They also heat swimming pools. Panels on the roof of a building contain water pipes. When the sun hits the panels and the pipes, the sunlight warms them. A common collector is called a flat-plate collector, and is usually mounted on the roof. This collector is a rectangular box with a transparent cover that faces the sun.
Small tubes run through the box, carrying the water or other fluid such as antifreeze to be heated.
The tubes are mounted on a metal absorber plate, which is painted black to absorb the sun's heat. The back and sides of the box are insulated to hold in the heat. Heat builds up in the collector, and as the fluid passes through the tubes, it heats up. That warmed water can then be used in a swimming pool or your shower. The mirror focuses the sunlight to strike the pipe, and it gets so hot that it can boil water into steam.