Nearly all of our electricity generation options involve converting mechanical energy into electrical energy—usually using a dynamo or turbine. The significant exception is photovoltaics, in which sunlight is converted directly into electricity—with no moving parts.
Photovoltaic (PV) cells use a phenomenon called the photovoltaic effect to generate electricity. A cell is made of a semiconductor material—a material that conducts electricity but whose electrical conductivity can be altered by adding small quantities of other elements (a process referred to as doping). Crystalline silicon is the most common semiconductor used, though other materials are being used as well, including amorphous silicon, cadmium-telluride (CdTe), gallium arsenide (GaAs), and copper indium gallium (di)selenide (CIGS). Most of these alternatives are being tried in an effort to reduce costs.
In manufacturing PV cells, thin wafers or strips of the semiconductor material are created, but these cells have two different layers. The top side is doped with an element that has an extra electron, usually phosphorous, to give it a negative charge (N type), while the back side is doped with a different element, usually boron, that is shy an electron, giving it a positive charge (P type). The cell junction separates these two layers.
When photons of sunlight strike the PV cell, electrons in the N layer are excited and jump across the P-N cell junction. This creates a charge imbalance, with electrons wanting to return to the N layer of the cell. By connecting the two sides of the cell with a wire, electrons are able to return to the N layer—and that electron flow is the electric current that we are able to make use of. While electrons move around in this process, there are no “moving parts” to wear out—as there are in nearly all other electrical generation systems.
Numerous individual PV cells are wired together in series to create a PV module, which increases the current flow, and then multiple modules can be combined to create a PV array. A PV system includes an array as well as various balance of system components, including charge controller, inverter (if direct current is to be converted into alternating current), and—depending on the application—batteries for storing the solar-generated electricity.
Refinement of the photovoltaic process and improvements in PV efficiency are the focus of a tremendous amount of research going on worldwide today. While the efficiency of the best crystalline silicon PV modules is now above 20%, much of the leading research is focused on lower-cost, thin-film technologies that have lower efficiencies, but that offer the promise of significantly lower-cost PV power.
Article source: BuildingGreen.com “Environmental Building News”
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