2. Life time
The film around the VIP
core provides a very efficient barrier against atmospheric gases
and moisture. It keeps the vacuum at a low level.
The best gas barrier is an aluminium laminate foil,
but it conducts a significant amount of heat around the edges. Therefore
it is only usable for large panel sizes. Special metallized high
barrier films avoid this heat bridge effect of aluminium and so may
provide a sufficient high gas barrier.
The increase of gas pressure within silica VIPs has
been monitored for more than a year as can be see in Fig.
3. According
to these results the metallized high barrier film used by va-Q-tec
keeps the increase of gas pressure at a value of 1 mbar per year.
As even at a gas pressure of 50 mbar the thermal
conductivity of microporous silica is below 0.008 W/mK, a life time
of up to 50 years may be extrapolated from these data. |
