Editing Solar thermal collector (section)

===Evacuated tube collectors===
[[Image:Vakuumroehrenkollektor 01.jpg|thumb|right|Evacuated tube collector]]
[[File:Vacuum collector double tube.png|alt=|thumb|Direct flow evacuated tube]]
[[File:Vacuum collector single tube.png|thumb|Heat pipe evacuated tube]]
[[Image:Solar vacuum tube collectors Thessaloniki.jpg|thumb|An array of evacuated tube collectors on a roof]]
Evacuated tube collectors are the most common solar thermal technology in the world.<ref name=":0" /> They make use of a [[glass tube]] to surround the absorber with [[Vacuum|high vacuum]] and effectively resist [[atmospheric pressure]]. The vacuum that surrounds the absorber greatly reduces [[convection]] and [[conduction (heat)|conduction]] heat loss, therefore achieving greater [[energy conversion efficiency]]. The absorber can be either metallic as in the case of flat plate collectors or being a second concentric glass tube ("Sydney Tube"). Heat transfer fluid can flow in and out of each tube or being in contact with a [[heat pipe]] reaching inside the tube. For the latter, heat pipes transfer heat to the fluid in a heat exchanger called a "manifold" placed transversely with respect to the tubes.{{citation needed|date=January 2020}} The manifold is wrapped in insulation ([[glass wool]]) and covered by a protective [[metal]] or [[plastic]] case also used for fixing to supports.

Glass-metal evacuated tubes are made with flat or curved metal absorber sheets same as those of flat plates. These sheets are joined to [[Pipe (fluid conveyance)|pipes]] or heat pipes to make "fins" and placed inside a single [[borosilicate glass]] tube. An anti-reflective coating can be deposited on the inner and outer surfaces of such tubes to improve transparency. Both selective and anti-reflective coating (inner tube surface) will not degrade until the vacuum is lost.<ref>{{cite web|url=http://ucsolar.org/files/public/documents/Poster-1.pdf|title=Solar Evacuated Tube Collectors|access-date=2013-10-06}}</ref> A high vacuum-tight [[Glass-to-metal seal|glass-metal seal]] is however required at one or both sides of each evacuated tube. This seal is cycled between ambient and fluid temperature each day of collector operation and might lead to failures in time.

Glass-glass evacuated tubes are made with two borosilicate glass tubes fused together at one or both ends (similar a [[vacuum bottle]] or dewar flask). The absorber fin is placed inside the inner tube at atmospheric pressure. Glass-glass tubes have a very reliable seal, but the two layers of glass reduce the amount of sunlight that reaches the absorber. The selective coating can be deposited on the inner borosilicate tube (high vacuum side) to avoid this, but heat has then to flow through the poorly conducting glass thickness of the inner tube in this case. Moreover, [[moisture]] may enter the non-evacuated area inside the inner tube and cause absorber [[corrosion]] in particular when made from dissimilar materials ([[galvanic corrosion]]).

A [[Getter|Barium flash getter]] pump is commonly evaporated inside the high vacuum gap in between tubes to keep the internal pressure stable through time.

The high temperatures that can occur inside evacuated tubes may require special design to prevent [[thermal shock]] and [[Optical overheating protection|overheating]]. Some evacuated tube collectors work as a thermal one-way valve due to their heat pipes. This gives them an inherent maximum [[operating temperature]] that acts as a safety feature.<ref>{{Cite patent|title=Heat pipe for a solar collector|gdate=2008-04-07|url=https://patents.google.com/patent/US8863740B2/en}}</ref> Evacuated tubes collectors can also be provided with low concentrating reflectors at the back of the tubes realising a CPC collector.<ref>{{Cite journal|last1=Kim|first1=Yong|last2=Han|first2=GuiYoung|last3=Seo|first3=Taebeom|date=April 2008|title=An evaluation on thermal performance of CPC solar collector|journal=International Communications in Heat and Mass Transfer|volume=35|issue=4|pages=446–457|doi=10.1016/j.icheatmasstransfer.2007.09.007|bibcode=2008ICHMT..35..446K }}</ref>