Photovoltaics (PV) is in many ways the most exciting and promising of solar technologies. If you've ever used a solar calculator, you've seen PV in action. As the name suggests, photovoltaic cells produce electricity directly from sunlight, a free and inexhaustible energy souce. PV is distinct from other kinds of solar energy in that it harnesses the sun's light, rather than its heat. Because the hardware needed for this is entirely solid-state electronics, photovoltaic cells are extremely low-maintenance and have very long lifespans.
Although very straight-forward in design, solar thermal panels need additional
hardware such as heat exchangers and pumps to reach maximum efficiency.
Converting this heat energy to electricity is much more complex, so complex
in fact that it is genrally only done on a large scale, where design and
maintenance costs are minimized. Although such huge, centralized operations
represent the peak efficiency for solar thermal, their high upkeep costs
prevent them from being competitive with fossil fuels.
The technology behind photovoltaics is fairly simple, but can be pretty confusing. The idea itself is a breeze: when light (in the form of a photon) hits silicon, electrical energy (in the form of an electron) is released. The tricky part is in harnessing these displaced electrons. The way that cell manufacturers generally do it is by combining two carefully-crafted layers of silicon. Other elements have been added to these layers,so that the top one is electron-rich and the bottom is electron-deficient. These layers are separated by a special junction, which allows electrons to flow from the bottom to the top, but not the other way.
When a photon sets an electron free in the bottom layer, the electron wanders around the silicon looking for a place to rest (a hole). Many of these electrons will find their way through the junction, replenishing the upper layer's supply of electrons.
Things get more interesting when an electron is freed from the top layer. Since the junction prevents it from finding its way to the holes in the bottom layer, and the upper layer is already crowded with electrons, it can't find a hole to settle in. These are the electrons that cell designers are looking for. Electrical contacts on the surface of the upper layer sweep them up by the millions, resulting in an electrical charge. The circuit is completed by another electrical contact on the bottom layer, which returns the electrons to the electron-deficient silicon, starting the cycle again.
Because there are no moving parts in PV solar cells, they are very easy to
keep running. In general, regular dusting is all that a PV panel needs to keep
it operating smoothly for thirty years.
Photovoltaics are thus perfectly suited for remote locations, far from technicians and tools. Of course, these are often also far from the electrical grid, making solar energy one of very few practical solutions.
Solar energy is often needed in places quite near the grid as well -- in villages too poor to appear as attractive investments to utilities companies. Several pilot programs have already made great progress in bringing electricity to rural villages across the globe -- with the help oh photovoltaics.
This odd insversion is one of the most fascinating things about PV: although the technology is far more complex than solar thermal, it is so easy to use that it's ending up in the hands of some of the poorest people in the world.
Introduction to Photovoltaics --
John Fraser