Influence of Spacer Systems on Heat Transfer in Evacuated Glazing

Abstract

One attractive possibility to essentially improve the insulation properties of glazing is to evacuate the space between the glass panes. This eliminates heat transport due to convection between the glass panes and suppresses the thermal conductivity of the remaining low pressure filling gas atmosphere. The glass panes can be prevented from collapsing by using a matrix of spacers. These spacers, however, increase heat transfer between the glass panes. To quantify this effect, heat transfer through samples of evacuated glazing was experimentally determined. The samples were prepared with different kinds of spacer materials and spacer distances. The measurements were performed with a guarded hot-plate apparatus under steady-state conditions and at room temperature. The measuring chamber of the guarded hot plate was evacuated to < 10⁻² Pa. An external pressure load of 0.1 MPa was applied on the samples to ensure realistic system conditions. Radiative heat transfer was significantly reduced by preparing the samples with a low-ε coating on one of the glass panes. In a first step, measurements without any spacers allowed quantification of the amount of radiative heat transfer. With these data, the measurements with spacers could be corrected to separate the effect of the spacers on thermal heat transfer. The influence of the thermal conductivity of the spacer material, as well as the distance between the spacers and the spacer geometry, was experimentally investigated and showed good agreement with simulation results. For mechanically stable matrices with cylindrical spacers, experimental thermal conductance values ≤0.44W·m⁻² ·K⁻¹ were found. This shows that U _g -values of about 0.5W · m⁻² · K⁻¹ are achievable in evacuated glazing, if highly efficient low-emissivity coatings are used.