Thermal Edge Effects


The lifetime and performance of Vacuum Insulation Panels (VIPs) depends upon the ability of the barrier film or envelope material to maintain a defined vacuum level during its lifetime. The major criteria in the selection of the appropriate barrier material for a particular VIP application is the compromise between the permeability of the barrier material itself, its cost and its thermal edge performance effects. Thermal edge effects result from a relatively high thermal transport region in the barrier material around the vacuum panel. This occurs to some extent with any barrier composition and thickness. The effective thermal performance of a vacuum panel is always lower than the value measured at the center of the panel. The “center-of-panel” value is the thermal performance value that is usually reported by panel manufacturers/suppliers since it is much easier to measure than the effective thermal conductivity. However, the effective conductivity is what describes the actual performance of the VIP in the final application.

“Thermal Edge Effects”, also known as thermal shunting or thermal short-circuiting, arise because the thermal performance of the highly porous vacuum panel inserts are very high as compared to the dense barrier materials.

This thermal edge effect is schematically illustrated below. The graph indicates the relative magnitude of the heat or energy flow by size of the flux arrows. Thus the flux at the panel’s center point is the lowest which implies that the VIP thermal performance is the highest at the center.

The effective thermal conductivity or performance requires consideration of the heat flux throughout the entire VIP surface. In other words, the intrinsic thermal performance depends on the VIP insert, the thickness and composition of the barrier, the boundary conditions around the VIP and most importantly, the size of the VIP.

Properties of barrier materials

In general, barrier materials for vacuum insulation panels can be selected from either plastics, metallized plastics (for example, produced by vapor depositions of metals such as aluminum), metal foil/plastic composites produced by lamination, or welded metal foils.

In most cases the barrier film structure is typically multilayer produced by lamination in order to impart a range of functionality (water and gas permeability, heat sealing, mechanical properties, etc.) to the film. For barriers using metal foil, aluminum foil is the metal of choice because of its ductility, availability, and cost. However, aluminum has very high thermal conductivity properties. In fact, the thermal conductivity of aluminum is approximately 1,000 times greater than that of common plastics used in barriers and 20,000 to 100,000 times greater than that of typical VIP filler materials.

Modeling of Thermal Edge Effects

For a square VIP of thickness equal to 25mm and of varying lengths, the effective thermal conductivity can be estimated as a function of VIP size by assuming a boundary condition model and fixing the VIP inner core thermal conductivity.

The figure below shows the effective thermal conductivity as a function of the side length of the square VIP for the case when the inner core has a thermal conductivity of 0.0036 W/(m · K) (3.6mW/(m · K)) or R/inch equal to 40).

Calculations have been performed for four different barrier material using typical thickness for the different layers.

The 50micron plastic exhibits negligible effects for all practical VIP sizes. However, for some VIP inserts/fillers which require extreme vacuum levels, plastic only barrier materials offer inadequate barrier performance.

Using a relatively large amount of aluminum metallization on the plastic, edge effects still disappear for panels larger than 150 mm (6.0 inch) on a side. Depending upon the desired VIP lifetime, the critical pressure of the VIP filler material and the number and type of metallization layers, metallized films may not provide sufficient barrier performance. However, for VIP inserts which operate at higher pressure levels, relatively inexpensive metallized barriers offer more than adequate VIP lifetimes. If better barrier performance is required for materials which must be maintained below several mbar pressure for the lifetime of the panel, than foils are used.

The thinnest available foil-based barrier has a foil thickness of approximately 7.5 microns. As shown in the figure, for a 300 mm · 300 mm · 25 mm (11.81” · 11.81 · 1”) VIP using that type of barrier, the effective performance would only be 1/3 of that if there were no edge effects.

For a 500 mm · 500 mm · 25 mm (19.68” · 19.68” · 1”) VIP, the performance would only be 1/2 of the performance with no edge effects. Due to the high cost of producing these very thin aluminum foil based laminates with a minimum of pinholes, thicker foils can be used which are actually cheaper. 25microns is typically taken as the thinnest foil which has no pinholes. However, as shown below, the edge effects associated with these thicker barriers practically precludes their use. Based on these results, it is clear that unless the panels being used are extremely large, the thermal performance of VIP’s can only offer dramatic improvement over conventional insulation in the end-use application if the plastic or metallized plastic barriers are used. The quantitative results presented above would be slightly different for different boundary conditions or for VIP insert materials with different intrinsic thermal conductivity. However, the same general trends would be observed. Therefore, when selecting a VIP insert material and the barrier material, it is important to balance the effects of the barrier on thermal performance, on VIP lifetime, and on overall cost of the VIP.