Photovoltaic Energy
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FIGURE 36-17
Photovoltaic effect.

Solar cells convert sunlight directly into electricity and are used in a wide variety of applications to supply electricity. They are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the photons knock electrons loose from their atoms, causing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage, which moves electrons) is called the photovoltaic (PV) effect Figure 36-17. When light energy strikes the surface of PV semiconductor materials, the materials emit electrons. These freed electrons can then be made to flow in a circuit where they are used to perform work. The principle is applied in some ignition systems and in vehicle speed sensors by creating signals used to time the ignition sparks or by telling the computer how fast the vehicle is moving. It is also used to sense the amount of daylight to operate automatic headlights and is used in heating, ventilation, and air-conditioning (HVAC ) systems to determine the sun load on the vehicle. In addition, it is the principle of the solar cell.

Technician Tip
The term photovoltaic comes from the Greek photos, meaning light, and the name of the Italian physicist Volta, after whom the volt (and consequently voltage) is named. Photovoltaic means literally “of light and electricity.”