Martin Meier, Werner Korner, Andreas Beck, Jochen Fricke

Bavarian Center for Applied EnergyResearch (ZAE Bayern) Thermal Insulation and Heat Transfer Division Wurzburg, Germany

Modern solar glazing units normally make use of low-e coatings, i. e. coatings with emissivities less than 0.05 in the IR spectral range. These coatings drastically reduce radiative heat transfer between the inner and the outer pane. Visible transmittance is hardly affected by these coatings. Unfortunately, due to their IR low-e characteristics these low-e coatings decrease the solar transmittance of the glazing, which can be a hindrance in solar applications. This paper demonstrates how the solar transmittance of solar glazing can be considerably improved by using micro-structured low-e-coatings, while the IR-emissivity of the system is not significantly increased.

SolarTransmittance of Low Emissivity Coatings

Low-e coatings mainly consist of one or two thin metal layers (usually silver) that determine the low-e behavior, and several additional layers for anti-oxidant, anti-reflex and adhesion-improving purposes.

As the main application of these coatings is in architectural glass, the coatings are designed in such a way that visible transmittance is hardly affected. Unfortunately, due to the decrease in transmission above 700 nm, these low-e coatings reduce the solar transmittance ofthe glazing, which can be a hindrance in solar applications.

For solar architecture, transparent insulations (Tl) and solar collectors, the solar and not the visible transmittance is the important factor. An ideal “solar low-e coating” for these applications would be extremely wavelength selective: a coating that reflects in the spectral range of 300 К heat radiation but is perfectly transparent in the range of solar radiation (see Figure 1).

Figure 1:

Reflectance of an ideal solar low-e coating. The coating is transparent in the spectral range ofsolar radiation (below a wavelength of2.5 pm), but reflects in the spectral range of heat radiation at a temperature of 300 K (above a wavelength of2.5 pm).

Considerations show that the solar transmittance can be increased by about 18 to 20 percent using this ideal solar low-e coating compared to a conventional coating. This value is calculated from the difference in transmittance of an uncoated pane of float glass and of a low-e-coated pane, each averaged over the solar spectrum [1] (see Figure 2).