| Photovoltaic (PV) systems convert sunlight into | | | | when the sun is not shining. |
| electricity. The photovoltaic effect is the basic physical | | | | A well-designed and properly installed PV system with |
| process through which this happens. Sunlight is | | | | a consistent maintenance schedule will operate for |
| composed of photons, or particles of solar energy. | | | | more than 20 years. The PV module, which has no |
| These photons contain various amounts of energy | | | | moving parts, has an expected lifetime of more than |
| corresponding to the different wavelengths of the | | | | 30 years. |
| solar spectrum. When photons strike a PV cell, they | | | | In the past 30 years, the photovoltaic industry |
| may be reflected or absorbed, or they may pass right | | | | maintained a growth rate of 20 percent on the |
| through. Only the absorbed photons generate | | | | average, while in the last five years, with an average |
| electricity. When this happens, the energy of the | | | | annual growth rate of as high as 35 percent. As of |
| photon is transferred to an electron in an atom of the | | | | 2007, the global PV power installed capacity is 9.1 |
| cell (which is actually a semiconductor). With its | | | | million kilowatts, the growth rate go up to 33 percent. in |
| newfound energy, the electron is able to escape from | | | | 2007, capacity of 2.2 million kilowatts is installed, and |
| its normal position associated with that atom to | | | | the growth rate is 40%.on the optimistic view , in the |
| become part of the current in an electrical circuit. By | | | | next 30 years ,the photovoltaic industry will maintain a |
| leaving this position, the electron causes a "hole" to | | | | growth rate of more than 25 percent, while the |
| form. Special electrical properties of the PV cell | | | | pessimistic view is that this opinion is not based on |
| provide the voltage needed to drive the current | | | | reality. |
| through an external load (such as a light bulb). | | | | Optimistic faction believes that as the technological |
| A PV system comprises several components. The | | | | progress and industrial expansion, photovoltaic power |
| basic building block of a PV panel is the PV cell, which | | | | generation costs will be quickly reduced, thereby it |
| is a solid state, or non-mechanical, device. A solar | | | | bring a fundamental demand for the expansion, the |
| system uses a number of PV panels, each made of | | | | process will run through the entire century. The latter |
| silicon, plus boron and phosphorous. The output of a | | | | part of the growth rate will decline because the base |
| single cell under direct sunlight is about one watt. To | | | | PV will be huge. |
| increase their effectiveness, dozens of individual cells | | | | Pessimistic view is base on the biggest obstacles for |
| are interconnected together in a sealed, weatherproof | | | | photovoltaic industry is the high cost. In the Western |
| glass package called a module. Modules come in a | | | | developed countries, they mainly go through various |
| range of wattages, and their nature allows for great | | | | kinds of financial subsidies to support the development |
| flexibility in designing systems that meet a variety of | | | | of the industry, such as United States allow advance |
| electrical needs. | | | | photovoltaic project with the financial and tax |
| Since PV modules are only capable of producing direct | | | | incentives, and support the Internet price of |
| current (DC) electricity, an inverter is required to | | | | photovoltaic for 21.29 cents / unit. |
| convert the direct current (DC) output produced by the | | | | According to our opinion, base on the cost of |
| PV array into alternating current (AC) power. AC | | | | photovoltaic, the next three to five years, in some time |
| electricity is needed to run computers, refrigerators | | | | and some areas, photovoltaic will have cost |
| and other appliances, and lighting. | | | | advantage. U.S. Environmental Co-op non-profit |
| A utility PV system, such as those installed under the | | | | organization says, the cost of solar power will be equal |
| Sun4Schools project, generate electricity which is | | | | with the traditional fossil energy for power generation |
| supplemented by the energy provided by the existing | | | | costs. With the decline of the cost for solar power, |
| utility grid. A PV system requires neither battery | | | | while coal, natural gas and the rising cost of nuclear |
| storage nor an emergency back-up system since it is | | | | power, United States will come to the intersection by |
| connected directly to the utility grid, which is used as | | | | 2015.From Comprehensive opinion, the photovoltaic |
| the storage medium. Systems that are not connected | | | | industry will have a high growth period. |
| to the utility grid use batteries to store energy for use | | | | |