Here is a compilation of essays on ‘Solar Cell’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Solar Cell’ especially written for school and college students.
Essay on Solar Cell
Essay Contents:
- Essay on the Introduction to Solar Cell
- Essay on Photon Energy
- Essay on the Theory of Solar Cells
- Essay on the Types of Solar Cells
- Essay on the Operation and Performance Characteristics of Solar Cells
- Essay on the Solar Energy Utilization
- Essay on the Solar Cell Efficiency and Losses
- Essay on the Methods of Increasing Cell Efficiency
- Essay on the Performance Analysis of Solar Cells
- Essay on the Solar Cell Materials
- Essay on the Solar Modules and Arrays
- Essay on the Solar Cell Power Plants
- Essay on the Energy Storage in Solar Cells
- Essay on the Design of a Solar Power Plant
- Essay on the Applications of Solar Photovoltaic Systems
- Essay on the Advantages of Photovoltaic Solar Energy Conversion
- Essay on the Solid-State Principles
- Essay on the Limitations of Solar Cells
Essay # 1. Introduction to Solar Cell:
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A Solar cell is based on photovoltaic energy conversion phenomenon. It is a direct energy conversion technology. Electricity is directly produced from sunlight without the use of a working fluid such as steam or gas. There is also no need of a mechanical cycle such as Rankine or Brayton Cycle.
Photovoltaic systems are simple, convenient dependable without the need of moving parts. The basic unit of a photovoltaic system is called a solar cell. These are assembled in arrays of identical modules to produce different power capacities. The power plants may range from small residential systems installed on roof tops to large central systems.
Essay # 2. Photon Energy:
Light is radiant energy. This is transferred in discrete pieces and not continuously. The smallest unit of energy is called quantum. The quantum of radiant energy is called a photon. The photon energy is proportional to the frequency of radiation. This is called Planck’s law.
where,
Ep = photon energy
h = Planck’s constant
= 6.6256 × 10-34 [J.s]
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= 4.13576 × 10-15 [eVs]
v = frequency of radiation, [herz]
c = speed of light
= 2.997925 × 108 m/s
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λ = wavelength of radiation, [m]
Light has dual characteristics. It consists of photons which have energy and it is also waves having a frequency and wavelength. The radiation from the sun is composed of the photons, each carrying a quantity of energy exactly equal to its frequency times Planck’s constant.
Example:
Calculate the photon energy of a mono energetic radiation beam having a wave length of half a micrometer.
Photon Flux:
It is the number of photons crossing a unit area perpendicular to the beam radiation per unit line. The photon flux, фp is a useful quality in the photo-voltaic cell calculations.
The solar energy flux E” can be calculated as:
Where i is the frequency or energy range.
Using average values of frequency and corresponding wave length.
The terrestrial insolation and average photon energy decrease as air mass (m) and angel of latitude (ф) increase.
When the sun’s photon flux strike a pn semiconductor junction, there is an electric current flowing through a load connected across the junction. The electric power produced is proportional to photon flux.
Essay # 3. Theory of Solar Cells:
A solar cell is composed of pn semiconductor junction as shown in Fig. 12.1. Solar cells can be manufactured from different semiconductor materials and their combinations. The voltage generated by a solar cell depends on the semiconductor material.
The electric current produced depends upon the intensity of solar radiation and the cell surface area receiving the radiations. The maximum achievable power is about 100 W/m2 of solar cell surface area. Pure semiconductor materials are needed for achieving high energy conversion efficiency.
Direct conversion of solar radiation into electricity can be studied with the help of solid-state principles.
Essay # 4. Types of Solar Cells:
The main types of solar cells along with brief performance specifications are:
1. Mono crystalline silicon cells.
Band gap: 1.12 eV
Maximum efficiency = 24%
2. Polycrystalline silicon cells.
Band gap: 1.12 eV
Maximum efficiency: 17.8%
3. Amorphous silicon cells.
Band gap: 1.75 eV
Maximum efficiency: 13%
4. Gallium arsenide (GaAs).
5. Copper indium diselenide (CID) Cells.
6. Multi-junction Cells.
7. Cadmium telluride (CdTe) Cells.
Band gap: 1.44 eV
Maximum efficiency: 15.8%
8. Concentrator Cells.
Maximum efficiency: 32.3%
At present, silicon solar cells occupy 60% of the world market.
For single-crystal silicon, p is obtained by doping silicon with boron and is typically 1 µm thick; n is obtained by doping silicon with arsenic and is typically 800 µm thick. Thin film cells are composed of copper sulphide for p, typically 0.12 µm thick, and cadmium sulphide for n, typically 200 µm thick.
Essay # 5. Operation and Performance Characteristics of Solar Cell:
Operation of Cell:
The sun’s photons strike the cell on the microthin p side and penetrate to the junction. There they generate electron-hole pairs. When the cell is connected to the load, the electron will diffuse from n to p. The direction of current I is conventionally in the opposite direction of the electrons.
Performance Characteristics of Cell:
Typical voltage-current characteristics are shown in Fig. 12.2 at two different solar radiation levels.
For each,
Vo = Open-circuit voltage
Io = Short-circuit voltage
Pm = Point of maximum power,
= (VI)max
Essay # 6. Solar Energy Utilization:
Table 12.1 shows the breakdown of solar energy wavelength distribution. It also shows fraction utilized by a cell and percent solar energy converted to electricity.
Solar cells do not convert all solar radiation falling upon them to electricity. Weak, low-frequency (long-wavelength) photons do not possess sufficient energy to dislodge electrons. On the other hand, strong, high-frequency (short-wavelength) photons are too energetic and although they dislodge electrons, some of their energy is left over unused.
Essay # 7. Solar Cell Efficiency and Losses:
Efficiency is defined as the ratio of electric power output of the cell, module, or array to the power content of sunlight over its total exposed area. The maxi-mum theoretical efficiency of solar cells is around 47 percent. Efficiencies of modules or arrays are therefore lower than those of the cells because of the areas between the individual cells. The fraction of cell to total areas is called the packing factor.
The actual efficiency of a solar cell is low because of the following losses:
i. Part of the solar energy is reflected back to the sky.
ii. Some portion of solar energy is absorbed by non-photovoltaic surfaces.
iii. Some solar energy is converted into heat.
iv. At high temperatures there is recombination of the electron-hole pairs, Cells are usually laboratory rated at 1000 W/m2 and 28°C. The cells normally operate at 50 to 60°C. This may reduce the efficiency by 1 to 2%.
v. The efficiency is also affected by various electrical losses.
vi. There are additional losses due to mismatch between individual cells in a module and between modules in an array.
vii. Mass production of modules also lowers the cell efficiency.
Typical energy balance of a concentrating silicon photovoltaic conversion array is shown in Fig. 12.3.
The resultant array efficiency is only 8 percent after deducting various losses.
Essay # 8. Methods of Increasing Cell Efficiency:
The following methods are used to improve solar cell efficiency:
1. Alternative:
Use of alternative materials.
2. Concentration of Incidence Solar Energy:
The cell efficiency can be improved by the use of concentrated sunlight. The cells are located at the foci of parabolic or trough concentrators. Cell efficiency of 25 percent can be achieved with a concentration of about 500 suns. Concentrators are cost-effective.
3. Thermo Photovoltaic Systems:
Highly concentrated light is used to heat a refractory material. The hot material reradiates the solar energy to silicon cells at longer wavelengths. Such wavelengths are more effective to generate electricity by silicon cells. Although there are material problems as the temperatures required is 1870 to 1925°C.
4. Cascade Systems:
Multi-junction solar systems are used. Each cell is subjected to different region of solar spectrum at which it operates most efficiently. Such a system can yield conversion efficiencies even higher than 25%.
Essay # 9. Performance Analysis of Solar Cells:
A solar cell uses a p-n junction as shown in Fig. 12.1.
The current and voltage relationship is given as:
where,
I0 = saturation current of diode. This is also called dark current when a large negative voltage is applied across the diode.
V = voltage across junction [V]
e = electronic charge
= 1.60219 × 10-9 Coulomb
k = Boltzmann constant
= 1.38066 × 10-23 J/K
T = Temperature [K]
The equivalent electrical circuit of a solar cell is shown in Fig. 12.4.
When light impinges on the p-n junction, electron hole pairs are created at a constant rate resulting in an electrical current flow across the junction. The net current (I) is the difference between the normal diode current (IL) and light generated current (IL). The internal series resistance (Rs) is due to high series resistance of the diffused layer which is in series with the junction.
The photovoltaic current or light generated current IL is branched into current I in the external circuit and Ij the diode current. The current-voltage characteristic curve is shown in Fig. 12.2. The following current-voltage relationship is valid at a constant cell temperature and a constant intensity of solar radiation.
Essay # 10. Solar Cell Materials:
Silicon solar cells occupy more than 60% of photovoltaic market.
Basic types of silicon solar cells are:
1. Monocrystalline silicon solar cells,
2. Polycrystalline silicon solar cells, and
3. Thin film or amorphous silicon solar cells.
1. Monocrystalline Silicon Solar Cells:
Silicon is doped with boron to produce p type semiconductor. A single crystal of 2m length and 10 to 15 cm diameter is drawn from the molten silicon. It is sliced into 0.3 to 0.5 mm thick wafers or discs. The upper layer of the wafers is doped with phosphorous up to 3 to 4 μm depth to produce n type semiconductor. This becomes a p-n junction. A copper layer is deposited for full area on the backside and 10% area on the front side. These metallic surfaces are used to take off the electric current.
A silicon solar cell of size 10cm × 10cm produces a voltage of 0.5V and power output of IW at a solar radiation intensity of 1000 W/m2. The solar cells are formed into modules by enclosing in an air tight 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.
2. Polycrystalline Silicon Solar Cell:
A polycrystalline silicon block (containing many small silicon crystals) is sliced into wafers in the similar manner as in case of single crystal solar cells. The highest efficiency of solar module is 12 to 14%.
3. Thin-Film Solar Cells:
The crystalline solar cells are labour and energy intensive in manufacturing. The thin film cells are produced from amorphous silicon. It has the capacity to absorb more solar radiation due to irregular atom arrangement. A thin 1 mm thick solar cell is used. Gaseous Si H4 is deposited on a glass sheet, the silicon sheet is simultaneous doped to produce p-n junction. The efficiency is 5 to 8%. These are very cheap to manufacture.
Thin film solar cells are also manufactured from the following materials:
1. Gallium Arsenide, (GaAs)
2. Cadmium Telliride (Cd Te)
3. Copper-Indium-Selenide (Cu In Se)
Essay # 11. Solar Modules and Arrays:
i. Solar Modules:
Solar cells are joined in series to form a module. The number of cells will depend upon the module voltage. A 12 V solar module consists of 33 to 36 cells. The cells are mounted together under an airtight, mechanically rigid, transparent cover.
The technical specifications of Siemen’s solar module M55 containing 36 solar cells connected in series are as under:
Maximum peak power = 53W
Open circuit voltage, V0 = 21.7V
Short-Circuit current, Is = 3.4A
Optimum operating current, I = 3.05A
Optimum operating voltage = 17.4 V
Dimensions = 1293 × 329 × 36 mm
Mass = 5.7 kg
The Central Electronics Ltd. Sahibabad (U.P.) has developed a solar photovoltaic panel with following specifications:
Maximum peak power = 300W
System voltage = 60V dc nominal
Dimensions = 2400 × 2500 mm
ii. Solar Array or Generator:
It is made by interconnecting solar modules to supply electricity. The modules are connected both in series as well as parallel. Connected in series will give a 24V solar array. In order to increase current and power output, the modules are connected in parallel.
Essay # 12. Solar Cell Power Plants:
There are two types of power plants:
1. Autonomous Solar Power Plants.
These are used for local networks.
2. Combined Solar-Wind-Diesel Power Plants.
These are used for external electrical networks.
1. Autonomous Solar Power Plant:
These are used for the following:
i. Home power supply in rural areas.
ii. Solar Water Pumps.
iii. Telecommunication and relay stations.
iv. Cattle fencing.
v. National parks for street lighting.
vi. Tourist facilities for remote and hilly areas.
The block diagram is shown in Fig. 12.5.
The system consists of the following:
i. Solar generator or array.
ii. Storage battery.
iii. Charge controller.
iv. Inverter.
v. Standby diesel engine-generator.
vi. Consumer network.
Solar generators can be designed for required electric supply.
2. Combined Solar-Wind-Diesel Power Plant:
The combined power plant of solar-wind and diesel engine is shown in Fig. 12.6. The solar generator produces electricity. A wind power plant and/or diesel generator driven by biogas from a sewage plant work as backup unit. A storage battery, an inverter and power distribution electric grid make the power supply system complete.
The capacities of different power units can be as follows:
i. Solar generator = 140 kW (40 modules)
ii. Wind power plant = 250 kW (Rotor diameter 25m)
iii. Diesel engine = 30 kW
Essay # 13. Energy Storage in Solar Cells:
Solar cell power systems must share conventional power grids or must use electrical storage if their output is to last longer than sunlight.
Some of the storages schemes are:
1. Battery:
This is storage of electric energy by conversion to chemical energy in batteries. The most common and highly developed is the lead-acid battery. Large electric energy storage in lead-acid batteries or other batteries is not economically feasible. Other battery systems with higher energy-to-mass ratios are under development. Currently available and suitable storage batteries have capacities between 40 to 280 Ah.
A storage battery cycle operation consists of a charging process and discharging process.
The amount of energy stored during charging process:
PS = CVB [Wh]
where,
C = Storage capacity [Ah]
VB = Nominal battery voltage [V]
A storage battery with a capacity of 200 Ah and a rated voltage of 12V, can store 2400 Wh of electric energy.
The amount of energy recovered from a battery during discharging process:
Pd = Vd Id td [Wh]
where,
Vd = discharge voltage [V]
Id = discharge current [A]
td = discharge duration [h]
In order to get highly reliable power supply a diesel engine should also be used as back-up unit in addition to a storage battery.
2. Pumped-Hydro Storage:
This method is more suitable for large power plants. The surplus energy is used to pump water into high reservoirs during sunny periods or periods of low demands and extraction of power during evening or cloudy periods or periods of high demands by running the stored water through water turbines.
The main limitation of this system is to find sites with suitable topography near solar power plants. Such power plants are mostly located in desert like flat terrains.
3. Cryogenic Storage:
The electric energy is directly stored in large underground electrical coils at liquid-helium temperatures of 4K. The electrical resistivity is almost zero.
Essay # 14. Design of a Solar Power Plant:
An autonomous solar power plant consists of:
1. PV generator or array,
2. Storage battery,
3. Charge control unit,
4. Inverter,
5. Back-up diesel generator, and
6. Electricity consumer network.
The power balance equation for a period of plant utilization can be written as:
PG + PB + PD – PIL – PL [Wh]
where,
PG = Electrical energy supplied from solar generator
PB = Electrical energy supplied from battery
PD = Electrical energy supplied from diesel engine
PIL = Internal energy losses & system consumption
PL = Energy requirement of consumer network
The energy supplied by solar generator over a given time duration:
PG= ʃ AG S Ƞg dt [Wh]
Where,
AG = Total solar cell surface area [m2]
S = Mean solar radiation [W/m2]
ȠG = Average solar generator efficiency
t = Time duration [h]
i. Size of Solar Array:
The following input data is needed:
1. Daily solar radiation at site,
2. Ambient temperature,
3. Network load and voltage,
4. Specification of solar generator including its efficiency, and
5. Specification of storage battery.
The daily electrical load is calculated as:
ii. Solar Panel Tilt:
The tilt angel θ of a solar panel is selected so that the generator receives maximum solar radiation at the time of maximum load.
where,
ф = Latitude of site in degrees
iii. Storage Battery Capacity:
The performance parameters of a storage battery are:
1. Capacity [Ah]
2. Rated voltage [V]
3. Rated current [A]
4. Efficiency
The battery efficiency is the energy recovered to energy charged. The usual values of efficiency are 75 to 90%.
The capacity of a storage battery can be calculated as:
where,
F = Reserve factor = 1.2
PL = Daily power load [Wh/day]
ND = Number of required storage days
Dmax = Max allowable depth of battery discharge = 0.8
VB = Rated battery voltage [V]
ȠB = Battery efficiency
For large power storage, batteries are connected in series and parallel in a storage bank.
Number of batteries to be connected in parallel is the ratio of total energy capacity and energy capacity of each battery.
The number of batteries to be connected in series is the rated voltage of the network load divided by the rated voltage of a battery.
The total number of batteries is the product of batteries in parallel and batteries in series.
Essay # 15. Applications of Solar Photovoltaic Systems:
The solar photovoltaic systems are classified as per field of application, type of system, rated capacity, etc.
I. Autonomous system:
1. Amorphous silicon solar cells of very small capacity are used in watches, pocket calculators, etc.
2. Small capacity solar systems from 50W to 50 kW capacities are used for remote houses and villages for lighting for domestic use, street lighting telecommunications, community development, water pumping, etc.
3. Roof-mounted solar system of 1 kW to 5 kW can be used for residential houses.
4. In developing countries, solar systems can find applications for drinking water supply, irrigation, vaccine refrigeration, milk chilling, and rural power supply.
5. In developed countries, solar generators can be used for lighting, recreation centres, radios, TV sets, small refrigerators, etc.
6. Solar generators can be used for boats, lighting towers, radio buoys, traffic signals, parking lights.
7. Developing countries have high solar energy potential and solar generators are used for water pumping, domestic power supply, hospitals, schools, farm houses.
8. Other applications can be for catholic protection of oil pipelines, weather monitoring, railway signaling, and battery charging.
II. Solar Water Pumps:
The major application of photovoltaic systems lies in water is pumping for drinking water, irrigation in rural areas, cattle stock watering. The solar generators required for this purpose should have power capacity from 10W to 10 kW. The water discharge of pump ranges between 1 to 40 m3/h with delivery head from 2.5m to 120m. These generators usually work without storage battery.
The specifications of solar photovoltaic water pumping system standardized by Central Electronics Ltd. (U.P.) are given below:
The solar array consists of 5 rows of 4 modules each. Each module consists of 36 cells each 76mm diameter connected in series to generate about 16 peak watts of power at incident radiation of 100 mW /cm2 at 28°C. Four such modules are connected in series and five such series connected module chains are kept in parallel. The complete array is mounted on an angle iron structure which can be provided with storage batteries and power conditioning equipment.
The total cost of the system including solar photovoltaic 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. 5000 only for a small or marginal farmer.
III. Central Power Generation:
A solar photovoltaic power plant of 16 MW capacity has been developed to supply electricity to 2300 households Solar power plants with a total capacity of 100 MW are planned for future installation.
IV. Space Satellite Power Stations:
The space satellite power station (SSPS) has been conceived as the future solar power supply system. Large surface area of photovoltaic panels will be mounted on a space satellite which will be synchronically moving with the earth orbit so that it will look stationary from every point on the earth. The energy generated 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 area. The transmitting antenna of 1 km diameter and receiving antenna of 7 to 10 km diameter. The overall efficiency is expected to be 77 percent.
Essay # 16. Advantages of Photovoltaic Solar Energy Conversion:
1. Absence of moving parts.
2. Direct conversion of light to electricity at room temperature.
3. Can function unattended for long time.
4. Modular design: Voltage and power outputs can be manipulated by integration.
5. Low maintenance cost.
6. No environmental pollution.
7. Very long life.
8. Highly reliable.
9. Solar energy is free and no fuel required.
10. Can be started easily as no starting time is involved.
11. Solar cells can be made from microwatts to megawatts. These can be used to feed the utility grid with power conditioning circuitry.
12. Easy to fabricate.
13. These have high power-to-weight ratio, therefore very useful for space application.
14. Decentralized or dispersed power generation at the point of power consumption can save power transmission and distribution costs.
15. These can be used with or without sun tracking.
Essay # 17. Solid-State Principles:
Solid-state phenomenons are important for the study of photovoltaics.
i. Solid Phenomenon:
A metallic crystalline solid contains atoms that have nuclei surrounded by electrons, which are tightly bound to them (the solid model). There are outer electrons which are weakly bound to the nuclei. The outer electrons are called valence or conduction electrons. These are free to migrate in the interior of the metal as these have no forces acting on them by the other free electrons or bound electrons or ionized positively charged nuclei.
These free electrons:
(a) Have equipotential field in which these move;
(b) Each of them has constant electrostatic potential energy, Ei.
(c) The electrostatic potential energy is independent of the location inside the crystal.
(d) Do not belong to any particular atom.
(e) Belong to the electro crystal.
(f) Cause electric and heat conduction in the metals.
(g) Do not have attractive forces as there are no positive ions on the surface of the metal.
(h) Are easily moved by the electric fields inside the metal,
(i) Encounter an energy barrier at the surface, and
(j) Require more energy to get out of the surface of the metal.
The free electrons form electron gas which is confined inside the metal. The free electrons follow Pauli Exclusion Principle which states that no two electrons can exist in the same state, at the same time and in the same atom.
ii. Fermi Energy:
The electron gas does not follow the energy distribution law of ordinary gas given by the Maxwell-Baltzmann law. The energy distribution is given by the Fermi- Dirac Distribution Law as given below. This law holds good for temperatures below 3000 K.
iii. Energy Distribution of Electron Gas:
The energy distribution of an electron gas is shown in Fig. 12.7 for different temperatures (T2 > T1 > 0). The Fermi-Dirac Probability distribution is given by the function,
The quantity E1/2 gives the parabolic rise of the curve from E = 0. Free electrons do not have zero energy at absolute zero temperature. It has finite energies upto a maximum of EF, i.e., Fermi Energy.
iv. Fermi Energy:
The Fermi energy can be calculated from the following equation:
The Fermi energy can be expressed in electron volts (eV). An electron volt is a unit of energy equal to the energy acquired by or single electron charge when accelerated through a potential of 1.0 V.
The average energy of an electron at T = 0 is equal to 3/5 EF. This value is much higher than the kinetic energy of a particle in a classical gas even at high temperatures.
v. Behaviour of Free Electrons:
For solar cells, the behaviour of free electrons is important at temperature higher than T = 0. At moderate temperatures (see Fig. 12.7) there is slight rounding of zero temperature distribution curves. As the temperature increases, the value kT increases.
The free electrons whose energies are much higher than Fermi energy (EF) remain locked in the same energy state as those occupied at T = 0 irrespective of the temperature. There is a fraction of free electrons with energies within kT of the Fermi energy which occupy higher energy states than Fermi energy. These electrons can be elevated to high energies by collision with high-energy protons from the sun (solar cell) or by thermal excitations (thermionics).
Essay # 18. Limitations of Solar Cells:
1. Manufacture of silicon crystals is labour and energy intensive.
2. The principal limitation is high cost, which is being reduced through various technological innovations.
3. The insolation is unreliable and therefore storage batteries are needed. Good batteries are not available.
4. Solar power plants require very large land areas.
5. Electrical generation cost is very high.
6. The energy spent in the manufacture of solar cells is very high. The plant operation period during which photovoltaic plant recovers the spent energy varies from 4 to 7 years.
7. The initial cost of the plant is very high and still requires a long gestation period.