Solar energy refers to the energy that is created and radiated by Sol—the star at the center of the solar system—or, more significantly, the amount of that energy incident on the planet Earth. Solar energy not only is important for human technological efforts but is also the foundation for virtually all biological life on earth. Solar energy also refers to the processes by which light or heat from the sun is collected and converted to some usable form. Aside from geothermal energy, virtually every energy source commonly used by humans is directly or indirectly derived from solar energy. Historically, humans have relied on solar power for a wide number of applications. There are three general categories of solar energy systems in common use today: solar electric, solar thermal, and passive solar. The source of solar energy is the sun itself or, more specifically, the thermonuclear reactions that take place at the sun's core. Solar energy is the by-product of the nuclear fusion of…