The term "photovoltaic solar energy" may seem a bit redundant to those who understand the technology – and completely mysterious to those who don't.
It's not all that complicated, however.
Photos, Photographs, Photon Torpedoes and Photovoltaics
The common denominator of that subtitle is "photo." This word, which has come to refer to images taken with a camera or similar device, is simply an old Greek term meaning "light." When combined with another Greek word, grafikos – meaning "image" or "inscription" – we get the word photograph, which means "image from light."
A photon is simply a light particle. All due respect to Star Trek fans, it's difficult to know how such an insubstantial particle could be turned into a weapon by itself – and the writers are vague on just how a "photon torpedo" actually is supposed to work.
Volts of course are units of electrical energy. Therefore, photovoltaic technology is the science of producing electrical energy directly from light particles.
Of course, the original source of those light particles, or photons, is the sun.
It's Not All Photovoltaic Energy
Some might argue that all energy is ultimately solar energy – even coal and oil. This idea has some truth to it, but when it comes to photovoltaics, there is a neat difference.
You may recall that plants feed themselves through a process known as photosynthesis, in which they use photons (light particles) directly in order to synthesize (change, create) energy from simple sugars. This energy creates the plant tissues that you and I (or a cow or chicken) eats; when we ingest these foods, enzymes in our digestive tract convert the plant's energy into a form that allows our bodies to function.
By the way, this solar energy stored by plants is also released when wood or grasses or other plants are burned.
Coal and oil are made up of ancient plant matter – the remains of vegetation that was alive hundreds of millions of years ago. When these plants died, the stored solar energy was locked inside. Eventually, this dead, rotting vegetable matter was buried and subject to preternatural geologic pressures, and this eventually produced coal and oil, which releases this "ancient sunlight" when burned.
Not very efficient, is it?
Direct Sunlight
Photovoltaics use sunlight directly as it hits the surface of the earth. Using specially designed cells, photovoltaic technology converts photons into electrons – thus producing actual voltage, or electrical energy.
At the moment, this is not terribly cost-effective; another type of solar energy uses mirrors and magnifiers to concentrate sunlight, focusing it on water towers in order to produce steam. In essence, these kinds of solar generation plants are similar to coal, oil and gas-fired facilities. The only difference is that sunlight is used as a fuel instead of fossil fuels. Eventually however, as photovoltaic technology matures over the next decade, it will become a significant part of the world's clean energy grid.
What Is "Photovoltaic Solar Energy?"