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Details of Planck Cooler Operation

A rendering of one of the Planck sorption coolers is shown below. As previously noted, it performs cooling using Joule-Thomson (J-T) expansion employing hydrogen as the working fluid. The key element of the 20 K sorption cooler is the sorption compressor (SCC), an absorption machine that pumps hydrogen by thermally cycling sorbent compressor elements. The principle of operation of the sorption compressor is based on a unique sorption material (La1.0Ni4.78Sn0.22), which can absorb large amounts of hydrogen at pressures well below 1 atmosphere (14.7 psi), and which will desorb to produce high-pressure hydrogen (typically >30 atmospheres).


Rendering of one of the Sorption Coolers
This rendering shows one of the two Planck sorption coolers in the positions needed to mount onto the spacecraft. The sorption compressor assembly mounts onto a warm radiator on the side of the spacecraft bus. The precoolers mount on their respective V-groove radiators. The J-T expander and the liquid hydrogen reservoirs mount onto the two Planck instruments. (Image credit: Fig. 2 from Pearson et al., 2007)


The Planck sorption compressor assembly (SCC), shown schematically below, is composed of six identical sorption compressor elements, each filled with metal hydride and capable of being independently heated or cooled. Each compressor element is connected to both the high pressure and low-pressure sides of the plumbing system through check valves, which allow gas flow in a single direction only. The check valves are indicated on the schematic as single arrows, which indicate the direction of gas flow through them. To damp out oscillations on the high-pressure side of the compressor, a four liter volume high-pressure stabilization tank assembly (HPST) is utilized. On the low-pressure side, a low-pressure storage sorbent bed (LPSB), filled with hydride, primarily functions to store a large fraction of the H2 inventory required to operate the cooler during flight and ground testing while minimizing the pressure in the non-operational cooler during launch and transportation. The compressor assembly mounts directly onto the heat rejection radiator on the spacecraft.


Sorption Cooler Schematic Diagram
Schematic diagram of sorption compressor assembly (SCC). The sorption compressor is composed of six sorption compressor elements, each of which has a gas gap thermal switch, and checkvalves to enable continuous flow of the refrigerant.


As outlined earlier in the sorption cooler rendering, each sorption compressor element (i.e. sorbent bed) is taken through four steps (heat up to pressurize, desorption, cool down to depressurize, absorption) in a cycle. To produce a continuous stream of liquid refrigerant several such sorption beds are needed to in sequence so that at any given time, one is desorbing while the others are either heating, cooling, or re-absorbing low-pressure gas. In such a system, there is a basic clock time period over which each step of the process is conducted. Electrical resistance heaters accomplish heating of the sorbent while the cooling is achieved by thermally connecting the compressor element to a radiator at 270 K.

To not lose excessive amounts of heat during the heating cycle, a heat switch is provided to alternately isolate the sorbent bed from the radiator during the heating cycle, and to connect it to the radiator thermally during the cooling cycle. A single compressor element is comprised of two concentric cylinders closed with end caps. The inner of these tubes contains La1.0Ni4.78Sn0.22 metal hydride and the outer forms a vacuum jacket around the inner cylinder. This vacuum jacket is used as a gas-gap heat switch.[7] This gas-gap heat switch is operated using hydrogen gas and a second metal hydride, ZrNi.

The refrigerant travels from the SCC though the Piping and Cold End Assembly (PACE). The PACE[1] primarily consists of a series of heat exchangers attached to three V-groove radiators on the spacecraft (at 140, 90 and 46 K), which provide pre-cooling, followed by expansion through the J-T expander. The major components of this assembly are schematically represented in the figure above.

Upon expansion, hydrogen forms liquid droplets whose evaporation provides the cooling power. The liquid/vapor mixture then sequentially flows through the first two liquid vapor heat exchangers (LVHX). Each of these hydrogen refrigerant reservoirs is filled with a wicking material in order to retain the liquid in the reservoirs in the absence of gravity. The LVHXs are thermally and mechanically coupled to the corresponding instrument (LFI/HFI) interface. In addition to cooling the instruments, precooling for the 4.5 K RAL mechanical J-T cooler and the 100 mK dilution coolers are also provided. Finally, any remaining liquid vapor mixture flows through the third LVHX, which is maintained above the hydrogen saturated vapor temperature. This third LVHX serves to evaporate any excess liquid that reaches it, preventing flash boiling which helps maintain a nearly constant pressure in the low-pressure plenum. Heat from the sensors evaporates liquid hydrogen and the low-pressure gaseous hydrogen is re-circulated back to the cool sorbent beds for compression.

In the figure below, two Planck sorption coolers are seen during integration with the warm spacecraft radiators and the V-groove radiators. All regulation of the system is done by simple heating and cooling, with no active control of valves being necessary. Normal operation of the compressors can be done with only a minimum of active feedback: The heaters for the compressors are controlled by a simple timed on-off heater system. In addition to the on-off heater power, each heater will have up to 30 W of additional heating supplied by a proportional controller to compensate or any degradation of hydride or gas-gap properties that might occur.



Sorption Cooler Integration with Planck Spacecraft
Integration of the two Planck sorption coolers with the Planck spacecraft was a very complicated procedure. Here, they are shown after the sorption compressor assemblies and the sorption cooler electronics had been attached to the warm spacecraft radiators. The precooling heat exchangers are attached to the V-groove shields. (Image credit: ESA)


The flight sorption cooler electronics and software were developed by the AFF Laboratoire de Physique Subatomique et de Cosmologie (LPSC) in Grenoble France with funding from the French space agency, CNES. These electronics and their controlling software provide for the basic sequential operation of the compressor beds, temperature stabilization of the cold end and monitoring of many of the cooler performance parameters. In addition they automatically detect several kinds of failure modes and will correctly adapt operations accordingly. Finally they are sufficiently flexible to allow the operational parameters to be adjusted in flight to maximize the lifetime and performance of the sorption coolers.

The total input power to the sorption cooler at end of life (maximum average power) is 470 W. Another 110 W is available to operate the sorption cooler electronics.


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