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Vacuum insulation panels: How VIPs work

Heat transfer in an insulation material comprises

• thermal conduction through the porous solid
• thermal radiative transfer through the solid
• thermal conduction via the gas within the pores of the solid

The thermal conductivity of the insulation material can be decreased by reducing each of these components. Thermal conduction through the solid can be decreased by adding special materials and by reducing the density of the solid (high porosity). Thermal radiation can be decreased by adding an infrared opacifier. Thermal conduction via the gas can be influenced by altering the type of gas and gas pressure in the pores. The thermal conduction via the gas is virtually eliminated in the evacuation process.
The degree to which the residual gas pressure has to be reduced depends on the pore sizes of the VIP core. Nanoporous powder cores are particularly insusceptible to pressure increases (cf. graph below). The thermal conductivity of a VIP is only doubled when the pressure in the core reaches 100 mbar. Even if the envelope fails completely, the thermal conductivity of fumed pyrogenic silica is still under 20·10^-3 W/(m·K). The increase in thermal conductivity in coarser-structured porous cores starts at residual gas pressures of just 0.1 to 1 mbar; the envelope must therefore be particularly gas-tight, made, for instance, out of stainless steel.

Thermal conductivity of different porous, open-pored insulation materials depending on gas pressure (gas = nitrogen) in the material. Each of the materials was put under 1 bar external pressure load. The thermal conductivity was determined at 20° C. The materials differ in their average pore diameter (fibreglass and foams 40 - 70  µm and pressed powders 0.3 - 1 µm).