Solar Energy Vol. 63, No. 4, pp. 243–249, 1998 © 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0038-092X/98 $—see front matter PII: S0038-092X(98)00019-X FABRICATION OF EVACUATED GLAZING AT LOW TEMPERATURE P. W. GRIFFITHS,* M. DI LEO,*‡ P. CARTWRIGHT,** P. C. EAMES,*,§ P. YIANOULIS,***,§ G. LEFTHERIOTIS*** and B. NORTON*,§ * PROBE: Centre for Performance Research On the Built Environment, School of the Built Environment, University of Ulster, Newtownabbey, N. Ireland BT37 OQB, U.K. ** CERAM Research, Stoke-on-Trent, ST4 7LQ, U.K. *** Physics Department, University of Patras, Patras 26500, Greece Revised version accepted 8 December 1997 Abstract—An indium-based seal augmented with an adhesive, developed to maintain a vacuum between two sheets of glass, avoids the high temperatures required to produce a seal in evacuated glazings to date. An experimentally-validated three-dimensional transient model has been used to predict heat transfer for an indium/adhesive sealed 1 m2 area evacuated window with a highly insulating frame. An overall heat loss coefficient of 0.9 W m-2 K-1, with a midplane value of 0.36 W m-2 K-1, can be achieved with 0.72 visible transmittance for two 6 mm panes separated by 0.2 mm diameter pillars 40 mm apart. The conduc- tion through a 3 mm edge-seal was 1.14 W m-2 K-1. Detailed three-dimensional isothermal contour plots through the system are presented. © 1998 Elsevier Science Ltd. All rights reserved. 1. INTRODUCTION (Benson et al., 1990; Collins and Robinson, 1991) has produced mechanically stable evacu- Low heat loss and high total solar transmittance ated glazings. Steady-state midplane U-values can be obtained from a contiguously-sealed have been measured by imposing set temper- double glazing with an evacuated gap, low long- atures at glazing surfaces and measuring accu- wave emittance coatings and sufficiently large rately heat flux through the glazing (Collins spacer pillar separation, as illustrated in Fig. 1. et al., 1997). Fabrication of present-generation Research to date related predominantly to the evacuated glazings ( Katoh et al., 1997) requires integrity and stability of glass vacuum seals temperatures in excess of 450°C, thus incurring high embodied energy and restricting the choice of low-emittance film to reduce radiative heat transfer across the system to hard-coated types. To date hard coatings have higher emittance to long-wave radiation in the range 2.5–30 mm (Johnson, 1991) than soft-coated films, which have emittances <0.1. The emittance of the hard coatings also degrades at high edge seal fabrication temperatures, with emittance after fabrication measured at ~0.25 (Collins et al., 1993). 2. POTENTIAL LOW-EMISSIVITY FILMS FOR USE IN LOW-TEMPERATURE FABRICATED EVACUATED GLAZINGS Lower temperature fabrication at <200°C both reduces embodied energy and enables soft- coated low-emittance surfaces to be used. Glass samples of area 20×20 mm prepared with anti- reflection/metal/anti-reflection layered films have very low emittance (Granqvist, 1989). Soft Fig. 1. Indium-based edge-sealed evacuated glazing. coatings consisting of multiple dielectric/ ‡Present address: Consorzio Venezia Nuova, Palazze metal/dielectric layers on glass give optimal Morosini, San Marco 2803, Venice, Italy. §Member ISES. thermal and optical performance. A 100–200 A ˚ 243