It is possible to convert solar energy directly into electrical energy by means of silicon wafer photovoltaic cells, also called 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. 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 solar energy is basically free of cost. The main problem faced is the 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.
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, as shown in Fig.
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 spacecrafts 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, USA etc. toward reduction of cost.
The cadmium sulphide/cuperous 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 um) 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 lung 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.
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 make 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.
Advantages of Photovoltaic Solar Systems
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 is 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. System is noiseless and cheap.
15. Modularity in operation.
16. These can be employed with or without sun tracking.
17. Decentralized power generation at the point of power consumption can save power transmission and distribution costs.
Limitations of PV Solar Systems
1. Manufacture of silicon crystals is labour and energy intensive.
2. The insolation is unreliable and therefore, storage batteries are required.
3. Solar power plants need very large land areas.
4. The principal limitation is high cost, which is being reduced through various technological innovations.
5. Electrical generation cost is very high.
6. Low efficiency.
7. The initial cost of the plant is very high and still needs a long gastation period.
8. 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.