Glass has been widely used to glaze solar collectors because it can transmit as much as 90% of the incoming shortwave solar irradiation while transmitting virtually none of the longwave radiation emitted outward by the absorber plate (see Example 2.10). Window glass usually has high iron content and is not suitable for use in solar collectors. Glass with low iron content has a relatively high transmittance for solar radiation (approximately 0.85-0.90 at normal incidence), but its transmittance is essentially zero for the longwave thermal radiation (5.0-50 |im) emitted by sun-heated surfaces.
Plastic films and sheets also possess high shortwave transmittance, but because most usable varieties also have transmission bands in the middle of the thermal radiation spectrum, they may have longwave transmittances as high as 0.40. Additionally, plastics are generally limited in the temperatures they can sustain without deteriorating or undergoing dimensional changes. Only a few types of plastics can withstand the sun's ultraviolet radiation for long periods. However, they are not broken by hail or stones, and in the form of thin films, they are completely flexible and have low mass.
The commercially available grades of window and greenhouse glass have normal incidence transmittances of about 0.87 and 0.85, respectively (ASHRAE, 2007). For direct radiation, the transmittance varies considerably with the angle of incidence.
Antireflective coatings and surface texture can improve transmission significantly. The effect of dirt and dust on collector glazing may be quite small, and the cleansing effect of an occasional rainfall is usually adequate to maintain the transmittance within 2-4% of its maximum value. Dust is collected mostly during summertime, when rainfall is less frequent, but due to the high magnitude of solar irradiation during this period, the dust protects the collector from overheating.
The glazing should admit as much solar irradiation as possible and reduce the upward loss of heat as much as possible. Although glass is virtually opaque to the longwave radiation emitted by the collector plates, absorption of that radiation causes an increase in the glass temperature and a loss of heat to the surrounding atmosphere through radiation and convection. These effects are analyzed in Section 3.3.
Various prototypes of transparently insulated flat-plate collectors and compound parabolic collectors (see Section 3.1.2) were built and tested in the 1990s (Spate et al., 1999). Low-cost, high-temperature resistant transparent insulating (TI) materials have been developed so that the commercialization of these collectors becomes feasible. A prototype flat-plate collector covered by transparent insulation was developed recently (Benz et al., 1998) and tested, and the efficiency of the collector was proven comparable with that of evacuated tube collectors (see Section 3.1.3). However, no commercial collectors of this type are available on the market yet.
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