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Green Collar Careers - Silicon Solar Panels

Silicon semiconductor technology shared many of the same processes and materials used in silicon solar cells that gave silicon an economic advantage for early solar cell products.  Today the vast majority of the solar panels use photovoltaic cells of silicon structure however that is beginning to change with the onset of newer thin film devices.

A simple explanation of how they work is:
 

Light photons strike a thin (almost transparent) layer of metals that free up electrons Atoms with free electrons are compelled to exchange "holes" to balance their electrical charges A thin layer of material allows electrons to flow from the surface metal layer and holes to flow to the surface Both electrons and holes flow through the silicon semiconductor material like streams of ants moving 2 ways The bottom of the silicon has a layer that also controls flow direction of holes and electrons the base metal The base metal, usually aluminum carries the electrons to the negative wire The surface metal usually silver/palladium is connected to the positive wire Electrons flow through the circuit from negative to positive maintaining a balance between electrons and holes Each solar cell produces a voltage of 0.5-0.6 volts.  Most solar panels connect individual cells in series to increase voltage so every solar panel may have different voltage outputs; manufacturer's specifications will need to be consulted to determine the voltage and current levels.  Solar Photovoltaic cells have an efficiency ranging from 12-19%.

The sun emits a spectrum of different colors of light and each color has a different wavelength.  A limitation of a silicon solar cell is they cannot convert all colors of light into electricity which is a primary reason for lower efficiency.  Solar cells have a fixed "band gap" value that determines the optimal range of wavelengths or colors the cell will work with.  In the morning and evening the sun passes through different shapes of the atmosphere which acts both as a prism and a lens and changes the color value slightly;  morning and evening tend to 'red-shift" meaning more of the shorter wave (red) colors are available at the surface, while the remainder of the day is "blue shifted" and shorter wavelengths are predominant.  Shorter (blue) color range is favored for solar cell band gap design because this is where most of the daily energy will come from.  This creates two restrictions for silicon solar cells:

1. Cloud and fog tend to absorb shorter wavelengths (blue) more then longer wavelengths (red) dropping the output significantly on cloudy days.
2. With morning and evening sun "red shifted" energy doesn't have a significant benefit by adding a solar tracking system.  In addition since the sunlight is diffused the gains from a solar tracking system rarely offset the cost of adding the system.

In selecting a solar panel look over the manufacturers specifications; most are downloadable from and wholesale solar panel websites.  Specifications will include:

• Size
• Weight
• Voltage
• Current
• Peak Watts
• Photon Energy Spectrum Charts
• Mounting provisions
• Warranty
• Expected Operating Life

Most Solar Panels have a life of between 20-30 years.  After about 15-18 years they begin to slowly degrade with reduced output that reaches about 75% after 25 years.