- Introduction
Solar cells function by converting solar light energy into electricity. It functions on the basis of the photovoltaic effect. “Photo” stands for light and “voltaic” means electricity (light–electricity). A solar cell is capable of providing DC current. This DC current can be used to light bulbs and power DC motors. Rechargeable batteries also use this DC current, which can be utilized later on when the sun is not available. Also, these batteries, when fully charged, are portable and can be used wherever and whenever you want them (De Vos, 1980).
Solar cells are comparable to batteries in providing DC current. Batteries differ, however, from solar cells in that they operate by the electrochemical reaction. The chemical reaction that takes place within the battery will provide the electricity to power other appliances (electro–chemical). DC current is derived from AC current, which can then power TVs, refrigerators, and other appliances. DC current cannot be converted to AC when required.
Conversion of light energy into electrical energy requires a material that is capable of absorbing solar energy (photon). Absorption then raises an electron to the high-energy state that then flows into an external circuit. Silicon is generally used in the construction of solar cells.
Figure 1: Solar Cell
Figure 2 Solar Panel.
Two processes are involved in the generation of electricity through the use of solar cells.
- Electron-hole pairs are first created by the absorption of photons that are incident on the material used. The energy of the incident photon has to be greater than that of the band gap so that electron-hole pairs are formed in the solar cell. It must be noted that electors present in the p-type of material and holes present in the n-type of material are only metastable. They will exist for the period equal to the minority carrier lifetime. This is followed by their recombination, due to which the electron-proton pair is lost, and hence, electricity cannot be generated.
- If these carriers are collected by a p-n junction, then the recombination of the protons and the holes is prevented. The electric field that exists in the p-n junction separates the electron and the holes. When the emitter and base of the cell remain connected, then there is the flow of the light-generated carriers through this external circuit.
2 A p-n Junction
When semiconductor materials of the p-type and n-type are joined, a p-n junction is formed. When n-type electrons move toward the p-type, then positive ion cores are exposed in the n-type side. The movement of n-type holes exposes negative ion cores in the p-type side. Thus, electron fields are generated at the junction and a depletion region is also formed.
- Photovoltaic Effect
Power is not generated with the mere collection of light-generated carriers. In order for the power to be generated, the current as well as a voltage must be generated. The photovoltaic effect is the means by which voltage is generated in the solar cell. Holes move to the p-type side of the junction and electrons move toward the n-type side of the junction, with the collection of light-generated carriers at the p-n junction. The carriers move out of the device as light-generated current.
- Manufacture
As solar cells are semiconductor devices, the same manufacturing processes used for computers and memory chips can be used for them also. The control for semiconductor fabrication and cleanliness are more relaxed while constructing solar cells than in conventional devices. Single crystalline silicon or screen-printed poly-crystalline solar cells are the kind of solar cells manufactured these days (Pearce, 2002).
Block cast silicon ingots are wire-sawed into very thin wafers or slices. To construct the poly-crystalline silicon wafers, the wafers are normally lightly p-type doped. Later, n-type dopants are surface diffused on the wafer’s front side. This helps in the formation of the p-n junction.
For enhancing the light collection in the solar cell, anti-reflection coatings are normally applied. Silicon nitride is the commonly used anti-reflection coating these days.
Carrier recombination at the surface of the solar cell is prevented by silicon nitride. Plasma-enhanced chemical vapor deposition (PECVD) is a technique used to apply the silicon nitride up to a few hundred nanometers thick on the solar cell. Other solar cells have textured surfaces that function as the anti-reflection coatings in helping to collect the light rays into the solar cells. These textured surfaces have been recently formed on multicrystalline silicon surfaces, which are recent developments (Wynn, 2007).
The wafer then comes to consist of a metal contact (full area) constructed on the back surface. A metal contact resembling a grid consisting of larger "bus bars" and fine "fingers" are screen-printed, using a silver paste, onto the front surface (Peacock, 2012). The metal paste, which is normally aluminum, is used to screen protons at the rear contact. In some cells, the rear contact extends over the entire back surface, while in some other cells; it is printed in a grid-like fashion. Metal electrodes which are now in ohmic contact with silicon are formed by firing the paste at hundreds of degrees Celsius. Once the metal contacts have been constructed, flat wires are used to make the interconnections in the solar cells. They are then assembled into solar panels or modules. A polymer encapsulation is fixed onto the back of the glass sheet.
- Applications of Solar Power
Solar power can be used to run your bath fans, floor fans, and ceiling fans at home. Utility bills can be cut down by solar power for your fans at homes that are used for cooling purposes.
Heat Your Swimming Pool with Solar Energy
Swimming pools are a joy to dive into; however, when they are too cold, people are reluctant to dive. This problem can be sorted out by adding a solar blanket to the pool, which would warm the water. The heating needs only the blanket and not any other extra installations. Other advances of use of solar power to heat swimming pool water would be the use of the solar hot water heating system. This system uses solar hot water heating panels located on the house roof to collect heat from the sun, which is then circulated to the pool.
Solar Energy Can Heat Your Water
Water used at home can be heated using solar panels, rather than having to use gas or the electric water heaters.
Power Pumps with Solar Energy
Water is used to transport heat energy in your home, to some appliances, using solar power. A pump is needed to circulate the water for accomplishing this task. A DC motor can be operated by the solar energy, which will then help to circulate the water through your house or through that water heater. This way, water used for homes can be heated as well rooms can be heated.
Solar Energy for Battery Charging
Batteries can be charged with solar power. These batteries can be utilized to power hot water pumps, sump pumps, and ceiling fans in your home. Video game batteries are charged by battery chargers using solar power as well. Thus, a reserve battery that can be used in the nighttime and can be charged when the sun is up in the mornings is very useful to be used in conjunction with various appliances at home.
Solar Energy For Cooking
Cooking with the aid of solar energy is very easy. A solar oven can be used for cooking purposes. Materials need for building a solar oven are thermometer, duct tape, cooking bag, aluminum foil, and box and pan.
Solar Energy for Indoor Lighting
Lighting is used in all homes. With the arrival of the light-emitting diodes or LEDs, it is possible to have optimal lighting in your homes that consumes power only minimally. These electronic sets of lights are connected to a battery charge system during the day and batteries during the night. The battery charge system charges the backup battery using solar energy in the mornings, while the batteries help to charge the backup battery and supply the necessary lighting, when the sun is done at night.
Solar Energy Used For Outdoor Lighting
Lighting up porches or verandas in the night can help in a number of ways; especially, to enhance visibility when the sun is not shining and also to increase security. Solar panels charge batteries during the day using solar power, and at night, these batteries can help to illuminate homes. The lighting comes at no cost at all, and thus, is very economical to use.
- New Developments in Solar Cells
A major breakthrough is the transparent conductor, a mixture of titanium dioxide nanoparticle and silver nanowires (Gupta, 2009). This conductor is quickly replacing the metal electrodes of the past. Solar cells can now be fabricated economically by using these composite electrodes by solution processing. A four-percent power conversion efficiency can be achieved for polymer solar cells with this combination (Lunt, 2012).
However, this development is still at their infancy; hence, it would be several years before it becomes possible to power buildings through the use of light passing the windows. This technology, when it matures and becomes applicable, will enable cost savings to a large extent, and the renewable energy efficiency will be astounding.
Solar cells are being developed to increase their efficiency such that the cost of using solar energy to generate electricity is reduced. The total plant cost of a solar panel depends on the amount of area being used per unit. Efficiency-enhancing techniques may attempt to reduce the panel area and thus, increase the efficiency of the cell.
- Disadvantages in the Use of Solar Power
Harnessing of solar energy can be done only in the daytime and not at nights. This is a disadvantage with this type of energy.
Solar panels, collectors, and cells are expensive to manufacture. However, as this technology is fast being embraced by many people to convert solar power to electricity and use it at economical costs, the costs for the manufacturing have been found to be decreasing.
Solar-powered stations are also very expensive to build. Also, the outputs from such solar power stations are too low relative to the conventional power stations of similar size.
Solar energy is derived when the sun is bright and shining. In countries such as the UK, the climate is unreliable. This means the possibitles of harnessing solar power are also unreliable. Cloudy skies hamper the collection of solar energy by solar panels. This is a major disadvantage in trying to use solar power to drive everyday functions.
The areas of land utilized to collect solar energy are large. Many collectors are placed together, if the energy is to be collected and used in the same location.
Solar power derived from the sun is converted to electricity and stored in batteries. These batteries can power fans and lights in houses at nights. However, these batteries are very heavy and need additional storage space.
Thus, solar energy although cheap, economical, and easy to obtain is not without disadvantages. These drawbacks must be considered while setting up solar panels at home.
8. Conclusion
Other advantages of using solar energy are that they can be used in remote areas, where electricity is not available. Apart from this, many everyday appliances, such as solar-powered calculators and low-power consuming devices, can run on solar energy thus, saving costs. It is a known fact that the world oil reserves are becoming depleted. However, the solar energy can be described as infinite energy.
References
De Vos, (1980) "Detailed balance limit of the efficiency of tandem solar cells", Journal of Physics D: Applied Physics Vol. 13, Issue 5, p. 839-846.
"Solar cell hits new world record with 44.7 percent efficiency". Retrieved 25 May 2014.
Gupta, N.; Alapatt, G. F.; Podila, R.; Singh, R.; Poole, K. F. (2009). "Prospects of Nanostructure-Based Solar Cells for Manufacturing Future Generations of Photovoltaic Modules". International Journal of Photoenergy.
Wynn, Gerard (19 October 2007). "Solar power edges towards boom time". Reuters. Retrieved 25 May 2014.
Peacock, Matt (20 June 2012) Solar industry celebrates grid parity, ABC News. Retrieved 25 May 2014.
"Sunny Uplands: Alternative energy will no longer be alternative". The Economist. 21 November 2012.
Pearce, J.; Lau, A. (2002). "Net Energy Analysis for Sustainable Energy Production from Silicon Based Solar Cells". Solar Energy. p. 181
Falling silicon prices shakes up solar manufacturing industry. Down To Earth (19 September 2011). Retrieved 25 May 2014.
Lunt, R. R. (2012). "Theoretical limits for visibly transparent photovoltaics". Applied Physics Letters Vol. 101 (4), p. 043902.
