A flat plate solar collector is an insulated box covered by glass or plastic with a metal absorber plate on the bottom. The weatherproofed collectors are usually glazed with a coating to better absorb and retain heat. Heat transfer fluid flows through metal tubes lying below the absorber plate. The fluid then flows through a heat exchanger before entering the storage tank. Unglazed flat-plate collectors (without insulation or absorber coatings) do not operate in cool or windy climates but are excellent for heating water in a pool.
Photo Credit: Physics World
Flat plate solar collectors suffer more heat loss than other types of solar collectors, and are thus not the most efficient type. However, flat plate collector are suited to a wider range of climates. Because these collectors can use a heat transfer fluid such as antifreeze, rather than directly heating the domestic water supply, this type of solar thermal panel can be used in colder climates.
Cost and Durability
While they are not the most efficient solar collectors, flat plates last for over 25 years and are one of the most cost-effective options for a residential solar water heating system. Typically, one flat plate collector can supply adequate energy to heat about 40 gallons of water for domestic hot water use. According to EnergyStar.gov, a family of four can obtain about half its hot water from two collectors.