Overview

Photovoltaic systems converts solar energy into useable energy in the form of electricity. The "photovoltaic" devices are composed of material (typically silicon crystal) that when exposed to solar energy, becomes excited resulting in electron flow, which is drawn away from metal contacts on either surface of the device resulting in current. This technology has permeated everyday life in the form of solar powered calculators, watches, road-side assistance telephones, stand-alone parking ticket machines, and which also appear on satellites in orbit above the earth.

In technical terms, the conventional solar cell is made from inorganic crystalline semi-conducting material such as silicon, which is "doped" (slightly contaminated with appropriate elements) to form a p-n junction. The p side of the junction contains an excess of positive charges (holes), the n side, an excess of negative charges (electrons). This creates an electric field across the junction preventing electrons from the n-side (where electrons are in the majority) from crossing over easily; though it does accelerate loose electrons from the p-side if they happen to be near the junction (and this is the most important point). When sunlight is applied to this, its electromagnetic energy can break electroncs free from their lattice bonds, creating loose electrons and positive holes on both sides of the joined crystal lattice sheets. The incoming sunlight creates an electical potential difference at the junction, and therefore, by the difference between the energy levels of free verses bound electrons (approximately 0.5V), a direct electrical current flow is created. By wiring the solar cells in series, the voltage can be increased, or if preferred, in parallel to increase amperage, or in both ways to increase both voltage and amperage. These solar cells are wired together to form solar panels, and solar panels can be wired together to form solar arrays.

An instructive animation, sourced from Energex, Australia, cleverly depicts the concepts of solar energy being captured and converted into useful electrical energy can be viewed by clicking "Photovoltaic Energy - Animation".

Application of Solar Cell Technology

Most residential photovoltaic applications are installed on the roof, however, in many cases, the roof's orientation or angle of inclination is not optimal to take advantage of the available solar energy.

Pearson Prentice-Hall, Geoscience Animations Earth-Sun Relations Animation

However, for the average homeowner, interested in reducing their dependency on traditional energy sources, setting the angle of inclination between 5 and 45 degrees will provide a relatively effective photovoltaic system. The panels can undergo slight changes to its angle manually twice annually to accomodate the sun's relative position to the horizon dependent upon the season. The illustration on the left when clicked offers a visually informative animation of the earth's position and orientation relative to the sun for the seasons of summer (the highest point of the sun relative to the horizon) and the winter (the lowest point of the sun relative to the horizon). Click on the image on the left to view this animation.

The most common errors during the examination of a potential site or location, followed by the installation of solar arrays is missing on the potential impact that shade will have on the array. The enemy of solar arrays is shade. Partial shading of an array can result in dramatic reduction of solar panel output. One entirely shaded solar cell can reduce the solar panel's output by as much as 75%. Three solar cells shaded can decrease up to 93% of the panel's output. Periodically, during installations, in particular, during the "off-season" of flora, when the spring and later, the summer arrives, shade which wasn't an issue during the autumn and winter months, becomes problematic when the trees return to their leafy state. Careful and intelligent site surveys will reduce, if not eliminate these potentially expensive errors in installations.

References

1. Solar Energy Society of Canada Inc.. "Photovoltaic Energy". 2003.
2. Energex, Australia. "Photovoltaic Energy - Animation". Energy and the Environment. 2006.
3. Pearson - Prentice Hall Inc.. "GeoScience Animations". The Prentice Hall Geoscience Animation Library. 2005.
4. Dr. Mae-Wan Ho. "Solar Power for the Masses". The Institute of Science in Society. Jan 2006.
5. A. Hunter Faney, Brian P. Dougherty, Mark W. Davis. "Measured Performance of Building Integrated Photovoltaic Panels". National Institute of Standards and Technology, Forums 2001. April 2001.
6. The Solar Server. "Follow the Sun: tracking solar power arrays can increase energy yield by up to 30%". 09 Feb 2006.