Solar thermal collectors and applications

Glazing materials

2.1.1.1. 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. 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 mm) 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. Plastics are also 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 green-house glass have normal incidence transmittances of about 0.87 and 0.85, respectively. For direct radiation, the transmittance varies considerably with the angle of incidence [21].

Antireflective coatings and surface texture can also 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.

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 collector plates, absorption of that radiation causes an increase in the glass temperature and a loss of heat to the surrounding atmosphere by radiation and convection. These are analysed in more details in Section 3.

Various prototypes of transparently insulated FPC and CPC have been built and tested in the last decade [22,23]. Low cost and high temperature resistant transparent insulating (TI) materials have been developed so that the commercialisation of these collectors becomes feasible. A prototype FPC covered by TI was developed by Benz et al. [24]. It was experimentally proved that the efficiency of the collector was comparable with that of ETC. However, no commercial collectors of this type are available in the market.