How solar cell are better & generate electricity?
What is a solar cell? How is the current generated by it and what are its applications?
The solar cell is a semiconductor device that directly converts sunlight into electricity. A solar cell converts sunlight into electricity by the photovoltaic effect. Hence, they are also called photovoltaic cells.
The solar cell generates current and voltage at its terminals when sunlight falls on it. The amount of electricity generated by a solar cell depends on the amount of sunlight incident on it. The electricity generated by a solar cell depends upon the intensity amount of light, the area of a cell, and the angle at which light falls on it. The higher is the intensity of sunlight, the more is electricity generated by solar cell
If the area of a solar cell is increased, the current generated by it increases. The power generated by the solar cell is optimum when sunlight falling is perpendicular to the front side of the solar cell. In common, all solar cells, irrespective of the technology and material used have only two terminals positive and negative terminals as output.
Typical solar cells have front contact at the top, emitter-base junction or p-n junction in the middle, and back contact at the bottom. At the emitter-base junction, the separation of negative and positive charges takes place. Electricity is supplied to a load by connecting its terminals to the front and back contacts of a solar cell or solar module Or solar panel
How do solar cells generate electricity?
The sunlight falling on the earth is basically bundles of photons or bundles of small energy. Each photon in a bundle has a finite amount of energyIn the solar spectrum, there are many photons of different energy. For the generation of electricity, photons must be absorbed by solar cells. The absorption of a photon depends upon the energy of the photon and the band-gap energy of the semiconductor material of the solar cell.
The photon energy and the band-gap energy of the semiconductor are expressed in terms of electron-volt (eV). The eV is a unit of energy.
So, the working of a solar cell can be explained as follows:
- Photons in the sunlight falling on the solar cell’s front face are absorbed by semiconducting materials.
- Free electron-hole pairs are generated. Electrons are considered negative charges and holes are considered positive charges. When a solar cell is connected to a load, electrons and holes near the junction are separated from each other. The holes are collected at the positive terminal anode and electrons at the negative terminal cathode. Electric potential is built at the terminals due to the separation of negative and positive charges. Due to the difference between the electric potentials at the terminals, we get voltage across the terminals.
- Voltage developed at the terminals of a solar cell is used to drive the current in the circuit. The current in the circuit will be direct current or DC current.
So, the solar cell with daylight falling on it can directly drive DC electrical appliances. But the amount of electricity generated is proportional to the amount of light falling.
So, the amount of electricity generated throughout the day is not constant.
Parameters of solar cell ?
How a solar cell does the conversion of sunlight into electricity is determined by the parameters of solar cells. There are several parameters of solar cells that determine the effectiveness of sunlight to electricity conversion. The list of solar cell parameters is following:
1.Short circuit current Isc:-
It is the maximum current a solar cell can produce. The higher the Isc better is the cell. It is measured in A or milli-ampere. The value of this maximum current depends on cell technology, cell area, amount of solar radiation falling on the cell, the angle of the cell, etc. Many times, people are given current density rather than current. The current density is obtained by dividing Isc by the area of the solar cell. The current density is normally referred to by the symbol J, therefore, the short circuit current density Jsc = 1sc /A
2 Open Circuit Voltage Voc:-
It is the maximum voltage that a solar cell produces. The higher the Voc, better is the cell. It is measured in volts (v) or sometimes milli-volts (mV). The value of this maximum open circuit voltage mainly depends on cell technology and operating temperature.
3.Maximum power point Pmax:-
It is the maximum power that a solar cell produces under STC. The higher the Pmax, the better the cell. It is given in terms of a watt (W). since it is the maximum power or peak power, it is sometimes also referred to as Watt peak. Solar cell can operate at many current and voltage conditions. But a solar cell will produce maximum power point only when operating at a certain current and voltage. Normally, the maximum power point for the I-V curve of a solar cell occurs at the knee or bend of the curve.
Pmax = Vm×Im
4.Current at the maximum power point Im:
This is the current which solar cell will produce when operating at maximum power point.
5.Voltage at maximum power point Vm:
This is the voltage which solar cell will produce when operating at maximum power point. The Vm will always be lower than Voc.
6.Fill factor (FF):
FF is the ratio of the area covered by Im Vm rectangle with the area covered by Isc – Voc rectangle. It indicates the square-ness of I-V curve. The higher the FF, the better the cell. The FF of a cell is given in terms of percentage (%). Cell with squre I-V curve is a better cell.
FF = Vm Im / Voc Isc =Pm / Voc Isc
7.Efficiency η :
The efficiency of a solar cell is defined as the maximum output power Pmax divided by the input power Power in. The efficiency of a cell is given in terms of percentage (%), which means that this percentage of radiation input power is converted into electrical power. Power in for STC is considered as 1000 W/m². This input power is power density (power divided by area), therefore, in order to calculate the efficiency using Pin at STC, we must multiply by solar cell area.
These parameters can be best understood by the current-voltage curve (I-V curve) of the solar cell.
Solar Cell Technologies
In the market, a wide variety of solar cells are available. These cells are made using different materials.
The name of a particular solar cell or solar cell technology depends on the name of the material used in that technology.
The properties of materials used in different types of solar cells are different, hence, different types of solar cells have different values of solar cell parameters like efficiency, short circuit current density, open circuit voltage and fill factor.
Factors affecting electricity generation
Factors affecting electricity generated by a solar cell
There are five common factors that affect the power generated by solar cells.
They are as follows:
1.The conversion efficiency.
Only a fraction of the light energy gets converted into electrical energy by the solar cell. The ratio of electrical energy generated to the input light energy is referred to as the conversion efficiency of solar cells. Once a solar cell of a given material is manufactured, its efficiency value becomes fixed and can not be changed. The efficiency of a solar cell is given in terms of the maximum power that a solar cell can generate for a given input of solar radiation.
The maximum power output of solar cells depends on the voltage developed across the cell terminal and the current it can supply. The cell area also affects the power output. If the instantaneous solar radiation or power density is Power input, the expression for the efficiency of a solar cell can be given as:
η = Power output /Power input x Area
Po = Pin X η X A
For given input power, the value of the output power is directly determined by the value of the solar cell’s conversion efficiency and solar cell area. The solar cells with higher efficiency values will always give better performance. The unit of solar cell efficiency is the percentage(%), the unit of Po is normally in watts, the unit of Pin is normally in W/m² or W/cm² and the unit of the cell area is in m² or cm²
The solar cell efficiency is given for standard test condition STC and under the STC, the value of input power density Pin is taken as 1000 W/m² or 0.1 W/cm²
2.The amount of light.
The amount of sunlight (intensity of sunlight) falling on solar cells keeps changing from morning to evening. The current and voltage output of a solar cell depends on the amount of light falling on it. The electric current generated by the solar cells is directly proportional to the amount of light falling on it. Suppose a solar cell produces 1 A current under 1000 W/m² input solar radiation. Then under 500 W/m², the cell will only produce 0.5 A because the input radiation is half. As the amount of sunlight falling on the solar cell increases from morning to afternoon, the current output of a solar cell also increases. From afternoon till evening the amount of sunlight falling on the solar cell decreases and hence the current output of a solar cell also decreases.
The output voltage of a solar cell is not affected strongly by changes in the amount of light. If a solar cell produces 1 V at noon time, its voltage will roughly remain the same in the morning as well as in the evening hours. The solar cell current output is proportional to the amount of solar radiation and voltage is relatively not affected by the variation in sunlight intensity.
The amount of power generated by the solar cell throughout the day keeps changing (it is not constant). So, a solar cell gives high power when the intensity of light falling is high, similarly, less power is generated when the intensity of light falling is low.
3.The solar cell area.
The amount of maximum output current (Isc) of a solar cell depends on the area of a solar cell. The current output is directly proportional to the cell area. So, when the solar cell area is large, the amount of electric current generated by it will be large. Similarly, less amount of electric current will be generated when the solar cell area is small.
For a given amount of input sunlight if a 100 cm² cell produces 2 A current, then a 200 cm² cell will produce 4 A current, and a 50 cm² cell will produce 1 A current under the same input sunlight intensity. When we divide the generated current by the area of solar cells, we get current/area or current per unit area, which is also referred to as current density. The current density is given in units of A/ cm² or mA/cm². The current density of a solar cell does not depend on the area and for a given sunlight intensity the current density of the solar cell is also fixed.
The output voltage of solar cells does not change with the change in the solar cell area. The output voltage is independent of the cell area. Thus, at a given input sunlight intensity, if a 100 cm² cell produces 0.5 V, then a cell of 100 ст², or 200 cm² or 50 cm² or 10 cm², etc will produce same 0.5 V. Current density of a solar cell is always fixed or constant.
4.The angle at which daylight falls.
The angle of sunlight with respect to solar cell greatly affects the output power. The Solar cell produces maximum power for given light intensity when sunlight falls perpendicular to the surface of the solar cells. When the light does not perpendicular to solar cells, it always gives less output power than the maximum possible output power. This is because when light falls at some angle some part of light falling on solar cell is reflected.
Hence, the actual light utilized by a solar cell is less than the amount of light falling on it. So, the output power generated is less when light is not falling perpendicular to the solar cell. Therefore, one should always try to install a solar cell or module in such a way that most of the time sunlight is close to perpendicular, especially in the afternoon time when the intensity of sunlight is high.
5.The operating temperature.
The solar cell output voltage, power, and efficiency ratings are given at standard test condition (STC=1000 W/m² and 25°C). The cell output voltage, cell efficiency, and output power depend on cell temperature. In practical applications, the operating temperature of solar cells may be different than 25°C. The cell temperature varies due to ambient temperature and in practice, the solar cell is encapsulated in PV module with glass which results in Heating of solar cells.
Due to encapsulation also solar cell temperature increases. The change in temperature from the standard operating temperature directly affects the output voltage, efficiency, and power. Normally, when a solar cell operates at a temperature above 25°C temperature; the output voltage, cell efficiency, and output power of a solar cell reduce. The decrease in voltage, power, and efficiency with temperature is different for different types of solar cells.
For crystalline Si solar cells, for every 1 °C increase in temperature above 25 °C, the decrease in value of voltage, power, and efficiency is given in the table
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