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.