Generating Power from Solar Energy
How Sunlight is Converted to Electricity Using Photovoltaic Process
Electricity generated from solar power makes up a growing portion of the worldwide power generation market.
Solar power is electricity that is generated from sunlight, and is a common choice of renewable energy for households and for large companies. There are two basic forms of solar power in current use: photovoltaics and solar thermal power.
Photovoltaic Solar Power
Photovoltaic systems, such as conventional solar panels, directly convert sunlight into energy using the principles of the photovoltaic effect. The photovoltaic effect takes advantage of the properties of semiconductor materials, with silicon being the primary material used in photovoltaic solar cells. When photons strike the solar cell, electrons in the semiconductor material are shaken loose, allowing them to flow as electricity. This electricity is direct current (DC), and can be directly used to charge batteries, or can be connected to an inverter to power alternating current (AC) components, or to be connected to the local electrical grid.
Traditional photovoltaic systems are based on silicon. Silicon ingots are sliced into wafers that are fabricated into cells. Cells are combined into modules, which are packaged into end-user systems. Silicon-based solar cells have efficiencies of approximately 14-19%. However, newer systems that use gallium arsenide, another semiconductor material, can be made into thinner and more flexible modules. These "thin film" modules can presently produce efficiencies up to 30%, but currently cost more to fabricate than traditional silicon-based modules.
Solar Thermal Systems
Solar thermal systems use mirrors to reflect sunlight to a central location. The intense heat of the focused sunlight on the central location is either used to heat a fluid used to create steam, which in turn powers turbine engines, or is used directly to power a Stirling cycle engine. The fluids used in these systems can be high-pressure steam, synthetic oils, or molten salt. Alternatively, the heated fluid can be stored and the heat used to generate electricity at a later time.
There are two types of reflectors used in solar thermal systems. The first type of reflector is a flat panel reflector. An array of flat panel reflectors are used to focus the sunlight onto a central tower. The panels are motorized and their motion is coordinated to track the path of the sun throughout the day, maximizing the sunlight at the central tower.
The second type of reflector is a parabolic reflector. These can take the form of parabolic troughs that reflect and focus sunlight on a tubular collector that runs along the axis of the trough, or as a round dish, where the sunlight is reflected to a central collector. The latter configuration is often coupled with a Stirling engine.
Solar-based electricity, whether generated directly by photovoltaic cells or indirectly by solar thermal systems, is clean, requires little maintenance, has no byproducts, and requires no fuel other than the sun.
Sources
O. Schultz, R. Preu & S.W. Glunz; A. Mette, "Silicon Solar Cells with Screen-Printed Front Side Metallization Exceeding 19% Efficiency", 22nd European Photovoltaic Solar Energy Conference and Exhibition, July 2007.
Lorenzo, E., Solar Electricity: Engineering of Photovoltaic Systems, Progensa, 1994.
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