In remote
areas the sun is a cheap source of electricity because instead of hydraulic
generators it uses solar cells to produce electricity. While the output of
solar cells depends on the intensity of sunlight and the angle of incidence. It
means to get maximum efficiency; the solar panels1 must remain in front of sun
during the whole day. But due to rotation of earth those panels can’t maintain
their position always in front of sun. This problem results in decrease of
their efficiency. Thus to get a constant output, an automated system is
required which should be capable to constantly rotate the solar panel.
The
Automatic Solar Tracking System (ASTS) was made as a prototype to solve the
problem, mentioned above. It is completely automatic and keeps the panel in
front of sun until that is visible. The unique feature of this system is that
instead of take the earth as in its reference, it takes the sun as a guiding
source. Its active sensors constantly monitor the sunlight and rotate the panel
towards the direction where the intensity of sunlight is maximum. In case the
sun gets invisible e.g. in cloudy weather, then without tracking the sun the
ASTS keeps rotating the solar panel in opposite direction to the rotation of
earth. But its speed of rotation is same as that of earth’s rotation. Due to
this property when after sometime e.g. half an hour when the sun again gets
visible,the solar panel is exactly in front of sun. Moreover the system can
manage the errors and also provides the error messages on the LCD display. In
manual mode, through the software (GUI) at computer, the solar panel can be
rotated at any desired angle.
STRUCTURE OF ASTS
ASTS is a hybrid hardware/software project.
The software includes:
• VB 6.0 based GUI.
• Microsoft Access Database.
• Embedded Software (written in C) for microcontroller AT89c52.
The hardware includes:
• Solar panel assembly structure containing six
functional sensors, stepper motor and solar cells.
• System Control Unit containing LCD, Keypad, Error
Indicators and Emergency Stop switch.
• Complete PCB containing two microcontrollers (89c52). First one is the
“Master Microcontroller” which controls the automatic operation of ASTS.
While second one, the “Slave Microcontroller” serially communicates (RS232)
with VB software in
computer.
MOTOR
SELECTION
There are
many types of motor can be selected in ASTS design. Currently, several types of
motors being used in the area of ASTS around the world are Step- motor,
Servo-motor, AC asynchronous motor, permanent magnetic DC servo motor,
permanent magnetic brushless synchronous motor, etc. Generally speaking, as the
gear ratio is high for the transmission system, motor control precision has
very small impact to the tracking precision. For example, for a system with the
gear ratio of 20000:1, the tracker only covers an angle of 0.314mrad when a one
complete circle is finished by the motor. Therefore, all kinds of the motor can
satisfy the precision of the tracking system. However the feature of each type
of motor is different.
The step-motor
has a simple controlling mode and is also low in price. AC servo-motor has the
best performance and wide power range. Its price is also the highest. As for
the performance and price for permanent magnetic DC brushless motor, they are
both rated between step- motor and AC servo-motor. Its performance is close to
servomotor. For the situations that the output torque is not very high (less
than 2 NM), permanent magnetic DC brushless motor is a good option.
SOLAR SENSOR
Dish type
tracking controller and PV tracking controller can be both applied as four-
quadrants solar sensor to correct tracking bias. It is known that solar sensor
will lose its functionality temporally when it’s cloudy. In the area of solar
thermal generation, solar sensor system usually follows the equation based on
the astronomic formula to locate the position of the sun. When a MPU (micro-
processor unit) is applied to calculate the sun’s position, because of its low
process speed and low precision, it’s necessary to include a solar sensor to
make a closed loop system. If the tracking system uses a PC or a
high-performance DSP as the controller, the bias for the calculated sun
position will be within one percent of mrad (milliradian), when the system
clock is precisely set (Direct time from GPS is an option). No solar sensor is
needed to track the sun, especially when the slope error and the gear-diastema
are all small. Exception happens when the motor is a step- motor and the output
torque is not enough. The situation can lead to a blockage of the motor (For
example, a windy weather), which will fail the tracking system to track the sun
precisely. As such, a closed loop solar sensor is recommended in such system. There
are many kinds of solar sensor.
In four quadrant sensor system, the Photovoltaic current will be bigger for the
quadrant of bigger solar facular area, which will indicate whether the sun’s incident
ray is parallel with the axial direction of the sensor, then to adjust the
angle by the motor. It should be stressed during the solar sensor design that
the inside wall of the solar sensor needs to be blacken to avoid misjudgment by
the reflection of sunray inside the solar sensor
WORKING OF ASTS Basic Principle:
The basic functional blocks of this system are
four sensors1, and their operation depends upon the intensity of light falling
on solar panel. All sensors (each with different functionality) send their
output to microcontroller AT89c52. Then the microcontroller executes predefined
task in its software. These sensors are being used with following names and
functionality.
• Step-1
shows that when the sun is in front of solar panel, both sensors i.e. STS-1 and
STS-2 are getting same amount of light.
• In step-2, after some time as the earth rotates the solar panel gets
repositioned with respect to sun and STS-1 obtains less amount of light. At
this point the LDR i.e. STS-1 sends signal to the microcontroller (figure 3).
Then the controller1 rotates motor, resulting the rotation of solar panel
towards the sun.
Night
Time Fault Detector (NTFD) in routine work of the system if a general fault2
occurs during nighttime then the next morning it would not work. So at the next
sunrise, this sensor detects whether the solar panel is ready for tracking or
not. As shown in figure-8, the NTFD is mounted in east of the solar panel so in
normal conditions it does not work because it gets lesser intense light
(predefined) as compared to the middle sensors i.e. STS-1 and STS-2, but as the
fault arises, it starts working. Day Time Fault Detector (DTFD) except some
special conditions e.g. cloudy weather etc, the ASTS is supposed to track the
sun the whole day. If the panel stops rotation then DTFD detects this type of
fault. The mounting strategy of this sensor is same as that of NTFD except that
it is mounted in the west. Night and Cloud Detection In a cloudy day light
intensity is less than a normal day.
CONTROL STRATEGY OF ASTS
For a successful operation, the ASTS has two types of
control approach.
• Automatic Control
• Manual Control
Automatic Control
With the help of an efficient algorithm (written in C) only
one Master Microcontroller1 is being used to manage the automatic operation of
ASTS. This controller has following functions.
• Senses all of six sensors.
• Drives stepper motor.
• Drives LCD.
• Controls the warning indicators e.g. buzzer, LED’s etc.
• Communicates (by parallel port) with the slave microcontroller. The central
driving components of automatic control are only six sensors. Their operation
has been explained on the previous page.
Manual Control
As no human made system is so perfect so an unpredictable
fault may occur in the any system. That is why a manual control option was also
kept in ASTS. While designing this part of control two objectives were kept in
mind:
• The manual control should work efficiently.
• It should be as user friendly as possible. Following two approaches have been
used to accomplish the manual control.
• Stand Alone System Control Unit
• Computer based control unit
Stand Alone System Control Unit
It is a simple user interface, which includes onboard LCD,
Keypad, Buzzer and a complete PCB of the system circuit. The LCD (Hitachi
HD44780) displays different messages, which can help the user in manually
operating the system. While the keypad includes keys of Numeric Digits,
Emergency Stop, clock wise rotation and counter clockwise rotation. Using
keypad a user can manually rotate the solar panel by entering angle from 0o to
180o. The angle value is limited to only 180 values because after sunrise, the
earth almost rotates only 180 degrees and then the sun disappears. The
advantage of this unit is that to run the system it does not need computer but its
disadvantage is that at a time it controls only one solar panel.
ADVANTAGES:
• This automatic solar tracker is easy to
implement since its construction is simple.
• With the implementation the proposed system the additional energy generated
is around 25% to 30% with very less consumption by the system itself.
• The solar panel with the sun in order to extract maximum energy falling on it
renewable energy is rapidly gaining importance as an energy resource as fossil
fuel prices fluctuate.
DISADVANTAGES:
• This
system cannot be used in rainy season.
• Initial cost is high.
APPLICATIONS
• This
system software and hardware can be used to drive a real and very huge solar
panel.
• The computer and System Control Unit would have a wireless communication with
the mechanical structure of solar panel.
• To make emergency control better more powerful microcontrollers e.g. PIC
16F877A would be used.
CONCLUSION
The
designed that system which ensures 25 to 30% of more energy conversion than the
existing static solar module system. Although ASTS is a prototype towards a
real system, but still its software and hardware can be used to drive a real
and very huge solar panel. A small portable battery can drive its control
circuitry.
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