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Photovoltaics basics

Solar cells can be thought of as solar batteries because they act much like car batteries except they use sunlight instead of chemical energy to produce electricity. Solar cells are technically named photovoltaic cells because they change light (or in Greek, photo) into electricity.
Photovoltaic (PV) cells use the same technology that is used to make computer chips and other solid state electronic components. Solar cells are not very exciting to watch, as they simply sit still in the sun. The excitement happens at the molecular level, where the photons from sunlight knock electrons from the atoms to produce electricity. No moving parts!
Solar cells are connected together to form PV modules or solar panels that produce direct current (DC) much like car batteries. PV modules come in sizes from 10 Watts to 300 Watts. When grouped together, solar panels form a solar array.

Grid-tied PV system

To connect the system to the utility grid requires an inverter that changes the direct current (DC) produced by the solar panels to alternating current (AC) that we find in the outlets in our home. A photovoltaic system consists of one or more arrays connected together and mounted on a roof or any other support structure, along with the necessary switch gear, inverter, and control equipment, usually mounted on a wall in the garage. The solar systems connected to the utility grid are called grid-tied PV systems. For houses that are not connected to the utility grid solar arrays is connected to a charge controller, batteries, and an optional inverter. These stand-alone systems are used to supply electricity when a utility connection is too expensive.

Miscellaneous facts about PV systems

Orientation: It is not necessary to have the system utilizing the optimum tilt and direction. Systems that are oriented generally south and on a 4/12 pitched roof (18.5 degree tilt) or steeper produce at least 95% of the electricity of an optimally oriented system (see contour performance plots).
Area: On a clear day around noon, incident solar radiation is approximately 100 Watts/square foot (1,000 W/square meter). Mono-crystalline (single crystal) and multi-crystalline solar panels change about 15-18% of the incident sunlight into electricity. Therefore it takes roughly 70 to 60 square feet of roof area respectively for a 1 kW solar electric system. Thin film technologies such as amorphous silicon panels are approximately 8% efficient and a 1 kW array would take up twice the area of a crystalline array.
KiloWatt Hours Produced: Each watt of a PV system produces between 1 and 1.6 kWhrs annually depending on where one lives in the Northwest. Therefore a 1 kW array would produce between 1000 and 1600 kWhrs per year.
Shading: Shading on solar cell in a panel will shutdown the production of solar cells strung together in series. This is like taking one battery out of a series of batteries. Shading solar cells in panel in a string of panels essentially shut down all electricity from that string.
Inverter: An inverter is usually about 90% efficient in turning DC current to AC current. Other factors such as line losses and dirt on the array typically cause another 10% decrease in performance.

Below: a solar cell.


Below: a solar panel.


Below: a solar array.


Below: inverters.



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© 2006, UO Solar Radiation Monitoring Laboratory.
Last revised: November 10, 2006.


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