Editing Solar thermal collector (section)

===Evacuated flat plate collectors===
Evacuated flat plate solar collectors provide all the advantages of both flat plate and evacuated tube collectors combined. They surround a large area metal sheet absorber with high vacuum inside a flat envelope made of glass and metal. They offer the highest energy conversion efficiency of any non-concentrating solar thermal collector,<ref>{{Cite journal|title=Performance and operational effectiveness of evacuated flat plate solar collectors compared with conventional thermal, PVT and PV panels|pages=588–601|journal=Applied Energy|volume=216|doi=10.1016/j.apenergy.2018.01.001|date=2018-04-15|last1=Moss|first1=R.W.|last2=Henshall|first2=P.|last3=Arya|first3=F.|last4=Shire|first4=G.S.F.|last5=Hyde|first5=T.|last6=Eames|first6=P.C.|doi-access=free|bibcode=2018ApEn..216..588M }}</ref> but require sophisticated technology for manufacturing. They should not be confused with flat plate collectors featuring low vacuum inside. The first collector making use of high vacuum insulation was developed at [[CERN]],<ref>{{Cite journal|last=Benvenuti|first=C.|date=May 2013|title=The SRB solar thermal panel|journal=Europhysics News|volume=44|issue=3|pages=16–18|doi=10.1051/epn/2013301|issn=0531-7479|bibcode=2013ENews..44c..16B|doi-access=free}}</ref> while TVP SOLAR SA of Switzerland was the first company to commercialise Solar Keymark certified collectors in 2012.<ref>{{Cite web|url=https://www.dincertco.tuv.com/registrations/60081291|title=DIN CERTCO - Register-Nr. 011-7S1890 F|website=www.dincertco.tuv.com|access-date=2019-04-28}}</ref>

Evacuated flat plate solar collectors require both a glass-metal seal to join the glass plate to the rest of the metal envelope and an internal structure to support such plate against atmospheric pressure. The absorber has to be segmented or provided with suitable holes to accommodate such structure. Joining of all parts has to be high vacuum-tight and only materials with low [[Vapor pressure|vapour pressure]] can be used to prevent [[outgassing]]. Glass-metal seal technology can be based either on metallized glass<ref>{{Cite patent|title=Evacuable flat panel solar collector|gdate=2004-01-22|url=https://patents.google.com/patent/WO2005075900A1/en}}</ref> or vitrified metal<ref>{{Cite patent|title=Vacuum solar thermal panel with a vacuum-tight glass-metal sealing|gdate=2009-07-08|url=https://patents.google.com/patent/WO2010003653A2/en}}</ref> and defines the type of collector. Different from evacuated tube collectors, they make use of [[non-evaporable getter]] (NEG) pumps to keep the internal [[pressure]] stable through time. This getter pump technology has the advantage of providing some regeneration in-situ by exposure to sunlight. Evacuated flat plate solar collectors have been studied for solar air condition and compared to compact solar concentrators.<ref>{{Cite journal|last1=Buonomano|first1=Annamaria|last2=Calise|first2=Francesco|last3=d’Accadia|first3=Massimo Dentice|last4=Ferruzzi|first4=Gabriele|last5=Frascogna|first5=Sabrina|last6=Palombo|first6=Adolfo|last7=Russo|first7=Roberto|last8=Scarpellino|first8=Marco|date=February 2016|title=Experimental analysis and dynamic simulation of a novel high-temperature solar cooling system|journal=Energy Conversion and Management|volume=109|pages=19–39|doi=10.1016/j.enconman.2015.11.047|bibcode=2016ECM...109...19B }}</ref>

====Polymer flat plate collectors====
These collectors are an alternative to metal collectors. These may be wholly [[polymer]], or they may include metal plates in front of freeze-tolerant water channels made of [[silicone rubber]]. Polymers are flexible and therefore freeze-tolerant and can employ plain water instead of antifreeze, so that they may be plumbed directly into existing water tanks instead of needing heat exchangers that lower efficiency. By dispensing with a heat exchanger, temperatures need not be quite so high for the circulation system to be switched on, so such direct circulation panels, whether polymer or otherwise, can be more efficient, particularly at low [[solar irradiance]] levels. Some early selectively coated polymer collectors suffered from overheating when insulated, as stagnation temperatures can exceed the polymer's melting point.<ref>{{cite book |doi=10.1115/ISEC2005-76005 |id={{INIST|17036823}} |chapter=Polymeric Absorbers for Flat Plate Collectors: Can Venting Provide Adequate Overheat Protection? |title=Solar Energy |year=2005 |last1=Kearney |first1=Meghan |last2=Davidson |first2=Jane H.|author2-link=Jane H. Davidson |last3=Mantell |first3=Susan|author3-link= Susan Mantell |pages=253–257 |isbn=978-0-7918-4737-4 }}</ref><ref>{{cite book |doi=10.1007/978-3-540-75997-3_118 |chapter=Solar Thermal Collectors in Polymeric Materials: A Novel Approach Towards Higher Operating Temperatures |title=Proceedings of ISES World Congress 2007 (Vol. I – Vol. V) |year=2008 |last1=Mendes |first1=João Farinha |last2=Horta |first2=Pedro |last3=Carvalho |first3=Maria João |last4=Silva |first4=Paulo |pages=640–643 |isbn=978-3-540-75996-6 }}</ref> For example, the melting point of [[polypropylene]] is {{convert|160|C}}, while the stagnation temperature of insulated thermal collectors can exceed {{convert|180|C}} if control strategies are not used. For this reason, polypropylene is not often used in glazed selectively coated solar collectors. Increasingly, polymers such as high temperate silicones (which melt at over {{convert|250|C}}) are being used. Some non polypropylene polymer based glazed solar collectors are matte black coated rather than selectively coated to reduce the stagnation temperature to {{convert|150|C}} or less.

In areas where freezing is a possibility, freeze-tolerance (the capability to freeze repeatedly without cracking) can be achieved by the use of flexible polymers. Silicone rubber pipes have been used for this purpose in UK since 1999. Conventional metal collectors are vulnerable to damage from freezing, so if they are water filled they must be carefully plumbed so they completely drain using gravity before freezing is expected so that they do not crack. Many metal collectors are installed as part of a sealed heat exchanger system. Rather than having potable water flow directly through the collectors, a mixture of water and antifreeze such as propylene glycol is used. A heat exchange fluid protects against freeze damage down to a locally determined risk temperature that depends on the proportion of propylene glycol in the mixture. The use of glycol lowers the water's heat carrying capacity marginally, while the addition of an extra heat exchanger may lower system performance at low light levels.{{citation needed|date=January 2021}}

A pool or unglazed collector is a simple form of flat-plate collector without a transparent cover. Typically, polypropylene or [[EPDM rubber]] or silicone rubber is used as an absorber. Used for pool heating, it can work quite well when the desired output temperature is near the ambient temperature (that is, when it is warm outside). As the ambient temperature gets cooler, these collectors become less effective.{{citation needed|date=January 2021}}

====Bowl collectors====
A ''solar bowl'' is a type of solar thermal collector that operates similarly to a [[#Parabolic dish|parabolic dish]], but instead of using a tracking parabolic mirror with a fixed receiver, it has a fixed spherical mirror with a tracking receiver. This reduces efficiency but makes it cheaper to build and operate. Designers call it a ''fixed mirror distributed focus solar power system''. The main reason for its development was to eliminate the cost of moving a large mirror to track the sun as with parabolic dish systems.<ref name="solarbowl">{{cite book |url=https://books.google.com/books?id=giwEAAAAMBAJ&q=crosbyton%20solar%20bowl&pg=PA199 |title=Duel for the Sun |last=Calhoun |first=Fryor |series=[[Texas Monthly]] |date=November 1983 }}</ref>

A fixed parabolic mirror creates a variously shaped image of the sun as it moves across the sky. Only when the mirror is pointed directly at the sun does the light focus on one point. That is why parabolic dish systems track the sun. A fixed [[Curved mirror|spherical mirror]] focuses the light in the same place independent of the sun's position. The light, however, is not directed to one point but is distributed on a line from the surface of the mirror to one half radius (along a line that runs through the sphere center and the sun).{{citation needed|date=January 2021}}

[[File:Sphericalmirrorimage.jpg|thumb|Typical energy density along the 1/2 radius length focal line of a spherical reflector]]

As the sun moves across the sky, the aperture of any fixed collector changes. This causes changes in the amount of captured sunlight, producing what is called the ''sinus effect'' of power output. Proponents of the solar bowl design claim the reduction in overall power output compared with tracking parabolic mirrors is offset by lower system costs.<ref name="solarbowl" />

The sunlight concentrated at the focal line of a spherical reflector is collected using a tracking receiver. This receiver is pivoted around the focal line and is usually counterbalanced. The receiver may consist of pipes carrying fluid for thermal transfer or [[Solar cell|photovoltaic cells]] for direct conversion of light to electricity.

The solar bowl design resulted from a project of the Electrical Engineering Department of the Texas Technical University, headed by Edwin O'Hair, to develop a 5 MWe power plant. A solar bowl was built for the town of [[Crosbyton, Texas]] as a pilot facility.<ref name="solarbowl" /> The bowl had a diameter of {{convert|65|ft|m|abbr=on}}, tilted at a 15° angle to optimize the cost/yield relation (33° would have maximized yield). The rim of the hemisphere was "trimmed" to 60°, creating a maximum aperture of {{convert|3318|sqft|m2}}. This pilot bowl produced electricity at a rate of 10&nbsp;kW peak.{{Citation needed|date=August 2011}}

A {{convert|15|m|adj=on}} diameter Auroville solar bowl was developed from an earlier test of a {{convert|3.5|m|adj=on}} bowl in 1979–1982 by the [[Tata Energy Research Institute]]. That test showed the use of the solar bowl in the production of steam for cooking. The full-scale project to build a solar bowl and kitchen ran from 1996 and was fully operational by 2001.{{Citation needed|date=August 2011}}

In locations with average available solar energy, flat plate collectors are sized approximately 1.2 to 2.4 square decimeter per liter of one day's hot water use.