Sorption is cool!
For instruments that should "see" very sharp, any vibration can kill the performance. Present coolers tend to be noisy and cause disturbing vibrations. A cooler that can operate vibration free and has a acceptable cooling power would therefore be very welcome. Sorption coolers show no detectable vibrations, but research is necessary to increase their cooling power.
The soption coolers that have been developed at the University of Twente only have a cooling power in the order of 10mW. The E-ELT instruments will have demands that are 100-1000 times higher. And although the system can be scaled relatively simply, research is necessary to do this in a smart way optimizing the required energy, mass and volume.
Energy, Materials and Systems chair (EMS) of the University Twente.
Airbus Defense & Space Netherlands BV
E-ELT instruments will require some form of (cryogenic) cooling. Many current state-of-art coolers use compressor units which involve relatively large moving parts and export a considerable amount of vibrations. For E-ELT instruments, many of which require a very stable environment, these vibrations can be detrimental to the performance.
A sorption cooler uses the Joule-Thomson effect for cooling a gas by expanding it through a flow restriction. The flow of gas is sustained by a compressor consisting of one or more sorption cells which cyclically adsorb and desorb gas according to the fully reversible process of physical sorption.
Figure 1: 5mW at 4K Helium cooler developed for ESA
The technology has been shown to provide active cooling in the cryogenic temperature range without exporting vibrations or electromagnetic interference. Due to full reversibility of the process and the absence of moving parts (apart from check valves which open and close with a very low frequency) such a cooler has the potential for a very long life and high reliability. Another attractive feature of the technology is the ability to provide cooling over a wide range of temperatures (roughly 2K-200K) by using different working gases and the high thermal stability of the cooling tip.
The technology has been developed for over a decade by the University of Twente, with support from Airbus Defence & Space Netherlands (ADSN, formerly Dutch Space) and more recently also the spin-off companies Kryoz Technologies and Cooll SES. Initial development focused on the application of the technology for space missions with very stringent requirements for exported vibrations. More recently, developments are also ongoing for terrestrial science applications, among them the METIS instrument on E-ELT. This application requires a cooling power which is 1 – 2 order of magnitude greater than the cooling power achieved in previous developments, therefore current activities focus on demonstrating the up-scaling of the technology to higher power.
Figure 2: Schematic overview of a sorption cooler
All sorption coolers developed previously by University of Twente with the support of Airbus Defence & Space Netherlands have had cooling powers in the range 5-20mW at temperatures from 4-15K. E-ELT instruments such as METIS require a cooling power of 0.1-1W in this temperature range, and additionally cooling powers in the range 1-10W in the temperature range >30K.Although such a system can be realized by employing several low-power coolers in parallel, this is not the most compact or efficient solution.
Within the frame of a PhD programme the University of Twente will therefore perform an investigation into the characteristics of a high-power sorption cooling systems at various cooling temperatures, with a focus on system compactness and efficiency. Four temperature regimes corresponding to various optical and detector elements in the E-ELT will be considered:
- Ca. 150K (for near-IR instruments)
- Ca. 80K (for near-IR detectors)
- Ca. 30K (for mid-IR instruments)
- Ca. 5K (for mid-IR detectors)
The following possible optimization techniques are investigated:
Optimization of the working fluid: Aim is to select the best gas/liquid for a given cooling temperature
Minimizing the cycle-time: The cooling power is determined by the average net quantity of gas which is desorbed over the cycle time. One way of optimizing the cooling power is therefore to maximize the quantity of gas desorbed (see the next point), another way is to minimize the cycle time. The cycle time is determined to a large extent by the cool-down phase of the sorption cell (the adsorption phase). The time constant of this process is roughly equal to the thermal resistance to the cold sink multiplied by the heat capacity of the cell. While the heat capacity is determined to a large extent by the mass of the activated carbon in the cell, the thermal resistance allows for improvements. Such improvements might be different cell geometries (such as length/width ratio) or even forced convection.
Maximizing sorption capacity: In close cooperation with suppliers of activated carbon, the sorption characteristics of the carbon in relation to various working fluids will be investigated. The University of Twente possesses an instrument which can perform measurements of sorption isotherms.
Multi staging: The efficiency of a sorption compressor unit may be improved by using multiple stages, i.e. several compressors in series, in particular because in that way the effect of dead volume can be reduced. Also several phases in the cooler generate a higher cooling capacity than a single phase.
In parallel to this PhD programme Airbus Defence & Space Netherlands will perform a study to determine the design, development and verification steps which are needed to deliver a working sorption cooler for the METIS instrument.
The initial phase of this programme will consist of the definition of a “strawmans concept” for a full-scale METIS sorption cooler followed by a trade-off between this sorption-based system and a “conventional” system based on pulse-tube coolers. The trade-off will highlight the advantages and disadvantages of both systems with respect to cooling performance, exported vibrations, flexibility in relation to evolving requirements, costs and other critical factors.
The second phase of the programme will focus on the definition of a Design, Development and Verification (DD&V) plan which will serve as a “roadmap” and make clear the required technical activities until final delivery of a working sorption cooler. A rough-order-of-magnitude cost assessment is also performed as part of this study.
The results of the study, in particular the DD&V plan, are subject to review by the METIS instrument consortium.
As of 2015, the conceptual design consists of three stages thermally linked in parallel, to obtain cooling at 40 K by a neon, 25 K by a hydrogen and 8 K by a helium-based cooler stage as shown in the figure below.
The helium stage is driven by a single-stage compressor unit and uses four counter flow heat exchangers (CFHXs), followed by a JT restriction and a cold-tip heat exchanger. In order to facilitate the maximum achievable performance, pre-cooling heat exchangers are applied at 40 K, 25 K and 15 K. Because the cooling temperature of 8 K is above the critical temperature of helium, the gas will not liquefy during expansion and a well-designed gas heat exchanger is needed to transfer the heat load to the cold helium gas at the cold tip. The 25 K temperature level is established by a hydrogen cooler. Because of the higher pressure ratio, it operates with a two-stage compressor, and the cold stage uses a double JT expansion not only to reach 25 K, required both by the METIS instrument, but also to establish the 25 K and 15 K cooling interfaces solely for the helium stage pre-cooling. Finally, a neon-operated cooler delivers the required cooling power at 40 K. This cooler uses a single-stage compressor and its cooling capacity is split into cooling of the METIS L/M-detectors and pre-cooling of the helium and hydrogen stages.
For the design of the METIS cooler chain, a heat sink temperature of 70 K is considered, which can be realized by pumping a dedicated LN2 loop to reduce the pressure on the gas nitrogen exit line.
In october 2010 a PhD student started work at the University of Twente focusing on high-power sorption coolers for E-ELT. Currently research is focusing on optimalisation of the cooler working fluid. The existing test-setup to measure sorption isotherms has been significantly improved. Other activities are
- Investigation into various high-power concepts and corresponding experiments
- Design of prototype components (e.g. sorption compressors, cold stage) relevant for E-ELT
- Realisation of these prototypes and corresponding experiments
- Evaluation of prototypes, subsequent design adaptations and further experiments
- Writing of PhD thesis
In April 2012 a team at Airbus Defence & Space Netherlands started work on the study to define the DD&V plan for sorption cooling on METIS. A mid-term review was successfully held at Airbus Defence & Space Netherlands on 22-11-2012 where a strawman’s concept of the METIS cooler, a trade-off between sorption cooling and “conventional” pulse-tube cooling and a draft DD&V plan were presented.
On 21-05-2013 the Final Review was held at the premises of the University of Twente where the final DD&V plan was presented along with a rough-order-of-magnitude cost assessment. The METIS instrument consortium was represented as well as ESO in the review committee which determined there was sufficient basis to initiate Phase A developments outlined in the DD&V plan.
In June 2014, an artists impression was presented of how the METIS cooler on E-ELT would look like and how it could be fitted in the volume available. The cooler design is based on the latest design trades and in line with the technology which is implemented for the Neon demonstrator.
Figure 3: Artists impression of the METIS cooler on E-ELT. The colors (yellow, cyan and purple) represent the Helium, Hydrogen and Neon stages respectively.
Currently work is focusing on the design and manufacturing of a single stage, 1W at 40K Neon sorption cooler which will demonstrate the performance of the Neon stage of the METIS cooler and give a first indication of the feasibility of upscaling from powers in the order of 10mW to power in the order of 1W.
The detailed design of the Neon demonstrator cold stage was completed by the University of Twente. It was integrated and tested and delivered at Airbus Defense & Space Netherlands in January 2015 for integration into the cooler assembly.
The overall detailed design of the Neon demonstrator including the sorption cells is completed and is being integrated at the moment. The cooler is expected to be completed in March 2015 after which performance testing will be started soon after that.
Figure 4: Cold stage (top) and sorption cells + check valves (bottom) as used in the Neon Demonstrator.
Figure 5: Neon demonstrator compressor assembly in its current (not completed) state
In parallel to these E-ELT related developments there is also considerable interest in vibration-free sorption cooling for other terrestrial and space applications. Although these developments represent a separate line of activities, there is considerable potential for spin-off’s and spin-in’s to the E-ELT development.
These are summarized below:
At the Universitity Twente, the activities are conducted by the chair Energy, Materials and Systems (EMS). Partner of the University of Twente for these developments is Airbus Defence & Space Netherlands. Collaboration with various industries is sought at lower levels, e.g. activated carbon suppliers.
Wu, Y., Vermeer, C.H., Holland, H.J., Doornink, J., Benthem, B., and Brake, H.J.M. ter (2014), Advances in cryogenic engineering 59. (pp. 142-147). isbn. 978-0-7354-1201-9.
Y. Wu, T. Mulder, C.H. Vermeer, H.J. Holland, B. Benthem1 and H.J.M. ter Brake
"Switchless sorption-compressor design"
International Cryocooler Conference, Syracuse, USA, 9-12 June 2014
Y. Wu, T. Mulder, C.H. Vermeer, H.J. Holland, B. Benthem, and H.J.M. ter Brake
"Vibration-free cooler for the METIS instrument using sorption compressors"
International Cryogenic Engineering Conference, Enschede, NL, 7-11 July 2014
B. Benthem et al.
"Present status of developments in physical sorption cooling for space applications"
A cooling method by which a gas flow is circulated in a cooling system by means of a pressure difference between a cell that adsorbs the gas and one that is heated and therefore releases the gas.
The Joule-Thomson effect describes the temperature change of a gas or liquid when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment.