Solar Charger

A Project Report On

MICROCONTROLLER BASED SOLAR CHARGER

Submitted at

Rajiv Gandhi Proudyogiki Vishwavidyalaya, Bhopal

In partial fulfilment of the degree

Of

Bachelor of Engineering

In

Electrical & Electronics Engineering

Submitted by

VIKAS KUMAR SINGH

DHEERENDRA

MANOJ RAM


Shri Dadaji Institute of Technology and Science Khandwa

2009-2010

MICROCONTROLLER BASED SOLAR CHARGER 1

SHRI DADAJI INSTITUTE OF TECHNOLOGY AND SCIENCE

CERTIFICATE

SESSION 2009-2010

This is to certify that:

Name of student- VIKAS KUMAR SINGH (0823EX061053)

DHEERENDRA (0823EX061014)

MANOJ RAM (0823EX061030)

Have completed their project work, titled

‘‘MICROCONTROLLER BASED SOLAR CHARGER’’

As per the syllabus and have submitted a satisfactory report on this project as a partial fulfillment towards the degree of

Bachelor of Engineering

In


From

Rajiv Gandhi Proudyogiki Vishwavidyalaya, Bhopal

Mr.R K SHUKLA Miss Pooja Sharma Dr.S.R.Madan Examined ByProject Guide Head of Department Principal


ACKNOWLEDGEMENT

We feel immense pleasure & deep feeling of gratitude towards Miss Pooja Sharma H.O.D. (Electrical and Electronics) Shri Dadaji Institute of Technology & Science for her skillful guidance, constructive and valuable suggestions and encouraging cooperation for our project ,this not merely helped but enabled us to give effort towards this project. The suggestion given by her was timely valuable and helped in focussed orientation of desertion.

We wish to acknowledge our heartily regards to Mr. Ramakant Shukla for his kind cooperation and for suggesting the project and for his constant advice and encouragement during the course.

We owe a debt of gratitude to all the entire management of the institute for elderly advice, encouragement, constructive criticism and support at all the time

VIKAS KUMAR SINGH

DHEERENDRA

MANOJ RAM


Index

Introduction……………………………………………………………………………05

Photovoltaic cell………………….……………………………………………………06

Photo Generation of Charge Carrier…………………………………………………..12

Construction of Selenium & Silicon Solar Cell...........................................................15

Photovoltaic Module.........................................................…………………………...16

Photovoltaic Array…………………………………………………………………….21

Photovoltaic Cell at Glance….………………………………………………………..24

Photovoltaic Electricity…..……………………………………………………………25

Charge Controller.......…………………………………………………………………28

Microcontroller Solar Charger.....................................................................................32

Components of Solar Charger……………..…………………………………………..35

Parts List………………...……………………………………………………………..41

Project Creation………………………………………………………………………..43

Circuit Description…………………………………………………………………….46

Testing of Circuit………………………………………………………………………49

The Road Ahead……………………………………………………………………….50

References……………………………………………………………………………..51


INTRODUCTION

Non conventional power generation is one of the fastest growing sectors. Globally, all countries are busy developing and implementing non-conventional power to bridge the electricity demand and power supply gap.

Solar photovoltaic (PV) power is leading ahead of the other sources. In a solar power generation system, the PV cell plays a major role.

The sun is the ultimate source of limitless solar energy in the form of light and heat. Light of the sun is directly converted into electrical energy without any inter mediate step.

When the rays of the sun strike certain light-sensitive material like solar cell connected to an appropriate circuit, it exhibits a phenomenon called ‘photovoltaic effect’. The photovoltaic effect is the generation of an electrical current in a circuit containing a photosensitive device when the device is illuminated by visible or invisible light. In other words, light is directly converted into electricity. The photovoltaic effect can be achieved by using a variety of materials like silicon, selenium, cadmium sulphide, germanium, gallium arsenide or amorphous glass.


PHOTOVOLTAIC CELL

PV cells were invented in 1953 by Charles Fariett. A PV cell is used for converting photon into electron and with sun light incident, electrical energy is generated. The solar-based battery may be used to directly feed electricity to electronic equipment or for domestic heating. Solar batteries can also be used for satellites, communication equipment and domestic appliances.

A selenium-or silicon-based solar cell exhibits open-circuit voltage of only 0.5V and short-circuit cell current of the order of 1milliampere for 6.4cm² area of the cell at 6458 meter candles. Therefore a large number of such silicon or selenium solar cells need to be connected in series and parallel to provide any significant power. A telemetry system required to operate 24 hours a day requires a solar panel providing 5 watts at 12 volts used for recharging corresponding storage batteries during daylight hours.


TYPES OF PHOTOVOLTAIC CELL

At the present time, most commercial photovoltaic cells are manufactured from silicon, the same material from which sand is made. In this case, however, the silicon is extremely pure. Other, more exotic materials such as gallium arsenide are just beginning to make their way into the field.

The four general types of silicon photovoltaic cells are:

Single-crystal silicon. Polycrystal silicon (also known as multicrystal silicon). Ribbon silicon. Amorphous silicon (abbreviated as "aSi," also known as thin film silicon).

Single-crystal silicon

Most photovoltaic cells are single-crystal types. To make them, silicon is purified, melted, and crystallized into ingots. The ingots are sliced into thin wafers to make individual cells. The cells have a uniform color, usually blue or black

Typically, most of the cell has a slight positive electrical charge. A thin layer at the top has a slight negative charge.

The cell is attached to a base called a "backplane." This is usually a layer of metal used to physically reinforce the cell and to provide an electrical contact at the bottom.

Since the top of the cell must be open to sunlight, a thin grid of metal is applied to the top instead of a continuous layer. The grid must be thin enough to admit adequate amounts of sunlight, but wide enough to carry adequate amounts of electrical energy

Light, including sunlight, is sometimes described as particles called "photons." As sunlight strikes a photovoltaic cell, photons move into the cell.

When a photon strikes an electron, it dislodges it, leaving an empty "hole". The loose electron moves toward the top layer of the cell. As photons continue to enter the cell, electrons continue to be dislodged and move upwards

If an electrical path exists outside the cell between the top grid and the backplane of the cell, a flow of electrons begins. Loose electrons move out the top of the cell and into the external electrical circuit. Electrons from further back in the circuit move up to fill the empty electron holes.


Operation of a Photovoltaic Cell

Most cells produce a voltage of about one-half volt, regardless of the surface area of the cell. However, the larger the cell, the more current it will produce.

Current and voltage are affected by the resistance of the circuit the cell is in. The amount of available light affects current production. The temperature of the cell affects its voltage. Knowing the electrical performance characteristics of a photovoltaic power supply is important, and is covered in the next section.

Polycrystalline silicon

Polycrystalline cells are manufactured and operate in a similar manner. The difference is that a lower cost silicon is used. This usually results in slightly lower efficiency, but polycrystalline cell manufacturers assert that the cost benefits outweigh the efficiency losses.

The surface of polycrystalline cells has a random pattern of crystal borders instead of the solid color of single crystal cells

Ribbon silicon

Ribbon-type photovoltaic cells are made by growing a ribbon from the molten silicon


instead of an ingot. These cells operate the same as single and polycrystal cells.

The anti-reflective coating used on most ribbon silicon cells gives them a prismatic rainbow appearance.

Amorphous or thin film silicon

The previous three types of silicon used for photovoltaic cells have a distinct crystal structure. Amorphous silicon has no such structure. Amorphous silicon is sometimes abbreviated "aSi" and is also called thin film silicon.

Amorphous silicon units are made by depositing very thin layers of vaporized silicon in a vacuum onto a support of glass, plastic, or metal.

Amorphous silicon cells are produced in a variety of colors

Since they can be made in sizes up to several square yards, they are made up in long rectangular "strip cells." These are connected in series to make up "modules." Modules of all kinds are described

Because the layers of silicon allow some light to pass through, multiple layers can be deposited. The added layers increase the amount of electricity the photovoltaic cell can produce. Each layer can be "tuned" to accept a particular band of light wavelength.

The performance of amorphous silicon cells can drop as much as 15% upon initial exposure to sunlight. This drop takes around six weeks. Manufacturers generally publish post-exposure performance data, so if the module has not been exposed to sunlight, its performance will exceed specifications at first.

The efficiency of amorphous silicon photovoltaic modules is less than half that of the other three technologies. This technology has the potential of being much less expensive to manufacture than crystalline silicon technology. For this reason, research is currently under way to improve amorphous silicon performance and manufacturing processes.

In 2002, the highest reported efficiency for thin film solar cells based on CdTe is 18%, which was achieved by research at Sheffield Hallam University, although this has not been confirmed by an external test laboratory.

The US national renewable energy research facility NREL achieved an efficiency of 19.9% for the solar cells based on copper indium gallium selenide thin films, also known as CIGS

NREL has since developed a robot that builds and analyzes the efficiency of thin-film solar


cells with the goal of increasing the efficiency by testing the cells in different situations.

These CIGS films have been grown by physical vapour deposition in a three-stage co-evaporation process. In this process In, Ga and Se are evaporated in the first step; in the second step it is followed by Cu and Se co-evaporation and in the last step terminated by In, Ga and Se evaporation again.

Thin film solar has approximately 15% marketshare; the other 85% is crystalline silicon. Most of the commercial production of thin film solar is CdTe with an efficiency of 11%.

Crystalline Silicon

The highest efficiencies on silicon have been achieved on monocrystalline cells. The highest commercial efficiency (22%) is produced by SunPower, which uses expensive, high-quality silicon wafers. The University of New South Wales has achieved 25% efficiency on monocrystalline silicon in the lab technology that has been commercialized through its partnership with Suntech Power. Crystalline silicon devices are approaching the theoretical limiting efficiency of 29% and achieve an energy payback period of 1-2 years.

By far, the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer.

1. monocrystalline silicon (c-Si): often made using the Czochralski process. Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the four corners of the cells.

2. Poly- or multicrystalline silicon (poly-Si or mc-Si): made from cast square ingots — large blocks of molten silicon carefully cooled and solidified. Poly-Si cells are less expensive to produce than single crystal silicon cells, but are less efficient. US DOE data shows that there were a higher number of multicrystalline sales than monocrystalline silicon sales.

3. Ribbon silicon is a type of multicrystalline silicon: it is formed by drawing flat thin films from molten silicon and results in a multicrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots.

MICROCONTROLLER BASED SOLAR CHARGER10

Basic structure of a silicon based solar cell and its working mechanism

PHOTOGENERATION OF CHARGE CARRIERS


http://upload.wikimedia.org/wikipedia/en/d/d7/Silicon_Solar_cell_structure_and_mechanism.svg

When a photon hits a piece of silicon, one of three things can happen:

1. the photon can pass straight through the silicon — this (generally) happens for lower energy photons,

2. the photon can reflect off the surface, 3. the photon can be absorbed by the silicon, if the photon energy is higher than the silicon

band gap value. This generates an electron-hole pair and sometimes heat, depending on the band structure.

When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighbouring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one fewer electron — this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighbouring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs.

A photon need only have greater energy than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations — called phonons) rather than into usable electrical energy.

CHARGE CARRIER SEPARATION


http://en.wikipedia.org/wiki/Phonons

http://en.wikipedia.org/wiki/Earth

http://en.wikipedia.org/wiki/Black_body

http://en.wikipedia.org/wiki/Frequency_spectrum

http://en.wikipedia.org/wiki/Conduction_band

http://en.wikipedia.org/wiki/Valence_band

http://en.wikipedia.org/wiki/Band_gap

http://en.wikipedia.org/wiki/Photon

There are two main modes for charge carrier separation in a solar cell:

1. drift of carriers, driven by an electrostatic field established across the device 2. diffusion of carriers from zones of high carrier concentration to zones of low carrier

concentration (following a gradient of electrochemical potential).

In the widely used p-n junction solar cells, the dominant mode of charge carrier separation is by drift. However, in non-p-n-junction solar cells (typical of the third generation solar cell research such as dye and polymer solar cells), a general electrostatic field has been confirmed to be absent, and the dominant mode of separation is via charge carrier diffusion

.

THE P-N JUNCTION


http://en.wikipedia.org/w/index.php?title=Charge_carrier_diffusion&action=edit&redlink=1

http://en.wikipedia.org/wiki/Polymer_solar_cell

http://en.wikipedia.org/wiki/Dye_solar_cell

http://en.wikipedia.org/wiki/Third_generation_solar_cell

http://en.wikipedia.org/wiki/P-n_junction

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon. In practice, p-n junctions of silicon solar cells are not made in this way, but rather by diffusing an n-type dopant into one side of a p-type wafer (or vice versa).

If a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction). When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. The diffusion of carriers does not happen indefinitely, however, because charges build up on either side of the junction and create an electric field. The electric field creates a diode that promotes charge flow, known as drift current, that opposes and eventually balances out the diffusion of electron and holes. This region where electrons and holes have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. It is also known as the space charge region.

CONSTRUCTION OF SELENIUM & SILICON SOLAR CELL


http://en.wikipedia.org/wiki/Depletion_zone

http://en.wikipedia.org/wiki/Drift_current

http://en.wikipedia.org/wiki/Diode

http://en.wikipedia.org/wiki/Electric_field

http://en.wikipedia.org/wiki/Diffusion

The surface area perpendicular to the cell is maximum. The metallic ring connected to the

maximum number of photons of incident light reaches the junction. The spectral response of

silicon extends well into the infra red region. Hence its conversion efficiency is relatively high (aproximetaly14%).

The conversion efficiency of the solar cell is defined as:

O/P electrical power / I/P of electrical power

The selenium solar cell has a much longer life and withstands damaging radiation

environments. It is commonly used in photometric equipment, exposure meters and certain

cameras to control the automatic exposure diaphragm. However, one disadvantage is that the selenium solar cell is 1/20th of a silicon cell.

Array configuration:

PV cell can be connected in series as well as parallel for different applications.

Cell, module and array of a PV system:

A number of cells connected in single-board combination of series and parallel are called ‘module.’ A number of modules connected in single-board combination of series and parallel are called an ‘array.’

Solar PV array design:

Globally, following configurations of PV cells are available:

Array voltage (V) 12 24 36 48

O/P current (A) 41.67 20.83 13.89 10.42

PHOTOVOLTAIC MODULE


For almost all applications, the one-half volt produced by a single cell is inadequate. Therefore, cells are connected together in series to increase the voltage. Several of these series strings of cells may be connected together in parallel to increase the current as well.

These interconnected cells and their electrical connections are then sandwiched between a top layer of glass or clear plastic and a lower level of plastic or plastic and metal. An outer frame is attached to increase mechanical strength, and to provide a way to mount the unit. This package is called a "module" or "panel". Typically, a module is the basic building block of photovoltaic systems.

Describing Photovoltaic Module Performance:

To insure compatibility with storage batteries or loads, it is necessary to know the electrical characteristics of photovoltaic modules.

As a reminder, "I" is the abbreviation for current, expressed in amps. "V" is used for voltage in volts, and "R" is used for resistance in ohms.

A photovoltaic module will produce its maximum current when there is essentially no resistance in the circuit. This would be a short circuit between its positive and negative terminals.

This maximum current is called the short circuit current, abbreviated I(sc). When the module is shorted, the voltage in the circuit is zero.

Conversely, the maximum voltage is produced when there is a break in the circuit. This is called the open circuit voltage, abbreviated V(oc). Under this condition the resistance is infinitely high and there is no current, since the circuit is incomplete.

These two extremes in load resistance, and the whole range of conditions in between them, are depicted on a graph called a I-V (current-voltage) curve. Current, expressed in amps, is on the vertical Y-axis. Voltage, in volts, is on the horizontal X-axis.

The short circuit current occurs on a point on the curve where the voltage is zero.The open circuit voltage occurs where the current is zero.

The power available from a photovoltaic module at any point along the curve is expressed in watts. Watts are calculated by multiplying the voltage times the current (watts = volts x amps, or W = VA).

At the short circuit current point, the power output is zero, since the voltage is zero. At the open circuit voltage point, the power output is also zero, but this time it is because the current is zero.


The short circuit current occurs on a point on the curve where the voltage is zero.The open circuit voltage occurs where the current is zero.

The power available from a photovoltaic module at any point along the curve is expressed in watts. Watts are calculated by multiplying the voltage times the current (watts = volts x amps, or W = VA).

At the short circuit current point, the power output is zero, since the voltage is zero.

At the open circuit voltage point, the power output is also zero, but this time it is because the current is zero.

There is a point on the "knee" of the curve where the maximum power output is located. This point on our example curve is where the voltage is 17 volts, and the current is 2.5 amps. Therefore the maximum power in watts is 17 volts times 2.5 amps, equaling 42.5 watts.

The power, expressed in watts, at the maximum power point is described as peak, maximum, or ideal, among other terms. Maximum power is generally abbreviated as "I (mp)." Various manufacturers call it maximum output power, output, peak power, rated power, or other terms.

The current-voltage (I-V) curve is based on the module being under standard conditions of sunlight and module temperature. It assumes there is no shading on the module.

Standard sunlight conditions on a clear day are assumed to be 1000 watts of solar energy per square meter (1000 W/m2or lkW/m2). This is sometimes called "one sun," or a "peak sun." Less than one sun will reduce the current output of the module by a proportional amount. For example, if only one-half sun (500 W/m2) is available, the amount of output current is roughly cut in half


For maximum output, the face of the photovoltaic modules should be pointed as straight toward the sun as possible

Because photovoltaic cells are electrical semiconductors, partial shading of the module will cause the shaded cells to heat up. They are now acting as inefficient conductors instead of electrical generators. Partial shading may ruin shaded cells.

Partial module shading has a serious effect on module power output. For a typical module, completely shading only one cell can reduce the module output by as much as 80%. One or more damaged cells in a module can have the same effect as shading.

This is why modules should be completely unshaded during operation. A shadow across a module can almost stop electricity production. Thin film modules are not as affected by this problem, but they should still be unshaded.

Module temperature affects the output voltage inversely. Higher module temperatures will reduce the voltage by 0.04 to 0.1 volts for every one Celsius degree rise in temperature (0.04V/0C to 0.1V/0C). In Fahrenheit degrees, the voltage loss is from 0.022 to 0.056 volts


per degree of temperature rise.

This is why modules should not be installed flush against a surface. Air should be allowed to circulate behind the back of each module so it's temperature does not rise and reducing its output. An air space of 4-6 inches is usually required to provide proper ventilation.

The last significant factor which determines the power output of a module is the resistance of the system to which it is connected. If the module is charging a battery, it must supply a higher voltage than that of the battery.

If the battery is deeply discharged, the battery voltage is fairly low. The photovoltaic module can charge the battery with a low voltage, shown as point #1 in Figure 2-20. As the battery reaches a full charge, the module is forced to deliver a higher voltage, shown as point #2. The battery voltage drives module voltage.


Eventually, the required voltage is higher than the voltage at the module's maximum power point. At this operating point, the current production is lower than the current at the maximum power point. The module's power output is also lower.

To a lesser degree, when the operating voltage is lower than that of the maximum power point, the output power is lower than the maximum. Since the ability of the module to produce electricity is not being completely used whenever it is operating at a point fairly far from the maximum power point, photovoltaic modules should be carefully matched to the system load and storage.

Using a module with a maximum voltage which is too high should be avoided nearly as much as using one with a maximum voltage which is too low.

The output voltage of a module depends on the number of cells connected in series. Typical modules use either 30, 32, 33, 36, or 44 cells wired in series.

The modules with 30-32 cells are considered self regulating modules. 36 cell modules are the most common in the photovoltaic industry. Their slightly higher voltage rating, 16.7 volts, allows the modules to overcome the reduction in output voltage when the modules are operating at high temperatures.

Modules with 33 - 36 cells also have enough surplus voltage to effectively charge high antimony content deep cycle batteries. However, since these modules can overcharge batteries, they usually require a charge controller.

Finally, 44 cell modules are available with a rated output voltage of 20.3 volts. These modules are typically used only when a substantially higher voltage is required.

Another application for 44 cell modules is a system with an extremely long wire run between the modules and the batteries or load. If the wire is not large enough, it will cause a significant voltage drop. Higher module voltage can overcome this problem.

It should be noted that this approach is similar to putting a larger engine in a car with locked brakes to make it move faster. It is almost always more cost effective to use an adequate wire size, rather than to overcome voltage drop problems with more costly 44 cell modules.


PHOTOVOLTAIC ARRAY

In many applications the power available from one module is inadequate for the load. Individual modules can be connected in series, parallel, or both to increase either output voltage or current. This also increases the output power.

When modules are connected in parallel, the current increases. For example, three modules which produce 15 volts and 3 amps each, connected in parallel, will produce 15 volts and 9 amps.

If the system includes a battery storage system, a reverse flow of current from the batteries through the photovoltaic array can occur at night. This flow will drain power from the batteries

.

TYPES OF ARRAY


Portable arrays

A portable array may be as small as a one square foot module easily carried by one person to recharge batteries for communications or flashlights. They can be mounted on vehicles to maintain the engine battery during long periods of inactivity. Larger ones can be installed on trailers or truck beds to provide a portable power supply for field operations.

Personal Photovoltaic Array

Portable Power Supply

Tracking arrays

Arrays that track, or follow the sun across the sky, can follow the sun in one axis or in two. Tracking arrays perform best in areas with very clear climates. This is because following the sun


yields significantly greater amounts of energy when the sun's energy is predominantly direct. Direct radiation comes straight from the sun, rather than the entire sky.

Normally, one axis trackers follow the sun from the east to the west throughout the day. The angle between the modules and the ground does not change. The modules face in the "compass" direction of the sun, but may not point exactly up at the sun at all times.

Two axis trackers change both their east-west direction and the angle from the ground during the day. The modules face straight at the sun all through the day. Two axis trackers are considerably more complicated than one axis types.

One Axis and Two Axis Tracking Arrays

PV CELLS AT A GLANCE


ADVANTAGES

Salient and low maintenance Solid states with no moving parts High-quality powers Life times of 20 to30 years Flexible and reliable Modular and expandable

APPLICATIONS

Coast guard navigation Off-grid and grid- connected homes Rail road signaling Satellites and space station Out door lightning’s Telecommunication and water pumping, etc.

PV ELECTRICITY

DC ELECTRICITY:


Photovoltaic electricity is DC (Direct Current). The current has a polarity, that is, it flows in one direction. This has an impact on wiring methods and equipment. In photovoltaic systems, grounding methods must be complete and correct. Wire colour conventions are critical, not only to protect equipment from reverse polarity, but also to protect service personnel and system users.

CURRENT LIMITED SYSTEM :

When the electrical lines from a utility company's AC power supply are crossed, the resultant short circuit causes an almost infinite current flow. For this reason, fuses and circuit breakers are used to provide over-current protection. Photovoltaic modules are current-limited. A short-circuited photovoltaic module will produce current only up to a certain level. In fact, a common check of system performance is to deliberately short-circuit the photovoltaic modules and measure the current flow. This does not damage the modules.

Short circuit in AC circuit (top) & Photovoltaic Module (bottom)

LOW VOLTAGE DOES NOT MEAN HARMLESSNESS:

Whenever working on or around photovoltaic systems remember three very important points:

1) Even at low voltages, photovoltaic systems may be able to deliver substantial current. The amount of available current may be high enough to kill you.

2) Photovoltaic systems can have two power supplies, not just one. Both the batteries and the modules in a system can deliver current.


3) Small "harmless" shocks can still injure you. For example, an arc created when making a wiring connection can ignite the hydrogen gas given off by storage batteries, causing an explosion. Likewise, a small shock can startle you, resulting in a fall from a ladder.

VOLTAGE DROP:

Unlike most AC systems, photovoltaic systems can suffer from a substantial voltage drop between the power source and the load. Good design practices minimize this drop.

As an extreme example, the available voltage at the photovoltaic array might be 16 volts. After travelling through hundreds of feet of undersized wire, it could be as low as 11 volts.

The system would not be able to recharge a 12 volt storage battery. This is because the available voltage is not higher than the voltage of the battery.

Wire runs must be kept as short as possible. Wire must be large enough to minimize the voltage drop. Use the charts in Appendix C to determine these sizes. Notice that the wire sizes in photovoltaic systems are much larger than those in AC systems

CONNECT/DISCONNECT SEQUENCE:

Unlike AC systems, the sequence of connection and disconnection is critical to many photovoltaic system components.

It should be noted that explosive hydrogen gas may be present near batteries. Making the last connection at the battery may create a spark which could result in an explosion. The best sequence of battery terminal connection might be as follows:

1) positive connection at battery.

2) positive connection at load.

3) negative connection at battery.

4) negative connection at load.

Other components are equally sensitive to connect/disconnect sequences. Charge controllers, in particular, may need to be connected in the correct sequence to prevent damage.


Battery Terminal Connection Sequence

CHARGE CONTROLLERS

The primary function of a charge controller in a stand-alone PV system is to protect the battery from overcharge and over discharge. Any system that has unpredictable loads, user intervention, optimized or undersized battery storage (to minimize initial cost), or any


characteristics that would allow excessive battery overcharging or over discharging requires a charge controller and/or low-voltage load disconnect. Lack of a controller may result in shortened battery lifetime and decreased load availability.

Systems with small, predictable, and continuous loads may be designed to operate without a battery charge controller. If system designs incorporate oversized battery storage and battery charging currents are limited to safe finishing charge rates (C/SO flooded or C/1OO sealed) at an appropriate voltage for the battery technology, a charge controller may not be required in the PV system.

Proper operation of a charge controller should prevent overcharge or over discharge of a battery regardless of the system sizing/design and seasonal changes in the load profile and operating temperatures. The algorithm or control strategy of a battery charge controller determines the effectiveness of battery charging and PV array utilization, and ultimately the ability of the system to meet the load demands. Additional features such as temperature compensation, alarms, and special algorithms can enhance the ability of a charge controller to maintain the health, maximize capacity, and extend the lifetime of a battery.

BASICS OF CHARGE CONTROLLER THEORY

While the specific control method and algorithm vary among charge controllers, all have basic parameters and characteristics. Manufacturer's data generally provides the limits of controller application such as PV and load currents, operating temperatures, losses, set points,


and set point hysteresis values. In some cases the set points may be intentionally dependent upon the temperature of the battery and/or controller, and the magnitude of the battery current. A discussion of the four basic charge controller set points follows:

Regulation set point (VR): This set point is the maximum voltage a controller allows the battery to reach. At this point a controller will either discontinue battery charging or begin to regulate the amount of current delivered to the battery. Proper selection of this set point depends on the specific battery chemistry and operating temperature.

Regulation hysteresis (VRH): The set point is voltage span or difference between the VR set point and the voltage when the full array current is reapplied. The greater this voltage span, the longer the array current is interrupted from charging the battery. If the VRH is too small, then the control element will oscillate, inducing noise and possibly harming the switching element. The VRH is an important factor in determining the charging effectiveness of a controller.

Low voltage disconnect (LVD): The set point is voltage at which the load is disconnected from the battery to prevent over discharge. The LVD defines the actual allowable maximum depth-of-discharge and available capacity of the battery. The available capacity must be carefully estimated in the system design and sizing process. Typically, the LVD does not need to be temperature compensated unless the batteries operate below 00C on a frequent basis. The proper LVD set point will maintain good battery health while providing the maximum available battery capacity to the system.

Low voltage disconnect hysteresis (LVDH): This set point is the voltage span or difference between the LVD set point and the voltage at which the load is reconnected to the battery. If the LVDH is too small, the load may cycle on and off rapidly at low battery state-of-charge, possibly damaging the load and/or controller. If the LVDH is too large, the load may remain off for extended periods until the array fully recharges the battery. With a large LVDH, battery health may be improved due to reduced battery cycling, but this will reduce load availability. The proper LVDH selection will depend on the battery chemistry, battery capacity, and PV and load currents.

CHARGE CONTROLLER ALGORITHM

Two basic methods exist for controlling or regulating the charging of a battery from a PV module or array - series and shunt regulation. While both of these methods can be effectively used, each method may incorporate a number of variations that alter basic performance and applicability.


Following are descriptions of the two basic methods and variations of these methods.

Shunt controller

A shunt controller regulates the charging of a battery by interrupting the PV current by short-circuiting the array. A blocking diode is required in series between the battery and the switching element to keep the battery from being shortened when the array is shunted. This controller typically requires a large heat sink to dissipate power. Shunt type controllers are usually designed for applications with PV currents less than 20 amps due to high current switching limitations

Shunt-linear:

This algorithm maintains the battery at a fixed voltage by using a control element in parallel with the battery. This control element turns on when the VR set point is reached, shunting power away from the battery in a linear method (not on/off), maintaining a constant voltage at the battery. This relatively simple controller design utilizes a Zener power diode which is the limiting factor in cost and power ratings.

Block Diagram of Linear and Switching Shunt Charge Controllers

Shunt-interrupting:

This algorithm terminates battery charging when the VR set point is reached by short-circuiting the PV array. This algorithm has been referred to as "pulse charging" due to the pulsing effect when reaching the finishing charge state. This should not be confused with Pulse-Width Modulation (PWM).


Series Controller

Several variations of this type of controller exist, all of which use some type of control element in series between the array and the battery.

Series-interrupting: This algorithm terminates battery charging at the VR set point by open-circuiting the PV array.

MICROCONTROLLER BASED SOLAR CHARGER

As the sources of conventional energy deplete day by day, resorting to alternative sources of energy like solar and wind energy has become need of the hour.

Solar-powered lighting systems are already available in rural as well as urban areas. This includes solar lanterns, solar home lightning systems, solar street lights, solar garden lights


and solar power backs. All of them consist of four components: solar photovoltaic module, rechargeable battery, solar charge controller and load.

In the solar powered lightning system, the solar charge controller plays an important role as the system’s overall success depends mainly on it. It is considered as in indispensable link between the solar panel, battery and load.

The-microcontroller-based solar charge controller described here as the following features:

1. Automatic dusk-to-dawn operation of the load.2. Built-in digital voltmeter (0V-20V range).3. Parallel-or shunt-type regulation.4. Over charge protection.5. Low battery lock.6. Charging current changes to ‘pulse’ at full charge.7. Low current consumption.8. System status display on LCD.9. Highly efficient design based on microcontroller..


Circuit of microcontroller based solar charger


Actual size PCB layout for microcontroller based solar charger

Component layout of PCB

COMPONENTS OF SOLAR CHARGER


Microcontroller:

Microcontroller AT89C2051 is the heart of the circuit. It is a low voltage, high-performance, 8-bit microcontroller that features 2KB of flash, 128 bytes of RAM, 15 input/output (I/O) lines, two 16-bit timers/counters, a five-vector two-level interrupt architecture, on-chip oscillator and clock circuitry. A 12MHz crystal is use for providing the basic clock frequency. All I/O pins are reset to ‘1’ as soon as RST pin goes high. Holding RST pins high for two machine cycles, while the oscillator is running, reset the device. Power-on reset is derived from resister R1 and capacitor C4. Switch S2 is used for manual reset.

Pin Description


Block Diagram of Microcontroller


Serial ADC:

The microcontroller monitors the battery voltage with the help of an analogue-to-digital converter. The ADC0831 is an 8-bit successive approximation analogue-to-digital converter with a serial I/O and very low conversion time of typically 32 µs. The differential analogue voltage input allows increase of the common mode rejection and offsetting of the analogue zero input voltage. In addition, the voltage reference can be adjusted to allow encoding of any smaller analogue voltage span to the full eight bits of resolution. It is available in an 8-pin PDIP package and can be interfaced to microcontroller with only three wires.


Functional Block Diagram of ADC


Sequence of Operation of ADC:


LCD module:

The system status and battery voltage are displayed on an LCD based on HD44780 controller. The backlight feature of the LCD makes it readable even in low light conditions. The LCD is used here in 4-bit mode to save the microcontroller’s port pins. Usually the 8-bit mode of interfacing with a microcontroller requires eleven pins, but in 4-bit mode the LCD can be interfaced to the microcontroller using to the seven pins.

Solar Panel:

The solar panel used is meant to charge a 12V battery and the wattage can range from 10 to 40 watts. The peak unload voltage output of the solar panel will be around19 volts. Higher-wattage panel can be used with some modifications to the controller unit.

Rechargeable battery:

The solar energy is converted into electrical energy and stored in a 12V lead-acid battery. The ampere-hour capacity ranges from 5Ah to 100Ah.

Dusk-to-down sensor:

Normally, in a solar-photovoltaic-based installation the load switched on a dusk and switched off at down. During daytime, the load from the disconnected to the battery and the battery is recharged with current from the solar panel. The micro controller needs to know the presence of solar panel voltage to decide whether the load is to be connected to or disconnected from the battery, or whether the battery should be in charging mode or discharging mode. A simple sensor circuit is built using a potential divider formed around resistors R8 and R9, zener diode and transistor T1 for the presence of panel voltage.

Charge Control:

Relay RL1 connects the solar panel to the battery through diode D1. Under normal conditions, it allows the charging current from the panel to flow into battery. When the battery is at full charge (14.0V), the charging current becomes ‘pulsed.’ To keep the overall current consumption of the solar controller low, normally-closed (N/C) contacts of the relay are used and the relay is normally in de-energized state.

Load Control:


One terminal of the load is connected to the battery through fuse F1 and another terminal of the load to an n-channel POWER MOSFET T3. MOSFETs are voltage driven devices that required virtually no drive current. The load current should be limited to 10A. One additional MOSFET is connected to parallel for more than 10A load current. PARTS LIST

Semiconductors:

IC1 - AT89C2051 microcontroller

IC2 - ADC0831 analogue-to-digital converter

IC3 - MCT2E optocoupler

IC4 -7805, 5V regulator

T1 - BC547 NPN transistors

T2 - BS170 n-channel MOSFET

T3 - IRF540 n-channel MOSFET

D1 - 6A4 rectifier diode

D2-D4 - 1N4007rectifier diode

ZD1 - 7.5V zener diode

Resistors (all ¼-watt, ±5% carbon):

R1 - 8.2-kilo-ohm

R2 - 1.2-kilo-ohm

R3, R4, R6-R11 - 10-kilo-ohm

R5 - 20-kilo-ohm

R12 - 330-ohm


Capacitors:

C1 - 100µf, 63V electrolytic

C2 - 100µf, 16V electrolytic

C3, C7 - 0.1 µf, ceramic disk

C4, C9 - 10 µf, 16V electrolytic

C5, C6 - 33pf ceramic disk

C8 - 0.01 µf, ceramic disk

Miscellaneous:

S1 - On/Off switch

S2 - Push-to-on-switch

RL1 - 12V, 1C/O relay

Xtal - 12MHzcrystal

LCD - 16 2 line display

Solar Panel - 10-40W

- 10A fuse


PROJECT CREATION

PCB Designing:

The main purpose of printed circuit board is in the routing of electric current and signals through a thin copper layer. The copper layer substitute the basic wired configuration of the circuit. Thus making the circuit simpler and compact. The thin copper through which current pass is bounded firmly to an insulating base material sometimes called as substrate. This base is manufactured with an integral bonded layer of thin copper foil which has to be etched or other removed to arrive at the predesigned pattern to suit the circuit connections or whatever other application is needed, thus making the circuit compact.

From the construction point of view the main attraction of using PCB is its role as the mechanical support for small components.

Schematic Preparation:

Schematic is a circuit that is drawn either with the help of software or by manually on paper with standard symbols. If the circuit is big and complicated then multilayer schematic is made. The schematic is drawn with coloured pen to indicate the different layers, power lines, signal lines and ground lines.

Artwork Preparation:

After making the schematic on the paper, same is duplicated on copper cladded epoxy sheet i.e. PCB with the help of HP marker or nail paint or metallic paint. This circuit is called artwork. Before going to the next stage, check the whole pattern and cross check against the circuit diagram.

Etching Process:

This process requires the use of chemicals, acid resistant dishes and a running water supply. Ferric chloride is maximum used solution, but other enchants such as ammonium persulphate can be used. Nitric acid can be used but in general it is not used due to poisonous fumes.

The etching bath should be in a glass or enamel dish. Then ferric chloride is thoroughly dissolved in water to the proportion suggested. There should be 0.5lt of water for 125gm anhydrous ferric chloride.


Now dip the PCB in the solution and wait till all unused copper gets removed. During the process, the PCB should be stirred continuously. The board should not be left in a bath for a moment longer than is needed to remove just the right amount of copper. Inspite of there being a resist coating, there is no protection against etching away through exposed copper edges, this leads to over etching. Have running water ready and pouring it on the etched board for 5 minutes, so that all ferric chloride can be removed properly and rinsed, this halt etching immediately. The reaction that takes place in the solution is given below:

2FeCl3 + 2H2O + 3Cu ―› 3CuCl2 + 2Fe(OH)2

Drilling Process:

Now the paint is washed out by the petrol or acetone or nail paint remover. Now the copper layout on PCB is rubbed with smooth sand paper slowly and lightly such that only the oxide layer over the copper is removed. Now the holes are drilled at the respective places, according to component layout as required.

Drilling is one of the operation that call for great care, because most of the holes will be made with very small drill. The size of the drill should not be either more than the required or less than the required. If the hole is large then it will be difficult to solder and a lot of lead will be consumed. If the hole is small then component will not be inserted easily.

Tinning Process:

After drilling the marked sites the hole are quoted with the layer of tin. This process is called as tinning. In this process, a wire of tin is melted and is deposited around the whole drilled. This is done to ease the process of soldering the deposited tin melts and soldering becomes easy and neat.

Testing the PCB:

PCB is checked for all interconnections through multimeter, whether the tracks are broken or short at any place, thereby correction is done through soldering.

Assembling of the Unit:

Components are assembled in proper direction and avoid the touching of the components to one another. Heat sink is to be put wherever required with heat sink compound.


Soldering:

Soldering is the process of joining of metal part with the aid of a molten solder where the melting temperature is situated below that of a material joined and where by the surface are wetted without them becoming molten.

The entire solder alloy has a low melting point with the liquid temperature below the melting point of pure lead in any solder metal lead is mainly used to lower the cost. The wetting phenomenon depends on tin. There are various types of solder material available, the one which is commonly used is tin-lead. It is the mixture of 61.9% tin and 38% lead. It is used for temperature upto 183˚C.

During the soldering operation, an auxiliary medium is mostly used to increase the flow properties of molten solder or to improve the degree of wetting. Such a medium is called flux. It is used to provide a liquid cover over the material. It is also used to dissolve any oxide on the metal surface. They are applied on the tip of the components to be soldered.


CIRCUIT DESCRIPTION

Basically, there are two methods of controlling the charging current: series regulation and parallel (shunt) regulation. A series regulator is inserted between the solar panel and the battery. The series type of regulation ‘wastes’ a lot of energy while charging the battery as the control circuitry is always active and series regulator requires the input voltage to be 3-4 volts higher than the output voltage. The current and voltage output of a solar panel is governed by the angle of incidence of light, which keeps varying.

Parallel regulation, is preferred in solar field. In parallel regulation, the control circuitry allows the charging current (even in MA) to flow into the battery and stop the charging once the battery is fully charged. At, this stage, the charging current is wasted by converting into heat (current is passed through low value, high-wattage resister); this part of the regulation dissipate a lot of heat.

In this project, we have used parallel regulation but instead of wasting the charging current as heat, we have made it pulsed and applied to the battery to keep the battery topped-up.

After power-on, the microcontrollers read the battery voltage with the help of the ADC and display the values on LCD. It monitors the input signal from the dusk-to-dawn sensor and activates the load or charging relay RL1 accordingly. The digital voltmeter works up to 20V. As Vref of the ADC is converted to Vcc (5V), the input voltage to the ADC connect exceed +5V. A potential divider is used at pin 2 of the ADC (IC2) using resisters R5, R6 & R7 to scale down the voltage from ADC output is multiplied four times and displayed on the LCD as battery voltage.

When the solar panel voltage is present, the duck-to-down sensor provides a signal to the microcontroller, which the displays ‘charging’ message on the LCD. During charging, the battery voltage is continuously monitored. When the voltage reaches 14.0V, the microcontroller interrupts the charging current by energizing the relay, which is connected to the MOSFET BS170 (T2) , and start a 5-minute times. During this stage, The LCD shows “battery full.”

After 5-minutes, the relay reconnects the panel to the battery. This way, the charging current is pulsed at the intervals of five minutes and the cycle repeats until the panel voltage is present.

When the panel voltage falls below the zener diode (ZD1) voltage of the dusk-to-dawn sensor, the microcontroller senses this and activates the load by switching on MOSFET T3 via optocoupler IC3 and “load on” message is displayed.


In this mode, the microcontroller monitors for low battery. When the battery voltage drops below 10 volts, the microcontroller turns off the load by switching off MOSFET T3 and “Battery Low-Load Off” message is displayed.

Normally, when the load is switched off, the battery voltage tends to rise back and the load oscillates between ‘on’ and ‘off’ states. To avoid this, the microcontroller employs a hysteresis control by intering into a ‘lock’ mode during low-battery state and comes out of the lock mode when the duck-to-dawn sensor receives the panel voltage (the next morning). During lock mode, the microcontroller keeps converting the ADC value and displays the battery voltage on the LCD.


Flowchart of the source program


TESTING OF CIRCUIT

Pin configurations of transistorsBC547, MOSFET BS170 and MOSFET IRF540are shown. An actual-size, single-side PCB for the microcontroller-based solar charger is shown in fig.3 and its component layout in fig.4. Wire the circuit on the PCB. Prior to inserting the programmed microcontroller into the PCB, check the soldering mistakes like shorts, and for proper connections using a millimeters. Mount power MOSFET IRF540N on a mutable heat-sink. Before switching on the controller unit, load and solar panel at appropriate places on the board.

Switch on the unit and the message “Solar Charge Controller-EFY” is display on the LCD for two seconds. The system status messages are displayed on line 1 of the LCD and the battery voltage is displayed on line 2. A small graphic representing the battery status is also display on line 2 of the LCD.


THE ROAD AHEAD

In future, India will have to depend on renewable energy. The only source available around us is sunlight, and we can easily convert sunlight energy into electrical energy by using PV cells to meet our requirement.

Presently, solar systems are developing 2%. By the end of 2010, the growth rate may increase to 7%.

However to extend use of solar power energy to industrial and commercial areas, the price

of PV cells need to be brought down through low-cost manufacturing techniques.


REFERENCE

Solar Electricity Engineering of Photovoltaic System – by Lorezo E.

Solar Energy – by Maxwell J.F & McGown J.G

System Identification – by Solderstron T. & Stoica P.

Power Electronics – by Bhimbra P.S

Electronics and Circuits - by Allen Mottershead

Basic Electronics – by Miami A.K

www.electronicsforu.com

www.wikipedia.com www.atmel.com


http://www.wikipedia.com/

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