A photovoltaic (PV) cell, also known as a solar cell, can either reflect, absorb, or pass through light that strikes it. The PV cell is made of a semiconductor substance; the term “semi” denotes that the material can conduct electricity better than an insulator but not as well as a metal, which is a good conductor. In PV cells, a variety of semiconductor materials are employed.
What are Photovoltaic cells?
The solar cell is a transducer or semiconductor which converts the sun’s radiant energy directly into electrical energy and is a semiconductor diode capable of developing a voltage of 0.5-1 volt and a current density of 20-40 mA/cm^2 depending upon the material used and the conditions of sunlight.
The solar energy is directly converted into electrical energy by means of silicon wafer photovoltaic cells or solar cells. Solar cell operates on the principle of photovoltaic effect i.e. the process of generating an emf as a result of absorption of ionizing radiation.
Construction and working of Photovoltaic cells
The photovoltaic effect can be observed in pure semiconductors such as silicon. In silicon, there is no free charge carrier at ordinary temperature but if silicon is doped with phosphorous or arsenic then there will be one extra electron per atom of the impurity leading to an N-type semiconductor. Similarly, if another piece of silicon is doped with boron there will be a deficiency of electrons leading to P-type semiconductors. If these are connected by some means a junction at which the nature of the current carriers changes, is created. Thus the potential energy gap(Eg) is created at the junction.
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 the 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 the 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 the field created by the charge separation predominates the electric field created by the diffusion of free electrons from the N-region to the P-region and holes from P-region to N-region current will start flowing in the circuit.
Photovoltaic cell module
One cell produces about 1.5 watts of power. Individual cells are connected to form a solar panel or module, capable of developing 3 to 110 W power. Panels are connected in series and parallel to make solar arrays that can produce any amount of wattage. 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.
- Series combination of Photovoltaic cell module: If more than two cells are connected in series with each other then the output current of the cell remains the same and their output voltage becomes doubles.
- Parallel combination of Photovoltaic cell module: If more than two cells are connected in parallel with each other then the output voltage of the cell remains same and their output current becomes doubles.
- Series-parallel combination of Photovoltaic cell module: If more than two cells are connected in series-parallel with each other then voltage and current increases.
Types of Solar Cells
According to the types of crystal the solar cells are of three types:
- Monocrystalline silicon solar cells
- Polycrystalline silicon solar cells
- Thin film or amorphous silicon solar cells
- Monocrystalline silicon solar cells: It is the silicon doped with boron to produce P-type semiconductors. Monocrystalline 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 an N-type semiconductor. The solar cells are formed into modules by enclosing them 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.
- Polycrystalline silicon solar cells: Liquid silicon is poured into blocks that are sawed into plates. The silicon solar cells made from polycrystalline silicon are low cost but low efficiency. These modules possess high efficiency between 17-18%.
- Thin film or amorphous silicon solar cells: Silicon film deposited on glass or another substrate material. The efficiency of amorphous cells is much lower than that of other cells. They are primarily used in low power equipment’s such as watches, pocket calculators, etc. Thin film solar cells are also manufactured from gallium arsenide(GaAs),cadmium telliride (CdTe) and copper-indium-selenide(CUInSe).The maximum efficiency of this cell is 13%.
Advantages of Photovoltaic solar systems
- Direct conversion of light to electricity at room temperature
- voltage and power outputs can be manipulated by integration
- Low maintenance cost
- Pollution less
- Longer life span
- Highly reliable
- solar energy is free and no fuel is required
- can be started easily as no starting time is involved
- Solar cells can be made from microwatts to megawatts so it can be used to feed the utility grid with power conditioning circuitry
- Easy to fabricate
- Has high power to weight ratio and can be used for space application
- Noiseless and cheap system
Limitations of Photovoltaic cell solar systems
- Solar power plants need very large land areas.
- Electrical generation cost is very high.
- Low efficiency
- initial cost of the plant is very high
Applications of solar photovoltaic systems
- Amorphous silicon solar cells of very small capacity are employed in watches, pocket calculators etc.
- 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.
- Roof mounted solar system of 1 KW to 5 KW can be employed for residential houses.
- It can be used for lightning, recreation centers, radios, TV sets, small refrigerators, drinking water supply, irrigation, vaccine refrigerator, milk chilling, rural power supply, parking lights, traffic signals, railway signaling etc.
Solar water pumps:
- Used in water pumping for drinking water ,irrigation in rural areas, cattle stock watering.
Central Power Generation
Space Satellite Power Stations