Automated Solar Radiation Tracking System

8 Mar

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If we could configure a solar cell so that it faces the sun continually as it moves across the sky from east to west, we could get the most electrical energy possible. One way to do this, of course, is by hand. However, keeping a solar cell facing the sun throughout the day is not a very efficient use of a person’s time. Going outside to a solar cell every hour to turn it toward the sun might be possible, but this would still not be an efficient method. A photo sensor is employed to control the solar cell tracking system. For example, if the photo sensor is not aligned with sun rays, then it could turn on the motor until it is once again aligned. If the motor is attached to the frame holding the solar cell, then the solar cell could be moved to face the sun. As long as the photo sensor is in alignment with the sun, nothing happens. However, when the sun moves across the sky and is not in proper alignment with the photo sensor, then a motor moves the frame until the photo sensor is in the sun once more.

This could have the effect of keeping the solar cell facing the sun as it moves across the required human attention. So we need a tracking system that would automatically keep the solar cell facing the sun throughout the day. We have to build an automated system of our own, using a single motor. The system includes a frame on which a solar cell could be mounted. The frame is to move so that it faces the sun as it travels across the sky during the day. The frame could be driven by an electric motor that turns on and off in response to the movement of the sky. Here in this thesis work, panel itself work as a sensor.

Solar Energy

 One of the most important problems facing the world today is the energy problem. This problem is resulted from the increase of demand for electrical energy and high cost of fuel. The solution was in finding another renewable energy sources such as solar energy, wind energy, potential energy…etc. Nowadays, solar energy has been widely used in our life, and it’s expected to grow up in the next years.

Solar energy has many advantages:

1. Needs no fuel

2. Has no moving parts to wear out

3. Non-polluting & quick responding

4. Adaptable for on-site installation

5. Easy maintenance

6. Can be integrated with other renewable energy sources

7. Simple & efficient

Tracking systems try to collect the largest amount of solar radiation and convert it into usable form of electrical energy (DC voltage) and store this energy into batteries for different types of applications. The sun tracking systems can collect more energy than what a fixed panel system collects.

Introduction to Sun Tracker

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. There are many types of solar trackers, of varying costs, sophistication, and performance. One well-known type of solar tracker is the heliostat, a movable mirror that reflects the moving sun to a fixed location, but many other approaches are used as well. The required accuracy of the solar tracker depends on the application. Concentrators, especially in solar cell applications, require a high degree of accuracy to ensure that the concentrated sunlight is directed precisely to the powered device, which is at (or near) the focal point of the reflector or lens. Typically concentrator systems will not work at all without tracking, so at least single-axis tracking is mandatory. Non-concentrating applications require less accuracy, and many work without any tracking at all. However tracking can substantially improve the amount of power produced by a system. The use of trackers in non-concentrating applications is usually an engineering decision based on economics. Compared to photovoltaics, trackers can be relatively inexpensive. This makes them especially effective for photovoltaic systems using high-efficiency panels. For low-temperature solar thermal applications, trackers are not usually used, owing to the relatively high expense of trackers compared to adding more collector area and the more restricted solar angles required for winter performance, which influence the average year-round system capacity. Some solar trackers may operate most effectively with seasonal position adjustment and most will need inspection and lubrication on an annual basis.

 Tracking Techniques

There are several forms of tracking currently available; these vary mainly in the method of implementing the designs. The two general forms of tracking used are fixed control algorithms and dynamic tracking. The inherent difference between the two methods is the manner in which the path of the sun is determined. In the fixed control algorithm systems, the path of the sun is determined by referencing an algorithm that calculates the position of the sun for each time period. That is, the control system does not actively find the sun’s position but works it out given the current time, day, month, and year. The dynamic tracking system, on the other hand, actively searches for the sun’s position at any time of day (or night).Common to both forms of tracking is the control system. This system consists of some method of direction control, such as DC motors, stepper motors, and servo motors, which are directed by a control circuit, either digital or analog.

Relevance of Solar Trackers

 For people living in remote communities, often in third world countries, access to grid-connected electricity is not always possible. Often the nearest utility is a long distance from homes and the cost of developing the infrastructure that would allow for access to the grid is prohibitive. Remote communities in third world countries are of course not the only ones that suffer this dilemma. Australia is a large country with many farmers and communities that are remote from the local grid and in these cases alternative sources of electrical power must be obtained.

To understand the electronic behaviour of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behaviour is well known. An ideal solar cell may be modelled by a current source in parallel with a diode. In practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model.

Materials and Efficiency

 Various materials have been investigated for solar cells. There are two main criteria -efficiency and cost. Efficiency is a ratio of the electric power output to the light power input. Ideally, near the equator at noon on a clear day; the solar radiation is approximately 1000 W/m². So a 10% efficient module of 1 square meter can power a 100 W light bulb. Costs and efficiencies of the materials vary greatly. By far the most common material for solar cells (and all other semiconductor devices) is crystalline silicon.

Crystalline silicon solar cells come in three primary categories:

  1. Single crystal or monocrystalline wafers. Most commercial monocrystalline cells have efficiencies on the order of 14%; the sun power cells have high efficiencies around 20%. Single crystal cells tend to be expensive, and because they are cut from cylindrical ingots, they cannot completely cover a module without a substantial waste of refined silicon. Most monocrystalline panels have uncovered gaps at the corners of four cells.
  2.  Poly or multi crystalline made from cast ingots – large crucibles of molten silicon carefully cooled and solidified. These cells are cheaper than single crystal cells, but also somewhat less efficient. However, they can easily be formed into square shapes that cover a greater fraction of a panel than monocrystalline cells, and this compensates for their lower efficiencies.
  3. Ribbon silicon formed by drawing flat thin films from molten silicon and has a multi crystalline structure. These cells are typically the least efficient, but there is a cost savings since there is very little silicon waste and this approach does not require sawing from ingots.These technologies are wafer based manufacturing. In other words, in each of the above approaches, self supporting wafers of approximate 300 micro metres thick are fabricated and then soldered together to form a module.Thin film approaches are module based. The entire module substrate is coated with the desired layers and a laser scribe is then used to delineate individual cells.

Photovoltaic Cell

 A solar electric module (also known as a ‘panel’) is made up of many PV cells that  are wired together in a series to achieve the desired voltage. The thin wires on the front of the module pick up the free electrons from the PV cell.

  •  A solar cell or a photovoltaic cell, converts sunlight directly into electricity at the atomic level by absorbing light and releasing electrons. This behaviour is a demonstration of the photoelectric effect, a property of certain materials that produce small amounts of electric current when exposed to light.
  • A typical solar cell has two slightly different layers of silicon in contact with each other. When the sun shines on these layers, it causes electrons to move across the junction between the layers, creating an electric current.
  • The top silicon layer in a solar cell is very thin. It includes as a deliberate impurity some atoms of an element that has more electrons than silicon, such as phosphorus. These impurity atoms are called donors, because they can donate or release their extra electrons into the silicon layer as free electrons.
  • The bottom silicon layer in a solar cell is much thicker than the top layer. It has as an impurity some atoms of an element such as boron that has fewer electrons than silicon atoms. These impurity atoms are called acceptors, because relative to the silicon atoms they have “holes” where electrons can be accepted.
  • At the junction where these two layers come together, the donors next to the junction give up their electrons, which migrate across the junction to the adjacent acceptors. This gives the top layer with the donors a net positive charge (because they gave up their excess electrons), and the bottom layer a net negative charge (because the acceptors have their “holes” filled with the excess electrons).
  • When light shines on the layers, atoms in the bottom layer absorb the light and release electrons in accordance with the photoelectric effect. These electrons then migrate to the positively charged top layer. This movement of electrons creates the electrical current from a solar cell that can flow through a circuit with contacts at the two layers.
  • During the central part of the day, the output of the solar cell will be at or near its maximum because the sunlight is arriving at a more direct angle. At the beginning and at the end of the day, the output will fall off regardless of the orientation of the solar cell, mainly because the sunlight has to travel obliquely through the atmosphere at these times, arriving at a low angle. This decreases the intensity of the sunlight.
  • Due to the designing, a solar cell will develop a voltage that is fairly constant. However, the higher the intensity of the sunlight falling on the cell, the more electrical current is produced. This is why a voltmeter connected to a solar cell will have just about the same reading from midmorning to mid afternoon, while a motor connected to the solar cell will run faster during the middle of the day, when the output current is a maximum.

 Photovoltaic Module

 Photovoltaic (PV) modules are devices that cleanly convert sunlight into electricity and offer a practical solution to the problem of power generation in remote areas. They are especially useful in situations where the demand for electrical power is relatively low and can be catered for using a low number of modules. Running lights, a refrigerator and a television in a small home or the powering of water pumps on a remote farming property are examples of tasks that a small array of solar modules can cope with. It has high purchase cost and to keep the number of modules required to a minimum, it is important that the modules produce as much electricity during the hours that they are exposed to sunlight as possible. The solar tracker that has been designed and constructed in this project optimizes the power output of PV modules by making sure that they are pointed towards the sun at all times during the day. The tracker could be implemented in any situation where solar modules are used. It would be especially effective in situations where only a small number of modules are required and where efficiency is of a great importance. Analysis has shown that by using this solar tracker an efficiency increase of about 8% when compared to fixed panels can be obtained.

Solar Tracker Fundamentals

 A solar tracker is a device that is used to align a single P.V module or an array of modules with the sun. Although trackers are not a necessary part of a P.V system, their implementation can dramatically improve a systems power output by keeping the sun in focus throughout the day. Efficiency is particularly improved in the morning and afternoon hours where a fixed panel will be facing well away from the suns rays. P.V modules are expensive and in most cases the cost of the modules themselves will outweigh the cost of the tracker system. Additionally a well designed system which utilizes a tracker will need fewer panels due to increased efficiency, resulting in a reduction of initial implementation costs.

Types of Solar Trackers

There are many different types of solar tracker which can be grouped into single axis and double axis models.

Single Axis Trackers:

Single axis solar trackers can either have a horizontal or a vertical axle. The horizontal type is used in tropical regions where the sun gets very high at noon, but the days are short. The vertical type is used in high latitudes (such as in UK) where the sun does not get very high, but summer days can be very long. These have a manually adjustable tilt angle of 0 – 45 °and automatic tracking of the sun from East to West. They use the PV modules themselves as light sensor to avoid unnecessary tracking movement and for reliability. At night the trackers take up a horizontal position.

Dual Axis Trackers

Double axis solar trackers have both a horizontal and a vertical axle and so can track the Sun’s apparent motion exactly anywhere in the world. This type of system is used to control astronomical telescopes, and so there is plenty of software available to automatically predict and track the motion of the sun across the sky. Dual axis trackers track the sun both East to West and North to South for added power output (approx 40% gain) and convenience.

HARDWARE AND EMBEDDED SOFTWARE

Hardware components

The various hardware components used in this project are listed as follows:-

AT89C52,AT89C2051,MAX232,7805,7812,Crystal,TIP122,BC548,LM324, MCP3202,LCD(16X2).

Description of 8051 Microcontroller

The following are some of the capabilities of 8051 microcontroller.

  •   Internal ROM and RAM
  •   I/O ports with programmable pins
  •   Timers and counters
  •    Serial data communication
  • The 8051 architecture consists of these specific features:
  • 16 bit PC &data pointer (DPTR)
  • 8 bit program status word (PSW)
  • 8 bit stack pointer (SP)
  • Internal ROM 4k
  • Internal RAM of 128 bytes.
  • 4 register banks, each containing 8 registers
  • 80 bits of general purpose data memory
  • 32 input/output pins arranged as four 8 bit ports: P0-P3
  • Two 16 bit timer/counters: T0-T1
  • Two external and three internal interrupt sources Oscillator and    clock circuits.

Stepper motor characteristics

1. Stepper motors are constant power devices.
2. As motor speed increases, torque decreases.
3. The torque curve may be extended by using current limiting drivers and increasing the driving voltage.
4. Steppers exhibit more vibration than other motor types, as the discrete step tends to snap the rotor from one position to another.
5. This vibration can become very bad at some speeds and can cause the motor to lose torque.
6. The effect can be mitigated by accelerating quickly through the problem speeds range, physically damping the system, or using a micro-stepping driver.
7. Motors with a greater number of phases also exhibit smoother operation than those with fewer phases.
Stepper Motor Advantages and Disadvantages

Advantages:

1. The rotation angle of the motor is proportional to the input pulse.

2. The motor has full torque at standstill (if the windings are energized)

3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a step and this error is non cumulative from one step to the next.

4. Excellent response to starting/ stopping/reversing.

5. Very reliable since there are no contact brushes in the motor. Therefore, the life of the motor is simply dependant on the life of the bearing.

6. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.

7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.

8. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Disadvantages:

1. Resonances can occur if not properly controlled.
2. Not easy to operate at extremely high speeds.
Driving a Stepper Motor:

  • It has been seen that out of the five wires two are grouped as common. The other four are the windings that have to give supply to. Major difficulty here is to identify the common line. Just take the multimeter and check the resistance between the wires.Hold one wire as  common and if it bears a resistance of 75 ohms with all the other wires then that is the common wire.

Step Modes

Stepper motor “step modes” include Full, Half and Microstep..

FULL STEP
Standard hybrid stepping motors have 200 rotor teeth, or 200 full steps per revolution of the motor shaft. Dividing the 200 steps into the 360º of rotation equals a 1.8º full step angle. Normally, full step mode is achieved by energizing both windings while reversing the current alternately. Essentially one digital pulse from the driver is equivalent to one step.
HALF STEP
Half step simply means that the step motor is rotating at 400 steps per revolution. In this mode, one winding is energized and then two windings are energized alternately, causing the rotor to rotate at half the distance, or 0.9º. Although it provides approximately 30% less torque, half-step mode produces a smoother motion than full-step mode.
MICROSTEP
Microstepping is a relatively new stepper motor technology that controls the current in the motor winding to a degree that further subdivides the number of positions between poles. Omegamation microstepping drives are capable of dividing a full step (1.8º) into 256 microsteps, resulting in 51,200 steps per revolution (.007º/step). Microstepping is typically used in applications that require accurate positioning and smoother motion over a wide range of speeds. Like the half-step mode, microstepping provides approximately 30% less torque than full-step mode.

SOFTWARE COMPONENTS

The various software components and the programming language used in this project are listed as follows:-

  • Orcad Cadence
  • Keil Microvision Compiler
  • C Language
  • UcFlash+ Burner

Design of the schematic

We begin the working of our project by first creating the schematic diagram of the required circuit.

  1. Creation of stepper motor card by interfacing transistors (BC548,TIP122),and ICs like LM324, RS232, stepper motor, voltage  regulators(7812,7805), diodes , crystals, capacitors and resistances

Creation of main controller by interfacing LCD, A/D convertor, crystal, MCP3202 capacitors, resistances 

 Hardware Implementation of the schematic

The development of the solar radiation tracking involves five phases.

  • Stepper motor phase
  • Lcd Interfacing phase
  • Signal Generation from Panel
  • Analog to Digital Conversion
  • Integration of different modules
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