[email protected], [email protected]
A. Yazdian Varjani
Tarbiat Modares University,
Tehran, Iran
[email protected]

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
2011 2nd Power Electronics, Drive Systems and Technologies Conference
978-1-61284-421-3/11/$26.00 ©2011 IEEE 129
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:
I = w, Ð = I (1)
And the required power of the airplane is:
P
¡cq
= I × I (2)
Where V is Velocity
And (2) yields:
P
¡cq
= Ð × I =
1
2
pI
2
SC
Ð
× I (S)
Where p is air density, S is area of the wing and C
Ð
is drag
coefficient.
C
Ð
= C
Ð
0
+ kC
L
2
(4)
C
Ð
0
is zero-lift drag coefficient and calculated with digital
datcom software[2] and is equal to 0.024 (dimensionless).
C
L
is lift coefficient of the airplane and is depend on angle
of attack.
k =
1
ncAR
(S)
AR is aspect ratio of the wing and c is the Oswald
efficiency factor and can be calculated by this formula:

c = 1.78(1 - u.u4SAR
0.68
) - u.64 (6)
w = I =
1
2
pI
2
SC
L
= C
L
=
2w
pv
2
S
(7)
Putting (7) into (4):
C
Ð
= C
Ð
0
+ k(
2w
pv
2
S
)
2
(8)
Equation (8) and (3) result:
P
¡cq
=
1
2
pI
3
S _C
Ð
0
+ k [
2w
pv
2
S
¸
2
] =
1
2
pI
3
SC
Ð
0
+
2kw
2
pvS
(9)
Equation (9) shows that in cruise phase P
¡cq
is a function of
velocity and for our airplane the P
¡cq
-I 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:
P
¡cq
= P
ìn
× p
m
× p
p
- P
ìn
=
P
rcq
q
m
×q
p
(1u)


Figure 1. P
¡cq
- I diagram for the aircraft
0
5
10
15
20
0 5 10 15 20 25
P
r
e
q
Velocity
Preq- V
130
Where η
m
is motor efficiency and η
p
is propeller
efficiency. For our airplane η
m
= u.S and η
p
= u.8
So the produced power of the solar cells should not be less
than
3.9
0.5×0.8
= 9.7Swatts.
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:
P = RI
2
= u.1 × 1.S
2
= u.22Sw (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

0
5
10
15
20
25
0 5 10 15 20 25 30 35
O
u
t
p
u
t

P
o
w
e
r

(
W
)
Time (s)
Power to Time
132
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 P
¡cq
- I 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 - u.281 - S = 12.S19 (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 P
¡cq
- I
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.
0
20
40
60
80
100
0 5 10 15 20 25 30
P
r
e
q
Velocity
133
[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
P
¡cq
Power requirement
P
ìn
Input power
I Velocity
Ð Drag force
p Air density
S Surface
C
Ð
Drag coefficient
C
Ð
0
Zero lift Drag coefficient
C
L
Lift coefficient
c Oswald efficiency factor
AR Wing aspect ratio
w Weight
I Lift force
I Trust force
η
m
Motor efficiency
η
p
Propeller efficiency
q Dynamic pressure
I Current
R Resistance
P Power



Figure 6: The Power circuit in Aircraft


Figure 7: The experimental unmanned aerial vehicle
(UAV)
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