CONTENTS

ABOUT THIS REPORT.................................................................................................4PURPOSE:............................................................................................................4SCOPE:.................................................................................................................4AUDIENCE:...........................................................................................................4ORGANIZATION:..................................................................................................41.FUNDAMENTALS OF PV CELLS......................................................................5

1.1 PV TEHCHNOLOGY...............................................................................................51.2 HISTORY OF PHOTOVOLTAIC............................................................................51.3 HOW PV CELLS WORK.........................................................................................61.4: TYPES OF PV CELLS.............................................................................................71.5: PHOTOVOALTAIC MODULES............................................................................9

1.5.1:Describing photovoltaic module performance.................................................101.6: PHOTOVOLTAIC ARRAYS................................................................................151.7: TYPES OF ARRAYS.............................................................................................18

1.7.1: Flat-plate stationary arrays..............................................................................181.7.2: Portable arrays.................................................................................................191.7.3: Tracking arrays................................................................................................20

1.8: FACTORS EFFECTING OUTPUT OF MODULES............................................201.9:TYPES OF MOUNTING........................................................................................21

1.9.1: Bracket mounting............................................................................................211.9.2: Pole mounting..................................................................................................221.9.3 Ground mounting..............................................................................................231.9.4: Structure mounting..........................................................................................24

2. PV CELLS.......................................................................................................252.1: TYPES OF PV CELLS:.........................................................................................262.1.1: MONOCYRSTALLINE PV CELLS..................................................................26

2.1.2: MULTICYRSTALLINE PV CELLS:.............................................................282.1.2.1: Manufacturing Process.................................................................................282.1.3: THIN FILM SOLAR CELL:...........................................................................292.1.3.1: Design and fabrication..................................................................................302.1.4: AMORPHOUS SILICON PV CELLS:...........................................................31

3. SOLAR CHARGE CONTROLLER..................................................................333.1: IMPORTANCE OF SOLAR CHARGE CONTROLLER.........................................33

3.1.1: Overcharge Protection.....................................................................................343.1.2: Over discharge Protection...............................................................................35

3.2: Charge Controller Set Points..................................................................................363.2.1: Voltage Regulation (VR) Set Point.................................................................373.2.2: Array Reconnect Voltage (ARV) Set Point.....................................................383.2.3: Voltage Regulation Hysteresis (VRH)............................................................383.2.4: Low Voltage Load Disconnect (LVD) Set Point.............................................38

3.3: CHARGE CONTROLLER DESIGNS..................................................................393.3.1: Shunt Controller Designs.................................................................................40

3.3.2: Series Controller Designs....................................................................................41

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3.4: WORKING OF SOALR CHARGE CONTROLLER............................................433.5 MAXIMUM POWER POINT TRACKING...........................................................45

3.5.1: Working of MPPT:..........................................................................................453.5.2: Hardware:........................................................................................................473.5.2.1 Switch Mode Converter.................................................................................473.5.2.2Control Section...............................................................................................473.5.2.3: Software........................................................................................................48

3.6: PWM CHARGE CONTROLLER VS MPPT CHARGE CONTROLLER...........533.7: CHARGE CONTROLLER SELECTION..............................................................53

3.7.1: Sizing Charge Controllers...............................................................................544. UN-INTERRUPTIBLE POWER SUPPLY........................................................555. BATTERIES....................................................................................................56

5.1: GENERAL DESCRIPTION...................................................................................565.1.1: Battery Design and Construction:....................................................................565.1.2: Battery Types and Classifications...................................................................60

5.2: Battery Selection Criteria.......................................................................................635.3: FACTORS EFFECTING BATTERY PERFORMANCE:.....................................65

5.3.3: Battery Gassing................................................................................................665.3.4: Sulfation...........................................................................................................675.3.5: Stratification....................................................................................................68

5.4: Battery Auxiliary Equipment..................................................................................685.4.1: Enclosures........................................................................................................685.4.1.1: Passive Cooling Enclosures..........................................................................685.4.1.2: Ventilation....................................................................................................695.4.1.3: Catalytic Recombination Caps.....................................................................69

5.5: BATTERY SAFETY CONSIDERATIONS..........................................................695.5.1: Handling Electrolyte........................................................................................705.5.2: Personnel Protection........................................................................................705.5.3: Dangers of Explosion......................................................................................705.5.4: Battery Disposal and Recycling......................................................................71

6: ECONOMY OF SOLAR BASED UNINTERRUPTED POWER SUPPLY........726.1 Power costs:.........................................................................................................726.2 Grid parity:...........................................................................................................73

7. WORKING OF SOLAR BASED UN-INTERRUPTED POWER SUPPLY SYSTEM..............................................................................................................74

7.1: TYPES OF PV SYSTEM.......................................................................................747.1.1: GRID CONNECTED PV SYSTEMS:............................................................747.1.2: STAND ALONE PV SYSTEMS:...................................................................77

8. APPLICATIONS AND ADVANTAGE:..............................................................81

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ABOUT THIS REPORT

PURPOSE: The purpose of this manual is to provide explanation & operation of solar-based uninterrupted power supply.

SCOPE: This report provides details on pv cells and the history of development solar charge controller ups system being used and batteries used for energy storage. It does not provide details about particular brands of batteries.

AUDIENCE: This report is intended for anyone who wants to know the basic details of solar-based ups system and its components

ORGANIZATION:

This report is organized in 9 chapters.

Chapter 1: Fundamentals of PV cells

Chapter 2: Type of Pv cell, advantages, physical structure, upgraded technology.

Chapter 3: Solar Charge Controller

Chapter 4: UPS

Chapter 5: Batteries

Chapter 6: Economy of SPUPS

Chapter 7: working of solar based ups

Chapter 8: advantages and applications

Chapter 9: summary

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1.FUNDAMENTALS OF PV CELLS

1.1 PV TEHCHNOLOGY

Photovoltaic (PV) solar cells as they are often referred to, are semiconductor devices that convert sunlight into direct current (DC) electricity. Groups of PV cells are electrically configured into modules and arrays, which can be used to charge batteries, operate motors, and to power any number of electrical loads. With the appropriate power conversion equipment, PV systems can produce alternating current (AC) compatible with any conventional appliances, and operate in parallel with and interconnected to the utility grid.

1.2 HISTORY OF PHOTOVOLTAIC The first conventional photovoltaic cells were produced in the late 1950s, and throughout the 1960s were principally used to provide electrical power for earth-orbiting satellites.

In the 1970s, improvements in manufacturing, performance and quality of PV modules helped to reduce costs and opened up a number of opportunities for powering remote terrestrial applications, including battery charging for navigational aids, signals, telecommunications equipment and other critical, low power needs. In the 1980s, photovoltaic became a popular power source for consumer electronic devices, including calculators, watches, radios, lanterns and other small battery charging applications. Following the energy crises of the 1970s, significant efforts also began to develop PV power systems for residential and commercial uses both for stand-alone, remote power as well as for utility-connected applications. During the same period, international applications for PV systems to power rural health clinics, refrigeration, water pumping, telecommunications, and off-grid households increased dramatically, and remain a major portion of the present world market for PV products. Today, the industry’s production of PV modules is growing at approximately 25 percent annually, and major programs in the U.S., Japan and Europe are rapidly accelerating the implementation of PV systems on buildings and interconnection to utility networks.

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1.3 HOW PV CELLS WORK

A typical silicon PV cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorus-doped (N-type) silicon on top of a thicker layer of boron doped (P-type) silicon. An electrical field is created near the top surface of the cell where these two materials are in contact, called the P-N junction.

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.

Fig 1.3.1

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

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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 back plane 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.

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.The resistance of the circuit of the cell will affect the current and voltage. The amount of available light affects current production. The temperature of the cell affects its voltage.

Regardless of size, a typical silicon PV cell produces about 0.5 – 0.6 volt DC under open-circuit, no-load conditions. The current (and power) output of a PV cell depends on its efficiency and size (surface area), and is proportional to the intensity of sunlight striking the surface of the cell. For example, under peak sunlight conditions a typical commercial PV cell with a surface area of 160 cm^2 (~25 in^2) will produce about 2 watts peak power. If the sunlight intensity were 40 percent of peak, this cell would produce about 0.8 watts.

Fig1.3.2

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1.4: TYPES OF PV CELLS

The four general types of photovoltaic cells are:

Single-crystal silicon. Polycrystalline silicon (also known as multicrystalline 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.

Polycrystalline silicon:

Polycrystalline cells are manufactured and operate in a similar manner. The difference is that 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.

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Ribbon silicon :

Growing a ribbon from the molten silicon instead of an ingot makes ribbon-type photovoltaic cells. 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.

1.5: PHOTOVOALTAIC MODULES 

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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.

Groups of modules can be interconnected in series and/or parallel to form an "array." By adding "balance of system" (BOS) components such as storage batteries, charge controllers, and power conditioning devices, we have a complete photovoltaic system.

1.5.1:Describing photovoltaic module performance

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

Consider "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 (Figure 1).

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A typical current voltage curve

As you can see in Figure 1, 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.

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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 (Figure 2)

A Typical Current-Voltage Curve at One Sun and One-half Sun

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% (Figure 3). One or more damaged cells in a module can have the same effect as shading.

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A Typical Current-Voltage Curve for an Unshaded Module and for a Module with One Shaded Cell.

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 (Figure 4).

A Typical Current-Voltage Curve for a Module at 25 ฐC (77 ฐ F) and 85 ฐC (185 ฐ F)

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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 that 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 5. 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.

Operating Voltages During a Battery Charging Cycle

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 (point #1), 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.

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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.

As an example, if the module is sometimes forced to operate at high temperatures, it can still supply enough voltage to charge 12-volt battery.

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. 

1.6: PHOTOVOLTAIC ARRAYS 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 (Figure 6).

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Three Modules Connected in Parallel

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. A diode is used to stop this reverse current flow. Diodes are electrical devices which only allow current to flow in one direction (Figure 7). A blocking diode is shown in the array in Figure 7.

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Basic Operation of a Diode

Because diodes create a voltage drop, some systems use a controller which opens the circuit instead of using a blocking diode. If the same three modules are connected in series, the output voltage will be 45 volts, and the current will be 3 amps.

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If one module in a series string fails, it provides so much resistance that other modules in the string may not be able to operate either. A bypass path around the disabled module will eliminate this problem (Figure 8). The bypass diode allows the current from the other modules to flow through in the "right" direction.Many modules are supplied with a bypass diode right at their electrical terminals. Larger modules may consist of three groups of cells, each with its own bypass diode.Built in bypass diodes are usually adequate unless the series string produces 48 volts or higher, or serious shading occurs regularly.Combinations of series and parallel connections are also used in arrays (Figure 9). If parallel groups of modules are connected in a series string, large bypass diodes are usually required.

Three Modules Connected in Series with a Blocking Diode and Bypass Diodes

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Twelve Modules in a Parallel-Series Array with Bypass Diodes and Isolation Diodes

1.7: TYPES OF ARRAYS

1.7.1: Flat-plate stationary arrays

Stationary arrays are the most common. Some allow adjustments in their tilt angle from the horizontal. These changes can be made any number of times throughout the year, although they are normally changed only twice a year. The modules in the array do not move throughout the day (Figure 10). Although a stationary array does not capture as much energy as a tracking array that follows the sun across the sky, and more modules may be required, there are no moving parts to fail. This reliability is why a stationary array is often used for remote or dangerous locations.

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Adjustable Array Tilted for Summer and Winter Solar Angles

1.7.2: 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 (Figures 11)

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portable array

1.7.3: Tracking arrays

Arrays that track, or follow the sun across the sky, can follow the sun in one axis or in two (Figure 12). 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.

Figure 12: One Axis and Two Axis Tracking Arrays

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.

1.8: FACTORS EFFECTING OUTPUT OF MODULES

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The surface on which the PV array is mounted should receive as much light as possible. The more light the solar array receives the more electricity will be generated. The three issues, which affect how much light a surface receives, are:

1. Orientation: Due south is the best possible orientation. If the PV is to be mounted on a vertical façade the orientation should preferably be between South East and South West. If the PV is to be mounted at a tilt a wider range of orientations will still give a reasonable energy yield. North facing orientations should be avoided.

2. Tilt: A tilted array will receive more light than a vertical array. Any angle between vertical and 15o off horizontal can be used. A minimum tilt of 15o off horizontal is recommended to allow the rain to wash dust off the array. The optimal tilt angle is 30o - 60o for a south facing array in Europe. Shallower tilt angles are better for east or west facing arrays.

3. Shadowing: Shadows cast by tall trees and neighboring buildings must also be considered. Even minor shading can result in significant loss of energy. If shading is unavoidable, your system designer can advise on how to minimize the effect of shade on the amount of electricity produced.

The area required for mounting a PV array depends on the output power desired and the type of module used. An area of around 8 m2 will be required to mount an array with a rated power output of 1kW, if monocrystalline modules are used (the most efficient modules type). If multicrystalline modules are used an area of around 10 m2 will be required for a 1kWp system and if amorphous modules are used an area of about 20 m2 will be required.These areas can be scaled up or down depending on the output power desired. 1 - 3 kWp is a typical power output for a domestic system, although smaller or larger systems can be installed.

1.9:TYPES OF MOUNTING

There are various ways in which a PV array can be mounted on a building. The various options offer different appearances and vary in cost.

1.9.1: Bracket mounting

Small arrays of one or two modules can use simple brackets to secure the modules individually to a secure surface (Figure 13). The surface may be a roof, wall, post, pole, or vehicle. Brackets can include some method to adjust the tilt angle of the module.

The brackets are usually aluminum. If steel is used, it should be painted or treated to prevent corrosion. Galvanized steel is normally avoided, because the

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continuous grounding used on arrays aggravates the galvanic corrosion that occurs between galvanized steel and almost all other metals.

Fastener hardware should be stainless steel or cadmium plated to prevent corrosion. Identical metals should be used for components and fasteners whenever possible.

1.9.2: Pole mounting

Typically, up to four modules can be connected together and mounted on a pole (Figure 14).

Black iron or steel pipe can be used, if painted. Galvanized pipe, rarely available in this size, can be used if compatible fasteners are used. Larger arrays can be pole mounted, if hardware sizes are appropriately increased.

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The same types of materials used for bracket mounting should be used for pole mounting.

Pole Mount of Photovoltaic Array

1.9.3 Ground mounting

For arrays of eight or more modules, ground mounting is usually the most appropriate technique. The greatest concern is often the uplifting force of wind on the array. This is why most ground-mounted arrays are on some kind of sturdy base, usually concrete.

Concrete bases are either piers, a slab with thicker edges, or footings at the front and rear of the array (Figure 15). All three usually include a steel reinforcement bar. In some remote sites it may be more desirable to use concrete block instead of poured concrete. The best way to do this is to use two-web bond-beam block, reinforce it with steel, and fill the space between the webs with concrete or mortar.

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Pressure-treated wood of adequate size is sometimes used for ground mounting. This can work well in fairly dry climates, but only if the beams are securely anchored to the

The array's mounting hardware can be bolted to an existing slab. With extensive shimming, some mountaintop arrays are bolted to exposed rock. In either case, adequately sized expansion-type anchor bolts are used. The heads of the bolts should be covered with some type of weatherproof sealant. Silicone sealant is the best choice.

Concrete Bases

1.9.4: Structure mounting

Photovoltaic modules mounted on buildings or other structures are subjected to downward force when the wind hits their front surfaces. When the wind strikes the back of the modules, upward force is generated (Figure 16).

For this reason, the attachment to the building of modules with exposed backs is designed to resist both directions of force.

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Another consideration when modules are mounted to a structure is the trapped heat between the module and the structure. Remember that module voltage drops with increased temperature. Generally, photovoltaic arrays are mounted on structures in such a way that air can maturely circulate under the modules. This keeps the modules operating at the lowest possible temperature and highest possible output voltage. Access to the back of the modules also simplifies service operations.

FIGURE 16 Forces on a Photovoltaic Array

2. PV CELLS

A solar cell is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices

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intended specifically to capture energy from sunlight such as solar panels and solar cells, while the term photovoltaic cell is used when the light source is unspecified. Assemblies of cells are used to make solar panels, solar modules, or photovoltaic arrays. Photovoltaic’s is the field of technology and research related to the application of solar cells in producing electricity for practical use. The energy generated this way is an example of solar energy (also known as solar power).Photovoltaic cells are manufactured by using different materials with different process of making. Each type has it’s own advantages and disadvantages , giving the end user a lot of choices, so as to consider different parameters.

2.1: TYPES OF PV CELLS:

There are four types of photovoltaic cells: multicrystalline silicon, monocrystalline silicon, ribbon silicon, and thin-film.

2.1.1: MONOCYRSTALLINE PV CELLS

2.1.1.1: MANUFACTURING PROCESS

The starting material is lumps of chemically pure polycrystalline silicon, of a quality close to semiconductor-grade, produced by the Siemens process. The traditional route for monocrystalline wafers is the Czochralski process in which a single crystal of up to about 150mm diameter is pulled from molten Si held in a large heated quartz crucible.  In the more recently developed method, Si is cast in a re-useable graphite mould to produce blocks of multicrystalline silicon (cubes of over 0.5m dimensions).  When sawn into bars and then wafers (just bigger than a compact disc) using a wire saw, the cleaned product is ready for cell manufacturing.Single crystal or monocrystalline wafers are made using the Czochralski process.

Czochralski process: High-purity, semiconductor-grade silicon (only a few parts per million of impurities) is melted down in a crucible, which is usually made of quartz. Dopant impurity atoms such as boron or phosphorus can be added to the molten intrinsic silicon in precise amounts in order to dope the silicon, thus changing it into n-type or p-type extrinsic silicon. This influences the electronic properties of the silicon. A precisely oriented seed crystal, mounted on a rod, is dipped into the molten silicon. The seed crystal's rod is very slowly pulled upwards and rotated at the same time. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt. Investigating and visualizing the temperature and velocity fields during the crystal growth process can avoid occurrence of unwanted instabilities

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in the melt. This process is normally performed in an inert atmosphere, such as argon, and in an inert chamber, such as quartz.

Due to the efficiencies that can be gained by the adoption of common wafer specifications, the semiconductor industry has for some time used wafers with standardized dimensions. Currently, high-end device manufacturers use 200 mm and 300 mm diameter wafers. The crystal ingots from which these wafers are sliced can be up to 2 meters in length, weighing several hundred kilograms. Larger wafers allow improvements in manufacturing efficiency, as more chips can be fabricated on each wafer, so there has been a steady drive to increase silicon wafer sizes. The next step up, 450 mm, is currently scheduled for introduction in 2012. Silicon wafers are typically about 0.2–0.75 mm thick, and can be polished to a very high flatness for making integrated circuits, or textured for making solar cells.The process begins when the chamber is heated up to approximately 1500 degrees Celsius, to melt the silicon. When the silicon is fully melted, a small seed crystal mounted on the end of a rotating shaft is slowly lowered until it just dips below the surface of the red-hot molten silicon. The shaft rotates counterclockwise and the crucible rotates clockwise. The rotating rod is then drawn upwards very slowly, allowing a roughly cylindrical boule to be formed. The boule can be from one to two meters, depending on the amount of silicon in the crucible.In the early days of the technology, the boles were smaller, only a few inches wide. With increasing technology, nowadays up to 300 mm (12-inch)- wide boules can be grown. The width is controlled by precise control of the temperature, the speeds of rotation and how fast the seed holder is withdrawn. Widths of 400 mm (16 inches) are expected in the next several years. This is one reason for the rapidly decreasing cost of chips in recent years, because more LSI chips can be created from a single wafer with the same number of fabrication process steps.The electrical characteristics of the silicon are controlled by adding material like phosphorus or boron to the silicon before it is melted. The added material is called dopant and the process is called doping. When silicon is grown by the Czochralski method, the melt is contained in a silica (quartz) crucible. During growth, the walls of the crucible dissolve into the melt and Czochralski silicon therefore contains oxygen at a typical concentration of 1018 cm−3. Oxygen impurities can have beneficial effects. Carefully chosen annealing conditions can allow the formation of oxygen precipitates. These have the effect of trapping unwanted transition metal impurities in a process known as gettering. Additionally, oxygen impurities can improve the mechanical strength of silicon wafers by immobilizing any dislocations that may be introduced during device processing. It was experimentally shown in the 1990s that the high oxygen concentration is also beneficial for radiation hardness of silicon particle detectors used in harsh radiation environment. Therefore, radiation detectors made of Czochralski- and Magnetic Czochralski-silicon are considered to be promising candidates for many future high-energy physics experiments. It has

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also been shown that presence of oxygen in silicon increases impurity trapping during post-implantation annealing processes.However, oxygen impurities can react with boron in an illuminated environment, such as experienced by solar cells. This results in the formation of electrically active boron–oxygen complex that detracts from cell performance. Module output drops by approximately 3% during the first few hours of light exposure.

2.1.2: MULTICYRSTALLINE PV CELLS:

Techniques for the production of multicrystalline silicon are more simple, and therefore cheaper, than those required for single crystal material. However, the material quality of multicrystalline material is lower than that of single crystalline material due to the presence of grain boundaries. Grain boundaries introduce high localised regions of recombination due to the introduction of extra defect energy levels into the band gap, thus reducing the overall minority carrier lifetime from the material. In addition, grain boundaries reduce solar cell performance by blocking carrier flows and providing shunting paths for current flow across the p-n junction.

2.1.2.1: Manufacturing Process

The feedstock (made by purification of silicon or by alternative refining methods) is charged in a silicon nitride coated quartz crucible and heated until all the silicon is melted. Heat is then extracted from the bottom of the crucible by moving the heat zone up compared to the crucible and / or cooling the bottom of the crucible. Often the crucible is lowered away from the heat zone and simultaneously the bottom is revealed to a cooling source.

A temperature gradient is created in the melt and the solidification will start at the bottom and crystals will grow upwards, and grain boundaries will grow parallel to the solidification direction. To obtain a directional solidification the solidification heat must be transported through the steadily growing layer of solid silicon. It is necessary to maintain a net heat flux over the solid-liquid interface and the temperature at the lower part of the crucible must be decreased according to the increase in solid silicon thickness to maintain a steady growth rate. The growth rate is proportional to the temperature gradient difference between the solid and the liquid silicon.

Impurity Distribution in Directionally Solidified Ingots:

Due to the fact that most elements are more soluble in liquid than in solid silicon, impurities dissolved in the melt will segregate and the element concentration in the ingot will in most cases increase upwards in the ingot following Scheil’s equation when the melt solidifies from the bottom and up.

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The exponential distribution will create a heavily contaminated thin layer at the top of the resulting ingot.

The Scheil equation assumes no diffusion in the solid state, complete mixing in the liquid state and equilibrium at the solid/liquid interface. If convection is not sufficient to provide complete mixing in the liquid phase, solute atoms are rejected by the advancing solid at a greater rate than they can diffuse into the bulk of the melt. A concentration gradient is thus developed ahead of the solid. This enriched region will determine the rate of solute incorporation into the solid front. This region is called a diffusion boundary layer. Scheil’s equation is still valid if an effective distribution coefficient is used.

Forming of Precipitates:

Precipitates may form after saturation is met, and Scheil’s equation will no longer be valid. The amount of super saturation needed for precipitates to form will vary with the chemical composition and the growth conditions in the system.

Diffusion of Impurities:

In addition to the Scheil distribution the impurity distribution will depend on diffusion. Impurities will diffuse into the solidified silicon from the crucible walls and bottom as well as from the coating. Back-diffusion can also occur as impurities diffuse from the heavily contaminated top layer back into the bulk material after solidification, or from the boundary layer during solidification. Both in-diffusion from the crucible and coating and back-diffusion are temperature dependent and the impurity distribution varies with varying temperature profile during growth and the subsequent cooling.

Boron Doped Silicon:

Boron is an acceptor in silicon, and multicrystalline silicon ingots made by directional solidification are often pre-doped with boron. A small amount of boron is added together with the feedstock prior to melting and solidification. Boron is most commonly used because it is the doping element with the distribution coefficient closest to 1 (k0 = 0.8). The distribution profile will thus not vary as much with height as the other doping elements.

2.1.3: THIN FILM SOLAR CELL:

A thin-film solar cell (TFSC), also called a thin-film photovoltaic cell (TFPV), is a solar cell that is made by depositing one or more thin layers (thin film) of photovoltaic material on a substrate. The thickness range of such a layer is wide and varies from a few nanometers to tens of micrometers.

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Many different photovoltaic materials are deposited with various deposition methods on a variety of substrates. Thin-film solar cells are usually categorized according to the photovoltaic material used:

Amorphous silicon (a-Si) and other thin-film silicon (TF-Si) Cadmium Telluride (CdTe) Copper indium gallium selenide (CIS or CIGS) Dye-sensitized solar cell (DSC) and other organic solar cells

2.1.3.1: Design and fabrication

The silicon is mainly deposited by chemical vapor deposition, typically plasma-enhanced (PE-CVD), from silane gas and hydrogen gas. Other deposition techniques being investigated include sputtering and hot wire techniques.

The silicon is deposited on glass, plastic or metal, which has been coated with a layer of transparent conducting oxide (TCO).

Polysilicon deposition, or the process of depositing a layer of polycrystalline silicon on a semiconductor wafer, is achieved by pyrolyzing silane (SiH4) at 580 to 650 °C. This pyrolysis process releases hydrogen.

Polysilicon layers can be deposited using 100% silane at a pressure of 25–130 Pa (0.2 to 1.0 Torr) or with 20–30% silane (diluted in nitrogen) at the same total pressure. Both of these processes can deposit polysilicon on 10–200 wafers per run, at a rate of 10–20 nm/min and with thickness uniformities of ±5%. Critical process variables for polysilicon deposition include temperature, pressure, silane concentration, and dopant concentration. Wafer spacing and load size have been shown to have only minor effects on the deposition process. The rate of polysilicon deposition increases rapidly with temperature, since it follows Arrhenius behavior, that is deposition rate = A·exp(–qEa/kT) where q is electron charge and k is the Boltzmann constant. The activation energy (Ea) for polysilicon deposition is about 1.7 eV. Based on this equation, the rate of polysilicon deposition increases as the deposition temperature increases. There will be a minimum temperature, however, wherein the rate of deposition becomes faster than the rate at which unreacted silane arrives at the surface. Beyond this temperature, the deposition rate can no longer increase with temperature, since it is now being hampered by lack of silane from which the polysilicon will be generated. Such a reaction is then said to be 'mass-transport-limited.' When a polysilicon deposition process becomes mass-transport-limited, the reaction rate becomes dependent primarily on reactant concentration, reactor geometry, and gas flow.

When the rate at which polysilicon deposition occurs is slower than the rate at which unreacted silane arrives, then it is said to be surface-reaction-limited. A deposition process that is surface-reaction-limited is primarily dependent on

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reactant concentration and reaction temperature. Deposition processes must be surface-reaction-limited because they result in excellent thickness uniformity and step coverage. A plot of the logarithm of the deposition rate against the reciprocal of the absolute temperature in the surface-reaction-limited region results in a straight line whose slope is equal to –qEa/k.

At reduced pressure levels for VLSI manufacturing, polysilicon deposition rate below 575 °C is too slow to be practical. Above 650 °C, poor deposition uniformity and excessive roughness will be encountered due to unwanted gas-phase reactions and silane depletion. Pressure can be varied inside a low-pressure reactor either by changing the pumping speed or changing the inlet gas flow into the reactor. If the inlet gas is composed of both silane and nitrogen, the inlet gas flow, and hence the reactor pressure, may be varied either by changing the nitrogen flow at constant silane flow, or changing both the nitrogen and silane flow to change the total gas flow while keeping the gas ratio constant.

Polysilicon doping, if needed, is also done during the deposition process, usually by adding phosphine, arsine, or diborane. Adding phosphine or arsine results in slower deposition, while adding diborane increases the deposition rate. The deposition thickness uniformity usually degrades when dopants are added during deposition.

Depending upon the efficiency required, these dopants are removed using several techniques.

2.1.4: AMORPHOUS SILICON PV CELLS:

Amorphous Silicon cells use layers of a-Si only a few micrometers thick, attached to an inexpensive backing such as glass, flexible plastic, or stainless steel.  This means that they use less than 1% of the raw material (silicon) compared standard crystalline Silicon (c-Si) cells, leading to a significant cost saving.

2.1.4.1: MANUFACTURING PROCESS:

Amorphous silicon is gradually degraded, by exposure to light, by phenomena called the Staebler-Wronski Effect (SWE).  SWE affects the power output of a-Si modules by as much as 10%. This light induced degradation is reduced by depositing the layers of the cell using high hydrogen dilution and by making combinations (alloys) of different types of cells.  Because of SWE, a-Si cells are rated in the stabilized condition, which occurs after about 100 hours exposure to light.

Unlike crystal silicon, in which atomic arrangements are regular, amorphous silicon features irregular atomic arrangements.  As a result, the reciprocal action between photons and silicon atoms occurs more frequently in amorphous silicon than in crystal silicon, allowing much more light to be absorbed. Thus, an ultra-

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thin amorphous silicon film of less than 1µm can be produced and used for power generation. This film, because it is not crystalline and is so thin, will not break when it is flexed, thus allowing it to be deposited on flexible substrates.  Because of the flexability of the cell and the substrates a-Si producers are able to use automated "roll-to-roll" manufacturing processes in which the substrate and deposited material move through the production process as one continuous strip passing over several rolls in the process which maintain stability to the process as well as moving the product along its way.

Amorphous silicon films are fabricated using plasma vapor deposition techniques to apply silane (SiH4) to the substrate or other beneficial film, allowing large-area solar cells to be fabricated much more easily than with conventional c-Si.  Three amorphous silicon layers — p-layer, i-layer, and n-layer — are formed consecutively on the substrate. This p-i-n junction corresponds to the p/n junction of a c-Si solar cell. Amorphous silicon can be deposited onto a many substrates Including glass and ceramics, metals such as stainless steel, and plastics. 

Low temperature induced crystallization of amorphous silicon:

Amorphous silicon can be transformed to crystalline silicon using well-understood and widely implemented high-temperature annealing processes. This typical method is the typical method used in industry but requires high-temperature compatible materials, such as special high temperature glass that is expensive to produce. However, there are many applications for which this is an inherently unattractive production method. Flexible solar cells have been a topic of interest for less conspicuous-integrated power generation than solar power farms. These modules may be placed in areas where traditional cells would not be feasible, such as wrapped around a telephone pole or cell phone tower. In this application a photovoltaic material may be applied to a flexible substrate, often a polymer. Such substrates cannot survive the high temperatures experienced during traditional annealing. Instead, novel methods of crystallizing the silicon without disturbing the underlying substrate have been studied extensively. Aluminum-induced crystallization (AIC) and local laser crystallization are common in the literature, however not extensively used in industry.

In both of these methods, amorphous silicon (a-Si or a-Si: H) is grown using traditional techniques such as plasma-enhanced chemical vapor deposition (PECVD). The crystallization methods diverge during post-deposition processing.

In aluminum-induced crystallization, a thin layer of aluminum (50 nm or less) is deposited by physical vapor deposition onto the surface of the amorphous silicon. This stack of material is then annealed at a relatively low temperature between 140°C and 200°C in a vacuum. The aluminum that diffuses into the amorphous silicon is believed to weaken the hydrogen bonds present, allowing crystal nucleation and growth. Experiments have shown that polycrystalline

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silicon with grains on the order of 0.2 – 0.3 μm can be produced at temperatures as low as 150°C. The volume fraction of the film that is crystallized is dependent on the length of the annealing process.

Aluminum-induced crystallization produces polycrystalline silicon with suitable crystallographic and electronic properties that make it a candidate for producing polycrystalline thin films for photovoltaics. AIC can be used to generate crystalline silicon Nan wires and other nano-scale structures.

Another method of achieving the same result is the use of a laser to heat the silicon locally without heating the underlying substrate beyond some upper temperature limit. An excimer laser or, alternatively, green lasers such as a frequency-doubled Nd:YAG laser is used to heat the amorphous silicon, supplying energy necessary to nucleate grain growth. The laser fluence must be carefully controlled in order to induce crystallization without causing widespread melting. Crystallization of the film occurs as a very small portion of the silicon film is melted and allowed to cool. Ideally, the laser should melt the silicon film through its entire thickness, but not damage the substrate. Toward this end, a layer of silicon dioxide is sometimes added to act as a thermal barrier. This allows the use of substrates that cannot be exposed to the high temperatures of standard annealing, polymers for instance. Polymer-backed solar cells are of interest for seamlessly integrated power production schemes that involve placing photovoltaics on everyday surfaces.

A third method for crystallizing amorphous silicon is the use of thermal plasma jet. This strategy is an attempt to alleviate some of the problems associated with laser processing – namely the small region of crystallization and the high cost of the process on a production scale. The plasma torch is a simple piece of equipment that is used to thermally anneal the amorphous silicon. Compared to the laser method, this technique is simpler and more cost effective.

Plasma torch annealing is attractive because the process parameters and equipment dimension can be changed easily to yield varying levels of performance. A high level of crystallization (~90%) can be obtained with this method. Disadvantages include difficulty-achieving uniformity in the crystallization of the film. While this method is applied frequently to silicon on a glass substrate, processing temperatures may be too high for polymers.

3. SOLAR CHARGE CONTROLLER

3.1: IMPORTANCE OF SOLAR CHARGE CONTROLLER

The primary function of a charge controller in a stand-alone PV system is to maintain the battery at highest possible state of charge while protecting it from

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overcharge by the array and from overdischarge by the loads. Although some PV systems can be effectively designed without the use of charge control, any system that has unpredictable loads, user intervention, optimized or undersized battery storage (to minimize initial cost) typically requires a battery charge controller. 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, meters, remote voltage sense leads and special algorithms can enhance the abilityof a charge controller to maintain the health and extend the lifetime of a battery, as well as providing an indication of operational status to the system caretaker.

Important functions of battery charge controllers and system controls are:

Prevent Battery Overcharge: to limit the energy supplied to the battery by the PV array when the battery becomes fully charged.

Prevent Battery Over discharge: to disconnect the battery from electrical loads when the battery reaches low state of charge.

Provide Load Control Functions: to automatically connect and disconnect an electrical load at a specified time, for example operating a lighting load from sunset to sunrise.

3.1.1: Overcharge Protection

A remote stand-alone photovoltaic system with battery storage is designed so that it will meet the system electrical load requirements under reasonably determined worst-case conditions, usually for the month ofthe year with the lowest insolation to load ratio. When the array is operating under good-to-excellent weather conditions (typically during summer), energy generated by the array often exceeds the electrical load demand. To prevent battery damage resulting from overcharge, a charge controller is used to protect the battery. A charge controller should prevent overcharge of a battery regardless of the system sizing/design and seasonal changes in the load profile, operating temperatures and solar insolation.

Charge regulation is the primary function of a battery charge controller, and perhaps the single most important issue related to battery performance and life. The purpose of a charge controller is to supply power to the battery in a manner which fully recharges the battery without overcharging. Without charge control, the current from the array will flow into a battery proportional to the irradiance, whether the battery needs charging or not. If the battery is fully charged, unregulated charging will cause the battery voltage to reach exceedingly high levels, causing severe gassing, electrolyte loss, internal heating and accelerated grid corrosion. In most cases if a battery is not protected from overcharge in PV system, premature failure

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of the battery and loss of load are likely to occur.

Charge controllers prevent excessive battery overcharge by interrupting or limiting the current flow from the array to the battery when the battery becomes fully charged. Charge regulation is most often accomplished by limiting the battery voltage to a maximum value, often referred to as the voltage regulation (VR) set point.

Sometimes, other methods such as integrating the ampere-hours into and out of the battery are used. Depending on the regulation method, the current may be limited while maintaining the regulation voltage, or remain disconnected until the battery voltage drops to the array reconnect voltage (ARV) set point.

3.1.2: Over discharge Protection

During periods of below average insolation and/or during periods of excessive electrical load usage, the energy produced by the PV array may not be sufficient enough to keep the battery fully recharged. When a battery is deeply discharged, the reaction in the battery occurs close to the grids, and weakens the bond between the active materials and the grids. When a battery is excessively discharged repeatedly, loss of capacity and life will eventually occur. To protect batteries from overdischarge, most charge controllers include an optional feature to disconnect the system loads once the battery reaches a low voltage or low state of charge condition.

In some cases, the electrical loads in a PV system must have sufficiently high enough voltage to operate. If batteries are too deeply discharged, the voltage falls below the operating range of the loads, and the loads may operate improperly or not at all. This is another important reason to limit battery overdischarge in PV systems.

Overdischarge protection in charge controllers is usually accomplished by open-circuiting the connection between the battery and electrical load when the battery reaches a pre-set or adjustable low voltage load disconnect (LVD) set point. Most charge controllers also have an indicator light or audible alarm to alert the system user/operator to the load disconnect condition. Once the battery is recharged to a certain level, the loads are again reconnected to a battery. Non-critical system loads are generally always protected from overdischarging the battery by connection to the low voltage load disconnect circuitry of the charge controller. If the battery voltage falls to a low but safe level, a relay can open and disconnect the load, preventing further battery discharge. Critical loads can be connected directly to the battery, so that they are not automatically disconnected by the charge controller. However, the danger exists that these critical loads might overdischarge the battery. An alarm or other method of user feedback should be included to give information on the battery status if critical loads areconnected directly to the battery.

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Regulation or limiting the PV array current to a battery in a PV system may be accomplished by several methods. The most popular method is battery voltage sensing, however other methods such as amp hour integration are also employed. Generally, voltage regulation is accomplished by limiting the PV array current at a predefined charge regulation voltage. Depending on the regulation algorithm, the current may be limited while maintaining the regulation voltage, or remain disconnected until the battery voltage drops to the arrayreconnect set point.

While the specific regulation method or algorithm vary among charge controllers, all have basic parameters and characteristics. Charge controller manufacturer's data generally provides the limits of controller application such as PV and load currents, operating temperatures, parasitic losses, set points, and set point hysteresis values. In some cases the set points may be dependent upon the temperature of the battery and/or controller, and the magnitude of the battery current.

3.2: Charge Controller Set Points

The battery voltage levels at which a charge controller performs control or switching functions are called the controller set points. Four basic control set points are defined for most charge controllers that have battery overcharge and over discharge protection features. The voltage regulation (VR) and the array reconnect voltage (ARV) refer to the voltage set points at which the array is connected and disconnected from the battery. The low voltage load disconnect (LVD) and load reconnect voltage (LRV) refer to the voltage set points at which the load is disconnected from the battery to prevent over discharge. Figure 1 shows the basic controller set points on a simplified diagram plotting battery voltage versus time for a charge and discharge cycle. A detailed discussion of each charge controller set point follows.

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Controller set points

3.2.1: Voltage Regulation (VR) Set Point

The voltage regulation (VR) set point is one of the key specifications for charge controllers. The voltage regulation set point is defined as the maximum voltage that the charge controller allows the battery to reach, limiting the overcharge of the battery. Once the controller senses that the battery reaches the voltage regulation set point, the controller will either discontinue battery charging or begin to regulate (limit) the amount of current delivered to the battery. In some controller designs, dual regulation set points may be used. For example, a higher regulation voltage may be used for the first charge cycle of the day to provide a little battery overcharge, gassing and equalization, while a lower regulation voltage is used on subsequent cycles through the remainder of the day to effectively ‘float charge’ the battery.

Proper selection of the voltage regulation set point may depend on many factors, including the specific battery chemistry and design, sizes of the load and array with respect to the battery, operating temperatures, and electrolyte loss considerations.

An important point to note about the voltage regulation set point is that the values required for optimal battery performance in stand-alone PV systems are generally much higher than the regulation or ‘float voltages’ recommended by battery manufacturers. This is because in a PV system, the battery must be recharged within a limited time period (during sunlight hours), while battery manufacturers generally allow for much longer recharge times when determining

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their optimal regulation voltage limits. By using a higher regulation voltage in PV systems, the battery can be recharged in a shorter time period, however some degree over overcharge and gassing will occur. The designer is faced selecting the optimal voltage regulation set point that maintains the highest possible battery state of charge without causing significant overcharge.

3.2.2: Array Reconnect Voltage (ARV) Set Point

In interrupting (on-off) type controllers, once the array current is disconnected at the voltage regulation set point, the battery voltage will begin to decrease. The rate at which the battery voltage decreases depends on many factors, including the charge rate prior to disconnect, and the discharge rate dictated by the electrical load. If the charge and discharge rates are high, the battery voltage will decrease at a greater rate than if these rates are lower. When the battery voltage decreases to a predefined voltage, the array is again reconnected to the battery to resume charging. This voltage at which the array is reconnected is defined as the array reconnect voltage (ARV) set point.

If the array were to remain disconnected for the rest of day after the regulation voltage was initially reached, the battery would not be fully recharged. By allowing the array to reconnect after the battery voltage reduces to a set value, the array current will ‘cycle’ into the battery in an on-off manner, disconnecting at the regulation voltage set point, and reconnecting at the array reconnect voltage set point. In this way, the battery will be brought up to a higher state of charge by ‘pulsing’ the array current into the battery.

It is important to note that for some controller designs, namely constant-voltage and pulse-width-modulated (PWM) types, there is no clearly distinguishable difference between the VR and ARV set points. In these designs, the array current is not regulated in a simple on-off or interrupting fashion, but is only limited as the battery voltage is held at a relatively constant value through the remainder of the day.

3.2.3: Voltage Regulation Hysteresis (VRH)

The voltage span or difference between the voltage regulation set point and the array reconnect voltage is often called the voltage regulation hysteresis (VRH). The VRH is a major factor, which determines the effectiveness of battery recharging for interrupting (on-off) type controllers. If the hysteresis is to great, the array current remains disconnected for long periods, effectively lowering the array energy utilization and making it very difficult to fully recharge the battery. If the regulation hysteresis is too small, the array will cycle on and off rapidly, perhaps damaging controllers, which use electro-mechanical switching elements.

3.2.4: Low Voltage Load Disconnect (LVD) Set Point

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Overdischarging the battery can make it susceptible to freezing and shorten it’s operating life. If battery voltage drops too low, due to prolonged bad weather for example, certain non-essential loads can be disconnected from the battery to prevent further discharge. This can be done using a low voltage load disconnect (LVD) device connected between the battery and non-essential loads. The LVD is either a relay or a solid-state switch that interrupts the current from the battery to the load, and is included as part of most controller designs. In some cases, the low voltage load disconnect unit may be a separate unit from themain charge controller.

In controllers or controls incorporating a load disconnect feature, the low voltage load disconnect (LVD) set point is the voltage at which the load is disconnected from the battery to prevent overdischarge. The LVD set point defines the actual allowable maximum depth-of-discharge and available capacity of the battery operating in a PV system. The available capacity must be carefully estimated in the PV system design and sizing process using the actual depth of discharge dictated by the LVD set point.

The proper LVD set point will maintain a healthy battery while providing the maximum battery capacity and load availability. To determine the proper load disconnect voltage, the designer must consider the rate at which the battery is discharged. Because the battery voltage is affected by the rate of discharge, a lower load disconnect voltage set point is needed for high discharge rates to achieve the same depth of discharge limit. In general, the low discharge rates in most small stand-alone PV systems do not have a significant effect on the battery voltage. Typical LVD values used are between 11.0 and 11.5 volts, which corresponds to about 75-90% depth of discharge for most nominal 12 volt lead-acid batteries at discharge rates lower than C/30

3.3: CHARGE CONTROLLER DESIGNS

Two basic methods exist for controlling or regulating the charging of a battery from a PV module or array - shunt and series regulation. While both of these methods are effectively used, each method may incorporate a number of variations that alter their basic performance and applicability. Simple designs interrupt or disconnect the array from the battery at regulation, while more sophisticated designs limit the current to the battery in a linear manner that maintains a high battery voltage.

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 electrical load demands. Most importantly, the controller algorithm defines the way in which PV array power is applied to the battery in the system. In general, interrupting on-off type controllers require a

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higher regulation set point to bring batteries up to full state of charge than controllers that limit the array current in a gradual manner.

Some of the more common design approaches for charge controllers are described in this section.

3.3.1: Shunt Controller Designs

Since photovoltaic cells are current-limited by design (unlike batteries), PV modules and arrays can be short-circuited without any harm. The ability to short-circuit modules or an array is the basis of operation for shunt controllers.

Figure 12 shows an electrical design of a typical shunt type controller. The shunt controller regulates the charging of a battery from the PV array by short-circuiting the array internal to the controller. All shunt controllers must have a blocking diode in series between the battery and the shunt element to prevent the battery from short-circuiting when the array is regulating. Because there is some voltage drop between the array and controller and due to wiring and resistance of the shunt element, the array is never entirely shortcircuited, resulting in some power dissipation within the controller. For this reason, most shunt controllers require a heat sink to dissipate power, and are generally limited to use in PV systems with array currentsless than 20 amps.

Shunt controllerThe regulation element in shunt controllers is typically a power transistor or MOSFET, depending on the specific design. There are a couple of variations of the shunt controller design. The first is a simple interrupting, or on-off type controller design. The second type limits the array current in a gradual manner, by increasing the resistance of the shunt element as the battery reaches full state of charge.

Shunt-Interrupting Design

The shunt-interrupting controller completely disconnects the array current in an interrupting or on-off fashion when the battery reaches the voltage regulation set

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point. When the battery decreases to the array reconnect voltage, the controller connects the array to resume charging the battery. This cycling between the regulation voltage and array reconnect voltage is why these controllers are often called ‘on-off’ or ‘pulsing’ controllers. Shunt-interrupting controllers are widely available and are low cost, however they are generally limited to use in systems with array currents less than 20 amps due to heat dissipation requirements.

Shunt-Linear Design

Once a battery becomes nearly fully charged, a shunt-linear controller maintains the battery at near a fixed voltage by gradually shunting the array through a semiconductor regulation element. In some designs, a comparator circuit in the controller senses the battery voltage, and makes corresponding adjustments to the impedance of the shunt element, thus regulating the array current. In other designs, simple Zener power diodes are used, which are the limiting factor in the cost and power ratings for these controllers. There is generally more heat dissipation in shunt-linear controllers than in shunt-interrupting types.

3.3.2: Series Controller Designs

As the name implies, this type of controller works in series between the array and battery, rather than in parallel as for the shunt controller. There are several variations to the series type controller, all of which use some type of control or regulation element in series between the array and the battery. While this type of controller is commonly used in small PV systems, it is also the practical choice for larger systems due to the current limitations of shunt controllers.

Figure 13 shows an electrical design of a typical series type controller. In a series controller design, a relay or solid-state switch either opens the circuit between the array and the battery to discontinuing charging, or limits the current in a series-linear manner to hold the battery voltage at a high value. In the simpler series interrupting design, the controller reconnects the array to the battery once the battery falls to the array reconnect voltage set point. As these on-off charge cycles continue, the ‘on’ time becoming shorter and shorter as the battery becomes fully charged.

Because the series controller open-circuits rather than short-circuits the array as in shunt-controllers, no blocking diode is needed to prevent the battery from short-circuiting when the controller regulates.

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Series controller

Series-Interrupting Design

The most simple series controller is the series-interrupting type, involving a one-step control, turning the array charging current either on or off. The charge controller constantly monitors battery voltage, and disconnects or open-circuits the array in series once the battery reaches the regulation voltage set point. After a pre-set period of time, or when battery voltage drops to the array reconnect voltage set point, the array and battery are reconnected, and the cycle repeats. As the battery becomes more fully charged, the time for the battery voltage to reach the regulation voltage becomes shorter each cycle, so the amount of array current passed through to the battery becomes less each time. In this way, full charge is approached gradually in small steps or pulses, similar in operation to the shunt-interrupting type controller. The principle difference is the series or shunt mode by which the array is regulated.

Similar to the shunt-interrupting type controller, the series-interrupting type designs are best suited for use with flooded batteries rather than the sealed VRLA types due to the way power is applied to the battery

Series-Interrupting, 2-step, Constant-Current DesignThis type of controller is similar to the series-interrupting type, however when the voltage regulation set point is reached, instead of totally interrupting the array current, a limited constant current remains applied to the battery. This ‘trickle charging’ continues either for a pre-set period of time, or until the voltage drops to the array reconnect voltage due to load demand. Then full array current is once again allowed to flow, and the cycle repeats. Full charge is approached in a continuous fashion, instead of smaller steps as describedabove for the on-off type controllers.

Series-Linear, Constant-Voltage DesignIn a series-linear, constant-voltage controller design, the controller maintains the battery voltage at the voltage regulation set point. The series regulation element

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acts like a variable resistor, controlled by the controller battery voltage sensing circuit of the controller. The series element dissipates the balance of the power that is not used to charge the battery, and generally requires heat sinking. The current is inherently controlled by the series element and the voltage drop across it.Series-linear, constant-voltage controllers can be used on all types of batteries. Because they apply power to the battery in a controlled manner, they are generally more effective at fully charging batteries than on-off type controllers.

Series-Interrupting, Pulse Width Modulated (PWM) Design

This algorithm uses a semiconductor-switching element between the array and battery, which is, switched on/off at a variable frequency with a variable duty cycle to maintain the battery at or very close to the voltage regulation set point. Although a series type PWM design is discussed here, shunt-type PWM designs are also popular and perform battery charging in similar ways. Similar to the series-linear, constant-voltage algorithm in performance, power dissipation within the controller is considerably lower in the series interrupting PWM design.

By electronically controlling the high speed switching or regulation element, the PWM controller breaks the array current into pulses at some constant frequency, and varies the width and time of the pulses to regulate the amount of charge flowing into the battery. When the battery is discharged, the current pulse width is practically fully on all the time. As the battery voltage rises, the pulse width is decreased, effectively reducing the magnitude of the charge current.

The PWM design allows greater control over exactly how a battery approaches full charge and generates less heat. PWM type controllers can be used with all battery type, however the controlled manner in which power is applied to the battery makes them preferential for use with sealed VRLA types batteries over on-off type controls. To limit overcharge and gassing, the voltage regulation set points for PWM and constant voltage controllers are generally specified lower than those for on-off type controllers.

3.4: WORKING OF SOALR CHARGE CONTROLLER When connecting a solar panel to a rechargeable battery, it is usually necessary to use a charge controller circuit to prevent the battery from overcharging and to avoid power wastage.

Solar charger controller is typically configured for a three stage charging process, Bulk, Absorption and Float. The three-stage charge process provides a somewhat higher charge voltage to charge the battery quickly and safely. Once the battery is fully charged a somewhat lower voltage is applied maintain the battery in a fully charged state without excessive water loss. The three stage

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charge process charges the battery as quickly as possible while minimizing battery water loss and maintenance.

Figure 1: Bulk charge curveBulk charge:When charge starts the Solar charger controller attempts to apply the bulk charge voltage to the battery. The system will switch to Bulk charge if the battery is sufficiently discharged and/or insufficient charge current is available to drive the battery up to the bulk voltage set point. During the Bulk charge stage the unit delivers as much charge current as possible to rapidly recharge the battery. Once the charge control system enters Absorption or Float, the unit will again switch to Bulk charge if battery voltage drops below the present charge voltage set point.Absorption charge:During this stage, the unit changes to a constant voltage mode where the absorption voltage is applied to the battery. When charge current decreases to the float transition current setting, the battery is fully charged and the unit switches to the float stage.

Float charge:During this stage, the float voltage is applied to the battery to maintain it in a fully charged state. When battery voltage drops below the float setting for a cumulative period, a new bulk cycle will be triggered.

The above charging process ensures that the battery is not overcharged thus ensuring long life of operation. Recently developed charge controllers also use a new technology called the maximum power point tracking that allows the charge controller to keep of the maximum power voltage as operation conditions change thus maximizing the output.

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Solar charge controller essentially uses the property of the PV cell being a constant current type device as shown in fig 2.

3.5 MAXIMUM POWER POINT TRACKING

3.5.1: Working of MPPT:

Maximum Power Point Tracking, frequently referred to as MPPT, is an electronic system that operates the Photovoltaic (PV) modules in a manner that allows the modules to produce all the power they are capable of. MPPT is not a mechanical tracking system that “physically moves” the modules to make them point more directly at the sun. MPPT is a fully electronic system that varies the electrical operating point of the modules so that the modules are able to deliver maximum available power. Additional power harvested from the modules is then made available as increased battery charge current. MPPT can be used in conjunction with a mechanical tracking system, but the two systems are completely different.

Feature:The most outstanding feature of Maximum Power Point

Tracking controller is intelligent tracking input voltage from solar panel, which could let solar panel always working at Maximum Power Point of V-A curve. Compared with normal solar charge controller, this MPPT controller could increase 10%-30% electrical power using efficiency from solar panel.

Function:This MPPT controller is not only have above mentioned special function, at the same time including completely Protecting andControlling functions:

Overcharge protectionOver-discharge protectionBattery Reverse Current ProtectionOverloading Protection

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Short Circuit ProtectionReverse Polarity Connection Protection

Schematic Diagram:Picture 2 shows typical 12V battery solar charge system V-A curve.Normal Solar Charge Controller:Solar Panel works at point A state, the solar panel working voltage is a little higher than battery voltage.Charge Voltage: UA=13.2V Charge Current:: IA=9.8ACharge Power: PA=13.2*9.8=129.36wArea in drawing: ①＋③MPPT Solar Charge Controller:Solar Panel works at point B state, the solar panel working voltage much higher than battery voltage.Charge Voltage: UB=18.4V, Charge Current:: IB=9.3ACharge Power: PB=18.4*9.3=171.12wArea in drawing: ①＋②

Comparison: The power B is more than power A△P/ PA =（PB— PA）/ PA＝32.3%As a result of different manufacture of solar panels, different solar illumination intensity, different temperature, different efficiency of solar charge controller and so on. The effective power increase rate is 10-30%.

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3.5.2: Hardware:

The MPPT consists of two basic components: a switch mode converter and control section.

3.5.2.1 Switch Mode Converter

The switch-mode converter is the core of the entire supply. This allows energy at one potential to be drawn, stored as magnetic energy in an inductor, and then released at a different potential. By setting up the switch-mode section in various different topologies, either high-to-low (buck converter) or low-to-high (boost) voltage converters can be constructed. Normally, the goal of a switch-mode power supply is to provide a constant output voltage or current. In power trackers, the goal is to provide a fixed input voltage and/or current, such that the array is held at the maximum power point, while allowing the output to match the battery voltage.

3.5.2.2Control Section

The control section is designed to determine if the input is actually at the maximumpower point by reading voltage/current back from the switching converter or from the array terminal and adjust the switch-mode section such that it is. Depending on the application, different feedback control parameters are needed to perform maximum power tracking. Most commonly voltage and power feedback controls are employed to control the system and hence to find the MPP of the array.

Voltage Feedback Control

The solar array terminal voltage is used as the control variable for the system. The system keeps the array operating close to its maximum power point by regulating the array’s voltage and matches the voltage of the array to a desired voltage.

However, this has the following drawbacks:a) The effects of the insolation and temperature of the solar array are neglected.b) It cannot be widely applied to battery energy storage systems.

Therefore, this control is only suitable for use under constant insolation conditions such as a satellite system, because it cannot automatically track the maximum power point of the array when variations in insolation and temperature occur.

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Power Feedback Control

Maximum power control is achieved by forcing the derivative (dP/dV) to be equal to zero under power feedback control. A general approach to power feedback control is to measure and maximize the power at the load terminal. This has an advantage of unnecessarily knowing the solar array characteristics. However, this method maximizes power to the load not power from the solar array. Although a converter with MPPT offers high efficiency over a wide range of operating points, but for a bad converter, the full power may not be delivered to the load due to power loss. Therefore, the design of a high performance converter is a very importance issue.

3.5.2.3: Software

You can use many algorithms to perform MPPT. Some of the important factors to consider when choosing a technique to perform MPPT are sensors used, ability of an algorithm to detect multiple maxima, costs, and convergence speed.

Sensors Used:For a large-scale application, the number of sensors you use can affect its complexity and accuracy. Often, for more precise MPPT, you may need to use more sensors. The number and type of sensors required depend largely on your MPPT technique.Ability of an Algorithm to Detect Multiple Local Maxima:

It is common for the irradiance levels at different points on a solar panel’s surface to vary. This leads to multiple local maxima in one system. The efficiency and complexity of an algorithm determine if the true maximum power point or a local maximum power point is calculated. In the latter case, the maximum electrical power is not extracted from the solar panel.

Costs:

The numbers of sensors as well as the type of hardware you use to monitor and control the electrical tracking system affect the cost of implementing it. The type of algorithm you use largely determines the resources required to set up this application.

Convergence Speed:

For a high-performance MPPT system, the time taken to converge to the required operating voltage or current should be low. Depending on how fast you need to do this and your tracking system requirements, the system has to accordingly maintain the load at the maximum power point.

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A few popular and effective MPPT algorithms are described below.

There are many algorithms that are used to control the MPPT. The algorithms that are most commonly used are the perturbation and observation method, dynamic approach method and the incremental conductance algorithm.

Perturbation and Observation Method

Perturbation and Observation (P&O) method has a simple feedback structure andFewer measured parameters. It operates by periodically perturbing (i.e. incrementing or decreasing) the array terminal voltage and comparing the PV output power with that of the previous perturbation cycle. If the perturbation leads to an increase (decrease) in array power, the subsequent perturbation is made in the same (opposite) direction. In this manner, the peak power tracker continuously seeks the peak power condition.

Flow chart of P&O algorithmModified Perturb and Observe

The P&O method implements a hill climbing technique, which works well in slow changingEnvironment but has some limitations under rapidly changing atmospheric conditions [1]-[8]. The methods may lead to incorrect or slow maximum power point tracking. To overcome such problems the MP&O method shown in Figure

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2, isolates the fluctuations caused by the perturbation process from those caused by the irradiance or weather change.

Flow chart of MP&O algorithmThis method adds an irradiance-changing estimate process in every perturb process to measure the amount of power change caused by the change of atmospheric condition.Because the estimate process stops tracking maximum power point by keeping the PV voltage constant, the tracking speed of MP&O method is only half of the conventional P&O method.

Estimate, Perturb and Perturb:

The EPP algorithm was introduced in previous work by the authors in order to improve the speed of the MP&O algorithm while keeping its main features. WhenCompared with the MP&O algorithm, the EPP algorithm that uses one estimate mode for every two perturb modes increases significantly the tracking speed of the MPPT control, without reduction of the tracking accuracy.Comparing with the MP&O algorithm, the EPP algorithm has a tracking speed of 1.5 times faster but has the same delay time between the estimate process and the perturb process. Therefore the EPP algorithm has obvious advantages over the MP&O algorithm.

Dynamic Approach Method:

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This method employe the ripple at the array output to maximize the array power by dynamically extrapolate the characteristic of the PV array. The instantaneous behavior of array voltage v, current i and power p can be grouped into three cases: current below that for the optimum power, current near the optimum and current above the optimum [5]. The array performance is reflected in both shapes and phase relationships. The product of the derivatives p’ and v’ is negative if the current is below that for optimum power and positive if the current is above the optimum and zero when the maximum power point is being tracked [5]. Since p’ v’ is a chain rule derivative, it is actually equal to dp/dv. This implies that by driving dp/dv to zero, power will be effectively maximized.

Incremental Conductance Algorithm

This method uses the source incremental conductance method as its MPP searchAlgorithm. It is more efficient than Perturb and Observe method and independent onDevice physics. The output voltage and current from the source are monitored upon which the MPPT controller relies to calculate the conductance and incremental conductance, and to make its decision (to increase or decrease duty ratio output).

Mathematical of the Incremental Conductance algorithm is discussed below.The output power from the source can be expressed as P = VI----------------------------------------- (1)The fact that P = V I and the chain rule for the derivative of products yields

dP/dV = d (V I) / dV = I dV / dV + V dI / dV = I + V dI / dV

(1/V) dP/dV = (I/V) + dI/dV----------------- (2)

Let’s define the source conductance: G = I/V --------------------------------------- (3)And the source incremental conductance: G = dI/dV ---------------------------------- (4)

In general output voltage from a source is positive. Equation (2) explains that the operating voltage is below the voltage at the maximum power point if the conductance is larger than the incremental conductance, and vice versa. The job of this algorithm is therefore to search the voltage operating point at which the conductance is equal to the incremental conductance. These ideas are expressed by equation (5), (6), (7), and graphically shown in Figure2.

dP/dV > 0, if G > G --------------------------(5)

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dP/dV = 0, if G = G --------------------------(6)dP/dV > 0, if G > G --------------------------(7)

The P-V curve

Flowchart for an Incremental Conductance Tracking System

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3.6: PWM CHARGE CONTROLLER VS MPPT CHARGE CONTROLLER

More modern charge controllers use pulse width modulation (PWM) to slowly lower the amount of power applied to the batteries as the batteries get closer and closer to fully charged.  This type of controller allows the batteries to be more fully charged with less stress on the battery, extending battery life.  It can also keep batteries in a fully charged state (called “float”) indefinitely.  PWM is more complex, but doesn’t have any mechanical connections to break.

The most recent and best type of solar charge controller is called maximum power point tracking or MPPT.  MPPT controllers are basically able to convert excess voltage into amperage.  This has advantages in a couple of different areas.

Most solar power systems use 12 volt batteries, like you find in cars.  (Some use other voltages and the same advantages apply to these systems as well.)  Solar panels can deliver far more voltage than is required to charge the batteries.  By, in essence, converting the excess voltage into amps, the charge voltage can be kept at an optimal level while the time required to fully charge the batteries is reduced.  This allows the solar power system to operate optimally at all times.

Another area that is enhanced by an MPPT charge controller is power loss. Lower voltage in the wires running from the solar panels to the charge controller results in higher energy loss in the wires than higher voltage.  With a PWM charge controller used with 12v batteries, the voltage from the solar panel to the charge controller typically has to be 18v.  Using an MPPT controller allows much higher voltages in the wires from the panels to the solar charge controller.  The MPPT controller then converts the excess voltage into additional amps.  By running higher voltage in the wires from the solar panels to the charge controller, power loss in the wires is reduced significantly.

MPPT charge controllers are more expensive that PWM charge controllers, but the advantages are worth the cost.

3.7: CHARGE CONTROLLER SELECTION

The selection and sizing of charge controllers and system controls in PV systems involves the consideration of several factors, depending on the complexity and control options required. While the primary function is to prevent battery overcharge, many other functions may also be used, including low voltage load

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disconnect, load regulation and control, control of backup energy sources, diversion of energy to and auxiliary load, and system monitoring. The designer must decide which options are needed to satisfy the requirements of a specific application. The following list some of the basic considerations for selecting charge controllers for PV systems.

1. System voltage2. PV array and load currents3. Battery type and size4. Regulation algorithm and switching element design5. Regulation and load disconnect set points6. Environmental operating conditions7. Mechanical design and packaging8. System indicators, alarms, and meters9. Over current disconnects and surge protection devices10. Costs, warranty and availability

3.7.1: Sizing Charge Controllers

Charge controllers should be sized according to the voltages and currents expected during operation of the PV system. The controller must not only be able to handle typical or rated voltages and currents, but must also be sized to handle expected peak or surge conditions from the PV array or required by the electrical loads that may be connected to the controller. It is extremely important that the controller be adequately sized for the intended application. If an undersized controller is used and fails during operation, the costs of service and replacement will be higher than what would have been spent on a controller that was initiallyOversized for the application.

Typically, we would expect that a PV module or array produces no more than its rated maximum power current at 1000 W/m2 irradiance and 25 oC module temperature. However, due to possible reflections from clouds, water or snow, the sunlight levels on the array may be “enhanced” up to 1.4 times the nominal 1000 W/m2 value used to rate PV module performance. The result is that peak array current could be 1.4 times the nominal peak rated value if reflection conditions exist. For this reason, the peak array current ratings for charge controllers should be sized for about 140% or the nominal peak maximum power current ratings for the modules or array.

The size of a controller is determined by multiplying the peak rated current from an array times this “enhancement” safety factor. The total current from an array is given by the number of modules or strings in parallel, multiplied by the module current. To be conservative, use the short-circuit current (Isc) is generally used instead of the maximum power current (Imp). In this way, shunt type controllers that operate the array at short-circuit current conditions are covered safely.

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4. UN-INTERRUPTIBLE POWER SUPPLY

UPSis designed to provide un-interruptible power supply to critical load. The system is supplied with a single phase mains power supply, which is rectified to nominal DC voltage and used to drive inverter and charge a battery. In the event of mains power failure, the battery will continue to supply the inverter for a period of time to the load current.

Un-interruptible power supply comprises mainly of

1. Rectifier & battery charger2. Battery3. Inverter

1. Rectifier & battery charger:The rectifier –battery charger transforms the alternating voltage of the mains to DC supply to feed the inverter and charging the battery.the level of the DC voltage is decided upon the number of batteries connected and the charging current depends on the ampere hour (Ah) capacity of the battery.

2. Battery:The battery stores DC energy and delivers the same to the inverter whenever the main supply fails. Electrical energy is converted on to a chemical energy and it is stored when in charging. Similarly the stored chemical energy is converted into

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electrical energy while discharging. The backup duration depends on the ampere hour capacity of the battery and the connected load.

3. Inverter:The inverter charging changes the dc voltage from the rectifier or from the battery in a single –phase sinusoidal alternating (AC) voltage for feeding the external loads connected to it.

5. BATTERIES

5.1: GENERAL DESCRIPTION

To properly select batteries for use in stand-alone PV systems, it is important that system designers have a good understanding of their design features, performance characteristics and operational requirements. Because the demand for energy does not always coincide with its production, electrical storage batteries are commonly used in PV systems. The primary functions of a storage battery in a PV system are to:

1. Energy Storage Capacity and Autonomy: to store electrical energy when it is produced by the PV array and to supply energy to electrical loads as needed or on demand.

2. Voltage and Current Stabilization: to supply power to electrical loads at stable voltages and currents, by suppressing or 'smoothing out' transients that may occur in PV systems.

3. Supply Surge Currents: to supply surge or high peak operating currents to electrical loads or appliances.

5.1.1: Battery Design and Construction:

Battery manufacturing is an intensive, heavy industrial process involving the use of hazardous and toxic materials. Batteries are generally mass produced, combining several sequential and parallel processes to construct a complete battery unit. After production, initial charge and discharge cycles are conducted on batteries before they are shipped to distributors and consumers.

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Some important components of battery construction are described below.

Cell: The cell is the basic electrochemical unit in a battery, consisting of a set of positive and negative plates divided by separators, immersed in an electrolyte solution and enclosed in a case. In a typical leadacid battery, each cell has a nominal voltage of about 2.1 volts, so there are 6 series cells in a nominal 12 volt battery. Figure 1 shows a diagram of a basic lead-acid battery cell.

Active Material: The active materials in a battery are the raw composition materials that form the positive and negative plates, and are reactants in the electrochemical cell. The amount of active material in a battery is proportional to the capacity a battery can deliver. In lead-acid batteries, the active materials are lead dioxide (PbO2) in the positive plates and metallic sponge lead (Pb) in the negative plates, which react with a sulfuric acid (H2SO4) solution during battery operation.

Electrolyte: The electrolyte is a conducting medium, which allows the flow of current through ionic transfer, or the transfer of electrons between the plates in a battery. In a lead-acid battery, the electrolyte is a diluted sulfuric acid solution, either in liquid (flooded) form, gelled or absorbed in glass mats. In flooded nickel cadmium cells, the electrolyte is an alkaline solution of potassium hydroxide and water. In most flooded battery types, periodic water additions are required to replenish the electrolyte lost through gassing. When adding water to batteries, it is very important to use distilled or de-mineralized water, as even the impuritiesin normal tap water can poison the battery and result in premature failure.

Grid: In a lead-acid battery, the grid is typically a lead alloy framework that supports the active material on a battery plate, and which also conducts current. Alloying elements such as antimony and calcium are often used to strengthen the lead grids, and have characteristic effects on battery performance such as cycle performance and gassing. Some grids are made by expanding a thin lead alloy sheet into a flat plate web, while others are made of long spines of lead with the active material plated around them forming tubes, or what are referred to as tubular plates.

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Plate: A plate is a basic battery component, consisting of a grid and active material, sometimes called an electrode. There are generally a number of positive and negative plates in each battery cell, typically connected in parallel at a bus bar or inter-cell connector at the top of the plates. A pasted plate is manufactured by applying a mixture of lead oxide, sulfuric acid, fibers and water on to the grid. The thickness of the grid and plate affect the deep cycle performance of a battery. In automotive starting or SLI type batteries, many thin plates are used per cell. This results in maximum surface area for delivering high currents, but not much thickness and mechanical durability for deep and prolonged discharges. Thick plates are used for deep cycling applications such as for forklifts, golf carts and other electric vehicles. Thethick plates permit deep discharges over long periods, while maintaining good adhesion of the active material to the grid, resulting in longer life.

Separator: A separator is a porous, insulating divider between the positive and negative plates in a battery, used to keep the plates from coming into electrical contact and short-circuiting, and which also allows the flow of electrolyte and ions between the positive and negative plates. Separators are made from micro porous rubber, plastic or glass-wool mats. In some cases, the separators may be

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like an envelope, enclosing the entire plate and preventing shed materials from creating short circuits at the bottom of the plates.

Element: In element is defined as a stack of positive and negative plate groups and separators, assembled together with plate straps interconnecting the positive and negative plates.

Terminal Posts: Terminal posts are the external positive and negative electrical connections to a battery.

A battery is connected in a PV system and to electrical loads at the terminal posts. In a lead-acid battery the posts are generally lead or a lead alloy, or possibly stainless steel or copper-plated steel for greater corrosion resistance. Battery terminals may require periodic cleaning, particularly for flooded designs. It is also recommended that the clamps or connections to battery terminals be secured occasionally as they may loosen over time.

Cell Vents: During battery charging, gasses are produced within a battery that may be vented to the atmosphere. In flooded designs, the loss of electrolyte through gas escape from the cell vents it a normal occurrence, and requires the periodic addition of water to maintain proper electrolyte levels. In sealed, or valve-regulated batteries, the vents are designed with a pressure relief mechanism, remaining closed under normal conditions, but opening during higher than normal battery pressures, often the result of overcharging or high temperature operation. Each cell of a complete battery unit has some type of cell vent.

Flame arrestor vent caps are commonly supplied component on larger, industrial battery systems. The venting occurs through a charcoal filter, designed to contain a cell explosion to one cell, minimizing the potential for a catastrophic explosion of the entire battery bank.

Case: Commonly made from a hard rubber or plastic, the case contains the plates, separators and electrolyte in a battery. The case is typically enclosed, with the exception of inter-cell connectors which attach the plate assembly from one cell to the next, terminal posts, and vents or caps which allow gassing products to escape and to permit water additions if required. Clear battery cases or containers allow for easy monitoring of electrolyte levels and battery plate condition. For very large or tall batteries, plastic cases are often supported with an external metal or rigid plastic casing.

5.1.2: Battery Types and Classifications

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Many types and classifications of batteries are manufactured today, each with specific design and performance characteristics suited for particular applications. Each battery type or design has its individual strengths and weaknesses. In PV systems, lead-acid batteries are most common due to their wide availability in many sizes, low cost and well understood performance characteristics. In a few critical, low temperature applications nickel-cadmium cells are used, but their high initial cost limits their use in most PV systems. There is no “perfect battery” and it is the task of the PV system designer to decide which battery type is most appropriate for each application.In general, electrical storage batteries can be divided into to major categories, primary and secondary batteries.Primary Batteries:Primary batteries can store and deliver electrical energy, but cannot be recharged. Typical carbon-zinc and lithium batteries commonly used in consumer electronic devices are primary batteries. Primary batteries are not used in PV systems because they cannot be recharged.Secondary Batteries:A secondary battery can store and deliver electrical energy, and can also be recharged by passing a current through it in an opposite direction to the discharge current. Common lead-acid batteries used in automobiles and PV systems are secondary batteries. Table 1 lists common secondary battery types and their characteristics which are of importance to PV system designers.

Table 1. Secondary Battery Types and Characteristics

Lead-Acid Battery ClassificationsMany types of lead-acid batteries are used in PV systems, each having specific design and performance characteristics. While there are many variations in the design and performance of lead-acid cells, they are often classified in terms of one of the following three categories.

SLI BatteriesStarting, lighting and ignition (SLI) batteries are a type of lead-acid battery designed primarily for shallow cycle service, most often used to power automobile starters. These batteries have a number of thin positive and negative plates per cell, designed to increase the total plate active surface area. The large

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number of plates per cell allows the battery to deliver high discharge currents for short periods. While they are not designed for long life under deep cycle service, SLI batteries are sometimes used for PV systems in developing countries where they are the only type of battery locally manufactured. Although not recommended for most PV applications, SLI batteries may provide up to two years of useful service in small stand-alone PV systems where the average daily depth of discharge is limited to 10-20%, and the maximum allowable depth of discharge is limited to 40-60%.Motive Power or Traction Batteries: Motive power or traction batteries are a type of lead acid battery designed for deep discharge cycle service, typically used in electrically operated vehicles and equipment such as golf carts, fork lifts and floor sweepers. These batteries have a fewer number of plates per cell than SLI batteries; however the plates are much thicker and constructed more durably. High content lead-antimony grids are primarily used in motive power batteries to enhance deep cycle performance. Traction or motive power batteries are very popular for use in PV systems due to their deep cycle capability, long life and durability of design.Stationary Batteries: Stationary batteries are commonly used in un-interruptible power supplies (UPS) to provide backup power to computers, telephone equipment and other critical loads or devices. Stationary batteries may have characteristics similar to both SLI and motive power batteries, but are generally designed for occasional deep discharge, limited cycle service. Low water loss lead-calcium battery designs are used for most stationary battery applications, as they are commonly float charged continuously.

Lead-Acid Battery ChemistryNow that the basic components of a battery have been described, the overall electrochemical operation of a battery can be discussed. Referring to Figure 10-1, the basic lead-acid battery cell consists of sets positive and negative plates, divided by separators, and immersed in a case with an electrolyte solution. In a fully charged lead-acid cell, the positive plates are lead dioxide (PbO2), the negative plates are sponge lead (Pb), and the electrolyte is a diluted sulfuric acid solution. When a battery is connected to an electrical load, current flows from the battery as the active materials are converted to lead sulfate (PbSO4).

Lead-Acid Cell ReactionThe following equations show the electrochemical reactions for the lead-acid cell. During battery discharge, the directions of the reactions listed goes from left to right. During battery charging, the direction of the reactions are reversed, and the reactions go from right to left. Note that the elements as well as charge are balanced on both sides of each equation.

At the positive plate or electrode:

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PbO2 + HSO4- + 3 H+ + 2 e →PbSO4 +H2O

On the negative electrode

Pb + HSO4- →PbSO4 + H+ + 2 e-

Overall reaction:Pb + PbO2 + 2 H+ + 2 HSO4- →2 PbSO4 + 2 H2O

As the battery is discharged, the active materials PbO2 and Pb in the positive and negative plates, respectively, combine with the sulfuric acid solution to form PbSO4 and water. Note that in a fully discharged battery the active materials in both the positive and negative plates are converted to PbSO4, while the sulfuric acid solution is converted to water.

Battery Strengths and Weaknesses:Each battery type has design and performance features suited for particular applications. Again, no one type of battery is ideal for a PV system applications. The designer must consider the advantages and disadvantages of different batteries with respect to the requirements of a particular application. Some of the considerations include lifetime, deep cycle performance, tolerance to high temperatures and overcharge, maintenance and many others. The following table summarizes some of the key characteristics of the different battery types discussed in the preceding section.

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5.2: Battery Selection Criteria

A Battery system design and selection criterion involves many decisions and trade offs. Choosing the right battery for a PV application depends on many factors. While no specific battery is appropriate for all PV applications, common sense and a careful review of the battery literature with respect to the particular application needs will help the designer narrow the choice. Some decisions on battery selection may be easy to arrive at, such as physical properties, while other decisions will be much more difficult and may involve making tradeoffs between desirable and undesirable battery features. With the proper application of this knowledge, designers should be able to differentiate among battery types and gain some application experience with batteries they are familiar with. Table summarizes some of the considerations in battery selection and design.

Type of system and mode of operationCharging characteristics; internal resistanceRequired days of storage (autonomy)Amount and variability of discharge currentMaximum allowable depth of dischargeDaily depth of discharge requirements

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Accessibility of locationTemperature and environmental conditionsCyclic life and/or calendar life in yearsMaintenance requirementsSealed or unsealedSelf-discharge rateMaximum cell capacityEnergy storage densitySize and weightGassing characteristicsSusceptibility to freezingSusceptibility to sulfationElectrolyte concentrationAvailability of auxiliary hardwareTerminal configurationReputation of manufacturerCost and warranty.

In the individual case, the selection of battery depends upon very many factors and will be influenced by system management and climatic conditions. The special requirements made of the battery in operation can be broadly classified according to the operating time per year, the type of loads ( high or low power drain) and the number of cycles per week. However, it is still difficult to make generalizations about which battery type is best for which typical applications, as the basic conditions (such as cost, housing capabilities, maintenance capabilities and reliability requirements) can be deciding factors.

Batteries for use in a pv stand-alone system should have the following features:

1. Good Price/ Performance ratio2. Low maintenance requirements 3. Sufficiently long service life4. Low self-discharging and high energy efficiency5. Can be charged with small charge currents6. High energy and power density (space requirement and weight )7. Vibration resistant (Mobile use or for transportation)8. Protection against health and environmental hazards, Recyclable.

No storage type fulfills all the stated requirements to the same extent. It has to be decided which are the most important properties, according to the respective application.

In sporadically used stationary systems (holiday home, weekend home and summer house), a simple solar battery with fluid electrolyte meets the requirements for the PV accumulator and is an excellent choice. It is cost effective, has low maintenance and achieves a long service life if it is generously

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dimensioned. If it can be housed in a protected location, it is safe against external damage and spillage of acid.

In the use of systems, that are used all year around, the service life of solar and gel batteries is not sufficient. In this case, the use of stationary batteries of block batteries , type can be recommended.

When purchasing batteries, it should also be ensured that the manufacturers specifications listed below are available.

1. Capacity specification with the associated discharge: Since manufacturers base the nominal capacities on different discharge times at the very least the C10 and C100 values for the 10- and 100- hour power discharge should be specified in order to be able to compare various products.

2. Nominal acid density, acid volume or weight

3. A graph or table showing the cycle resilience or the expected service life in years, in relation to the discharge depth. With the aid of this information, it is possible to tell which the most cost-effective product is.

In general, transparent housings should be preferred as they enable better visual checking with regard to acid level, sedimentation, corrosion of terminals. Sealed batteries however, are not available with transparent housings.

5.3: FACTORS EFFECTING BATTERY PERFORMANCE:

5.3.1: Temperature Effects:

For an electrochemical cell such as a battery, temperature has important effects on performance. Generally, as the temperature increases by 10o C the rate of an electrochemical reaction doubles, resulting in statements from battery manufacturers that battery life decreases by a factor of two for every 10o C increase in average operating temperature. Higher operating temperatures accelerate corrosion of the positive plate grids, resulting in greater gassing and electrolyte loss. Lower operating temperatures generally increase battery life. However, the capacity is reduced significantly at lower temperatures, particularly for lead-acid batteries. When severe temperature variations from room temperatures exist, batteries are located in an insulated or other temperature-

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regulated enclosure to minimize battery temperature swings.

Temperature effects on battery life

5.3.2: Corrosion

The electrochemical activity resulting from the immersion of two dissimilar metals in an electrolyte, or the direct contact of two dissimilar metals causing one material to undergo oxidation or lose electrons and causing the other material to undergo reduction, or gain electrons. Corrosion of the grids supporting the active material in a battery is an ongoing process and may ultimately dictate the battery's useful lifetime. Battery terminals may also experience corrosion due to the action of electrolyte gassing from the battery, and generally require periodic cleaning and tightening in flooded lead-acid types. Higher temperatures and the flow of electrical current between two dissimilar metals accelerates the corrosionProcess.

5.3.3: Battery Gassing

Gassing occurs in a battery during charging when the battery is nearly fully charged. At this point, essentially all of the active materials have been converted to their fully charged composition and the cell voltage rises sharply. The gas products are either recombined internal to the cell as in sealed or valve regulated batteries, or released through the cell vents in flooded batteries. In general, the overcharge or gassing reaction in batteries is irreversible, resulting in water loss. However in sealed lead-acid cells, an internal recombinant process permits the reforming of water from the hydrogen and oxygen gasses generated under normal charging conditions, allowing the battery to be sealed and requiring no electrolyte maintenance. All gassing reactions consume a portion of the charge current which can not be delivered on the subsequent discharge, thereby reducing the battery charging efficiency.

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The onset of gassing in a lead-acid cell is not only determined by the cell voltage, but the temperature as well. As temperatures increase, the corresponding gassing voltage decreases for a particular battery. Regardless of the charge rate, the gassing voltage is the same, however gassing begins at a lower battery state of charge at higher charge rates. The grid design, whether lead-antimony or lead-calcium also affects gassing.

The charge regulation voltage, or the maximum voltage that a charge controller allows a battery to reach in operation plays an important part in battery gassing. Charge controllers are used in photovoltaic power systems to allow high rates of charging up to the gassing point, and then limit or disconnect the PV current to prevent overcharge. The highest voltage that batteries are allowed to reach determines in part how much gassing occurs. If charge regulation voltages in a typical PV system were set at the manufacturer’s recommended float voltage, the batteries would never be fully charged.

5.3.4: Sulfation

Sulfation is a normal process that occurs in lead-acid batteries resulting from prolonged operation at partial states of charge. Even batteries which are frequently fully charged suffer from the effects of sulfation as the battery ages. The sulfation process involves the growth of lead sulfate crystals on the positive plate, decreasing the active area and capacity of the cell. During normal battery discharge, the active materials of the plates are converted to lead sulfate. The deeper the discharge, the greater the amount of active material that is converted to lead sulfate. During recharge, the lead sulfate is converted back into lead dioxide and sponge lead on the positive and negative plates, respectively. If the battery is recharged soon after being discharged, the lead sulfate converts easily back into the active materials.However, if a lead-acid battery is left at less than full state of charge for prolonged periods (days or weeks), the lead sulfate crystallizes on the plate and inhibits the conversion back to the active materials during recharge. The crystals essentially “lock away” active material and prevent it from reforming into lead and lead dioxide, effectively reducing the capacity of the battery. If the lead sulfate crystals grow too large, they can cause physical damage to the plates. Sulfation also leads to higher internal resistance within the battery, making it more difficult to recharge. Sulfation is a common problem experienced with lead-acid batteries in many PV applications. As the PV array is sized to meet the load under average conditions, the battery must sometimes be used to supply reserve energy during periods of excessive load usage or below average insolation. As a consequence, batteries in most PV systems normally operate for some length of time over the course of a year at partial states of charge, resulting in some degree of sulfation. The longer the period and greater the depth of discharge, the greater the extent of sulfation.

To minimize sulfation of lead acid batteries in photovoltaic systems, the PV array is generally designed to recharge the battery on the average daily conditions during the worst insolation month of the year. By sizing for the worst month’s

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weather, the PV array has the best chance of minimizing the seasonal battery depth of discharge. In hybrid systems using a backup source such as a generator or wind turbine, the backup source can be effectively used to keep the batteries fully charged even if the PV array cannot. In general, proper battery and array sizing, as well as periodic equalization charges can minimize the onset of sulfation.

5.3.5: Stratification

Stratification is a condition that can occur in flooded lead-acid batteries in which the concentration or specific gravity of the electrolyte increases from the bottom to top of a cell. Stratification is generally the result of undercharging, or not providing enough overcharge to gas and agitate the electrolyte during finish charging. Prolonged stratification can result in the bottom of the plates being consumed, while the upper portions remaining in relatively good shape, reducing battery life and capacity. Tall stationary cells, typically of large capacity, are particularly prone to stratification when charged at low rates. Periodic equalization charges thoroughly mix the electrolyte and can prevent stratification problems.

5.4: Battery Auxiliary EquipmentBattery auxiliary equipment includes any systems or other hardware necessary to safely and effectively operate a battery system. Some of the more important battery auxiliary systems and equipment are discussed below.

5.4.1: Enclosures

Batteries are generally required by local electrical codes and safety standards to be installed in an enclosure separated from controls or other PV system components. The enclosure may also be insulated, or may have active or passive cooling/heating mechanisms to protect the batteries from excessive temperatures. Battery enclosures must be of sufficient size and strength hold the batteries, and can be located below ground if needed to prevent freezing. If the enclosure is located above ground, care should be taken to limit the direct exposure to sunlight, or some type of shading or reflective coating should be provided.

5.4.1.1: Passive Cooling Enclosures

We have shown that temperature is a critical factor affecting battery performance and life expectancy. Any actions taken by the system designer to reduce temperature swings will be rewarded with better battery performance, longer life, and lower maintenance.

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One approach to moderating the influence of ambient temperature swings on battery temperature is the use of passive cooling enclosures, without the need for active components such as motors, fans or air conditioners. The use of active temperature regulation means generally requires additional electrical power, and adds unnecessarily to the complexity, size and cost of the PV system. By using a thermodynamically passive approach, maximum benefits are gained with minimal complexity and maximum reliability – key features of any PV system installation.

5.4.1.2: Ventilation

Batteries often produce toxic and explosive mixtures of gasses, namely hydrogen, and adequate ventilation of the battery enclosure is required. In most cases, passive ventilation techniques such as vents or ducts may be sufficient. In some cases, fans may be required to provide mechanical ventilation. Required air change rates are based on maintaining minimum levels of hazardous gasses in the enclosure. Under no circumstances should batteries be kept in an unventilated area or located in an area frequented by personnel.

5.4.1.3: Catalytic Recombination Caps

A substitute for standard vented caps on lead-antimony batteries, catalytic recombination caps (CRCs) primary function is to reduce the electrolyte loss from the battery. CRCs contain particles of an element such as platinum or palladium, which surfaces adsorb the hydrogen generated from the battery during finishing and overcharge. The hydrogen is then recombined with oxygen in the CRC to form water, which drains back into the battery. During this recombination process, heat is released from the CRCs as the combination of hydrogen and oxygen to form water is an exothermic process. This means that temperature increases in CRCs can be used to detect the onset of gassing in the battery. If CRCs are found to be at significantly different temperatures during recharge (meaning some cells are gassing and others are not), an equalization charge may be required. The use of CRCs on open-vent, flooded lead-antimony batteries has proven to reduce electrolyte loss by as much as 50% in subtropical climates.

5.5: BATTERY SAFETY CONSIDERATIONS

Due to the hazardous materials and chemicals involved, and the amount of electrical energy which they store, batteries are potentially dangerous and must be handled and used with caution. Typical batteries used in stand-alone PV systems can deliver up to several thousand amps under short-circuit conditions, requiring special precautions. Depending on the size and location of a battery installation, certain safety precautions are be required.

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5.5.1: Handling Electrolyte

The caustic sulfuric acid solution contained in lead-acid batteries can destroy clothing and burn the skin. For these reasons, protective clothing such as aprons and face shields should be worn by personnel working with batteries. To neutralize sulfuric acid spills or splashes on clothing, the spill should be rinsed immediately with a solution of baking soda or household ammonia and water. For nickel-cadmium batteries, the potassium hydroxide electrolyte can be neutralized with a vinegar and water solution. If electrolyte is accidentally splashed in the eyes, the eyes should be forced open and flooded with cool clean water for fifteen minutes. If acid electrolyte is taken internally, drink large quantities of water or milk, followed by milk of magnesia, beaten eggs or vegetable oil. Call a physician immediately.

If it is required that the electrolyte solution be prepared from concentrated acid and water, the acid should be poured slowly into the water while mixing. The water should never be poured into the acid. Appropriate nonmetallic funnels and containers should be used when mixing and transferring electrolyte solutions.

5.5.2: Personnel Protection

When performing battery maintenance, personnel should wear protective clothing such as aprons, ventilation masks, goggles or face shields and gloves to protect from acid spills or splashes and fumes. If sulfuric acid comes into contact with skin or clothing, immediately flush the area with a solution of baking soda or ammonia and water. Safety showers and eye washes may be required where batteries are located in close access to personnel. As a good practice, some type of fire extinguisher should be located in close proximity to the battery area if possible. In some critical applications, automated fire sprinkler systems may be required to protect facilities and expensive load equipment. Jewelry on the hands and wrists should be removed, and properly insulated tools should be used to protect against inadvertent battery short-circuits.

5.5.3: Dangers of Explosion

During operation, batteries may produce explosive mixtures of hydrogen and oxygen gasses. Keep spark, flames, burning cigarettes, or other ignition sources away from batteries at all times. Explosive gasses may be present for several hours after a battery has been charged. Active or passive ventilation techniques are suggested and often required, depending on the number of batteries located in an enclosure and their gassing characteristics. The use of battery vent caps with a flame arrester feature lowers the possibility of a catastrophic battery explosion. Improper charging and excessive overcharging may increase the possibility of battery explosions. When making and breaking connections to a battery from a charging source or electrical load, ensure that the charger or load is switched off as to not create sparks or arcing during the connection.

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5.5.4: Battery Disposal and Recycling

Batteries are considered hazardous items as they contain toxic materials such as lead, acids and plastics which can harm humans and the environment. For this reason, laws have been established which dictate the requirements for battery disposal and recycling. In most areas, batteries may be taken to the local landfill, where they are in turn taken to approved recycling centers. In some cases, battery manufacturers provide guidelines for battery disposal through local distributors, and may in fact recycle batteries themselves. Under no circumstances should batteries be disposed of in landfills, or the electrolyte allowed to seep into the ground, or the battery burned.

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6: ECONOMY OF SOLAR BASED UNINTERRUPTED POWER SUPPLY

The energy payback time of a power generating system is the time required to generate as much energy as was consumed during production of the system. In 2000 the energy payback time of PV systems was estimated as 8 to 11 years and in 2006 this was estimated to be 1.5 to 3.5 years for crystalline silicon PV systems and 1-1.5 years for thin film technologies.

Another economic measure, closely related to the energy payback time, is the energy returned on energy invested (EROEI) which is the ratio of electricity generated divided by the energy required to build and maintain the equipment. (This is not the same as the economic return on investment (EROI), which varies according to local energy prices, subsidies available and metering techniques.) With lifetimes of at least 30 years, the EROEI of PV systems are in the range of 10 to 30, thus generating enough energy over their lifetimes to reproduce themselves many times (6-31 reproductions) depending on what type of material, balance of system (BOS), and the geographic location of the system.

6.1 Power costs:

The PV industry is beginning to adopt levelized cost of energy (LCOE) as the unit of cost. For a 10 MW plant in Phoenix, AZ, the LCOE is estimated at $0.15 to 0.22/kWh in 2005.

The table below is a pure mathematical calculation. It illustrates the calculated total cost in US cents per kilowatt-hour of electricity generated by a photovoltaic system as function of the investment cost and the efficiency, assuming some accounting parameters such as cost of capital and depreciation period. The row headings on the left show the total cost, per peak kilowatt (kWp), of a photovoltaic installation. The column headings across the top refer to the annual energy output in kilowatt-hours expected from each installed peak kilowatt. This varies by geographic region because the average insolation depends on the average cloudiness and the thickness of atmosphere traversed by the sunlight. It also depends on the path of the sun relative to the panel and the horizon.

Panels can be mounted at an angle based on latitude, or solar tracking can be utilized to access even more perpendicular sunlight, thereby raising the total energy output. The calculated values in the table reflect the total cost in cents per kilowatt-hour produced. They assume a 10% total capital cost (for instance 4% interest rate, 1% operating and maintenance cost, and depreciation of the capital outlay over 20 years).

Physicists have claimed that recent technological developments bring the cost of solar energy more in parity with that of fossil fuels. In 2007, David Faiman, the director of the Ben-Gurion National Solar Energy Center of Israel, announced

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that the Center had entered into a project with Zenith Solar to create a home solar energy system that uses a 10 square meter reflector dish. In testing, the concentrated solar technology proved to be up to five times more cost effective than standard flat photovoltaic silicon panels, which would make it almost the same cost as oil and natural gas. A prototype ready for commercialization achieved a concentration of solar energy that was more than 1,000 times greater than standard flat panels.

6.2 Grid parity:

Grid parity, the point at which photovoltaic electricity is equal to or cheaper than grid power, is achieved first in areas with abundant sun and high costs for electricity such as in California and Japan.

Grid parity has been reached in Hawaii and other islands that otherwise use fossil fuel (diesel fuel) to produce electricity, and most of the US is expected to reach grid parity by 2015.

General Electric's Chief Engineer predicts grid parity without subsidies in sunny parts of the United States by around 2015. Other companies predict an earlier date the cost of solar power will be below grid parity for more than half of residential customers and 10% of commercial customers in the OECD, as long as grid electricity prices do not decrease through 2010.

The fully loaded cost (cost not price) of solar electricity is $0.25/kWh or less in most of the OECD countries. By late 2011, the fully loaded cost is likely to fall below $0.15/kWh for most of the OECD and reach $0.10/kWh in sunnier regions. These cost levels are driving three emerging trends:

1. vertical integration of the supply chain;2. origination of power purchase agreements (PPAs) by solar power

companies;3. Unexpected risk for traditional power generation companies, grid

operators and wind turbine manufacturers.

Abengoa Solar has announced the award of two R&D projects in the field of Concentrating Solar Power (CSP) by the US Department of Energy that total over $14 million. The goal of the DOE R&D program, working in collaboration with partners such as Abengoa Solar, is to develop CSP technologies that are competitive with conventional energy sources (grid parity) by 2015. Concentrating photovoltaics (CPV) could reach grid parity in 2011.

Due to the growing demand for photovoltaic electricity, more companies enter into this market and lower cost of the photovoltaic electricity would be expected.

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7. WORKING OF SOLAR BASED UN-INTERRUPTED POWER SUPPLY SYSTEM

7.1: TYPES OF PV SYSTEM

Photovoltaic power systems are generally classified according to their functional and operational requirements, their component configurations, and how the equipment is connected to other power sources and electrical loads. The two principal classifications are grid-connected or utility-interactive systems and stand-alone systems. Photovoltaic systems can be designed to provide DC and/or AC power service, can operate interconnected with or independent of the utility grid, and can be connected with other energy sources and energy storage systems.

7.1.1: GRID CONNECTED PV SYSTEMS:

Grid-connected or utility-interactive PV systems are designed to operate in parallel with and interconnected with the electric utility grid. The primary component in grid-connected PV systems is the inverter, or power conditioning unit (PCU). The PCU converts the DC power produced by the PV array into AC power consistent with the voltage and power quality requirements of the utility grid, and automatically stops supplying power to the grid when the utility grid is not energized. A bi-directional interface is made between the PV system AC output circuits and the electric utility network, typically at an on-site distribution panel or service entrance. This allows the AC power produced by the PV system to either supply on-site electrical loads, or to back-feed the grid when the PV system output is greater than the on-site load demand. At night and during other periods when the electrical loads are greater than the PV system output, the balance of power required by the loads is received from the electric utility This safety feature is required in all grid-connected PV systems, and ensures that the PV system will not continue to operate and feed back into the utility grid when the grid is down for service or repair.

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Figure 1. Diagram of grid-connected photovoltaic system

Grid-Intertied Solar-Electric Systems with Battery Backup

Array DC disconnect: The DC disconnect is used to safely interrupt the flow of electricity from the PV array. It’s an essential component when system maintenance or troubleshooting is required.The disconnect enclosure houses an electrical switch rated for use in DC circuits. It also may integrate either circuit breakers or fuses, if needed.

Charge Controller: A charge controller’s primary function is to protect your battery bank from overcharging. It does this by monitoring the battery bank—when the bank is fully charged, the controller interrupts the flow of electricity from the PV panels. Batteries are expensive and pretty particular about how they like to be treated. To maximize their life span, you’ll definitely want to avoid

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overcharging or undercharging them. Most modern charge controllers incorporate maximum power point tracking (MPPT), which optimizes the PV array’s output, increasing the energy it produces. Some battery based charge controllers also include a low-voltage disconnect that prevents over discharging, which can permanently damage the battery bank.

Battery Bank: When there is no power supply from the grid, the power stored in the battery bank is used for supplying the loads connected to the system. The batteries are to be chosen carefully considering the performance characteristics of the other equipment connected to the system like inverters.System meter: System meters measure and display several different aspects of your solar-electric system’s performance and status, tracking how full battery bank is; how much electricity your solar panels are producing or have produced; and how much electricity is in use..

Main DC disconnect: In battery-based systems, a disconnect between the batteries and inverter is required. This disconnect is typically a large, DC-rated breaker mounted in a sheetmetal enclosure. This breaker allows the inverter to be quickly disconnected from the batteries for service, and protects the inverter-to-battery wiring against electrical fires.

Inverter: The feeding of electricity into the grid requires the transformation of DC into AC by a special, grid-controlled inverter.In kW sized installations the DC side system voltage is as high as permitted (typically 1000V except US residential 600V) to limit ohmic losses. Most modules (72 crystalline silicon cells) generate about 160W at 36 volts. It is sometimes necessary or desirable to connect the modules partially in parallel rather than all in series. One set of modules connected in series is known as a 'string'

Grid connected inverter

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On the AC side, these inverters must supply electricity in sinusoidal form, synchronized to the grid frequency, limit feed in voltage to no higher than the grid voltage including disconnecting from the grid if the grid voltage is turned off.

On the DC side, the power output of a module varies as a function of the voltage in a way that power generation can be optimized by varying the system voltage to find the 'maximum power point'. Most inverters therefore incorporate 'maximum power point tracking'.The inverters are designed to connect to one or more strings.

AC Breaker Panel & Inverter AC Disconnect: The AC breaker panel is the point at which all of a home’s electrical wiring meets with the “provider” of the electricity, whether that’s the grid or a solar-electric system.

Kilowatt- Hour Meter: Most homes with a grid-tied solar electric system will have AC electricity both coming from and going to the electric utility grid. A bidirectional KWH meter can simultaneously keep track of how much electricity flows in each of the two directions.

In some countries, for installations over 30kWp a frequency and a voltage monitor with disconnection of all phases is requiredFor safety reasons a circuit breaker is provided both on the AC and DC side to enable maintenance. The AC output usually goes through across an electricity meter into the public grid.

Using grid-connected PV power can have economic as well as environmental advantages. Where utility power is available, consumers can use a grid-connected PV system to supply some of the power they need and use utility-generated power at night and on very cloudy days. When the PV system supplies power to the grid as well as to a specific building or piece of equipment, the utility becomes a kind of storage device or battery for PV-generated power.

7.1.2: STAND ALONE PV SYSTEMS:

Stand-alone PV systems are designed to operate independent of the electric utility grid, and are generally designed and sized to supply certain DC and/or AC electrical loads. These types of systems may be powered by a PV array only, or

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may use wind, an engine-generator or utility power as an auxiliary power source in what is called a PV hybrid system. The simplest type of stand-alone PV system is a direct-coupled system, where the DC output of a PV module or array is directly connected to a DC load (Figure 5). Since there is no electrical energy storage (batteries) in direct-coupled systems, the load only operates during sunlight hours, making these designs suitable for common applications such as ventilation fans, water pumps, and small circulation pumps for solar thermal water heating systems. Matching the impedance of the electrical load to the maximum power output of the PV array is a critical part of designing well-performing direct-coupled system. For certain loads such as positive displacement water pumps, a type of electronic DC-DC converter, called a maximum power point tracker (MPPT) is used between the array and load to help better utilize the available array maximum power output.

In many stand-alone PV systems, batteries are used for energy storage. The following figure shows a diagram of a typical stand-alone PV system powering DC and AC loads. Shows how a typical PV hybrid system might be configured.

Diagram of stand-alone PV system with battery storage powering DC and AC loads

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Diagram of photovoltaic hybrid system

WORKING OF STAND ALONE SYSTEMS:

As said above the direct coupled system does not use an energy storage system in between the pv array and the load. Hence it is capable of supplying to the load only during the day hours when there is sunlight available. The working here is quite simple. The sunlight is converted to electrical energy by the pv array, which is directly supplied to the load. If the load is ac, then a converter might have to be used. In order to maximize the power transfer from the array to the load impedance matching is to be optimized. This is explained below:

Load matching factor:

This is defined as the ratio of the load energy to the array maximum energy in a one day period. It is used as a measure for load matching of the pv array. Optimum matching of parameters can be done by carefully selecting the array parameters with respect to the load parameters. The temperature of the array has little effect on the optimum matching factor. However the optimum matching parameters are greatly affected by the array temperature. A battery of selected parameters can be included if the load characteristics results in poor matching performance. Matching the impedance of the electrical load to the maximum power output of the PV array is a critical part of designing well-performing direct-coupled system. Because this influences the power transfer power transfer between the pv array and the load connected directly as for the maximum power to take place between the source and load which is the main requirement in a pv system, transfer of maximum power.

Several studies were conducted for optimum load matching and it’s characteristics with different loads.

It was found out that Resistive loads are optimized by selecting the load rate power, voltage and resistance with respect to those of the array. The optimum matching can be as high as 94.34% at 25 C.

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Electrolytic loads are found compatible with the array. The optimum matching factor is 99.83% at 25C. This can be achieved by carefully selecting the number of series and parallel cells of the array with respect to those of the electrolyte so that the internal emf and resistance of the load are close to the optimum values. The optimum values of power and voltage must be reduced with the increase of the array operating temperature at approximately the same rate as Pmax and Vmax.

Several models were developed for finding the optimum load matching factor are developed.

STAND ALONE PV SYSTEMS (SAPV) vs THERMAL:

A comparison between stand alone systems economy and thermal power economy is presented in the tabular form below:

s.no Functions SAPV (million )

Thermal ( million )

1 Installation cost of Power Plants

17 104.35 12 274.44

2 Transmission and distribution

Nil 5521.45

3 Production cost Nil 5809.37

Cost comparison of SAPV panels versus conventio

8. APPLICATIONS AND ADVANTAGE:

1. Saves you money

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After the initial investment has been recovered, the energy from the sun is practically FREE.

The recovery/ payback period for this investment can be very short depending on how much electricity consumption.

Financial incentives are available form the government that will reduce cost.

If the system produces more energy than usage, utility company can buy it from user, building up a credit on the user account. This is called net metering.

It will save money on the user’s electricity bill. Solar energy does not require any fuel. It's not affected by the supply and demand of fuel and is therefore not

subjected to the increasing price of conventional sources of energy. The savings are immediate and for many years to come. The use of solar energy indirectly reduces health costs.

2. Environmentally friendly

Solar Energy is clean, renewable (unlike gas, oil and coal) and sustainable, helping to protect our environment.

It does not pollute our air by releasing carbon dioxide, nitrogen oxide, sulphur dioxide or mercury into the atmosphere like many traditional forms of electrical generations does.

Therefore Solar Energy does not contribute to global warming, acid rain or smog.

It actively contributes to the decrease of harmful green house gas emissions.

It's generated when it is needed. By not using any fuel, Solar Energy does not contribute to the cost and

problems of the recovery and transportation of fuel or the storage of radioactive waste.

3. Independent/ semi-independent

Solar Energy can be utilized to offset utility-supplied energy consumption. It does not only reduce your electricity bill, but will also continue to supply your home/ business with electricity in the event of a power outage.

A Solar Energy system can operate entirely independent, not requiring a connection to a power or gas grid at all. Systems can therefore be installed in remote locations (like holiday log cabins), making it more practical and cost-effective than the supply of utility electricity to a new site.

The use of Solar Energy reduces our dependence on foreign and/or centralized sources of energy, influenced by natural disasters or international events and so contributes to a sustainable future.

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Solar Energy supports local job and wealth creation, fuelling local economies.

4. Low/ no maintenance

Solar Energy systems are virtually maintenance free and will last for decades.

Once installed, there are no recurring costs. They operate silently, have no moving parts, do not release offensive

smells and do not require you to add any fuel. More solar panels can easily be added in the future when your family's

needs grow.

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