C H A P T E R 3 . B A C K G R O U N D T H E O R Y
S. M. Khaled Ferdous
Eastern University Bangladesh.
Before beginning the design of the tracking system, it was necessary to obtain some background information on solar cells and methods of energy collection. It was equally important to research the various tracking systems available. To obtain this information a study of relevant literature was conducted. This study involved a review of solar cell theory, an investigation into the sources of loss in solar systems and an examination of current tracking methods.
3.2 GENERAL CONCEPT
In 1968 Dr. Peter Glaser in the U.S. Published an idea that centered on the fact that in orbit close to earth, 1.43 KW of solar energy illuminates may one square meter which is considerably greater and one more continuous than an anyone square meter on the Earth which, even when perpendicular to the sun can receive only a maximum of 1 kw. His idea was, converting sunlight to electricity to convert to a radio frequency signal and beamed down to the earth carrying significant levels of energy. This electricity is by establishing a very large array of solar cells in geostationary orbit. A receiving antenna station on the earth would convert this radio frequency back into an alternate current which would be fed into a local grid.
The applications of solar energy which are enjoying most success today are: 1) heating and cooling of residential buildings
C H A P T E R 3 . B A C K G R O U N D T H E O R Y
3) Solar drying of agriculture and animal products 4) Solar distillation on a small community scale
5) Salt production by evaporation of seawater or inland brines 6) Solar cookers
7) Solar engines for water pumping 8) Food refrigeration
9) Bio conversion and wind energy, which are indirect source of solar energy 10) Solar furnaces
11) Solar electric power generation
3.3 Solar Cell Theory
3.3.1 Solar cell physics analysis
Nuclear fusion reactions on the sun's surface supply earth with solar energy. This energy is primarily released in the form of electromagnetic radiation in the ultraviolet, infrared and radio spectral regions (wavelengths from 0.2 to 3m).1 Presently, the most
efficient means of harnessing this power source is the solar cell, which converts solar radiation directly into electricity.
Solar cells are fabricated from various semiconductor materials using numerous device configurations and selecting single-crystal, polycrystalline, and amorphous thin-film structures.1 To following theory considers the silicon p-n junction cell, because it acts
as a reference device for all solar cells.2
The solar cell has a single energy band gap Eg, shown in Figure 1(a). When the cell is
exposed to the solar spectrum, a photon with energy less than Eg makes no contribution
to the cell output. A photon with energy greater than Eg contributes an energy Eg to the
cell output, and the remaining energy is wasted as heat. The idealized equivalent circuit of the cell is shown in Figure 1(b), where a constant-current source is in parallel with the junction. The source I results from the excitation of excess carriers by solar
C H A P T E R 3 . B A C K G R O U N D T H E O R Y
Figure 1. (a) Energy-band diagram of a silicon p-n junction solar cell under solar irradiation.
(b) Idealized equivalent circuit of a solar cell
A typical schematic representation of a solar cell is shown in Figure 2. "It consists of a shallow p-n junction formed on the surface (e.g., by diffusion), a front ohmic contact stripe and fingers, a back ohmic contact that covers the entire back surface, and an anti-reflection coating on the front surface.
Figure: Figure 2. Schematic representation of a silicon p-n junction solar cell.
3.3.2 PRINCIPLES OF OPERATION SOLAR
The solar energy can be directly converted into electrical energy by means of photovoltaic effect, i.e. conversion of light into electricity. Generation of an electromotive force due to absorption of ionizing radiation is known as photovoltaic effect.
The energy conversion devices which are used to convert sunlight to electricity by use of the photovoltaic effect are called solar cells.
Photo voltaic energy conversion is one of the most popular nonconventional energy source. The photovoltaic cell offers an existing potential for capturing solar energy in a way that will provide clean, versatile, renewable energy. This simple device has no moving parts, negligible maintenance costs, produces no pollution and has a lifetime equal to that of a conventional fossil fuel.
Photovoltaic cells capture solar energy and convert it directly to electrical current by separating electrons from their parent atoms and accelerating them across a one way electrostatic barrier formed by the function between two different types of semiconductor material.
3.3.2 Photo Voltaic Effect On Semiconductors
Semi conductors are materials which are neither conductors nor insulators. The photo voltaic effect can be observed in nature in a variety of materials but semiconductors has shown best performance.
When photons from the sun are absorbed in a semiconductor they create for electrons with higher energies than the electrons which provide the boarding in the base
Once these electrons are created, there must be an electric field to induce these higher energy electrons to flow out of the semiconductor to do useful work. The electric field in most solar cells is provided by a junction of materials which have different electrical properties.
To understand more about the functioning and properties of semiconductors, let us briefly discuss. Semi conductors are classified into 1) Extrinsic semiconductor 2) Intrinsic semiconductor. Semiconductors in its purest form are called intrinsic and when impurities are added it is called extrinsic. Further extrinsic semiconductors are divided into p type and N type semiconductor.
3.3.3 P-Type Semiconductor
When a small amount of pentavalent impurities (e.g. Gallium, Indium, Aluminum, and Boron) are added to intrinsic semiconductor, it is called as p type semiconductor.
In p type semiconductor, when an electric potential is applied externally, the holes are directed towards the negative electrode. Hence current is produced.
3.3.4 N- Type Semiconductor
When a small amount of pentavalent impurities (e.g. Antimony, Arsenic, Bismuth, Phosphorus) are added to intrinsic semiconductors it is called N type semiconductor. When an external electrical field is applied the free electrons are directed towards positive electrode. Hence current is produced.
3.3.5 PN Junction Silicon Solar Cell
materials to one half side and N type materials to other half side.
It consists of both types of semiconductor materials. The N type layer is situated towards the sunlight. As N type layer is thin, light can penetrate through it.
The energy of the sunlight will create free electron in the N type material and holes in the p type material. This condition built up the voltage with in the crystal. Because the holes will travel to the +ve region and the holes will travel to the –ve region. This conduction ability is one of the main technical goals in fabricating solar cells.
3.3.6 Purification And Reformation Into Wafers
The purification process basically entails high temperature melting of the sand and simultaneous reduction in the presence of hydrogen. This results in a very pure polycrystalline form of silicon.
The next step is to reform this silicon into a single crystal and then cut the crystal into a single crystal and then cut the crystal into individual wafers. There are two methods namely czochralski growth method and film fed growth. The former method produces single, cylindrical crystals and later produces continuous ribbon of silicon crystals.
Then this cylindrical crystal and ribbon crystal is transformed into disc shaped cells and rectangular cells by slicing. After that one side is doped by exposure to high temperature phosphorus, forming a thin layer of N type material. Similarly p type is made. Electrical contacts are applied to the two surfaces, an anti-reflection coating is added to the entire surface and the entire cell is then sealed with protective skin.
Antireflective coating (arc) is an important part of a solar cell since the bare silicon has a reflection coefficient of 0.33 to 0.54 in the spectral range of 0.35 to 1.1 cm. The arc not only reduces the reflection losses but also lowers the surface
recombination velocity. A single optimal layer of ARC can reduce the reflection to 10 percent and two layers can reduce the reflection up to 3 percent in desired range of wavelengths.
Generally, Arc’s are produced on the solar cell by vacuum evaporation process and the coatings which are tried are SiO2, SiO, Al2O3, TiO2, Ta2O5 and Si3N4. Other
methods of deposition are sputtering, spin-on, spray-on or screen printing. Only the vacuum evaporation sputtering give good results but are expensive. The average reflection can be further reduced by using two antireflective coatings instead of one where the outside (exposed side) coating has an index of refraction 1.3 to 1.6 and the second layer between silicon and the first layer has an index of refraction 2.2 to 2.6. This two layer ARC gives a better impedance match between the index of silicon and the index of air.
3.4 Properties of Photon(http://physics.about.com/od/lightoptics/f/photon.htm)
Under the photon theory of light, a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy. Photons are always in motion and, in a vacuum, have
a constant speed of light to all observers, at the vacuum speed of light (more commonly just called the speed of light) of c = 2.998 x 108 m/s.
The term photon was coined by Gilbert Lewis in 1926, though the concept of light in the form of discrete particles had been around for centuries and had been formalized in Newton's construction of the science of optics.
In the 1800s, however, the wave properties of light (by which I mean electromagnetic radiation in general) became glaringly obvious and scientists had essentially thrown the particle theory of light out the window. It wasn't until Albert Einstein explained the photoelectric effect and realized that light energy had to be quantized that the particle theory returned.
3.5 Maximizing Power Obtained from Solar Cells
Through experiments conducted during research, it was concluded that the current obtained from solar cells is influenced by the angle at which incident rays strike the cell surface. By using a stationary light source and adjusting the angle at which the light rays strike the cell, a plot of current delivered vs. angle of incidence can be created. This property of solar cells is confirmed by the data contained in Table 1, and illustrated by Figure 3. ANGLE OF CURRENT INCIDENCE DELIVERED IN DEGREES IN mAs 0 55 5 54 10 53 15 52 20 51 25 49 30 47 35 44 40 41 45 38 50 35 55 31 60 25
Figure 3. Graph of data contained in Table 1.
After considering the experimental data obtained, it can be stated that, to maintain maximum power output from a solar array, the angle of incidence must be held at zero degrees. Hence the array must constantly face the sun. This requires a tracking system that can continuously align the array into the desired position
Solar Tracker theory:
Introduction of Solar Tracking Techniques
Most of the panel installations that are done in our country are all fixed arrays. As the day passes, the sun moves away from the facing position of the panel and thus the power output of the panel decreases. The easiest way to overcome this problem is to adapt a moveable solar panel using sun tracking mechanism. I have adopted this system to improve the efficiency for photovoltaic cell applications.
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.
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.
Despite the unlimited solar energy, harvesting it is a challenged mainly because of the inefficiency of the panels. Recent works shows that different types of methodology have been proposed to improve the efficiency of solar panels -.
Definition of solar tracker
A solar tracker is a device for rearranging the position of solar PV panel’s face towards the sun to make close possible right angle to sun rays. The position of sun varies in the sky both with time of day as the sun moves across the sky and season. The maximum efficiency of Solar powered equipment depend on the right angle incident of sun ray, more preciously the sun ray’s element photon .So the solar tracker can increase the efficiencies of such equipments over any fixed position solar system, at the considerable cost for additional system complexities.
2.3 Types of tracker
There are various types of solar tracker; some of them are as mentioned below: • Active trackers
• Passive trackers • Chronological tracker • Multi-mirror reflective unit • Horizontal axle solar tracker • Vertical axle solar tracker • Altitude azimuth solar tracker • Two axis mount solar tracker
2.3.6 Active tracker
It uses motors and gear trains to direct the tracker in the direction of the sun. a controller is used to control the motors and the gear trains so that it moves accordingly and the panel faces the sun in the right direction required. The active two axis tracker uses a heliostat – movable mirror that reflects the sunlight towards the absorber of a central power station, or a light sensor to track the sun.
2.3.7 Passive tracker
Use a low boiling point compressed gas fluid that is driven to one side or the other to cause the tracker to move in response to an imbalance. As this is a non-precision orientation it is unsuitable for certain types of concentrating photovoltaic collectors but works fine for common PV panel types.
2.3.8 Chronological tracker
It counteracts the earth’s rotation at an equal rate as the earth, but in the opposite
direction. These trackers are very simple but yet potentially very accurate solar trackers specifically for use with a polar mount. The drive method may be as simple as a gear motor that rotates at a very slow average rate of one revolution per day (15 degrees per hour).
2.3.5 Multi-mirror reflective unit
This device uses multiple mirrors in a horizontal plane to reflect sunlight upward to a high temperature photovoltaic or other system requiring concentrated solar power. Only two drive systems are required for each device. Because of the configuration of the device it is especially suited for use on flat roofs and at low altitudes.
2.3.1 Horizontal axle solar tracker
In this type of tracking system a long horizontal tube is supported on bearing mounted upon the tube and the tube will rotate on the axis to track the apparent motion of the sun through the day. As they do not tilt towards the equator so therefore they are not that much effective in during the winter midday (unless located near the equator), but these tracking system are very much productive in during the spring and summer season when the solar path is high in the sky. The devices are less effective at higher latitudes. The principle advantage is the inherent robustness of the supporting structure and the simplicity of the mechanism. Due to the characteristics of being horizontal the panels can be compactly placed on the axle tube without danger of self-shading and are also readily accessible for cleaning. A single control and motor may be used to actuate multiple rows of panels for active mechanisms.
2.3.2 Vertical axle solar tracker
In this type of tracking system the panels are mounted on a vertical axle at a fixed, adjustable or tracking elevation angle. Such trackers with fixed or (seasonably)
adjustable angles are suitable for high altitudes. This is because at high latitudes the apparent solar path is not especially high but which leads to long days in summer, with the sun traveling through a long arc.
2.3.3 Altitude azimuth solar tracker
Here the mounting is done in such a way so that it supports the entire weight of the solar tracker and allows it to move in both directions and locate a specific target. The horizontal axis (called the azimuth) allows the telescope to move up and down, the axis, vertical, (called the azimuth), allows the telescope to swing in a circle parallel to the ground. This mechanism makes it easy as the telescope can swing around in a circle and then lift to the target. As tracking an object from the earth is more complicated due to the rotational movement of the earth. For this reason computer controlling is required.
2.3.4 Two axis mount solar tracker
In two axis mount, one axis is a vertical pivot shaft or horizontal ring mount that allows the device to be swung to a compass point. The second axis is a horizontal elevation pivot mounted upon the azimuth platform. Using this combination of the two axis any location in the upward hemisphere can be pointed. Such system needs computer control or tracking sensor to control motor drives that orient the panels toward the sun.
2.4 My project mechanism
I have designed and developed duel axis tracker according to azimuth tracking theory with a simple but effective an analogue controller circuit for a two axis solar tracker that will maintain the solar panel orthogonal to the sun, it can rack the sun’s position swiftly is in the sky. The model consists of two parts; the upper part operates through horizontal axis while the lower part operates through vertical axis. Since I arranged the tracker mechanism to operate independently for particular axis, I included two Dc motors for controlling each axis. The design also includes four sensors, comparator with required circuits for controlling the motion and direction of the motor to ensure the direction of the panel towards the sun and position resetting control. The difference in voltage output from the sensors arrangement is fed into the comparator to drives the dc motor in the required specified direction for mechanical action to follow on.
THEORY OF OPERATION
The light tracking system consists of DC motor, a directional light detecting circuit, and an H-bridge to drive the motor. Refer to the Fig (5.1) for the simple block diagram representation.
Fig 4: The Block diagram representation for the circuit.
Two photo resistors are physically mounted on the system so that when the panel (the aluminum plate) is perpendicular to the light source, each receives an equal amount of light. When one receives more light than the other, the panel is not aligned properly and an error voltage results. The error voltage is used as a command to an amplifier circuit to drive the motor and align the panel to be perpendicular to the light source beam. The following subsections describe in detail the mechanical and electrical components of the model.
3.8 Power Supply
To make the light tracking model self sufficient, a plus and minus 12 volt DC power supply is necessary for the electronic components, figure 5.2 shows the electrical circuit which handles the conversion of AC to DC power.
Photo detecting Circuit Op-Amp based Cooperative Output unit H-Bridge & Motor control
Set Point control Two axis solar array mechanism
Fig 5: Electrical circuit used to convert AC to DC Power
In a typical solar application, this DC power source is obtained directly from the PV panels or batteries charged by the PV panels. However, in this case, the DC power is obtained by converting the standard 220 AC (alternating current) power from the wall socket.
The circuit can be divided into three sections; transformation, rectification, and regulation.
1. Transformation is accomplished by the transformer (TR1) which steps down the 220 volts to two 15 volt peak AC sources. In a properly designed circuit, the secondary voltage should be 20 volts peak. The lower voltage level causes some ripple voltage on the output under load conditions of 1.5 amps or greater. The two secondary are tied together at one end to form the common ground of the dc source.
2. Rectification is accomplished by the diode bridge. Although the circuit seems to indicate a bridge rectifier, it is actually two full-wave rectifiers. The right side of the bridge provides positive full-wave rectification while the left side provides negative full-wave rectification.
3. The function of the regulators (7812 and 7912) is to provide a constant output voltage (+12 & -12 respectively. The input voltage required to maintain line regulation is given as 14.6 volts. This makes is clear why the transformer must have a peak voltage of 20 volts. If the peak voltage is 20 volts and the ripple is 5 volts, the lowest voltage that will occur is 15 volts which is still above the specified input voltage to the regulator.
3.9 Concluding Remarks
After careful consideration of the forms of tracking available and the methods of implementing each, it was decided that the preferred tracking system involved an analogue control based duel tracking system using dc motors for alignment. This system has been proposed as it appears offers the satisfied and cost effective and almost maintenance free method of maintaining maximum power output from a solar array.
References are not given since it is only a part of my project documents. Hope all will be provided when I give the full document.