Design of a Variable-Conductance Vacuum Insulation 940315

This paper describes one approach to the design of a variable-conductance vacuum insulation. In this design, the vacuum insulation consists of a permanently sealed, thin sheet steel, evacuated envelope of whatever geometry is required for the application. The steel envelope is supported internally against the atmospheric pressure loads by an array of discrete, low-conductance, ceramic supports, and radiative heat transfer is blocked by layers of thin metal radiation shields. Thermal conductance through this insulation is controlled electronically by changing the temperature of a small metal hydride connected to the vacuum envelope. The hydride reversibly absorbs/desorbs hydrogen to produce a hydrogen pressure typically within the range from less than 10^-6 to as much as 1 torr. Design calculations are compared with results from laboratory tests of bench scale samples, and some possible automotive applications for this variable-conductance vacuum insulation are suggested.