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Passive cooling has shown to be a very dependable cryogenic cooling method for space missions. Several missions
employ passive radiators to cool down their delicate sensor systems for many years, without consuming power, without
exporting vibrations or producing electromagnetic interference. So for a number of applications, passive cooling is a
good choice. At lower temperatures, the passive coolers run into limitations that prohibit accommodation on a spacecraft.
The approach to this issue has been to find a technology able to supplement passive cooling for lower temperatures,
which maintains as much as possible of the advantages of passive coolers.
Sorption cooling employs a closed cycle Joule-Thomson expansion process to achieve the cooling effect. Sorption cells
perform the compression phase in this cycle. At a low temperature and pressure, these cells adsorb the working fluid. At
a higher temperature they desorb the fluid and thus produce a high-pressure flow to the restriction in the cold stage. The
sorption process selected for this application is of the physical type, which is completely reversible. It does not suffer
from degradation as is the case with chemical sorption of e.g. hydrogen in metal hydrides. Sorption coolers include no
moving parts except for some check valves, they export neither mechanical vibrations nor electromagnetic interference,
and are potentially very dependable due to their simplicity. The required cooling temperature determines the type of
working fluid to be applied. Sorption coolers can be used in conjunction with passive cooling for heat rejection at
different levels.
This paper starts with a brief discussion on applications of passive coolers in different types of orbits and the limitations
on passive cooling at low cooling temperatures.
Next, the working principle of sorption cooling is summarized. The DARWIN mission is chosen as an example
application of sorption and passive cooling and special attention is paid to the reduction of the radiator area needed by
the sorption cooler.
By examining the performance of alternative working fluids suitable for different cooling temperatures, the application
field of this type of sorption cooling is currently expanded.
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