Effect of Cavity Vacuum Pressure Diminution on Thermal Performance of Triple Vacuum Glazing

Abstract

Featured Application: Triple Vacuum glazing is regarded as a smart window technology and a robust solution of retrofitting it to existing buildings for the reduction of demand side carbon emissions and improving the energy efficiency of buildings.Abstract: Long-term durability of the vacuum edge seal plays a significant part in retrofitting triple vacuum glazing (TVG) to existing buildings in achieving progress towards a zero-energy building (ZEB) target. Vacuum pressure decrement with respect to time between panes affects the thermal efficiency of TVG. This study reports a 3D finite element model, with validated mathematical methods and comparison, for the assessment of the influence of vacuum pressure diminution on the thermal transmittance (U value) of TVG. The centre-of-pane and total U values of TVG are calculated to be 0.28 Wm −2 K −1 and 0.94 Wm −2 K −1 at the cavity vacuum pressure of 0.001 Pa. The results suggest that a rise in cavity pressure from 0.001 Pa to 100 kPa increases the centre-of-pane and total U values from 0.28 Wm −2 K −1 and 0.94 Wm −2 K −1 to 2.4 Wm −2 K −1 and 2.58 Wm −2 K −1 , respectively. The temperature descent on the surfaces of TVG between hot and cold sides increases by decreasing the cavity vacuum pressure from 50 kPa to 0.001 Pa. Nonevaporable getters will maintain the cavity vacuum pressure of 0.001 Pa for over 20 years of life span in the cavity of 10-mm wide edge-sealed triple vacuum glazing, and enable the long-term durability of TVG.Triple vacuum glazing (TVG) has the ability to curtail heat flow between the warm side and cold side of a window, i.e., to provide preeminent thermal insulation, so-called thermal transmittance (U value) [6]. TVG consists of three panes of 4 mm thick glass, with an evacuated gap 0.13 mm high, separated by 0.13 mm high and 0.3 mm diameter stainless-steel support pillars. TVG edges are sealed with airtight material such as solder glass, indium alloy, or Cerasolzer CS186. The reason TVG has the potential for retrofitting to existing buildings is because its U value is less than 0.5 Wm −2 K −1 , compared to triple air-filled glazing, which has a U value of 2.8 Wm −2 K −1 . It also would allow increasing the window-to-wall area ratios because of its U value being closer to that of cavity wall insulation. Nevertheless, TVG has associated challenges; this paper presents the findings of the effects of diminution of vacuum pressure on the U value. The U value of TVG primarily depends on the thickness of the glass panes, vacuum pressure in the space between the glass panes, emissivity of the coatings on the glass panes, frame, and type of spacers that separate the panes of glass and type of window frame. The total U value consists of the influence of the frame and the area of the edge seal. The edge seal area is dependent on the width and the type of edge-sealing material. The airtight edge seal of a TVG must be able to perpetuate a vacuum pressure below 0.1 Pa for quelling gaseous heat conduction for long-term duration [7]. Benson et al. [8] investigate...

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