Editing Solar thermal collector (section)

==Heating water==
Flat-plate and evacuated-tube solar collectors are mainly used to collect heat for space heating, domestic hot water, or [[cooling]] with an [[absorption chiller]]. In contrast to solar hot water panels, they use a circulating fluid to displace heat to a separated reservoir. The first solar thermal collector designed for building roofs was patented by William H. Goettl and called the "[[Solar heat collector and radiator for building roof]]".<ref>{{Cite patent|title=Solar heat collector and radiator for building roof|gdate=1977-02-07|url=https://patents.google.com/patent/US4098260A/en}}</ref>

Evacuated flat-plate solar collectors are a more recent innovation and can be used for Solar Heat for Industrial Cooling (SHIC) and [[Solar air conditioning|Solar Air Conditioning]] (SAC), where temperature in excess of {{convert|100|C}} are required.<ref>{{Cite web|url=http://task49.iea-shc.org/|title=IEA SHC {{!}}{{!}} Task 49 {{!}}{{!}} IEA SHC {{!}}{{!}} Task 49|website=task49.iea-shc.org|access-date=2019-04-28}}</ref><ref>{{Cite web|url=http://task48.iea-shc.org/|title=IEA SHC {{!}}{{!}} Task 48 {{!}}{{!}} IEA SHC {{!}}{{!}} Task 48|website=task48.iea-shc.org|access-date=2019-04-28}}</ref> These non-concentrating collectors harvest both diffuse and direct light and can make use of [[steam]] instead of water as fluid.

===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}}.

===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>

===Comparisons of flat plate and evacuated tube collectors===
A longstanding argument exists between proponents of these two technologies. Some of this can be related to the structure of evacuated tube collectors which have a discontinuous absorbance area. An array of evacuated tubes collectors on a roof has space between the individual tubes and a vacuum gap between each tube and its absorber inside, covering only a fraction of the installation area on a roof. If evacuated tubes are compared with flat-plate collectors on the basis of the area of roof occupied (gross area), a different conclusion might be reached than if the absorber or aperture areas were compared. The recent revision of the ISO 9806 standard<ref>ISO 9806:2017. Solar energy – Solar thermal collectors – Test methods [[International Organization for Standardization]], Geneva, Switzerland</ref> states that the efficiency of solar thermal collectors should be measured in terms of gross area and this might favour flat plates in respect to evacuated tube collectors in direct comparisons.
[[File:MT-Power Masdar City.jpg|thumb|An array of evacuated flat plate collectors next to compact solar concentrators]]
[[File:SolarCollectorsCompare1.jpg|thumb|A comparison of the energy output (kW.h/day) of a flat plate collector (blue lines; Thermodynamics S42-P{{dubious|date=April 2011}}; absorber 2.8 m<sup>2</sup>) and an evacuated tube collector (green lines; SunMaxx 20EVT{{dubious|date=April 2011}}; absorber 3.1 m<sup>2</sup>. Data obtained from SRCC certification documents on the Internet.{{dubious|date=April 2011}} Tm-Ta = temperature difference between water in the collector and the ambient temperature. Q = insolation during the measurements. Firstly, as (Tm-Ta) increases the flat plate collector loses efficiency more rapidly than the evac tube collector. This means the flat plate collector is less efficient in producing water higher than 25 degrees C above ambient (i.e. to the right of the red marks on the graph).{{dubious|date=April 2011}} Secondly, even though the output of both collectors drop off strongly under cloudy conditions (low insolation), the evac tube collector yields significantly more energy under cloudiness than the flat plate collector. Although many factors obstruct the extrapolation from two collectors to two different technologies, above, the basic relationships between their efficiencies remain valid{{dubious|date=April 2011}}.]]
[[File:panelcomp2.jpg|thumb|A field trial<ref name="its-Honeyborne">{{Cite web|url=http://gogreenheatsolutions.co.za/sites/default/files/Difference%20between%20Flat%20plate%20&%20Evac%20Tube%20-%20Residential_0.pdf|title=Flat plate versus Evacuated tube solar collectors|last=Honeyborne|first=Riaan|date=14 April 2009|website=Go Green Heat Solutions, via Internet Archive|url-status=live|archive-url=https://web.archive.org/web/20171004085243/http://gogreenheatsolutions.co.za/sites/default/files/Difference%20between%20Flat%20plate%20%26%20Evac%20Tube%20-%20Residential_0.pdf|archive-date=4 October 2017|access-date=2017-10-04}}</ref> illustrating the differences discussed in the figure on the left. A flat plate collector and a similar-sized evacuated tube collector were installed adjacently on a roof, each with a pump, controller and storage tank. Several variables were logged during a day with intermittent rain and cloud. Green line = solar irradiation. The top maroon line indicates the temperature of the evac tube collector for which cycling of the pump is much slower and even stopping for some 30 minutes during the cool parts of the day (irradiation low), indicating a slow rate of heat collection. The temperature of the flat plate collector fell significantly during the day (bottom purple line) but started cycling again later in the day when irradiation increased. The temperature in the water storage tank of the evac tube system (dark blue graph) increased by 8 degrees C during the day while that of the flat plate system (light blue graph) only remained constant. Courtesy ITS-solar.<ref name="its-Honeyborne" />{{dubious|date=April 2011}}]]

Flat-plate collectors usually lose more heat to the environment than evacuated tubes because there is no insulation at the glass side. Evacuated tube collectors intrinsically have a lower absorber to gross area ratio (typically 60–80% less) than flat plates because tubes have to be spaced apart. Although several European companies manufacture evacuated tube collectors (mainly glass-metal type), the evacuated tube market is dominated by manufacturers in China, with some companies having track records of 15–30 years or more. There is no unambiguous evidence that the two designs differ in long-term reliability. However, evacuated tube technology (especially for newer variants with glass-metal seals and heat pipes) still needs to demonstrate competitive lifetimes. The modularity of evacuated tubes can be advantageous in terms of extensibility and maintenance, for example, if the vacuum in one heat pipe tube is lost it can be easily be replaced with minimal effort.

[[File:Comparison 1000.png|thumb|right|Chart showing flat-plate collectors outperforming evacuated tubes up until {{convert|120|F-change|C-change|disp=flip}} above ambient and, shaded in gray, the normal operating range for solar domestic hot water systems.<ref>{{cite book | title=Solar Hot Water Systems: Lessons Learned, 1977 to Today | author=Tom Lane | page=5 }}</ref>]]
In most climates, flat plate collectors will generally be more cost-effective than evacuated tubes.<ref>{{cite conference|last=Trinkl|first=Christoph|author2=Wilfried Zörner|author3=Claus Alt|author4=Christian Stadler|date=2005-06-21|title=Performance of Vacuum Tube and Flat Plate Collectors Concerning Domestic Hot Water Preparation and Room Heating|url=http://www.thermo-dynamics.com/pdfiles/technical/Solar_Performane_VTvsLFP.pdf|publisher=CENTRE OF EXCELLENCE FOR SOLAR ENGINEERING at Ingolstadt University of Applied Sciences|access-date=2010-08-25|book-title=2nd European Solar Thermal Energy Conference 2005 (estec2005)}}
</ref> However, evacuated tube collectors are well-suited to cold ambient temperatures and work well in situations of low solar irradiance, providing heat more consistently throughout the year. Unglazed flat plate collectors are the preferred devices for heating swimming pool water. Unglazed collectors may be suitable in tropical or subtropical environments if domestic hot water needs to be heated by less than {{convert|20|C-change}} over ambient temperature. Evacuated tube collectors have less aerodynamic drag, which may allow for a simpler installation on roofs in windy locations. The gaps between the tubes may allow for snow to fall through the collector, minimizing the loss of production in some snowy conditions, though the lack of radiated heat from the tubes can also prevent effective shedding of accumulated snow. Flat plate collectors might be easier to clean. Other properties, such as appearance and ease of installation are more subjective and difficult to compare.

===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.

=== Applications ===
The main use of this technology is in residential buildings where the demand for hot water has a large impact on energy bills. This generally means a situation with a large family or a situation in which the hot water demand is excessive due to frequent laundry washing. Commercial applications include laundromats, car washes, military laundry facilities and eating establishments. The technology can also be used for space heating if the building is located off-grid or if utility power is subject to frequent outages. [[Solar water heating]] systems are most likely to be cost effective for facilities with water heating systems that are expensive to operate, or with operations such as laundries or kitchens that require large quantities of hot water. Unglazed liquid collectors are commonly used to heat water for swimming pools but can also be applied to large-scale water pre-heating. When loads are large relative to the available collector area, the bulk of the water heating can be done at low temperature, lower than swimming pool temperatures where unglazed collectors are well established in the marketplace as the right choice. Because these collectors need not withstand high temperatures, they can use less expensive materials such as plastic or rubber. Many unglazed collectors are made of polypropylene and must be drained fully to avoid freeze damage when air temperatures drop below {{convert|44|F}} on clear nights.<ref>Tom Lane, Solar Hot Water Systems, Lessons Learned 1977 to Today p7</ref> A smaller but growing percentage of unglazed collectors are flexible meaning they can withstand water freezing solid inside their absorber. The freeze concern only needs to be the water-filled piping and collector manifolds in a hard freeze condition. Unglazed solar hot water systems should be installed to "drainback" to a storage tank whenever solar radiation is insufficient. There are no thermal shock concerns with unglazed systems. Commonly used in swimming pool heating since solar energy's early beginnings, unglazed solar collectors heat swimming pool water directly without the need for antifreeze or heat exchangers. Hot water solar systems require heat exchangers due to contamination possibilities and in the case of unglazed collectors, the pressure difference between the solar working fluid (water) and the load (pressurized cold city water). Large-scale unglazed solar hot water heaters, like the one at the Minoru Aquatic Center in Richmond, BC operate at lower temperatures than evacuated tube or boxed and glazed collector systems. Although they require larger, more expensive heat exchangers, all other components including vented storage tanks and uninsulated plastic PVC piping reduce the costs of this alternative dramatically compared to the higher temperature collector types. When heating hot water, we are actually heating cold to warm and warm to hot. We can heat cold to warm as efficiently with unglazed collectors, just as we can heat warm to hot with high-temperature collectors.{{citation needed|date=January 2021}}