Variable pressure thermal insulating jacket

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In a preferred form of the invention shown in FIG.
8 the gas pressure in a thermal jacket 10 of a thermal source 11 (for example, high temperature fuel cells, mechanical, chemical, or electrochemical processes and the like, including, but not limited to, endothermic or exothermic chemical reactions, and other sources of thermal energy) is controlled to range over a broad range of pressure.
In the most preferred form of the invention shown in FIG.
5 the gas pressure in a thermal jacket 10 of a bipolar battery 12 is controlled to range over a pressure of 1-1000 Pa (10.
sup.
-2 to 10 torr).
The thermal jacket 10 comprises loosely wrapped multifoil insulation.
Gas pressure control is exerted for the pressure within the spaces between the foil layers and thus allows control of the rate of heat loss from the jacket 10.
In particular, at the low pressure end of the range, the mean free path of the gas molecules greatly exceeds the size of the interstices in the insulation, thus resulting in low conductivity through the gas.
The variation in pressure in the jacket 10 can be achieved by the admission of air to raise the heat loss rate, and the use of a small mechanical vacuum pump to lower the cooling rate.
A particularly effective, and most preferred, method of varying the pressure is accomplished by changing the temperature of a conventional getter material which reversibly absorbs and desorbs hydrogen (or other gas) in the gas space.
A getter such as Zr-70%, V-24.
6%, Fe-5.
4% (St 707 alloy supplied by SALES Getters/U.
S.
A.
, Inc.
), is normally used to remove gases in vacuum systems and is effective for control of nitrogen, oxygen, and hydrogen gases, the preferred gaseous constituents in the thermal jacket 10.
Alloys of the same type, but at different constituent concentrations, or other conventional gettering alloys will irreversibly getter nitrogen and/or oxygen but permit variation in the hydrogen pressure over the desired range by varying the getter temperature