Editing Solar thermal collector (section)

===Flat plate collectors===
[[File: Flat-plate solar thermal collector, viewed from roof-level.png|alt=|thumb|Two flat plate solar collectors side-by-side]]
Flat-plate collectors are the most common solar thermal technology in [[Europe]].<ref name=":0">{{Cite web|url=http://www.iea-shc.org/solar-heat-worldwide|title=IEA SHC {{!}}{{!}} IEA SHC {{!}}{{!}} Solar Heat Worldwide Markets and Contribution to the Energy Supply|website=www.iea-shc.org|access-date=2019-04-28}}</ref> They consist of an (1) enclosure containing (2) a dark-colored absorber plate with fluid circulation passageways, and (3) a transparent cover to allow transmission of solar energy into the enclosure. The sides and back of the enclosure are typically insulated to reduce heat loss to the ambient. A heat transfer fluid is circulated through the absorber's fluid passageways to remove heat from the solar collector. The circulation fluid in tropical and sub-tropical climates is typically water. In climates where freezing is likely, a heat transfer fluid similar to an automotive [[antifreeze]] solution may be used instead of water, or in a mixture with water. If a heat transfer fluid is used, a [[heat exchanger]] is typically employed to transfer heat from the solar collector fluid to a hot water storage tank. The most common absorber design consists of copper tubing joined to a high conductivity metal sheet (copper or aluminum). A dark coating is applied to the sun-facing side of the absorber assembly to increase its absorption of solar energy. A common absorber coating is black enamel paint.

In higher performance solar collector designs, the transparent cover is tempered [[Soda–lime glass|soda-lime glass]] having reduced [[iron oxide]] content same as for [[Solar panel|photovoltaic solar panels]]. The glass may also have a [[stippling]] pattern and one or two [[anti-reflective coating]]s to further enhance [[Transparency and translucency|transparency]]. The absorber coating is typically a selective coating, where selective stands for having the special optical property to combine high [[Absorption (electromagnetic radiation)|absorption]] in the [[Visible spectrum|visible]] part of the [[electromagnetic spectrum]] coupled to low [[Radiant exitance|emittance]] in the [[infrared]] one. This creates a [[selective surface]], which reduces [[black body]] [[energy]] emission from the absorber and improves performance. Piping can be [[laser]] or [[ultrasound]] welded to the absorber sheet to reduce damage to the selective coating, which is typically applied prior to joining to large coils in a [[Roll-to-roll processing|roll-to-roll process]].

Absorber [[piping]] configurations include:

*[[harp]]: traditional design with bottom pipe risers and top collection pipe, used in low pressure [[Thermosiphon|thermosyphon]] and pumped systems;
*[[Serpentine shape|serpentine]]: one continuous S-shaped pipe that maximises [[temperature]] but not total energy yield in variable flow systems, used in compact solar domestic hot water only systems (no space heating role);
* flooded: consisting of two sheets of metal [[Molding (process)|molded]] to produce a wide circulation zone that improves [[heat transfer]];
*[[boundary layer]]: consisting of several layers of transparent and opaque sheets that enable absorption in a boundary layer. Because the energy is absorbed in the boundary layer, heat conversion may be more efficient than for collectors where absorbed heat is conducted through a material before being accumulated in the circulating liquid.{{Citation needed|date=October 2011}}

A flat plate collector making use of a [[honeycomb structure]] to reduce heat loss also at the glass side too has also been made available commercially. Most flat plate collectors have a life expectancy of over 25 years.{{Citation needed|date=April 2019}}.