The history of PV's dates back to 1839. Solar
Electric or Photovoltaic Systems convert some of the energy in sunlight directly
into electricity. Photovoltaic (PV) cells are made primarily of silicon, the
second most abundant element in the earth's crust, and the same semiconductor
material used for computers. When the silicon is combined with one or more other
materials, it exhibits unique electrical properties in the presence of sunlight.
Electrons are excited by the light and move through the silicon. This is known
as the photovoltaic effect and results in direct current (DC) electricity. PV
modules have no moving parts, are virtually maintenance-free, and have a working
life of 20 - 30 years.
There are three basic categories of photovoltaic
systems with several types in each category.
- Crystalline Photovoltaic Materials
flat plate collectors are the most developed and
prevalent type in use today. These include single crystal silicon and
polycrystalline silicon which is either grown or cast from molten silicon
and later sliced into its cell size. They are then assembled onto a flat
surface; no lenses are used.
- Thin Film systems
are inherently cheaper to produce than crystalline
silicon but are not as efficient. They are produced by depositing a thin
layer of photovoltaic material to a substrate like glass or metal. This
group includes amorphous silicon like the kind found in calculators and
use much less of a specialized photovoltaic material and
employ a lens or reflectors to concentrate sunlight on the photovoltaic cell
and increase its output. They can be produced more cheaply than either of
the other type due to the reduced amount of expensive PV material. But they
can only use direct sun, so they must track the sun precisely and do not
work when it is cloudy.