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TRANSCRIPT
Portable Solar Tracker
Tri Bui, Tuyen Bui, Christopher Davis,
Stephen Holman
Department of Electrical Engineering and
Computer Science, University of Central
Florida, Orlando, 32816
Abstract – In order to be a part of the “green
technology” revolution, a goal was established to
design and create an optimized device that would
capture, store, and eventually distribute solar energy.
A concentrator is attached in order to focus a wide
area of light on a small area of solar panels. The
device tracks the path of the sun so that the sun’s rays
are always orthogonal to the solar panels, thus
maximizing the energy captured. A microcontroller
“conducts the symphony” of the peripheral devices
that work in tandem; these devices include a display,
compass, photoresistors, motors, and battery charging.
Once charged, the energy stored in the battery is
capable of powering and charging a small electronic
device such as a cell phone or an iPod®. The device
was also designed to be structurally stable and
relatively lightweight such that it may be considered
portable.
Index terms – Reverse Current/Overcharge
Protection, Trough Concentrator, Mylar,
Photoresistor, Voltage Regulation, Dual Axis Rotation
I. INTRODUCTION
The portable solar tracker is an optimization based
project that involves challenges with power system
monitoring and management, feedback control systems,
circuit design, programming, structural reliability and
practicality, amongst other residual challenges. After
conducting research, it was discovered that there is a
significant difference in the voltage that is output
depending on the angle of the panel with respect to the
sun’s rays, and was thus decided that a primary source
of the optimization would involve the panels tracking
the path of the sun. An efficient, low power system
would be required to move and monitor the panels such
that they remained in a constant perpendicular
orientation. It was also hypothesized that an optical
concentration system would increase energy intake, so a
lightweight focusing contraption needed to be designed.
A picture of the final prototype is shown in Figure 1.
Figure 1a) Side View
Figure 1b) Top View
The device utilizes a two-button interface, one to
reset the stored information and sweep to determine the
sun’s position and another to display the information on
an LCD. The user can then use this information to
determine if there is enough energy to charge his or her
electronic device.
II. THE EFFECTS OF SUNLIGHT ON EXPOSED
SURFACES
The solar radiance given at Earth’s distance from the
Sun, is about 1,368 watts of energy in the form of EM
radiation per square meter. By positioning the
photovoltaic panel on a tracking system, you can
optimize the area that is being exposed to its fullest.
Utilizing the tracking system you can achieve an
increase in the output power by up to 20%. Ideally, the
sun and the panel would have to be orthogonal to each
other in order to achieve that efficiency. The reason is
that at any other angle, the amount of area exposed will
disperse the amount of radiance exposure on it. Figure
2 below shows the effect of exposure on a given area.
Figure 2: Effects of sunlight at 90 degree and 30 degree
III. COMPONENTS
A. Solar Panels
The solar panel array will consist of 16
monocrystalline photovoltaic cells arranged in two
ways, each will consist of 4 cells. The first is an
arrangement of 4 cells that are in parallel with each
other to produce 8 V and 220 mA. As for the other
configuration, it will be 2 cells in series in parallel with
another 2 cells that is in series, making up 16 V with
110 mA.
This is the energy conversion efficiency of the
photovoltaic cells shown in equation 1. Where P
power produced by the cell, E is the irradiance which is
1000 W/m2, and Ac is the area of the cell. With this
equation, the calculated efficiency of each solar panel
14.59 %. The solar tracker’s aim is to help either
maintain peak performance of the solar panel array by
keeping the panel orthogonal to the sun’s ray, or to
further increase the performance with the solar
concentrator. The aim is to try to reach
increase of 20 %.
Equation 1:
This is the energy conversion efficiency of the
photovoltaic cells. Where Pm is the power produced by
the cell, E is the irradiance, and Ac is
cell. The solar panel for the tracker’s fill factor came
out to be .89, which is typical for commercial grade
cells ( usually >.7).
Equation 2:
The LM2598 adjustable switching regulator, an
efficient component, will be used to regulate an input of
16V to 7.3V. A similar switching regulator was used to
light at 90 degree and 30 degree
The solar panel array will consist of 16
photovoltaic cells arranged in two
ways, each will consist of 4 cells. The first is an
arrangement of 4 cells that are in parallel with each
0 mA. As for the other
configuration, it will be 2 cells in series in parallel with
n series, making up 16 V with
This is the energy conversion efficiency of the
photovoltaic cells shown in equation 1. Where Pm is the
power produced by the cell, E is the irradiance which is
he cell. With this
equation, the calculated efficiency of each solar panel is
The solar tracker’s aim is to help either
maintain peak performance of the solar panel array by
keeping the panel orthogonal to the sun’s ray, or to
performance with the solar
he aim is to try to reach a minimum
This is the energy conversion efficiency of the
is the power produced by
is the area of the
cell. The solar panel for the tracker’s fill factor came
out to be .89, which is typical for commercial grade
The LM2598 adjustable switching regulator, an
efficient component, will be used to regulate an input of
16V to 7.3V. A similar switching regulator was used to
regulate an input voltage from the battery to the USB
output, the second switching regulator will
The reason why the switching regulator is used was
because it proves to be more efficient then the linear
regulator. Unlike the linear regulator, the constant
switching (at 150 kHz) that is produce
minimizes wasted energy in the form of heat given off
by the heat sink. Figure 3 shows the switching regulator
waveform and the characteristic switching that occurs
by the regulator. As you can see, it switches from peak
value to zero and then cycles over again.
Figure 3: Oscilloscope picture of the D
Switching regulator.
B. Microcontroller
The components of the solar tracker
controlled by the Atmega168 microcontroller
microcontroller will be powered directly by one of the
solar panel arrays and will be regulated to 5 V
operating voltage. A reset switch is attached for
necessary cases and situations. This IC has
pins, reserved for the photoresistors, and 14
that will control all the other components.
microcontroller was chosen because it has the precise
number of pins for the components that needed it. The
MCU also has 14 KB of flash memory
programming.
C. Motors
In order for the sun's rays to be orthogonal to the sun
at all times and thus provide the most power from the
photovoltaic cells, motors were required to be able to
adjust and move the solar tracker to follow the sun or
any other light source. The advantages and
disadvantages of various motor types and
configurations were analyzed including DC, Servo, and
Stepper motors. Servo motors and in particular two (2)
regulate an input voltage from the battery to the USB
output, the second switching regulator will output 5 V.
The reason why the switching regulator is used was
because it proves to be more efficient then the linear
regulator. Unlike the linear regulator, the constant
switching (at 150 kHz) that is produced by the regulator
form of heat given off
shows the switching regulator
waveform and the characteristic switching that occurs
by the regulator. As you can see, it switches from peak
value to zero and then cycles over again.
Oscilloscope picture of the D-D Adjustable
Switching regulator.
the solar tracker will be
controlled by the Atmega168 microcontroller. The
microcontroller will be powered directly by one of the
arrays and will be regulated to 5 V, the ideal
A reset switch is attached for
This IC has 6 analog
ed for the photoresistors, and 14 digital pins
that will control all the other components. This
microcontroller was chosen because it has the precise
number of pins for the components that needed it. The
flash memory to store the
In order for the sun's rays to be orthogonal to the sun
ll times and thus provide the most power from the
photovoltaic cells, motors were required to be able to
adjust and move the solar tracker to follow the sun or
any other light source. The advantages and
disadvantages of various motor types and
ns were analyzed including DC, Servo, and
Stepper motors. Servo motors and in particular two (2)
Hitec HS-322HD fixed rotation servos were selected.
Specifications that made the HS-322HD ideal for use in
this project include it's low power consumption of
7.4mA current draw idle, and 160mA/60º at 4.8V.
Because of the nature of the solar tracker project, speed
was not a consideration when selecting a motor while
torque on the other hand (3 kg.cm/41.66 oz.in at 4.8V
and 3.7 kg.cm/51.38 oz.in at 6.0V) was a major factor
in its selection because the optical configuration needed
to be moved and then held at specific angles depending
on the location of the light source. There are two
motors used in the solar tracker, one used for
controlling direction along the X-Axis, and one for
controlling direction across the Y-Axis. This is referred
to as a pan and tilt configuration. Figure 4 below shows
the motors and how the pan and tilt works on the solar
tracker.
Figure 4: Motors Demonstrating Pan and Tilt
D. Optical Configuration
The purpose of the optical configuration was to
increase the amount of light that is received incident
onto the photovoltaic cells and thus increasing the
power output by the cells. Different designs were
considered including Plane, Parabolic, and Trough
(open-cylinder) mirror configurations along with lenses
used for focusing light onto a focal point positioned
precisely on the solar panels. The trough design was
eventually determined to be the most efficient method
of increasing the power output of the solar panels after
testing several different methods and combinations of
lenses and mirrors. Figure 5 shows several terms and
properties of the concave mirror design of the trough
shape configuration chosen. The final optical
configuration was constructed using a reflective layer
of Mylar for the mirror like surface which was then laid
on a lightweight wire mesh to allow the frame to be
adjustable so that the focal point could be adjusted if
required. Once the trough was finished the solar panels
were then added both facing the vertex and facing
towards the sun. This configuration allowed the
efficiency of the optical configuration to measure at all
times. Figure 6 below shows the final optical
configuration of the solar tracker. Initial tests concluded
that depending on the time of day and the amount of
cloud cover the efficiency of the optical configuration
ranges from -5% to +5%.
Figure 5: Concave Mirror Properties and Terms
Figure 6: Final Optical Configuration
E. Compass
An additional feature of the solar tracker is to be
able to determine the direction that the tracker is facing
and display that information to the user. The compass
used in this project was the HMC6352 which works
using 2-axis magnetic sensors. Operating anywhere
from 2.7V to 5.2V the HMC6352 features heading
repeatability of 1º, heading resolution of 0.5º, and a
selectable update range from 1Hz to 20Hz. The
compass communicates using the simple I2C
communication protocol which allowed it to be
implemented using only two data lines, serial data
(SDA) and serial clock (SCA). Another reason that the
HMC6352 was the ideal candidate for the solar tracker
was the option of three different unique modes of
operation including Standby, Query, and Continuous.
Using standby mode the power consumption of the
compass was greatly reduced because in standby mode
the compass only performs calculations when data is
requested from the user by the push of a button as
opposed to continuously calculating the direction and
consuming much more power. The compass is
positioned on the frame so that the user easily knows
the direction that the tracker is facing at the push of a
button.
F. Battery
A 7.2V 3300 mAh Nickel Metal-Hydride battery
was chosen to store the energy collected by the solar
panels. Nickel Metal-Hydride exhibited qualities that
were more ideal for this project. It was the second most
lightweight battery type and exhibited the second
highest capacity both next to lithium-ion type batteries.
It was also a relatively cost effective battery type and
was obtained for about half the price of comparable
lithium-ion batteries. A lead-acid battery would have
been a little bit cheaper to purchase, but it would have
been much heavier and detracted from the “portable”
aspect of the device. A major drawback of lithium-ion
batteries that erased them from consideration was that
they exhibit thermal runaway in high ambient
temperatures, not ideal for an outdoor solar device. A
picture of the Ni-MH battery is shown in Figure 7.
Figure 7: Battery Close-up
The negative aspect of this battery type is that it
loses its overall capacity very quickly compared to
other types if it is not charged and discharged correctly.
Therefore the battery needed protection in the forms of
reverse current and overcharge protection. A diode was
installed to prevent reverse current and a relay
controlled by a signal from the microcontroller prevents
overcharge. The microcontroller must be powered on
for the battery to charge and will receive information
from the DS2438 to determine if it should. Two
methods, -∆V and dT/dt, are used to determine when
the battery is fully charged, and at this point the relay
creates an open circuit for the battery. A conceptual
diagram of this protection circuitry is shown in Figure
8.
Figure 8: Relay and Overcharge Protection
The battery has four solar panels dedicated to
powering it, with two in series and two in parallel to
guarantee that the input voltage meets the 7.2V
threshold. This is the threshold set by the adjustable
regulator, and since the sun’s availability is variable it
is best to have a nice cushion of voltage availability
before cut out.
G. Battery Monitoring
The status of the battery is monitored by the DS2438
battery monitoring chip from MAXIM-IC. The device
is an 8 pin surface mount IC that can measure the
battery’s voltage, the amount of current that comes in or
out, and other secondary information such as
temperature and relative humidity. Interfacing it with
the MCU is through a One Wire interface that requires
only a single connection, thus allowing more I/O ports
and read/write commands to perform one at a time. The
line is tied to a 4.7K pull up resistor with 5V power
supply making it an active high when idle. In this One
Wire interfacing scheme, message is sent by producing
low-duration time pulses. To send a 1, the connection is
held low for 15µs and returned to high again. For a 0,
the signal is held low 60µs.
Figure 9: One Wire interface (Reprinted with the
permission of MAXIM-IC)
Figure 10 below shows the circuit configuration needed
for battery monitoring. The positive terminal of the
battery connects to the VDD pin and the negative
terminal is wired to the ground of the DS2438. The
negative terminal of the load connects to the Vsense+
pin making all the current coming into or out of the
battery travel through the current sensing resistor before
going to ground. The voltage across the sense resistor is
use to calculate the current. CF (0.1µF ) and RF (100kΩ)
is a low pass filter that help prevent current spike for an
accurate current accumulation calculation.
Figure: 10 Battery monitoring circuit
(Reprinted with the permission of MAXIM-IC)
H. Photoresistors
To maintain the solar panels perpendicular with the
sun’s rays, photoresistors are used to determine whether
the sun has shifted from its last detected position.
Photoresistor (or photocell) is a type of device that
changes its resistance depending on the amount of
illumination exert on it. As light intensity increases, the
amount of resistance decreases. With this characteristic,
two photoresistors can be used to calculate the sun’s
position in one direction by differentiating their values.
If one resistor has a smaller value than the other
resistor, it means that the sun is locates closer to the
smaller value resistor. If their values are the same, then
it means that the sun is directly above. The solar tracker
contains four photoresistors (one for each direction) to
ensure that the panels are facing in an optimized
direction. To interface the photoresistors with the
MCU, each resistor are place in a voltage divider
configuration. The VT83N1 was chosen because it
outperformed the P203 photoresistor in sensitivity tests
as shown in Figure 11.
Figure 11: Photoresistor comparison
A 5V DC is supplies across the photoresistor and a
1kΩ resistor. The MCU’s A/D input measures the
voltage drop across the 1kΩ, quantize it and compare
with the other photoresistor quantized value
I. Pololu Push-Button Switch
Minimizing the amount of power consumed by the
device is partially handled by a Pololu Push-Button
Switch. The switch serves the purpose of powering the
LCD display and the HMC6352 electronic compass
when the data is requested from the user. Two pins on
the switch are connected to the microcontroller, an off
pin and a voltage out pin. When the off pin is driven
high the LCD display and the compass both receive
power, then are initialized and calculations are
performed on the compass. These calculations are
performed only once, then displayed for approximately
8 seconds. After the 8 seconds have elapsed the off pin
is driven low and power is turned off. The
microcontroller then checks the voltage out pin to
verify that the power is off by turning the voltage out
pin low. The microcontroller then rechecks for the
button to be pressed by the user before driving the off
pin high and performing the calculations and display
again.
J. LCD
To allow the users to know the status of the solar
tracker, an LCD screen is used to display information
such as the battery’s voltage and capacity, the direction
and angle the panels are facing. This type of LCD is a
simple 2X16 character display with no backlight and
has a Hitachi HD44780 controller. The reasons it was
chosen because of its ease in communicating with any
type of microcontrollers and its minimal power
requirements at 2.7V. Normal interfacing of the LCD
requires eight data connections for 8-bit mode, but with
software initialization, it can be interfacing in 4-bit
mode to save I/O pins of the microcontroller.
K. USB Output
To utilize the battery, it is use to charge electronic
devices through a Universal Serial Bus (USB) port. The
reason for having a USB port as a charger because
nowadays many portable electronic devices such as cell
phones, MP3 player and camera use the connection to
simultaneously transfer data and draw power from the
computer. The USB port has four connection pins, they
are: ground, D+, D-, and 5Vdd (see Figure 12). The D+
and D- are used to transfer data and they produce 2.5V
and 2V, respectively. To build a USB port that has
these characteristics, a simple voltage divider circuit
can be used. From the Figure 12b below, the voltage
across 100kΩ and 50kΩ produce 2.5V and 2V, and they
can to power the D+ and D- pin.
Figure 12: a) USB pin assignment b) Circuit
IV. PROGRAMMING
The ATmega168 microcontroller was programmed
using the Arduino development environment. Coding to
control the LCD, motors, and peripheral devices was
made easy because the Arduino environment is built to
support many of these devices and the majority of the
code is open source.
Every Arduino “sketch” (or program) consists of a
‘setup’ and ‘loop’ function. The setup runs once at the
initialization of the device, and this function contains
all of the initializations to which pins are interfacing
with which. It was also desired to have the tracker lock
on to the sun’s position upon reset of the device
(initialized by one of its two buttons) so an algorithm
was written into the setup function to accomplish this.
The loop function constantly cycles once the setup is
finished and continues as long as the microcontroller is
powered. This function contains the code to constantly
monitor the position of the sun and adjust the
orientation of the panels and concentrator when
necessary, it also calls to separate functions to perform
calculations for the temperature and battery state so that
they can be displayed and monitored. Every 20 cycles
the code will monitor the state of the battery and store
the information into an array, the new elements in the
array will be compared with old elements to determine
whether a signal needs to open or close the relay. The
code will use the aforementioned -∆V and dT/dt
methods simultaneously to determine whether or not
the battery should be charging.
voltagearray[x] = MeasADC(_1W_Pin, V_AD);
for(i = x; i > 0; i--)
if(voltagearray[i] < voltagearray[i-1])
dropping++;
temparray[y] =
MeasTemperature_2438(_1W_Pin);
for(j = y; j > 0; j--)
if(temparray[j] > pow(voltagearray[j-1], 1.25))
rising++;
if(dropping >= 3 || rising >= 3)
digitalWrite(relaypin, LOW);
A separate function will also be called upon to
display information to the LCD when the pololu switch
is initiated (the round button labeled “Display”). This
function will convert compass and battery monitoring
information to quantities that can be output in a
pleasing and useful manner to the user. A sample of the
compass code follows:
if(int(headingValue / 10) >= 337 ||
int(headingValue / 10) < 22)
lcd.print("North");
else if(int(headingValue / 10) >= 22 &&
int(headingValue / 10) < 67)
lcd.print("NorthEast");
Information about the state of the battery will also be
useful for the user so he or she can effectively charge
their electronic device via the USB output. The
following code illustrates how the current accumulated
will be calculated to determine and display the
percentage of the remaining battery capacity:
crntacc = CrntAccumulation(_1W_Pin);
capacity = crntacc / (2048 * .05);
battpercent = (capacity / 3.3) * 100;
lcd.print(crntacc);
The capacity variable is returned in amp hours.
Other various factors such as cloud cover and
position resetting in anticipation of sunset will be
anticipated by the code, but some aspects will be part of
the user’s responsibility to maintain optimal operation
of this device. A conceptual flow diagram of the entire
code structure is shown in Figure 13.
Figure 13: Program Concept Diagram
V. PRINTED CIRCUIT BOARD
To interface all the components for a clean and
efficient design, a printed circuit board was designed
and created. The group went to 4pcb.com and had the
Advanced Circuits corporation take care of the
fabrication of the board once schematics were
completed and error checked. Figures 15 and 16 exhibit
samples of the overall schematic. The board serves to
contain and easily interface all the components of the
solar tracker. The printed circuit board consists of 2
layers, a top layer for wire routing, and a bottom layer
which serves as a ground (GND) plane for the board.
Power is routed into the board in two separate places,
on one side to power the microcontroller, lcd, compass,
and servo motors. On the opposite side of the board
power comes in which then distributes to the battery
monitoring chip, the LM2598 switching voltage
regulators, the 7.2V NiMH battery, and eventually
flows to the 5.0V USB DC output. Throughout the
board are also various analog components placed in a
manner to minimize space but also allow for enough
space for soldering and additional component
integration if necessary. The board measures
approximately 9.5" x 4". Figure 14 below shows the
printed circuit board for the solar tracker. Figures
Figure 14: Printed Circuit Board
VI. STRUCTURE DESIGN
Structurally, the solar tracker will take the form of
the trough system. Instead of using the trough system
for solar-thermal energy, this design will be used for
solar concentration. The solar panel will be placed at
the focal point of the concentrator. For this design,
Mylar will be used as the reflective material. The
structure as a whole will be made from acrylic. This
material was tested to determine if it is structurally
sound. The trough will be supported by two support
Figure 15: Microcontroller Schematic close-up
Figure 16: Input Voltage Regulation close-up
bars of acrylic on each side. It will then be situated on
another beam of the same, but thicker material. The
support bar will raise the panel 6.5” from the beam. The
beam is 13” long by 2.5” wide. From there, wheels are
attached to help the base servo rotate while helping to
support the structure utilizing weight distribution. As
for the placement of various components such as the
PCB, they will be situated in a custom-made enclosure
with dimensions (14.5x8.5x4.5)”. Figure 1 depicts the
structure of the solar tracker.
1.<http://www.womdpws/icar/edu/tour/link=/earth/clim
ate/sunradiation_at_earth.html> . 3/31/10
2. Arduino Datasheet
<http://arduino.cc/en/Main/ArduinoBoardDuemilanove
>. 4/2/10
3. Equation 2 provided by: Jenny Nelson. The Physics
of Solar Cells (2003). Imperial College Press.
THE ENGINEERS
Tuyen Bui is a graduating
Electrical Engineer, and has
been working at an electronics
repair shop in recent years. After
graduating, he hopes to find a
job in digital communications.
Christopher Davis is a senior
graduating with a BSEE. After
graduation Chris plans on
traveling the world and honing
his skills as an engineer to work
in the analog devices industry
and/or pursue his Master's
degree in Electrical Engineering.
Stephen Holman is a graduating
Electrical Engineer. He wants to
eventually obtain an MBA. He
hopes to dedicate his career to
the advancement of renewable
energy sources or music
electronics.
Tri Bui is a graduating Electrical
Engineer from UCF. His career
interest is in hardware design. In
the future he hopes to obtain an
MBA or a master’s degree as an
Electrical Engineer.
4. Equation 2 provided by: Introduction to Thermal
Science : Thermodynamics, fluid dynamics, heat
transfer. Frank W,. Schmidt, Robert E. Henderson, Carl
H. Wolgemuth. --2nd ed. ISBN 0-471-54939-8. Page
92.
5. DS2438 Datasheet
<http://pdfserv.maximic.com/en/ds/DS2438.pdf>
REFERENCES