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How does solar photovoltaic (PV) cell work?

photovoltaic principle.jpgPhotovoltaics is the direct conversion of light into electricity at the atomic level. Some materials exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. When these free electrons are captured, an electric current results that can be used as electricity.

The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in 1839, who found that certain materials would produce small amounts of electric current when exposed to light. In 1905, Albert Einstein described the nature of light and the photoelectric effect on which photovoltaic technology is based, for which he later won a Nobel prize in physics. The first photovoltaic module was built by Bell Laboratories in 1954. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. In the 1960s, the space industry began to make the first serious use of the technology to provide power aboard spacecraft. Through the space programs, the technology advanced, its reliability was established, and the cost began to decline. During the energy crisis in the 1970s, photovoltaic technology gained recognition as a source of power for non-space applications.

The diagram above illustrates the operation of a basic photovoltaic panel, also called a solar panel. The Solar cells of the panel are made of the same kinds of semiconductor materials, such as silicon, used in the microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current -- that is, electricity. This electricity can then be used to power a load, such as a light or a tool.

A number of solar cells electrically connected to each other and mounted in a support structure or frame is called a photovoltaic module. Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system. The current produced is directly dependent on how much light strikes the module.

Multiple modules can be wired together to form an array. In general, the larger the area of a module or array, the more electricity that will be produced. Photovoltaic modules and arrays produce direct-current (dc) electricity. They can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination.

Costs, savings and maintenance

An average system is 2.9 kWp and will cost around 13,500 euros (including VAT). Most domestic PV systems cost around 4,000 euros to 5,000 euros per kWp installed, though small systems cost proportionately more. Costs vary between installers, so it is important to get several quotes. Other factors are:

- the more electricity the system can generate, the more it costs but the more it could save
- larger systems are usually more cost-effective than smaller systems (up to 4 kWp)
- PV panels are all around the same price per kWp, but PV tiles cost much more than a typical system made up of panels
- panels built into a roof are more expensive than those that sit on top.

A 2.9kWp system can generate up to 2,500 kilowatt hours of electricity a year - that's around three quarters of a typical household's electricity needs - and will save over a ton of carbon dioxide every year.

Solar PV needs little maintenance - you'll just need to keep the panels relatively clean and make sure trees don't begin to overshadow them. The panels should last 25 years or more, but the inverter is likely to need replacing some time during this period, at a current cost of around 1,500 euros.