A long standing argument exists between protagonists of these two technologies. Some of this can be related to the physical structure of evacuated tube collectors which have a discontinuous absorbance area. An array of evacuated tubes on a roof has 1) open space between collector tubes and 2) (vacuum-filled) space occupied between the two concentric glass tubes of each collector tube. Consequently, a square meter of roof area covered with evacuated tubes (collector gross area) is larger than the area comprising the actual absorbers (absorber plate area). If evacuated tubes are compared with flat-plate collectors on the basis of area of roof occupied, a different conclusion might be reached than if the areas of absorber were compared. In addition, the way that the ISO 9806 standard[7] specifies the way in which the efficiency of solar thermal collectors should be measured is ambiguous, since these could be measured either in terms of gross area or in terms of absorber area. Unfortunately, power output is not given for thermal collectors as it is for PV panels. This makes it difficult for purchasers and engineers to make informed decisions.
[dubious – discuss] [dubious – discuss]
A comparison of the energy output (kW.h/day) of a flat plate collector (blue lines; Thermodynamics S42-P[dubious – discuss]; absorber 2.8 m2) and an evacuated tube collector (green lines; SunMaxx 20EVT[dubious – discuss]; absorber 3.1 m2. Data obtained from SRCC certification documents on the Internet.[dubious – discuss] 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 – discuss] 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 – discuss]. A field trial[who?] 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[who?].
Flat-plate collectors usually lose more heat to the environment than evacuated tubes and this loss increases with temperature difference. So they are usually inappropriate choice of solar collector for high temperature commercial applications such as process steam production. Evacuated tube collectors have a lower absorber plate area to gross area ratio (typically 60-80% of gross area) compared to flat plates. (In early designs the absorber area only occupied about 50% of the collector panel. However this has changed as the technology has advanced to maximize the absorption area.) Based on absorber plate area, most evacuated tube systems are more efficient per square meter than equivalent flat plate systems. This makes them suitable where roof space is limiting, for example where the number of occupants of a building is higher than the number of square metres of suitable and available roof space. In general, per installed square metre, evacuated tubes deliver marginally more energy when the ambient temperature is low (e.g. during winter) or when the sky is overcast for long periods. However even in areas without much sunshine and solar heat, some low cost flat plate collectors can be more cost efficient than evacuated tube collectors. Although several European companies manufacture evacuated tube collectors, the evacuated tube market is dominated by manufacturers in the East. Several Chinese companies have long favorable track records of 15–30 years. There is no unambiguous evidence that the two collector technologies (flat-plate and evacuated tube) differ in long term reliability. However, the evacuated tube technology is younger and (especially for newer variants with sealed heat pipes) still need to prove equivalent lifetimes of equipment when compared to flat plates. The modularity of evacuated tubes can be advantageous in terms of extendability and maintenance, for example if the vacuum in one particular tube diminishes.

Chart showing flat-plate collectors outperforming evacuated tubes up until 120°F above ambient and, shaded in gray, the normal operating range for solar domestic hot water systems.[8]
For a given absorber area, evacuated tubes can therefore maintain their efficiency over a wide range of ambient temperatures and heating requirements. In most climates, flat-plate collectors will generally be a more cost-effective solution than evacuated tubes. When employed in arrays, when considered instead on a per square metre basis, the efficient but costly evacuated tube collectors can have a net benefit in winter and also give real advantage in the summer months. They are well suited to cold ambient temperatures and work well in situations of consistently low sunshine, providing heat more consistently than flat plate collectors per square metre. On the other hand, heating of water by a medium to low amount (i.e. Tm-Ta) is much more efficiently performed by flat plate collectors. Domestic hot water frequently falls into this medium category. Glazed or unglazed flat collectors are the preferred devices for heating swimming pool water.[9] Unglazed collectors may be suitable in tropical or subtropical environments if domestic hot water needs to be heated by less than 20°C. A contour map can show which type is more effective (both thermal efficiency and energy/cost) for any geographic region.
Besides efficiency, there are other differences. EHPT's work as a thermal one-way valve due to their heat pipes. This also gives them an inherent maximum operating temperature which may be considered a safety feature. They have less aerodynamic drag, which may allow them to be laid onto the roof without being tied down. They can collect thermal radiation from the bottom in addition to the top. Tubes can be replaced individually without shutting down the entire system. There is no condensation or corrosion within the tubes. One hurdle to wider adoption of evacuated tube collectors in some markets is their inability to pass internal thermal shock tests where ISO 9806-2 section 9 class b is a requirement for durability certification.[10] This means that if unprotected evacuated tube collectors are exposed to full sun for too long prior to being filled with cold water the tubes may shatter due to the rapid temperature shift. There is also the question of vacuum leakage over their lifetime. Flat panels have been around much longer, and are less expensive. They may be easier to clean. Other properties, such as appearance and ease of installation are more subjective.
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