Optimising the Thermal Performance of Triple Vacuum Glazing with Low-emittance Coatings

The thermal performance of the triple vacuum glazing with one to four internal glass surfaces coated with a low-e (emittance) coating was simulated using a finite volume model. The simulated triple vacuum glazing comprises three, 4 mm thick glass panes with two vacuum gaps, sealed with indium metal and separated by an array of stainless steel pillars, 0.2 mm high, 0.3 mm diameter and spaced at 25 mm. The simulation results show that decreasing the emittance of the four low-e coatings from 0.18 to 0.03 decreases the heat transmission U-values at the centre-of-glazing area from 0.41 W.m-2 .K-1 to 0.22 W.m-2 .K-1 for a 0.4 m by 0.4 m TVG rebated by 10 mm within a solid wood frame. When using three low-e coatings in the TVG in a heating dominated climate, the vacuum gap with two low-e coatings should be set facing the warm environment, while the vacuum gap with one coating should face the cold environment. When using two low-e coatings with emittance of 0.03, the U-values at the centre-of-glazing area with one coating in both vacuum gaps is 0.25 W.m-2 .K-1 ; that with two coatings in the cold facing environment vacuum gap is 0.50 W.m-2 .K-1 and that with two low-e coatings in the warm facing environment vacuum gap is 0.33 W.m-2 .K-1 . Thus setting one low-e coating in both vacuum gaps is better than setting two coatings in the same vacuum gap. The thermal performance of fabricated 0.4 m by 0.4 m TVGs with two and three low-e coatings were experimentally characterised and were found to be in very good agreement with simulation results.