Solar Energy Backgrounder

by Dave

a knol by William Pentland

In 2004, solar energy accounted for only 0.039 percent of the world’s total primary energy supply of 11,059 million metric tons of oil equivalent, according to the International Energy Agency. In other words, solar energy provided about 4 terawatt-hours of electricity generation, out of an estimated overall total production of some 17,450 terawatt-hours (1 terawatt = 1 trillion watts). The strength of the solar energy available at any point on the earth depends on the day of the year, the time of day, and the latitude of at which it hits the Earth.

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 photovoltaic cell, they may be reflected, pass right through, or be absorbed. Only the absorbed photons provide energy to generate electricity. When enough sunlight is absorbed by the material, electrons are dislodged from the material’s atoms. Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to free electrons, so the electrons naturally migrate to the surface.

When the electrons leave their position, holes are formed. When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell’s front and back surfaces creates a voltage potential like the negative and positive terminals of a battery. When the two surfaces are connected through an external load, electricity flows. To increase power output, cells are electrically connected into a packaged weather-tight module. Modules can be further connected to form an array. The term array refers to the entire generating plant, which can consists of as few as one solar module or several thousand modules. The number of modules connected together in an array depends on the amount of power output needed.

Several technologies have been developed to harness that energy, including concentrated solar-power systems; passive solar heating and daylighting, photovoltaic systems, solar hot water, and solar process heat and space heating and cooling.

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