The sunlight that passes through a building's windows not only lights the interior of the building but also transmits a solar energy through the glass, often heating the interior unnecessarily. Several energy technology companies are working to develop solar windows, systems that will convert that solar energy to electricity, thereby serving the building's energy needs at the same time that its cooling needs are reduced.
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Photovoltaic Glass Unit
A solar window system being developed by Pythagoras Solar uses optical technologies coupled with traditional silicon photovoltaic cells in a window panel the company calls a Photovoltaic Glass Unit (PVGU). The PVGU uses an optical system sandwiched between layers of glass to control the passage of sunlight through the window, allowing the PVGU to control the amount of heat and light that is transmitted through the window and, at the same time, refract sunlight so it is directed toward the photovoltaic cells housed outside the transparent area of the window.
New Energy Technologies Inc. is developing a product it calls SolarWindow, a photovoltaic system that uses solar cells embedded in a thin film that can be sprayed onto window glass. These cells were the world's smallest as of 2010, about a quarter the size of a grain of rice, and made use of organic materials rather than the metal contacts in traditional solar cells. These organic materials have a higher degree of transparency than traditional cells, allowing them to be applied to windows without significantly reducing the transparency of the glass.
Nanoparticle Solar Cells
The Norwegian company EnSol is developing a solar window product that employs energy-generating metal nanoparticles embedded in a thin film that, like SolarWindow, can be sprayed directly on window glass. EnSol says these particles will produce only a slight tint in window glass, allowing an adequate amount of light to pass through. The company also suggests that its film could be applied to all exterior surfaces of a building, turning the entire building into an energy-generating solar cell.
Passive Solar Windows
The simplest form of solar window technology is passive solar window design. Passive solar design involves considering climate when choosing window placement -- putting windows in south-facing walls in climates where winter heating is most important, for example -- and using roof overhangs and landscaping to block sunlight when necessary. The use of insulated, tinted, reflective or spectrally selective glass is also an option in passive solar designs.