Current status of the science and technology of vacuum glazing

Abstract

This paper reviews the current state-of-the-art of the science and technology of vacuum glazing. The construction of vacuum glazing, and its method of manufacture in the laboratory, is described. Experimental data are presented on the magnitude of heat flows through vacuum glazing. Gaseous heat transfer is negligible, and the internal vacuum is shown to be stable over many years, in well-manufactured glazing. Values of air-to-air, centre-of-glazing thermal conductance have been achieved ranging from 3 W m −2 K −1 (for vacuum glazing with no internal low emittance coating) to 0.8 W m −2 K −1 (for samples with two internal low emittance coatings). The overall heat transport rate through 1 m×1 m samples of vacuum glazing has been measured in accurately calibrated guarded hot box instruments. The results obtained agree to within experimental error (±6%) with those estimated on the basis of local measurements of heat transfer due to radiation, pillar conduction and lateral heat flow through the edge seal. Sources of mechanical tensile stress in vacuum glazing are identified. Stresses due to atmospheric pressure occur in the vicinity of the pillars, and (in poorly designed glazing) near the edge seal. Stresses due to temperature differences are influenced by many factors including the external heat transfer coefficients, level of insulation of the glazing, edge insulation, and edge constraints. Methods of estimating these stresses are discussed. It is shown that vacuum glazing can be designed with adequately low stresses, and low thermal conductance.