Sizing of Lithium-ionCapacitor/Solar Array Hybrid Power Supplyto Electrify Mi...
Solar Cells Power Model Aircraft
1. 1
Solar Cells in a Model Solar Aircraft
Jeffery Liu
Energy Systems Lab
2014-2015
2. 2
Table of Contents
Introduction:
• Rationale 3
o Statement of Problem
o Importance of Topic
• Goal of Project 4
• Literature Review 3
Project Brief
• Requirements 4
• Overview 4
• Limitations 5
• Reiterative Evaluation Plan 5
• Developmental Procedures 6
Testing
• Purpose 7
• Criteria 7
• Testing Methods 7
Results 8
Discussion 8
Conclusion 8
Literature Cited 9
Acknowledgements 9
Appendix 10-19
Introduction:
3. 3
In today’s society, aviation has become vastly important. From
transportation of hundreds of people at time with the Boeing 747 to dominating
the skies with a superior fighter jet such as the F-22 Raptor, aircrafts influence
the way we live. Some problems with existing aircraft are that they burn lots of
fuel, pollute (releasing two percent of all carbon dioxide emissions), and cannot
stay in the air for extended durations of time. The development of highly efficient
solar aircraft can address these issues. One of the major challenges that must be
faced while designing solar aircraft is implementing the solar panels.
Solar cells convert light into electricity through the photovoltaic effect.
Photovoltaic cells are a type of photoelectric cells, which are materials whose
electrical characteristics change when put under light. Silicon solar cells work by
first having photons hit the silicon atoms of the solar cell, transferring their energy
to loose electrons. The loose electrons must be put into an electric current by
making an electrical imbalance within the cell. Silicon atoms on solar cells are
arranged in two different types: n-type, which has spare electrons, and p-type,
which has missing electrons. The n-type’s spare electrons jump to fill the gaps of
the p-type’s. As a result the n-type silicon atoms become positively charged and
the p-type silicon atoms become negatively charged, giving an electric field for
the loose solar cells to cross. As silicon is a semiconductor, it can maintain the
electrical imbalance because it can act as an insulator. (Green, 1982)
Currently, there exist several models of solar aircraft, with flying wing-
designed aircraft such as the NASA Helios and Pathfinder(Figure 1.) (Dunbar,
2014) After doing research on these aircraft, I have gained insight on how the
shape of the wing should be designed to best suit the placement of the
photovoltaic cells. For example, both the solar aircraft have U-shaped wings that
are flexible. However, the RC plane we worked with did not have a flying-wing
design, so the specifics of the NASA aircraft could not be applied directly. I have
also done research on specific solar cells, like the Alta solar panels that are
extremely thin, lightweight, and flexible (Figure 2). These solar panels would be
perfect for the airfoil as the array of solar panels could bend in the shape of the
airfoil and would not add much mass to the structure of the RC aircraft. However,
such cells would be much more expensive so therefore not attainable. In addition
to the solar cells themselves, I have done background research on how to
position the solar cells so they are not as prone to flexing and breaking. This can
be done by segmenting the aircraft into multiple sections and then connecting
each segment with a spur. Also, the wing can be layered to protect the cells.
(Gripp and Rawdon, 2013)
In the future, sturdy and sustainable solar aircraft can be used for many
applications. As solar aircraft can fly high and stay independently in the air for
long periods of time, the planes can be used for surveillance, weather data, and
atmospheric monitoring. Solar aircraft are also more environmentally friendly
than conventional aircraft, as they do burn jet fuel or emit exhaust (airplane’s
carbon dioxide emissions accounted for 2 percent of all pollution by humans in
1992). By engineering an efficient model solar aircraft, data and insights on how
the plane functions can be obtained without having to construct a full-size,
expensive one.
4. 4
Purpose of Project
The ultimate goal of this project is to create a RC aircraft that flies better
with solar panels as its sources of energy as compared to with what the original
kit came with. My part of this goal will be the implementation of the solar panels.
To do this, the voltage requirements of the system must be calculated, a
setup of the cells must be determined based off the plane’s dimensions, and the
cells must be soldered and attached on.
To power the plane, 11.1 volts must run through which will be regulated
by a charge controller. So, the goal was to have the cells run around 11 volts in
sunny and clear and conditions.
I need to wire the solar cells in series across the wingspan of the plan in
order to generate sufficient voltage. The solar panels must be arranged in a
manner so that they are not prone to flexing and are protected from conditions
such as temperature changes. I must also test the solar panels under various
circumstances (different temperatures, different altitudes, different amounts of
shading) to determine how much more efficient/inefficient the solar panels make
the aircraft.
Project Brief:
Requirements
The success of the project will be determined by whether or not the plane
with solar cells can fly for a longer period of time than the plane without the cells
attached. If the plane with cells fails to fly or flies for a shorter duration, then we
have failed in creating an efficient model solar plane. For my part specifically, the
cells can be considered successfully implemented if the cells generate enough
voltage to power the system and the can be wired onto the wing properly.
Basically, the project is deemed successful if the motor can run off the cells’
power alone and the panels do not disturb with the structure and flight of the
plane.
Overview
In order to construct the balsa plane, a kit was ordered online and the
instructions were followed. However, we had to make some adjustments in order
to account for the solar cells. After the single body wing was constructed, I could
start soldering my solar cells. The cells were all connected by copper tape in
series. A series of holes were then drilled in the plane to run wire through to
connect all the cells to a charge controller. A layer of monokote will then be put
over the cells and wings to hold everything in place. The plane will be controlled
by a handheld radio controller, which moves the servos in the plane. There are
5. 5
two servos, one for the elevator( to control height), and one for the rudder, (to
control direction). The speed of the motor will also by synced to the radio
controller.
Limitations
One of the biggest limitations on the project was the fragility and
brittleness of the solar cells. A large quantity of cells ( a minimum of 30) was
needed and to keep the cost within the 100 dollar budget, flexible cells were not
viable. Given unlimited resources, flexible cells of around 3 by 6 inches would
have been perfect to fit between the ribs of the wing and bend to the contour of
the shape. The cells purchased were multicrystalline untabbed ones that cracked
easily with the slightest drop or bend. Time was also a substantial limitation as
the body of the plane and wings were not completed until recently so there was
not much time to attach the cells to the wings. Additionally, while testing the
plane outside, a huge gust of wing knocked the wind over and cracked over half
the cells, resulting in a big setback. (Figure 3)
Reiterative Evaluation
After soldering a few cells together, a voltmeter was used to ensure that
cells were properly connected in series. A single cell generates around 0.5 volts,
so to test if a set of four cells were wired correctly, a voltmeter measurement was
taken to see if the voltage was around 2 volts. In addition, after soldering
together a large group of cells, a multimeter was used to test for cold solder
connections. This was done by using the continuity measurement on the
multimeter, if the piezo beeper made a sound, the connection was stable. If not,
the copper tape would need to be resoldered onto the bus bars of the cell.
Developmental Procedures
In the beginning, the focus of the project was constructing the balsa plane
as the aircraft kit was the first material to come in. Also, the electronic
components and the solar cells relied on the general structure of the plane. After
half of the wing was completed, the process for implementation of the wings
started. The general steps are as listed:
• Compare and analyze dimensions of plane with dimensions of cells
• Try different configurations and see if they meet specifications for
plane
• Find way to connect all the cells in a circuit
• Solder all cells together into circuit to connect to charge controller
• Attach cells on plane so they do not fall off during flight
The area of the wing was 675 square inches. The dimensions of a single solar
cell were 3.25 x 6 inches, giving an area of 19.5 square inches. Theoretically, 34
cells could fit on the wing. The maximum theoretical voltage outputted by an
6. 6
array of solar cells can be calculated by finding the open circuit voltage( where
no current flows):
𝑉!" =
𝑘! 𝑇
𝑞
ln
𝐽!"
𝐽!
+ 1
𝐽!" 𝑆ℎ𝑜𝑟𝑡 𝐶𝑖𝑐𝑢𝑖𝑡 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =
𝐼!"
𝐴𝑟𝑒𝑎
Isc is the short circuit current, where the terminals are connected to each
other for zero load resistance. T is the temperature in Kelvin. J0 is a constant
and q is the electron charge. Although this formula can give a general voltage
range, specifications from the manufacturer are needed for additional analysis of
the solar panels as many other factors such as quality of build can affect the
actual efficiency of the voltage output. As a result, the theoretical best set up can
be calculated with formulas while the actual data for building the solar panels
should be calculated with the experimental data provided by the manufacturer.
Because the shape of the wing was like the airfoil documented in Figure
4., the cells could not bend, so putting 34 cells on the wing was unviable.
Instead, having 22 cells laying parallel to the ribs of the plane across the
wingspan of the plane was the configuration chosen, as pictured in Figure 5. With
this configuration, a maximum of 11 volts and 3.5 amps could be generated in
sunny conditions. This configuration added over 132 grams to the back of the
wing and plane. To balance this, some electronic components were moved
closer to the front of the plane like the servos and charge controller.
To achieve proper voltage, all of the cells were connected in series. At
first, wires were to be used. However, using wires would make the long length of
cells more disorganized and not lay flat on the backside of the wings. Instead,
copper tape (one side adhesive, one side not) was used. A series connection
was made by putting the adhesive side on the back of the solar cell on the
negative terminal and then soldering the non-adhesive side to the positive bus
bar of the solar cell (Figure 6).
After all the cells were connected in series, they must be connected to the
charge controller at the center of the plane. To do this two strands of wire were
cut out, each having the length of half the wingspan, 39.25 inches. One of them
was soldered to the positive bus bar and the other was soldered to the negative
terminal on the cell on the other side of the solar array. To run the wire through
the wing, holes were drilled through each rib of the plan, as displayed in Figure 7.
Once the cells were on the wing, data collection started. The wing and
cells were brought outdoors on a cloudless day. A LabQuest was then used to
measure voltage over a fifteen second period. Current and voltage readings were
then measured for electronic components like the servo and the motor. To
properly attach the cells to the wing and to make the plane aerodynamic, a layer
of monokote will be applied to the entirety of the wing. A major concern for this
step is the fragility of the cells. As applying monokote will put some pressure and
heat on the cells, cracking of the cells may occur.
7. 7
Testing
The characteristics that must be tested are voltage, amperage, and
wattage of the cells. The wattage is just equal to the voltage multiplied by the
current. The voltage must be calculated as if the threshold of around 11 volts is
not achieved, the motor will not generate enough lift for the plane to obtain flight.
To measure voltage, a LabQuest with a voltage probe was used. The black clip
connected to the wire attached to the negative terminal and the red clip attached
to the wire connected to the positive bus bars. The brushless motor we are using
has a motor velocity constant of 2618RPM/ V. So at 11 volts, the propeller will be
rotating at 28,798 revolutions per minute, which is necessary to generate enough
force to maintain flight in the air. Similarly, sufficient current is needed to maintain
RPM. The current of an individual cell is 3.5 amps. Since the cells were wired in
series, the amperage should stay at a constant 3.5A. To measure current, a
current probe will be connected to the LabQuest. To test individual components
like the motor and servos, a power supply was used to provide a certain voltage
and current to see the amperage draw and performance of the motors and
servos under certain conditions. When testing the motor, a clamp was used to
hold the motor in place while it ran. When testing the servos, the full range of
movement for the elevator and rudder were used.
To test the aircraft altogether, a flight must be completed to determine how
long the plane can fly using the energy generated by the solar cells. This can
then be compared with a vanilla flight of the plane where the solar cells are not
used. Testing must be done on days where there are essentially no clouds in the
sky and the wind speed is less than five miles per hour. The materials required
for testing are a multimeter, power supply, alligator clips, timer, current probe,
voltage probe, and a LabQuest.
Results and Discussion
The first measurement done was the voltage readings. The LabQuest
recorded the voltage for 15 seconds, taking a reading every 0.1 seconds. The
first measurement recorded a consistent 10V as graphed in Figure 8. However,
the voltage probe can only read from ranges to -10 to 10 volts. Therefore, the
graph indicated that the cells were consistently generating over 10 volts,
probably around 11 volts since there were 22 cells (0.5 V each). As a result, the
first measurement of the voltage indicated that the cells were outputting enough
voltage to power the plane. The weather conditions the readings were taken in
were extremely ideal, with high sunlight no clouds, and little to no cloud
coverage. The temperature was around 80 degrees Fahrenheit. The only
downside to the weather was the periodic gusts of wind.
Another voltage recording was done with the LabQuest after the first in
similar weather conditions, as graphed in Figure 9. In this run the voltage spiked
between nearly 0 and 11V. This was probably due to the fact that some of the
cells were cracked and tape was covering some of the cells to hold them onto the
8. 8
plane. In this situation, the charge controller would regulate the voltage to a
consistent 11.1V as the battery would have excess power from when the cells
perform as well as they did in the run done in Figure 8.
In Figure 10, a power supply was set to 11 volts and 3.5 amps to see how
the current would change as the motor and propeller sped up. With 3.5 amps, the
RPM of the motor did not seem to be high enough to lift the plane. The current
draw stayed at around 3.5 amps and spiked at a little over 5 amps. When running
on 10 amps, the RPM of the motor was significant enough to fly the plane.
Although the power supply was set to 10 A, the motor only used 7.5 A for the
majority of the run, peaking at a high of 8.5 A. This indicates that the motor runs
most efficiently at 7.5 amps. A higher current will not be put to use and a lower
current does not generate torque in the motor. To test the current draw of the
servos that control the rudder and elevator, 11 volts and no current was set on
the power supply. The results are displayed in Figure 11.
There were several other senior technology projects completed this year
that were concerned with environmental problems. For example, using a bike to
generate power, a solar power backpack, and a hydrogen fuel cell bike all utilize
alternative energy sources. Outside of the energy systems lab, solar planes have
been built before. Manned one propeller solar gliders like the Sunseeker II have
flown across the United States and the Alpine mountain range. On the other
hand, high altitude unmanned solar aircraft like the NASA Pathfinder are capable
of collecting data for scientific studies and weather monitoring. Through this
model solar aircraft project, it has been demonstrated that solar cells are capable
of supplying sufficient energy to power flight. However, the solar cells take up a
great deal of surface area, add electronic components, and were fragile.
Conclusion
The overall goal of the project from the beginning was to create an
efficient model solar aircraft. The two main requirements of the project included:
generating enough voltage to store into the 11.1 V battery and fitting the cells
securely on the wing of the plane. The first requirement was met. On the other
hand, securing the cells on the wing was a much greater challenge as the cells
did not fit between the ribs of the plane well and were prone to breaking easily. A
heat gun will be used to apply a layer of monokote over the cells to properly
attach them. Success of the plane is determined by whether or not the plane flies
for a longer period of time than without the solar cells. To improve upon the
project, flexible cells should have used instead.
A full-scale solar aircraft can be used for a variety of purposes including
surveillance, atmospheric monitoring, and data connection (provide Wifi to
inaccessible areas). Further down the road, more advanced solar cells could
allow for solar aircraft that are made to transport people. For further research,
flexible cells could be used to make an improved solar aircraft
9. 9
Literature Cited
Dunbar, B. (2014, February 28). NASA Armstrong Fact Sheet: Pathfinder Solar-
Powered Aircraft. Retrieved September 10, 2014, from
http://www.nasa.gov/centers/armstrong/news/FactSheets/FS-034-DFRC.html
Flittie, K., & Curtin, B. (1998). Pathfinder solar-powered aircraft flight
performance. American Institute of Aeronautics and Astronautics, 618-620.
http://dx.doi.org/10.2514/6.1998-4446
Green, M.A. (1982). Solar cells: operating principles, technology, and system
applications. United States: Prentice-Hall, Inc.,Englewood Cliffs, NJ.
Gripp, Robert E., and Blaine R. Rawdon. Segmented Aircraft Wing Having Solar
Arrays. The Boeing Company, assignee. Patent US 20130099063 A1. 25 Apr.
2013. Print.
Hibbs, Bart D., Peter Lissaman, Walter R. Morgan, and Robert L. Radkey.
Aircraft. Bart Hibbs, assignee. Patent 5810284. Sept.-Oct. 1998. Print.
"Open-Circuit Voltage." PVEducation. N.p., n.d. Web. 02 Oct. 2014.
"StevensAero - Helium MG2, RES Motor Glider Kit." Model Airplane & Boat Kits.
N.p., n.d. Web. 02 Oct. 2014.
Sunseeker II - Europe Tour and First Alps Crossing. (n.d.). Retrieved May 28,
2015, from SolarFlight website: http://www.solar-flight.com/home/
Technology Brief: Single Solar Cell. (2014, January 1). Retrieved September 12,
2014, from http://www.altadevices.com/pdfs/single_cell.pdf
Acknowledgements
Thanks to Mr.Kemp for helping with the project throughout the year and guiding
the way
Thanks to classmates for supporting our project
Thanks to my partners, Christian and Wilson, for being great to work with
Appendix
16. 16
Original Project Proposal
Jeffery Liu
Pd. 6
Calculations, Design, and Implementation of Solar Cells in a Model Solar Aircraft
Description of the Problem:
In today’s society, aviation has become vastly important. From transportation of
hundreds of people at time with the Boeing 747 to dominating the skies with a superior
fighter jet such as the F-22 Raptor, aircrafts influence the way we live. Some problems
with existing aircraft are that they burn lots of fuel, pollute (releasing two percent of all
carbon dioxide emissions), and cannot stay in the air for extended durations of time. The
development of highly efficient solar aircraft can address these issues. One of the major
challenges that must be faced while designing solar aircraft is implementing the solar
panels.
Currently, there exist several models of solar aircraft, with flying wing-designed
aircraft such as the NASA Helios and Pathfinder. After doing research on these aircraft, I
have gained insight on how the shape of the wing should be designed to best suit the
placement of the photovoltaic cells. For example, both the solar aircraft have U-shaped
wings that are flexible. However, the RC plane we will be working with will not have a
flying-wing design, so the specifics of the NASA aircraft cannot be applied directly. I
have also done research on specific solar cells, like the Alta solar panels that are
extremely thin, lightweight, and flexible. These solar panels would be perfect for the
airfoil as the array of solar panels could bend in the shape of the airfoil and would not add
much mass to the structure of the RC aircraft. The research on these solar panels will help
us to decide which solar panels to purchase. In addition to the solar cells themselves, I
have done background research on how to position the solar cells so they are not as prone
to flexing and breaking. This can be done by segmenting the aircraft into multiple
sections and then connecting each segment with a spur. Also, the wing can be layered to
protect the aircraft.
In the future, sturdy and sustainable solar aircraft can be used for many
applications. As solar aircraft can fly high and stay independently in the air for long
periods of time, the planes can be used for surveillance, weather data, and atmospheric
monitoring. Solar aircraft are also more environmentally friendly than conventional
17. 17
aircraft, as they do burn jet fuel or emit exhaust (airplane’s carbon dioxide emissions
accounted for 2 percent of all pollution by humans in 1992). By engineering an efficient
model solar aircraft, data and insights on how the plane functions can be obtained without
having to construct a full-size, expensive one.
Objectives:
The ultimate goal of this project is to create a RC aircraft that flies better with
solar panels as its sources of energy as compared to with what the original kit came with.
My part of this goal will be the implementation of the solar panels.
I must calculate the highest efficiency setup theoretically (unlimited financial
resources, perfect conditions) to determine the range of voltage that my set up must
generate. The calculations will be based off some of the formulas listed in the approaches
section and data will come from the manufacturer of the solar panels that are purchased.
Also the dimensions of the RC plane must be known.
Right now, I know that the electronic components will require 5.3 V and the
servos and motors will require 11.1 V. However, the speed controller of the motor will
generate the required voltage for the Arduino electronics so the arrays of solar panels will
need to need to output voltage regulated to 11.1 volts. I must also figure out how to
I need to wire the solar cells in series to generate enough voltage into a single
array and then wire the arrays of solar cells in parallel. The solar panels must be arranged
in a manner so that they are not prone to flexing and are protected from conditions such
as temperature changes. I must also test the solar panels under various circumstances
(different temperatures, different altitudes, different amounts of shading) to determine
how much more efficient/inefficient the solar panels make the aircraft.
Approach:
To start, I must determine the solar panels I will be using and the dimensions of
the RC plane. Although the specific RC plane has not been determined yet, a plane that is
being looked at has wing area of around 645 sq. in. This area will allow me to get a
general idea of how many solar cells I can put on the wings of the aircraft. However, all
645 sq. in. cannot be covered by solar panels as space is needed for the structure of the
wing and too much additional mass should not be added.
After getting the number of solar cells that I can fit on the win, I must calculate
the theoretical maximum voltage outputted by the connected arrays of solar cells. This
can be done by finding the voltage of the open circuit (where no current flows):
𝑉!" =
𝑘! 𝑇
𝑞
ln
𝐽!"
𝐽!
+ 1
𝐽!" 𝑆ℎ𝑜𝑟𝑡 𝐶𝑖𝑐𝑢𝑖𝑡 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =
𝐼!"
𝐴𝑟𝑒𝑎
Isc is the short circuit current, where the terminals are connected to each other for zero
load resistance. T is the temperature in Kelvin. J0 is a constant and q is the electron
charge. Although this formula can give a general voltage range, specifications from the
manufacturer are needed for additional analysis of the solar panels as many other factors
such as quality of build can affect the actual efficiency of the voltage output. As a result,
18. 18
the theoretical best set up can be calculated with formulas while the actual data for
building the solar panels should be calculated with the experimental data provided by the
manufacturer.
I will also need to determine which solar panels to buy. I need to consider pricing,
voltage and amp rating, strength, and flexibility of the panels. After purchasing the solar
panels, I will wire up and connect the solar panels to the physical aircraft. I will test many
configurations until the longest flight time is obtained.
Components List:
• Solar Cells
• Battery to store power (2100 mAh 11.1V 15C discharge LiPoly)
• Charge Controller
• Power Inverter
Schematics
Solar cells are wired in parallel and then the arrays of the solar cells will be wired in
series and connected to a voltage regulator. This pattern will be repeated throughout the
645 in wing.
The wing will have spur going through it prevent
flexing so the solar panels do not bend
Project Management:
Because the lab will down for the majority of October, I want to complete my
theoretical calculations and purchase the necessary materials before the end the thirty first
of October.
After the materials have arrived, I will help to construct the plane so we can get
control data for the setup without the solar panels. The original plane should be fully
Voltage
Regulator
11.1V
19. 19
constructed and ready to fly before the end of November (11/26). Data for the plane will
be conducted during December (12/19).
During January and February, I will implement the solar panels and test a variety
of configurations and find the most efficient one. For March, I will then compare this
data with the original control data and research on how the solar panels improved or
negatively affected the aircraft. Afterwards, in late spring, I will work on the paper and
presentation for TjStar.
