solar energy - photo voltaic cell

8/3/2019 Solar Energy - Photo Voltaic Cell

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ARJUN PRATAP SINGH

DEVICES AND FIELDS ASSIGNMENT

PHOTOVOLTAIC CELL


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Introduction:

Electricity is generated from sunlight with the help of photovoltaic cell which is majorly known

as solar cell. This is useful in many applications as sunlight is easy to tap to generate fair amount

of electricity (its a renewable source of energy) and overall cost of operation is reduced.

Report Includes:

1. Photovoltaic design, operation, types, advantages, disadvantages and application.

2. The calculation of number of modules and battery needed for the design of a solar energy

system with a given conditions to operate a garden light.

3.

Design of a 12v regulated D.C solar power including the charge and discharge controller.


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Photovoltaic Panels:

Photovoltaic panels are designed to receive energy from sunlight and convert it to electricity.

These panels consist of cells which operate on the principle of photovoltaic action (conversion of

light energy into electrical energy). The radiation of the sunlight falling on this type of panel will

determine its correspond electrical output.

Construction:

Each solar cell basically consists of two wafers of doped P - type and N - type semiconductor

materials. These P and N type materials together form a PN Junction. Each cell is capable of

producing up to 0.7 volts.


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As we can see from above detailed diagrams of photovoltaic cell .That the P-N

diode/semiconductor/wafer is sandwiched two layers of front contact and back contact. Over thefront contact is an anti-reflective coating to increase the radiation absorption, over which is a

superstrate or conducting material for absorbing radiations from the sun and protection. Between

the wafer and the front contact, and wafer and back contact is an electric connection grid, which

connects one cell and to another. Below the back contact is the backsheet for protection from

below.


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Theory of Operation:

When solar cell is radiated by sunlight, the semiconductor or p-n junction comes into play. An

electric field is established near the p-n junction by the positive and negative ions created due to

the production of electron - hole pairs, which leads to the development of potential across the

junction. Since the number of number of electron hole pairs far exceeds the number, needed for

thermal equilibrium, many of the electrons are pulled across the junction by the force of an

electric field. Those that cross the junction will contribute to the current in cell and through the

external load. The terminal voltage of the cell is directly proportional to the intensity of the

incident light.

Each cell voltage is around 0.7V depending on the external load. In order to obtain higher

voltages and currents, a large number of cells are arranged together in series and in parallel.


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Types ofPhoto Voltaic Panels:

There are numerous kinds ofPV panel some are as follow:

1. Monocrystalline Silicon Panels

2. Polycrystalline Silicon Panels

3. String Ribbon or Thin-Layer Silicon Panels

4. Amorphous Silicon or Thin Film Panels

5. Group III-V Technologies

1. Monocrystalline silicon panels:

15-18% efficiency

Monocrystalline panels use crystalline silicon produced in large sheets which can be cutto the size of a panel and integrated into the panel as a single large cell. Conducting metal

strips are laid over the entire cell to capture electrons in an electrical current.

These panels are more expensive to produce than other crystalline panels but have higherefficiency levels and, as a result, are sometimes more cost-effective in the long run.


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2. Polycrystalline Silicon Panels:

12-14% efficiency

Polycrystalline, or multi - crystalline, photovoltaics use a series of cells instead of onelarge cell. These panels are one of the most inexpensive forms of photovoltaics available

today, though the costs of sawing and producing wafers can be high. At the same time,they have lower conversion efficiencies than mono - crystalline panels.

For this technology, several techniques can be used:

Cast Polysilicon:

In this process, molten silicon is first cast in a large block which, when cooled, is in the

form of crystalline silicon and can be sawn across its width to create thin wafers to beused in photovoltaic cells. These cells are then assembled in a panel. Conducting metal

strips are then laid over the cells, connecting them to each other and forming a continuouselectrical current throughout the panel.


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Comparison:

MONOCRYSTALLINE POLYCRYSTALLINE

1. Made of large silicon crystals Made of lots of tiny silicon crystals

2. Efficiency (15 18%) Efficiency (12-14%)

3. Patchwork pattern Geometric pattern

4. More expensive Less expensive

3. String ribbon or Thin Layer Silicon Panels:

String ribbon photovoltaics use a variation on the polycrystalline production process,

using the same molten silicon but slowly drawing a thin strip of crystalline silicon out ofthe molten form. These strips of photovoltaic material are then assembled in a panel with

the same metal conductor strips attaching each strip to the electrical current.

This technology saves on costs over standard polycrystalline panels as it eliminates thesawing process for producing wafers. Some string ribbon technologies also have higher

efficiency levels (18%) than other polycrystalline technologies.


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4. Amorphous Silicon or Thin Film Panels:

5-6% efficiency

Thin-film panels are produced very differently from crystalline panels. Instead ofmolding, drawing or slicing crystalline silicon, the silicon material in these panels has nocrystalline structure and can be applied as a film directly on different materials.

Variations on this technology use other semiconductor materials like copper indiumdiselenide (CIS) and cadmium telluride (CdTe). These materials are then connected to

the same metal conductor strips used in other technologies, but do not necessarily use theother components typical in photovoltaic panels as they do not require the same level of

protection needed for more fragile crystalline cells.

The primary advantages of thin-film panels lie in their low manufacturing costs andversatility. Because amorphous silicon and similar semiconductors do not depend on the

long, expensive process of creating silicon crystals, they can be produced much morequickly and efficiently. As they do not need the additional components used in crystalline

cells, costs can be reduced further. Because they can be applied in thin layers to differentmaterials, it is also possible to make flexible solar cells.

However, thin-film panels have several significant drawbacks. What they gain in costsavings, they lose in efficiency, resulting in the lowest efficiency of any current

photovoltaic technology. Thin-film technologies also depend on silicon with high levelsof impurities. This can cause a drop in efficiency within a short period of use.

Thin-film panels have the potential to grow in use, and already figure in some of the most

exciting enhanced photovoltaic systems, including high-efficiency multi - junctiondevices and building integrated photovoltaics.


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5. Group III-V Technologies

25% efficiency

These technologies use a variety of materials with very high conversion efficiencies.

These materials are categorized as Group III and Group V elements in the Periodic Table.A typical material used in this technology is gallium arsenide, which can be combined

with other materials to create semiconductors that can respond to different types of solarenergy.

Though these technologies are very effective, their current use is limited due to their

costs. They are currently employed in space applications and continue to be researchedfor new applications.

Enhanced Systems

1. Building-Integrated Photovoltaics (BIPV)BIPV technologies are designed to serve the dual purpose of producing electricity and

acting as a construction material. There are many forms that this technology can take.One common structure is the integration of a semi-translucent layer of amorphous silicon

into glass, which can then be used as window panes that let controlled amounts of lightinto a building while producing electricity. Another common structure is the use of

shingle-sized panel of amorphous silicon as a roofing material.

Currently, BIPV technologies have very low efficiency levels due to their use ofamorphous silicon, but present the advantage of replacing other construction materials

and offering a wide variety of aesthetic choices for the integration of photovoltaics intobuildings.

2. Concentrator SystemsConcentrator systems are designed to increase the efficiency of solar photovoltaics. Thesesystems cover a standard photovoltaic panel with concentrating optics, or lenses that

gather sunlight and increase its intensity in hitting the photovoltaic panel. These systems


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reduce the amount of photovoltaics needed to produce electricity, and also reduce theamount of space needed for a photovoltaic installation.

Their main disadvantage is that they depend solely on direct light to produce electricity,

while stand-alone photovoltaic panels can use both direct and diffuse light. Many regions

do not receive enough direct light throughout the year for these systems to be practical.Another disadvantage is the complexity of their construction, which makes these systemsmore difficult to build and install than photovoltaic panels on their own.

3. High-Efficiency Multijunction DevicesMulti - junction devices receive their name from their use of multiple layers of cells, each

layer acting as a junction where certain amounts of solar energy are absorbed. Each layerin a multi - junction device is made from a different material with its own receptivity to

certain types of solar energy.

In a typical device, the top photovoltaic layer responds to solar waves that travel in shortwavelengths and carry the highest energy, absorbing this energy and creating an electrical

charge. As other solar waves pass through this layer, they are absorbed and translated intoelectricity by the lower layers. Typical materials used in this device include gallium

arsenide and amorphous silicon.Though some two-junction devices have successfully been built, these devices are still

largely in the research and development stage, with most research focused on three- andfour-junction devices.

Application ofPhoto Voltaic Panels:

1. In Transport:Solar panels are used to power cars, train and even it is employed in aerospace

technology and aviation system to power plane and jets. As its one of the most cost

effective ways of generating energy\fuel.


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2. Solar Roadways:

Road ways are now being integrated with solar power, where solar panels will be laid on

the road way and the movement/ pressure on the panels will produce the correspond

electricity which will later be used for specific operation


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3. Power Stations:

Solar power station is now becoming the major focus in electricity power generation. It is

being adopted by power stations where some uses sunlight heat to generate steam and

drive a generator to make electricity.

4. Rural Electrification:

In rural areas where electricity could not reach by other power generation sources .Solar

energy tapping with help of solar panels has played its part in getting the electricity to

these areas for low cost.


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Solar Energy System Design Calculations:

System requirement:

It is required to design a solar energy system to operate a garden light as following:

1. System nominal voltage is 12V DC.2. System nominal Current is 2A DC.3. Days of autonomy are 4 days.4. Daily energy consumption is 288 W.h.5. Daily anticipated peak sunshine hours are 4 hours.6. Cable and circuit operation losses are 0.25 .7. The PV panel power is 18.75 W.8. 100 A rechargeable battery


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Calculating:

System nominal Current = 2A DC.

Daily energy consumption = 288W.h (Ed)

Cable and circuit operation losses = 0.25 . (El)

Total daily energy consumption (ET) = (ED) + (El)

Converting the resistance loss to energy loss

We know that energy (E) = (I2RT)

El = 22**0.25*12 (daily operating hours)

El = 12W.h.

So,

ET = + 12 = 300W.h

Energy absorbed by each panel daily = energy absorbed by each panel x total peak

sunshine hours

= 18.75 x 4

= 75W.h.

=

= 4 modules

For clearance and extension, 33% margin is included.

= 33% of 4

= 1.32.


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Total number of panel required is 4 + 1.32 = 5.32 = 6 (approx.)

Battery

Days of autonomy = 4

Daily energy used = 300 W.h

Energy reqd. in 4 days = 4*300 = 1200 W.h

Q = E\V = 1200\12 = 100 A.h (charge to be stored in battery)

33 % * 100 = 33 (prevention method for battery from fully discharging)

100 + 33 = 133 A.h

So,

Battery we require should be of 150 A.h rating

Solar Panel Charger Circuit:

Working of the Circuit:

The circuit is a single transistor oscillator called a feedback oscillator, or more accurately a

blocking oscillator. It has 45 turns on the primary and 15 turns on the feedback winding. There is

no secondary as the primary produces a high voltage during part of the cycle and this voltage is

delivered to the output via a high-speed diode to produce the output.

The transistor is turned on via the 1 ohm base resistor. This causes current to flow in the primary

winding and produce magnetic flux. This flux cuts the turns of the feedback winding and

produces a voltage in the winding that turns the transistor on. This continues until the transistor

is fully turned on and at this point, the magnetic flux in the core of the transformer is a

maximum. But it is not expanding flux. It is stationary flux and does not produce a voltage in the

feedback winding. Thus the turn-on voltage from the feedback winding disappears and the


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transistor turns off slightly (it has the turn-on effect of the 1 ohm resistor).

The magnetic flux in the core of the transformer begins to collapse and this produces a voltage in

the feedback winding that is opposite to the previous voltage. This has the effect of working

against the 1 ohm resistor and turns off the transistor. The transistor continues to turn off until it

is fully turned off. At this point the 1 ohm resistor on the base turns the transistor on and cycle

begins at the same time.The collapsing magnetic flux is producing a voltage in the primary

winding. Because the transistor is being turned off during this time, we can consider it to be

removed from the circuit and the winding is connected to a high-speed diode. The energy

produced by the winding is passed through the diode and appears on the output as a high voltage

spike. This high voltage spike also carries current and thus it represents energy. This energy is

fed into the load and in our case the load is a battery being charged. When a magnetic circuit

collapses (the primary winding is wound on a ferrite rod and this is called a magnetic circuit), the

voltage produced in the winding depends on the quality of the magnetic circuit and the speed at

which it collapses. The voltage can be 5, 10 or even 100 times higher than the applied voltage

and this is why we have used it. This is just one of the phenomenons of a magnetic circuit. The

collapsing magnetic flux produces a voltage in each turn of the winding and the actual voltage

depends on how much flux is present and the speed of the collapse. The only other two

components are the electrolytics. The 100u across the solar panel is designed to reduce the

impedance of the panel so that the circuit can work as hard as possible.

The circuit is classified as very low impedance. The low impedance comes from the fact the

primary of the transformer is connected directly across the input during part of the cycle.

The circuit requires a very high current for part of the cycle. If the average current is 150mA, the

instantaneous current could be as 300mA or more. The panel is not capable of delivering this

current and so we have a storage device called an electrolytic to deliver the peaks of current.

The 10u works in a similar manner. When the feedback winding is delivering its peak of current,

the voltage (and current) will flow out both ends of the winding. To prevent it flowing out the

end near the 1R resistor, an electrolytic is placed at the end of the winding. The current will now

only flow out the end connected to the base of the transistor. It tries to flow out the other end but

in doing so it has to charge the electrolytic and this take a long period of time.

These two components improve the efficiency of the circuit considerably. The energy into the

battery will be delivered according to the voltage of each source.


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This circuit is only suitable if you have a constant, reliable, source of sun as any clouds will

reduce the output to below the regulated voltage. The BC 547 prevents the ZXT 851 oscillator

transistor turning on when the voltage is above 12v (charging battery of 12 v). The 10uF on the

output stores the reference voltage and keeps the BC 547 turned on during the time when the

output voltage is above 12v. This effectively stops the oscillator, but as soon as the output

voltage drops below 12v, the circuit comes back into operation, charge-pumping the 10uF on the

output.

The components used in the circuit are:

1.

Transistors amplify current, for example they can be used to amplify the small output

current from a logic IC so that it can operate a lamp, relay or other high current device. In

many circuits a resistor is used to convert the changing current to a changing voltage, so

the transistor is being used to amplify voltage. A transistor may be used as a switch

(either fully on with maximum current, or fully off with no current) and as an amplifier

(always partly on).The amount of current amplification is called the current gain.

2. Zener Diode is a special kind of diode which permits current to flow in the forward

direction as normal, but will also allow it to flow in the reverse direction when the

voltage is above a certain value - the breakdown voltage known as the Zener voltage.


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A zener diode can be used to make a simple voltage regulation circuit as pictured above.

The output voltage is fixed at the zener voltage of the zener diode used and so can be

used to power devices requiring a fixed voltage

3. Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol

shows the direction in which the current can flow. Diodes are the electrical version of a

valve and early diodes were actually called valves.

4. Transformers are solution to the voltage and frequency problem. What transformers do

is take the voltage and adjust it as it comes into the appliance or machine to the proper

level. They also push the electricity through the machine to keep it running correctly.


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5. Resistors

6. Capacitors

(Output of 5.9v was got while doing the practicals from solar panel producing 17.5v)


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Conclusion:

Advantages of Solar Panels

1. No pollution

2. It uses renewable source of energy

3. Sunlight is free of cost

4. Low maintenance cost

5. Its easy to use

Disadvantages of Solar Panels

1. Its initial installation cost is very high

2. It requires quite a large area for installation

3. Its output relies solely on amount of radiation from the sun

4. Weather plays a major factor in its output

5. Still quite new to the world

The energy generated from the solar panel is stored in a battery which can be connected to a load

when fully charged. In our case the load will be the garden lights. The battery storage capacity is

taken a little larger than required so that to protect battery from discharging completely.


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solar energy - photo voltaic cell

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