Solar water heating (SWH) is the conversion of sunlight into heat for water heating using a solar thermal collector. Evacuated Tube Collector (ETC) system is one of the products used in Solar Water Heating technology and widely used all over the world for its efficiency. ETC tubes are a way to reduce the heat loss. Typically, the water piping in an ETC is therefore surrounded by two concentric tubes of glass separated by a vacuum that admits heat from the sun (to heat the pipe) but that limits heat loss. The inner tube is coated with a thermal absorber. ETCs can gather energy from the sun all day long at low angles due to their tubular shape. The tubes are made of a material which is strong, resistant to high temperatures, and has a high transmittance for solar irradiation.
Flat Plate Collector (FPC) system is also one of the products used in Solar Water Heating technology and is the oldest one. Flat plate collectors are an extension of the idea to place a collector in an 'oven'-like box with glass directly facing the Sun. Most flat plate collectors have two horizontal pipes at the top and bottom, called headers, and many smaller vertical pipes connecting them, called risers. The risers are welded (or similarly connected) to thin absorber fins. Heat-transfer fluid (water or water/antifreeze mix) is pumped from the hot water storage tank into the collectors' bottom header and it travels up the risers, collecting heat from the absorber fins, and then exits the collector out of the top header.
Flat plate collectors are generally more efficient than ETC in full sunshine conditions, however, the energy output of flat plate collectors reduces more than ETCs in cloudy or extremely cold conditions.