Here is a term paper on ‘Solar Cells‘ for class 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Solar Cells‘ especially written for school and college students.
Term Paper on Solar Cells
Term Paper Contents:
- Term Paper on the Introduction to Solar Cells
- Term Paper on the Different Types of Solar Cells
- Term Paper on the Advantages of Photovoltaic Solar Systems
- Term Paper on the Limitations of Photovoltaic Solar Systems
- Term Paper on the Applications of Photovoltaic Solar Systems
1. Term Paper on the Introduction to Solar Cells:
It is possible to convert solar energy directly into electrical energy by means of silicon wafer photovoltaic cells, also called the solar cells, without any intermediate thermodynamic cycle. The solar cells operate on the principle of photovoltaic effect, which is a process of generating an emf as a result of the absorption of ionizing radiation.
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Thus a solar cell is a transducer, which converts the sun’s radiant energy directly into electrical energy and is basically a semiconductor diode capable of developing a voltage of 0.5-1 volt and a current density of 20-40 mA/cm2 depending on the materials used and the conditions of sunlight. The efficiency of the solar cells is as low as 15%, but that does not matter as the solar energy is basically free of cost.
The main problem faced is that cost (Rs 1,400 to Rs 7,000 per watt) of the solar cells and their maintenance. With the likelihood of a breakthrough in the large scale production of solar cells at low cost, this technology may compete with conventional methods of generation of electrical power, particularly as conventional sources of energy become scarce.
The photovoltaic effect can be observed in nature in a variety of materials but the materials having the best performance in sunlight are the semiconductors. In a piece of pure semiconductor like silicon, there is no free charge carrier at ordinary temperatures, but if this piece of silicon is doped with phosphorous or arsenic there will be one extra electron per atom of the impurity leading to N-type (negative type) semiconductor.
Similarly, if another piece of pure silicon is doped with boron (having one electron less than silicon) there will be deficiency of electrons (or excess of holes) leading to P-type (positive type) semiconductor. If these two pieces of silicon containing N-type and P-type impurities are connected by some means, a junction, at which the nature of the current carrier changes, is created. In fact, a potential energy gap (Eg) is created at the junction.
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When a photon of energy ‘hv’ is allowed to fall on the P-region, it is absorbed by an electron in the valence bond. If ‘hv’ exceeds energy gap Eg, the electron will migrate to the N-region. Similarly if ‘hv’ is less than Eg in the N-region, the photon will be absorbed by a hole which will migrate to P-region.
This charge separation creates an electric field opposite to the electric field created by the diffusion of free electrons of the N-region and in case the field created by charge separation predominates the electric field created by the diffusion of free electrons from N-region to P-region and holes from P-region to N-region current will start flowing in the circuit.
Photovoltaic cells generate a voltage proportional to electromagnetic radiation intensity and are called as such because of their voltage generating capability.
Single crystal silicon is most highly developed material for photovoltaic conversion. The physical properties of single crystal silicon are well known and the raw material is abundant. Single crystal silicon cells have been used for many years as power sources for spacecraft’s in sizes from a few watts to over 20 kW per satellite. However they are still very costly and many attempts have been initiated in Japan, France, West Germany, and USA etc. toward reduction of cost.
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The cadmium sulphide/cuprous sulphide solar cell is the only other commercially available solar cell. It is not a simple P-N junction. It provides a charge separation field by the junction of two dissimilar materials that have different band structures. The advantage of this cell is that it can be made very thin (about 20 µm) using chemical processing techniques and does not require single crystal material. Poor efficiency is their main drawback.
Typically, one cell produces about 1.5 watts of power. Individual cells are connected together to form a solar panel or module, capable of developing 3 to 110 W power. Pannels be connected together in series and parallel to make a solar array which can produce any amount of wattage as space will permit. Modules are usually designed to supply electricity at 12 V. Photovoltaic (PV) modules are rated by their peak watt output at solar noon on a clear day.
Some applications for PV systems are lighting for commercial buildings, outdoor (street) lighting, rural and village lighting, etc. Solar electric power systems can offer independence from the utility grid and offer protection during extended power failures. Solar PV systems are found to be economical especially in the hilly and far flung areas where conventional grid power supply will be expensive to reach.
PV tracking system is an alternative to the fixed, stationary PV panels. PV tracking systems are mounted and provided with tracking mechanisms to follow the sun as it moves through the sky. These tracking system run entirely on their own power and can increase output by 40%. Backup systems are necessary since PV systems only produce electricity during sunshine. The two most common methods of backing up solar electric systems are by connecting the system to the utility grid or storing excess electricity in batteries for use at night or on cloudy days.
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The performance of a solar cell is expressed in terms of its efficiency in conversion of sunlight into electricity. Only sunlight of certain energy will work efficiently to produce electricity and much of it is reflected or absorbed by the material that makes up the cell. Because of this, a typical commercial solar cell has an efficiency of 15% i.e., only about one-sixth of the sunlight striking the cell generates electricity. Low efficiencies mean that larger arrays are required and higher investment costs. It should be noted that the first solar cell, built in the 1950s, had efficiencies of less than 4%.
Compared to other ways of generating electricity, PV systems are expensive, but they are good means of producing electricity in remote areas. Some offshore platforms have begun using PV systems to generate electricity whenever required. One international energy company has designed solar systems to power radio communications, helicopter landing pad lights and navigation warning lights on offshore platforms. Ocean buoys and other monitoring equipment also have PV cells as a power source.
2. Term Paper on the Different Types of Solar Cells:
According to type of crystal, the solar cells are of three types:
1. Mono-crystalline silicon solar cells (band gap 1.12 eV).
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2. Polycrystalline silicon solar cells (band gap 1.12 eV).
3. Thin film or amorphous silicon solar cells (band gap 1.75 eV).
In mono-crystalline silicon cells, silicon is doped with boron to produce P-type semiconductor. Mono-crystalline rods are extracted from silicon and then sawed into thin plates or wafers. The upper layer of the wafers is doped with phosphorous to produce N-type semiconductor.
A silicon solar cell of size 10 cm × 10 cm develops a voltage of 0.5 V and power output of 1 W at a solar radiation intensity of 1,000 W/m2. The solar cells are formed into modules by enclosing in an airtight casing with a transparent cover of synthetic glass. These modules possess high efficiency between 15 and 18% and are used in medium and large size plants.
In polycrystalline silicon cells, liquid silicon is poured into blocks that are sawed into plates. During solidification of the material, crystal structures of varying sizes are formed. The size of crystals mainly depends upon the cooling condition. If the molten silicon is cooled very slowly, the crystals of larger size are obtained. The silicon solar cells made from polycrystalline silicon are low cost but low efficiency. Maximum efficiency is 17-18%.
If a silicon film is deposited on glass or another substrate material, this is so-called amorphous or thin layer cell. The layer thickness is less than 1 µm, so production costs are lower due to the low material costs. However, the efficiency of amorphous cells is much lower than that of other cells. Because of this, they are primarily used in low power equipment such as watches, pocket calculators etc. Maximum efficiency is 13%. Thin film solar cells are also manufactured from gallium arsenide (GaAs), Cadmium telliride (CdTe) and copper-indium-selenide (CuInSe).
Two systems namely roof top array system and satellite system for generation of electric power from solar cells are being considered:
1. Roof Top Array System:
This system is an earth-based solar cell system mounted on roofs. The main difficulty is the problem of energy storage since this system will work when there is sunshine.
The various possible alternatives for energy storage are:
(a) Electrochemical storage, but batteries of adequate capacity that can withstand frequent charging and discharging for several years are yet to be developed.
(b) Hydro storage in which water is pumped uphill when power is abundant and allowed to flow through hydro generators at a time of peak demand. This possibility is suitable only to a few regions of the country.
(c) Mechanical storage in high speed flywheels.
(d) To store energy in the form of hydrogen which could be re-converted into electricity in fuel cells. Hydrogen is obtained by the electrolysis of water by the output of the solar cells.
2. Satellite System:
A more long range system that would also avoid the need for major storage of power is a space power station in synchronous orbit around the earth. According to the proposal put forward by Dr. Glaser of Massachusetts (USA), a solar collector of 8 × 8 km will be fixed on the surface of the satellite. Electricity so produced will be used to produce a microwave beam.
This microwave power will be transmitted to antennae on earth, and converted back into electric power. Although this system could provide large amount of power, but the problems regarding the endurance of the components, the control of large structures in space, and the safety of the microwave radiation, are still to be solved. Such a system is shown in Fig. 7.19.
3. Term Paper on the Advantages of Photovoltaic Solar Systems:
i. Absence of moving parts.
ii. Direct conversion of light to electricity at room temperature.
iii. Can function unattended for long time.
iv. Modular design- voltage and power outputs can be manipulated by integration.
v. Low maintenance cost.
vi. No environmental pollution.
vii. Very long life.
viii. Highly reliable.
ix. Solar energy is free and no fuel is required.
x. Can be started easily as no starting time is involved.
xi. Solar cells can be made from microwatts to megawatts. These can be used to feed the utility grid with power conditioning circuitry.
xii. Easy to fabricate.
xiii. These have high power-to-weight ratio, therefore very useful for space application.
xiv. System is noiseless and cheap.
xv. Modularity in operation.
xvi. These can be employed with or without sun tracking.
xvii. Decentralized power generation at the point of power consumption can save power transmission and distribution costs.
4. Term Paper on the Limitations of Photovoltaic Solar Systems:
i. Manufacture of silicon crystals is labour and energy intensive.
ii. The insolation is unreliable and therefore, storage batteries are required.
iii. Solar power plants need very large land areas.
iv. The principal limitation is high cost, which is being reduced through various technological innovations.
v. Electrical generation cost is very high.
vi. Low efficiency.
vii. The initial cost of the plant is very high and still needs a long gestation period.
viii. The energy spent in the manufacture of solar cells is very high. The plant operation period during which photovoltaic plant recovers the span energy varies from 4 to 7 years.
5. Term Paper on the Applications of Photovoltaic Solar Systems:
The photovoltaic solar systems are classified according to field of application, type of system, rated capacity, etc.
1. Autonomous System:
a. Amorphous silicon solar cells of very small capacity are employed in watches, pocket calculators, etc.
b. Small capacity solar systems from 50 W to 50 kW capacity are used for remote houses and villages for lighting for domestic use, street lighting, telecommunications, community development, water pumping, etc.
c. Roof-mounted solar system of 1 kW to 5 kW can be employed for residential houses.
d. In developing countries, solar systems can be used for lighting, recreation centres, radios, TV sets, small refrigerators etc.
e. In developing countries, solar system can find applications for drinking water supply, irrigation, vaccine refrigeration, milk chilling, and rural power supply.
f. Solar generators can be employed for boats, lighting towers, radio buoys, traffic signals, parking lights etc.
g. Developing countries have high solar energy potential and solar generators are employed for water pumping, domestic power supply, hospitals, schools, farmhouses etc.
h. Other applications like catholic protection of oil pipelines, weather monitoring, railway signalling, battery charging etc.
2. Solar Water Pumps:
The major applications of PV systems lies in water pumping for drinking water, irrigation in rural areas, cattle stock watering. The solar generators required for such purpose should have power capacity from 10 W to 10 kW. The water discharge of pump ranges between 1 to 40 m3/hour with delivery head from 2.5 m to 120 m. These generators usually operate without storage batteries.
The total cost of the system including solar PV array, on/off switch, motor-pump set, accessories including suction and delivery valves and piping is about Rs 40,000, which is highly subsidised and installed at a cost of Rs 5,000 only for a small or marginal farmer.
3. Central Power Generation:
A solar PV power plant of 16 MW capacity has been developed to supply electrical energy to 2,300 households. Solar power plants with a total capacity of 100 MW are planned for future installation.
4. Space Satellite Power Stations:
The space satellite power station (SSPS) has been conceived as the future solar power supply system. Large surface area of PV panels will be mounted on a space satellite which will be synchronically moving with the earth orbit so that it will appear stationary from every point on earth. The energy produced will be converted into microwave energy and transmitted to the earth. It will be collected with antennas of a few square kilometer areas and then converted into commercial frequency electric power.
Satellite mounted solar power plants can be designed for power outputs from 3 to 20 GW. The plant will require solar battery of 20 km2 total areas, the transmitting antenna of 1 km diameter and receiving antenna of 7 to 10 km diameter. The overall efficiency is expected to be 77%.