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