TRANSPARENT EVACUATED INSULATION

Transparent evacuated insulation utilises the same operating principles as the Dewar flask-gas conduction and convection are essentially eliminated by the evacuated space, and radiative heat transport is small because of internal low emittance coatings. These insulating structures consist of two flat sheets of glass with a hermetic edge seal. An array of support pillars is necessary to maintain the separation of the glass sheets under the influence of atmospheric pressure. The extensive literature on transparent evacuated insulation is reviewed. The design of these devices involves trade-offs between the heat flow through the pillars, and the mechanical stresses. A design methodology for determining the dimensions of the pillar array is developed. An analytic method is described for calculating the stresses and bending produced by a temperature difference across the structure. The results are in reasonable agreement with experimental measurements. The stresses within the structure are shown to be less than conventionally accepted levels over a wide range of operating conditions. Many samples of transparent evacuated insulation have been built and tested in which the heat transport through the evacuated space is due entirely to radiation, to the limit of resolution of the measuring device (0.2 W m-2 K-1). No increase in heat transport has been observed over a period of 18 months. Much higher accuracy measurements have commenced. It appears likely that transparent evacuated insulation will achieve mid-plane insulating values of 0.6 W m-2 K-1, and possibly somewhat lower.