Study on envelopes for vacuum maintenance of vacuum insulation panels

Abstract

This study investigates envelopes for the vacuum maintenance of vacuum insulation panels (VIPs). The VIP is lately becoming a key factor to save the building energy, which accounts for nearly 40% of global energy consumption. VIPs must have good insulation performance and a long service lifetime. Al-metallized film can meet these requirements by suppressing thermal bridging and gas permeation. First, the gas permeation mechanism through the Al-metallized film is investigated. The gas permeabilities through an Al-metallized film for various gases are measured using a newly-developed experimental apparatus based on a pressure difference method. Its reliability is validated by measuring a few well-known gas permeabilities through uncoated 12 um polyethylene terephthalate (PET) film. Then, a 12 um PET film coated with a 33 nm aluminum layer is selected as the sample. It is found using a residual gas analyzer (RGA) that nitrogen, oxygen, carbon dioxide and water vapor are the major gases permeating through the Al-metallized film. These gases are then taken as the test gases. The measured gas permeabilities of these gases through the Al-metallized film are found to be only about 4.1% of those through uncoated PET film. This value is nevertheless higher than those of metal sheets and Al-foil films because of the abundant pin-holes which are formed during the production process. Pin-holes are further investigated in terms of their distribution and diameters using an optical microscope. The effective gas permeability including the pin-hole effect is calculated to be 3.6 to 4.0% of the uncoated PET film permeability, agreeing with the experimental results. This leads to the conclusion that the main passage of gas permeation through Al-metallized film is the pin-holes. Multiple Al-metallized layers must be therefore employed to form an effective gas barrier. A correlation pertaining to the degree of permeation through a layer of two-side Al-metallized film is also propos...