Heat transfer coefficients of translucent glazing units for daylighting purposes

The Bavarian Centre for Applied Energy Research has developed a double-pane glazing unit containing a light-scattering translucent fleece mounted at half distance between two glass panes. The fleece, which consists of synthetic fibres with diameters in the range 1-4 μm, suppresses convection between the panes and reduces the infrared radiative heat transfer. The resulting heat transition coefficient, or U value, is as low as for a triple-pane window; the weight, however, is close to that of a double-pane window. A model based on the radiative, conductive, and convective components of heat transfer has been developed to calculate the U value as a function of the properties of the glass panes, fleeces, gas fillings, and gap widths used. The model was validated by measurements of the U values for several assembled glazings. With two low-e coated window panels (emissivity e = 0.04), a krypton gas filling, and a width of the two gaps of 12 mm, a U value of 0.47 W m -2 K -1 is obtained. Transmission of direct sunlight can be avoided as a result of the scattering properties of the fleece. Optimised fleeces provide a directional-hemispherical transmittance for the glazing unit of about 50% in the visible and a solar heat gain coefficient or g value of 20%-35%. Because of the scattering of sunlight within the fleece, the illumination level in the room was quite uniform.