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Solar thermal collector
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===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 β Solar thermal collectors β 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 60β80% 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 15β30 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 ZΓΆrner|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.
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