To Create The PV Effect

To create the PV effect, radiation from the sun ('sunlight') hits a photovoltaic cell. These cells are made up of two layers of semi-conducting material, typically silicon, that have been chemically treated. The industry refers to these layers as P and N. The boundary between P and N acts as a diode allowing electrons to move from N to P, but not from P to N. When photons with sufficient energy hit the cell, they cause electrons to move (from N to P only) causing excess electrons in the N-layer and a shortage in the P layer.

This voltage difference is typically in the range of 0.5V for as long as the cell is in sunlight. If you short-circuit the upper and lower layer a current runs of about 3 Amps. If you arrange sufficient cells in series, the result is a PV module or PV panel. Let's say 36 cells in series produce 36 x 0.5V = 18V at 3 Amps = 54Watts.

The following graphic sets out the layers within the cell. The top layer is an Anti-Reflective-Coating (ARC) that enhances the light effect of the sun. The N layer is typically semi-conducting silicon doped with phosphorus that creates the free flow of electrons. The P layer is again typically semi-conducting silicon, but this time doped with boron which creates the free flow of positive charges called “holes”. As the holes and electrons are attracted and move towards each other, they create an electrical field across the P-N junction. Sunlight striking this electrical field separates the electrons and holes, creating the voltage.


The voltage pushes the flow of electrons or 'DC current' to contacts at the front and back of the cell where it is conducted away along the wiring circuitry that connects the cells together.

Solar Cells and Arrays

Solar cells are typically combined into modules that hold about 40 cells; about 10 of these modules are mounted in PV arrays that can measure up to several meters on a side. These flat-plate PV arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day. About 10 to 20 PV arrays can provide enough power for a household; for large electric utility or industrial applications, hundreds of arrays can be interconnected to form a single, large PV system.

Some solar cells are designed to operate with concentrated sunlight. These cells are built into concentrating collectors that use a lens to focus the sunlight onto the cells. This approach has both advantages and disadvantages compared with flat-plate PV arrays. The main idea is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible. But because the lenses must be pointed at the sun, the use of concentrating collectors is limited to the sunniest parts of the country.

Efficiency

The performance of a solar cell is measured in terms of its efficiency at turning sunlight into electricity. Only sunlight of certain energies will work efficiently to create electricity, and much of it is reflected or absorbed by the material that make up the cell. Because of this, a typical commercial solar cell has an efficiency of 15%-about one-sixth of the sunlight striking the cell generates electricity, although leading competitors are working towards 18%. The theoretical maximum efficiency of a solar cell using current techniques is in the 30% range

For this reason, implementations of photovoltaics are most effective in areas with a large amount of daily sunlight. Note that the solar cells are temperature-dependent, such that in a cold environment a photovoltaic cell performs better than in a hot environment. (0.3% increase per 1 degree C drop in temperature). Unfortunately, there aren't a lot of places on earth that are both cold and have long sunny days.

Low efficiencies mean that larger arrays are needed, and that means higher cost. Improving solar cell efficiencies while holding down the cost per cell is an important goal of the all participants in the solar energy industry, and they have made significant progress. The first solar cells, built in the 1950s, had efficiencies of less than 4%.

The diagram on the left is sets out the maximum power performance of a photovoltaic cell. The red curve is the voltage-performance graph of the cell and the green curve is the current-voltage graph. The best performance (in Watts) is obtained at that voltage at which the current definitely starts to decline: the maximum power point (MPP).

Source:

http://www.engineering.com/SustainableEngineering/RenewableEnergyEngineering/SolarEnergyEngineering/Photovoltaics/tabid/3890/Default.aspx

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