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Публикувано на: 11.02.2010 06:20Comparisons of flat plate and evacuated tube collectors
Flat plate and evacuated tube collectors side-by-side. Courtesy: ITS-solarA long standing argument has existed between protagonists of these two types of solar thermal collectors. Some of this can be related to the physical structure of evacuated tube collectors. An evacuated tube collector does not have an absorbance area that is continuous. 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 square meterage of roof occupied, a different conclusion might be reached that if the actual areas of absorber were compared. In addition, the way that the ISO 9806 standard [50]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 plate area.
Flat-plate collectors suffer from the disadvantage of losing heat to the environment when the liquid in the collector reaches a temperature far above the ambient (the delta-t effect discussed above). Evacuated tube collectors suffer the disadvantage that their absorber plate area to gross area ratio is smaller (typically 60-80 per cent of gross area) than for flat plates. Based on absorber plate area, many evacuated tube systems are more efficient than equivalent flat plate systems. In general, evacuated tubes work well when the ambient temperature is low (e.g. during winter) or when the sky is overcast for long period (when delta-t is expected to be high). On the other hand, in areas with large degree of sunshine and solar heat, a plat plate collectors may be more efficient especially if its price compares favourably with that of an evacuated tube collector. Although several European companies manufacture evacuated tube collectors, the evacuated tube market is dominated by manufacturers in the East. Several Chinese companies have long favourable track records and have been manufacturing these devices for 15-30 years. There is no unambiguous evidence that the two collector technologies (flat-plate and evacuated tube) differs in longer 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 yields clear advantages in terms of extendability and maintenance.
Flat plate and evacuated tube collectors side-by-side. Courtesy: ITS-solarA long standing argument has existed between protagonists of these two types of solar thermal collectors. Some of this can be related to the physical structure of evacuated tube collectors. An evacuated tube collector does not have an absorbance area that is continuous. 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 square meterage of roof occupied, a different conclusion might be reached that if the actual areas of absorber were compared. In addition, the way that the ISO 9806 standard [50]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 plate area.
Flat-plate collectors suffer from the disadvantage of losing heat to the environment when the liquid in the collector reaches a temperature far above the ambient (the delta-t effect discussed above). Evacuated tube collectors suffer the disadvantage that their absorber plate area to gross area ratio is smaller (typically 60-80 per cent of gross area) than for flat plates. Based on absorber plate area, many evacuated tube systems are more efficient than equivalent flat plate systems. In general, evacuated tubes work well when the ambient temperature is low (e.g. during winter) or when the sky is overcast for long period (when delta-t is expected to be high). On the other hand, in areas with large degree of sunshine and solar heat, a plat plate collectors may be more efficient especially if its price compares favourably with that of an evacuated tube collector. Although several European companies manufacture evacuated tube collectors, the evacuated tube market is dominated by manufacturers in the East. Several Chinese companies have long favourable track records and have been manufacturing these devices for 15-30 years. There is no unambiguous evidence that the two collector technologies (flat-plate and evacuated tube) differs in longer 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 yields clear advantages in terms of extendability and maintenance.
