ABSTRACT
A solar energy collecting surface performs best whenever it is faced to the sun. In this research, studies have been carried out to develop a microprocessor based automatic solar tracker locally with least cost. The system has been programmed to detect the intensity of sunlight by a differential arrangement of two photoresistors and subsequently actuate the motor to position the solar panel where it can receive maximum sunlight. This permits the solar panel moving from east to west and returning from west to the east, within the angle regulation range of 0-180° forward and reverse. The designed system can track the better sunlight position within 1 sec. This microcontroller based system with its auxiliaries and stepper motor drive arrangement made it interactive with high degree of dynamic and steady state stability.
Key Words: Renewable energy, photovoltaic, solar tracking system, microcontroller, stepper motor, maximum power capture, closed-loop control.
1. INTRODUCTION
Energy is one of the key building blocks of sustainable development. Even today one third of the world population does not have access to electricity and are not connected to the national grid. Solar, wind, and hydroelectric power not only produce minimal carbon emissions once the generating systems are in place, they also help reduce poverty through improved energy access in underserved areas. Photovoltaic (PV) system will need to play a significant role in the world’s energy mix in 2050 to help achieve global climate change goals at the lowest cost [1]. The International Energy Agency estimates that nearly 50% of global electricity supplies will need to come from renewable energy sources in order to halve carbon dioxide emissions by 2050 and minimize significant, irreversible climate change impacts [2].
Solar panels are generally statically aligned at lattitude tilt angle, they have a fixed position at that certain angle towards the sky; the time and intensity of direct sunlight falling upon the solar panel is greatly reduced, it results in low power output from the PV cells. In order to have maximum power capture throughout the day it is necessary that the photovoltaic panels change their orientation throughout the day following the path of the sun in the sky, this is
Corresponding Author: Dr. Enamul Basher
Email: enamul_basher@eee.buet.ac.bd
possible by means of an automatic solar tracking system. A solar tracker improves the efficiency of solar electric or thermal energy conversion system [3]. The main reason to use a solar tracker is to reduce the cost of the energy captured by the solar panel and other criterion for solar tracker is the efficiency. A tracker produces more power over a longer time than a stationary array with the same number of modules.
From the past decades, several techniques and experiment has been developed for solar tracking device for an alternative source of energy. In this work we present a single axis tracker. It helps maximizing the investment in a PV system. Embedded controlled solar tracker like [4] needs statistical data over a long period of time. Conversely presented prototype avoids all the complex calculations. This scheme can be easily converted to a dual axis tracker. Actually trackers need not point directly at the sun to be effective, and if the aim is off by ten degrees the output is still 98.5% of the full-tracking maximum. Dual axis tracking is effective only for seasonal tracking; therefore single axis tracking is sufficient for effective energy transformation. It has better accuracy. It has less dependence in hardware. It is more feasible than [5, 6]. It is very user friendly, easily reprogrammable and numerous features can be added to it, if required.
2. SOLAR TRACKERS
A Solar tracker is a device for orienting a solar photovoltaic panel or concentrating solar reflector or lens toward the sun. The sun's position in the sky varies both with the seasons (elevation) and time of day as the sun moves across the sky. Solar powered equipment works best when pointed at or near the sun, so a solar tracker can increase the effectiveness of such equipment over any fixed position, at the cost of additional system complexity. The additional output or “gain” can be quantified as a percentage of the output of the stationary array. Gain varies significantly with latitude, climate, and the type of tracker one chooses- as well as the orientation of a stationary installation in the same location. (The energy required to move the tracker is insignificant in these calculations.)
Climate is the most important factor. The more sun and less clouds, moisture, haze, dust, and smog, the greater the gain provided by trackers. At higher latitudes gain will be increased due to the long arc of the summer sun. In the cloudiest, haziest locations the gain in annual output from trackers can be in the low 20 percent range. In a given area the annual gain on any given day may vary from almost zero to nearly 100 percent. In general, a tracker adds most to output during the hours when a stationary array produces the least power.
There are several types of classification of solar tracker. Considering movement capability, three main types of sun trackers can be found-fixed surfaces [7], one axis trackers [8] and two axes trackers [9].The amount energy they produce is varied due to the ability of reducing the pointing error, increasing the daily irradiation that the solar cells receive. Different studies have been made to do a comparative study between the energy available to a two axes tracker, an east–west tracker and a fixed surface [10].
Fig 1: One and two axis tracking PV array [4, 11]
As main results, it concluded that the annual energy available to the ideal tracker is higher by 5–10% and 50% than the east–west tracker and the fixed surface, respectively. Another classification can be made regarding control units, the major types of solar trackers are [12]: passive, microprocessor and electro-optical controlled units. In the first one there is no electronic control or motor [13]. The second ones use mathematical formulae to predict the sun’s movement and need not sense the sunlight. An example of this kind of unit can be found in [9]. And in the last one, the electro-optical controlled units that use the sensing information from sensor like auxiliary bifacial solar cell panel, pyrheliometer to estimate the sun’s real position and use in the control algorithm [8, 14].
Closed-loop types of sun tracking systems
Closed-loop types of sun tracking systems are based on feedback control principles. In these systems, a number of inputs are transferred to a controller from sensors which detect relevant parameters induced by the sun, manipulated in the controller and then yield outputs (i.e. sensor-based). From performance test of sun-tracking systems [15] gain in energy production comparing with a non-tracking systems and error is compared between open-loop and close- loop control sun-trackers. Using open-loop control sun tracker maximum 41% gain has been achieved than the non-tracking modules. Energy production gain can be 10-75% for different type close-loop control sun trackers.
3. FRAMEWORK OF THE SOLAR TRACKER PROTOTYPE
The design we present is based on the following criterions-
· Simplicity
· Low cost
· Easy to construct
· Minimum maintenance
· Reliability
· Less steady state error
· Dynamic output
· High Convergence Speed
· Availability of components
· Independent of PV characteristics
Working methodology of designed solar tracker
This design is developed and implemented using a simplified horizontal-axis and active tracker method fitted to a solar panel. It is able to navigate to the best angle of exposure of
Fig 2: Sun path
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|
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Fig 3: Schematic diagram of single axis high precision solar tracker circuit
light, entailing control of one angle. This tracker implies tracking only in path 1 but not Path 2 (Fig 2); to comprise tracking in path-2 facilitation to control the azimuth and latitude angle is necessary.
The eyes of the Solar Tracker are taken care by the photo resistor. The photo resistors are imperative to detect the amount of illumination; two Cadmium Sulphide (CdS) light sensors are used as comparator of light intensity. When one of the sensors has higher intensity of light, the position of the sun is on the side of that light sensor. In this prototype, the two photocells have been positioned on a small straight piece of plastic; an additional small piece has been placed perpendicular to the straight piece to divide both the sensors. The purpose of doing so is that- if both the photocells are equally illuminated by the sun, their resistance level will be same; if one of the sensors comes under shadow, then the controller of the tracking system will sense the deviation of signal and it will generate actuating signals for drivers to correct misalignment between sun’s ray and the surface normal. The microcontroller is programmed so that it can attain signal from the two light dependent
resistors (LDR) and to move motor either clock wise or anti clock wise depending on which LDR is under shadow, to a position where equal light is being illuminated on both of them.
Obviously in real world solar trackers are not so simple. A solar tracker must be able to reset itself at sunset so it is ready for sunrise.For this two limit switches are placed at both sides of the stand. When the tracking is going on then the motor will rotate in one direction and when the sun goes set the stepper motor will rotate in reverse direction as one of the limit switch will be touched. This is done for tracking the sun for next day morning. The other limit switch is used to give signal when the panel touches it so that the microcontroller can generate a pulse to halts rotation until next sunrise is sensed by the light sensors.
Fig 4: Sensor module Fig 5: Operation of sensor module once an
LDR comes under shadow
Fig 6: System implementation flowchart
Fig 7: Proposed solar tracker prototype
Overall design considerations
In terms of optimal functionality, the barrier between two LDRs should be adjusted based on the location where to implement. To protect the photovoltaic array from damage, protection diodes were used. Two lead acid battery banks may be utilized- only one battery bank will be charged at a time, the other may be employed to run other components of the circuitry. In order to tickle charge the batteries, a voltage within cut off points must be fed to the bank. Voltage regulators may need a proper heat sink to operate smoothly.
4. DESCRIPTION OF MAJOR COMPONENTS
ATmega32 architecture detail
The ATmega32 [16] is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC architecture. By executing powerful instructions in a single clock cycle, the ATmega32 achieves throughputs approaching 1 MIPS per MHz allowing the system designer to optimize power consumption versus processing speed. The AVR core combines a rich instruction set with 32 general purpose working registers.
Configuring ATmega32
The AVR Microcontroller chosen for the system, has sufficient memory to meet the requirements of the design. The analog to digital converters (ADC) is integrated in the AVR which reduced the amount of additional external parts. As program compiler AVR studio is used and Extreme Burner is used to burn the microcontroller.
To configure the microcontroller unit (MCU), 5 Volt digital supply from the voltage regulator is fed to Vcc (Pin 10). Pin 32 is the analog reference signal for the A/D converter. Analog input from LDR-1 and LDR-2 is received by Pin 37 and Pin 38 of port A respectively and converted to digital signal. Port A receives supply voltage through AVCC and a low pass filter is used in this case to use port A as ADC. Built-in compare [5] is there to compare the input values. PB0- PB4 is connected with the driver to provide signaling information to the
motor. Port D is configured to conduct the limit switches through interrupt signals via PD2 (INT0) and PD3 (INT1). Inverting oscillator amplifier receives input signal from XTAL1 (Pin 12) and XTAL2 (Pin 13) of MCU and is configured to be employed as an On-chip Oscillator. Few delays are purposely introduced in order to avoid gratuitous assessment, tracking, rotation and signaling.
Relay
A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts [17]. In this design a relay is used as an automatic switch which enables the current flow from the battery at night and in the day when the sun light is available then the PV output is fed into the battery for charging [18].
High-voltage, high-current Darlington arrays
This is two transistors connected together so that the current amplified by the first is amplified further by the second transistor. The overall current gain is equal to the two individual gains multiplied together: Darlington pair current gain, hFE = hFE1 × hFE2 (hFE1 and hFE2 are the gains of the individual transistors).This gives the Darlington pair a very high current gain, such as 10000, so that only a tiny base current is required to make the pair switch on. A Darlington pair behaves like a single transistor with a very high current gain. In this work ULN2804 [19] is used. The eight NPN Darlington connected transistors in this family of arrays are ideally suited for interfacing between low logic level digital circuitry (such as TTL, CMOS or PMOS/NMOS) and the higher current/voltage requirements of lamps, relays, printer hammers or other similar loads for a broad range of computer, industrial, and consumer applications. All devices feature open–collector outputs and freewheeling clamp diodes for transient suppression.The ULN2803 is designed to be compatible with standard TTL families while the ULN2804 is optimized for 6 to 15 volt high level CMOS or PMOS.
Voltage regulator
A Voltage Regulator (also called a "regulator") has only three legs and appears to be a comparatively simple device but it is actually a very complex integrated circuit.
Fig 8: Schematic diagram of voltage regulator
A regulator converts varying input voltage and produces a constant "regulated" output voltage. Voltage regulators are available in a variety of outputs, typically 5 volts, 9 volts and 12 volts. Voltage regulators are very robust. They can withstand over-current draw due to short circuits and also over-heating. In both cases the regulator will shut down before damage occurs. The only way to destroy a regulator is to apply reverse voltage to its input. In this work LM7805 is used for its excellent thermal overload protection, short circuit protection and output transition so a protection.
4.7 Light detection theory
A light sensor is the most regular electronic equipment. The simplest optical sensor is a photo resistor or photocell which is a light sensitive resistor these are made of two types, cadmium sulfide (CdS) and gallium arsenide (GaAs) [20]. The sun tracker system designed here uses the cadmium sulfide (CdS) photocell for sensing the light. This photocell is a passive component whose resistance is inversely proportional to the amount of light intensity directed towards it. It is connected in series with capacitor. The photocell to be used for the tracker is based on its dark resistance and light saturation resistance. The term light saturation means that further increasing the light intensity to the CdS cells will not decrease its resistance any further [20, 21]. In this work VAC54 [19] is used.
5. TEST AND RESULT
From the experiment testing section, we varied the bulbs in four directions; 15, 35, 55 and 75° respectively. The result of time response and speed are as shown in Table 1.
Table 1: Experimental Result
|
Position of lamp (°) |
Time of positioning to the target (sec) |
Response time of the system (sec) |
|
15 |
1 |
.25 |
|
35 |
1 |
.35 |
|
55 |
1 |
.45 |
|
75 |
1 |
.55 |
Even for variation of small angle this system can response dynamically and no dead band is found. The speed of it position tracking is 1 sec. In order to verify light sensing ability an evaluation is done from the LDR outputs by means of digital display oscilloscope. Figure 9 shows the result when one of the Two LDRs is under shadow. Channel 1 and 2 represent the PA3 and PA4 of ATmega32 respectively. The indication of channel 1 illustrates that LRD1 is under shadow. The signal shown in the figure is sent to the motor drive from the ATmega32. The motor is therefore actuated, and it runs until the resistance on both LDRs is the same. Fig 10 illustrates that, when LDR 1 and LDR 2 both are under shadow i.e. equal sun light is received by both light detector microcontroller puts no comparative output to fed the motor to generate the next pulse. Thus the panel remain in the same position.
Fig 10: Generated ssignals while both LDRs are under shadow
6. FUTURE WORK
The goals of this project were purposely kept within what was believed to be attainable within the allotted timeline. As such, many improvements can be made upon this initial design. That being said, it is felt that this design represents a functioning miniature scale model which could be replicated to a much larger scale. The following recommendations are provided as ideas for future expansion of this project:
• Remedy the motor binding problems due to the photosensor leads. This could be done with some sort of slip ring mechanism, smaller gauge wire, a larger motor with more torque, or a combination of some or all of these ideas.
• Increase the sensitivity and accuracy of tracking by using a different light sensor. A phototransistor with an amplification circuit would provide improved resolution and more précised tracking.
• Utilize a dual-axis design instead of a single-axis to increase PV output to at least 40%.
7. CONCLUSION
Different study shows that the factors that influence the performance of the Solar Home System (SHS) in developing countries e.g. Bangladesh indicates that the overall efficiency is quite low. It makes energy cost significantly high and discourages rapid dissemination of the idea of green energy via solar PV modules.
The sun tracking system which is proposed in this article is able to track the sun under clear sky and partly cloudy sky. In worse case, this tracking system disable when there have not a sun under the overcast sky. Presented sun-tracking scheme results in a considerable saving in energy; use of the step-tracking scheme instead of continuous tracking keeps the motors idle for most of the time which also helps to save energy. It is easy to assemble, portable, light and long-lasting. Tracker’s competency depends upon the solar panel weight and the mechanical frame weight. The tracking system is slightly constrained by wind speed because of the lightness of overall scheme. Predetermining the necessary solar panel height from the base can avoid discretionary rotation of the solar panel. The system has 1 sec response time. But this response time can be reconfigured through programming. The short interval will utilize the full bright sunshine hours and will give better output of solar energy collectors. Its dynamic response, least steady state error and stability make the system a substantial one. A typical 100Wp module will give 21% more electricity as fixed at latitude tilt angle. This reveals that the proposed system is compatible with the additional energy production.
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