Environmental Engineering Reference
In-Depth Information
the electricity used on the base, where 12,000 people live and work. It
will save the federal government nearly $1 million a year and reduce
carbon pollution by 24,000 tons a year, the equivalent of removing
24,000 cars from American roads.
Photovoltaic cells function by separating electrons from their parent
atoms and accelerating them across a one-way electrostatic barrier
formed by the junction between two different kinds of semiconductors.
Photovoltaic cells are already widely used in small items such as calcula-
tors, watches, toys, and a variety of other consumer products, but getting
high conversion effi ciencies from large cells for electricity generation is
a challenging problem. Traditional solar cells are made of crystalline
silicon, which is expensive and requires a lot of energy to manufacture.
It takes about three years for a conventional photovoltaic panel and the
equipment associated with it (the rigid frame used to mount it and the
power-conditioning electronics that attach it to the grid) to produce
the amount of electrical energy required to manufacture this equipment
in the fi rst place.
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Experiments with dye-sensitive cells made of titanium dioxide have
shown promise in reducing costs—they cost one-tenth the price of pure
silicon—but they have low conversion effi ciencies. Experiments using
plastics have shown that they are poor conductors of electrons; cells
using them have effi ciencies of only 3 or 4 percent. However, because of
the low cost of plastic, an effi ciency of even 7 percent would make such
cells economically competitive with silicon.
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What is needed is a way to
boost the effi ciency of cells made from cheap materials.
One emerging technology that is promising is thin-fi lm technology,
which does not require silicon.
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It is cheaper to produce but has lower
effi ciencies than standard photovoltaic panels. Its global market share
doubled from 7 percent in 2006 to 14 percent in 2008 and is projected
to reach 31 percent by 2013. Thin fi lms are composed of very thin layers
of photosensitive materials and require less energy and materials to make
than conventional silicon-based solar cells. They can be integrated into
roof shingles, siding, and the windows of buildings. Conversion effi -
ciency for commercial uses is only 10 percent, but effi ciencies have
reached 20 percent in the laboratory.
Because of the cost and conversion effi ciency problems associated with
solar cells made of silicon, solar thermal technology, which has been
operating in California's Mojave Desert since 1985, has undergone a
rebirth in the United States. The technology is called concentrated solar
power (CSP) or solar thermal electricity (STE). CSP systems capture and
