There are generally three main types of solar thermal collectors: flat-plate collectors, evacuated tube collectors, and concentrating collectors. The term "solar collector" commonly refers to a device for solar hot water heating, but may also refer to large power generating installations such as solar parabolic troughs and solar towers, or to non- water -heating devices such as. Sunlight can cause a molecule to change structure, and then release heat later. In tower (or central receiver) plants, mirrors, known as heliostats, track the sun on two axes, with. Solar energy collectors are crucial for converting solar radiation into usable forms like heat or electricity.