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Present status of developments in physical sorption cooling for space applications
Cryogenics xxx (2014) xxx–xxx
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Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Present status of developments in physical sorption cooling for space applications B. Benthem a,⇑, J. Doornink a, E. Boom a, H.J. Holland b, P.P.P.M. Lerou c, J.F. Burger d, H.J.M. ter Brake b a
Dutch Space B.V., P.O. Box 32070, 2303 DB Leiden, The Netherlands University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands c Kryoz Technologies B.V., Pantheon 18-22, 7521 PR Enschede, The Netherlands d Cooll Sustainable Energy Solutions B.V., Hengelosestraat 298A, 7521 AM Enschede, The Netherlands b
a r t i c l e
i n f o
Article history: Available online xxxx
a b s t r a c t 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. 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. This paper starts with a recapitulation of the principles of physical sorption cooling followed by an overview of the strengths and weaknesses of the technology in relation to other space cooling technologies, such as pulse-tube cooling and Stirling cooling. Next, the present status of physical sorption cooling technology is presented based on developments previously and currently being performed by the University of Twente, Dutch Space and Kryoz Technologies. A summary will be given of the various existing demonstrator- and lab-models which have been built, along with an overview of the tests which have so far been performed. The central result of this paper is an assessment of the current Technology Readiness Level (TRL) of various sorption cooler configurations, along with their application range in terms of temperatures, heat loads and mission profile. Finally, an outline is given on the way forward currently being pursued by the developers to achieve full maturity of the technology. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Since the 1960s spacecraft have been flown with components that require cooling at temperature ranges anywhere from subKelvin to slightly above room temperature. Cooling can be required for various reasons, including: 1. Improvement of signal-to-noise ratio in detectors, especially in infrared (IR) instruments. The elements of the instruments that may require cooling are radiation detectors, optical components, baffles, or in some cases, the whole instrument. Cryogenic cooling is necessary to provide the required detector response, reduce pre-amplifier noise, and/or reduce background radiation [1].
⇑ Corresponding author. Address: P.O. Box 32070, NL-2303 DB Leiden, The Netherlands. Tel.: +31 (0)71 524 5212. E-mail address: [email protected] (B. Benthem).
2. Removal of waste heat and maintain components within operational limits, e.g. for dissipating electronic equipment or momentum wheels [2]. 3. Reduction of boil-off during long-term on-orbit storage of cryogenic fluids, e.g. for propulsion systems [3]. Various methods exist for providing this cooling, including passive cooling using radiators, usage of stored cryogens and closedcycle active cooling [1,4]. Passive cooling is particularly attractive in many of the above applications because it takes advantage of the cold space environment to reject heat via radiators without using consumables or power, and without exporting vibrations or producing electromagnetic interference, while simultaneously providing high cooling reliability and long-term performance [5]. However, at temperatures below typically 40–60 K passive coolers run into fundamental limitations that prevent their practical accommodation on a spacecraft, although there are exceptions
http://dx.doi.org/10.1016/j.cryogenics.2014.02.010 0011-2275/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Benthem B et al. Present status of developments in physical sorption cooling for space applications. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.02.010
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B. Benthem et al. / Cryogenics xxx (2014) xxx–xxx
(e.g. Spritzer, where a radiator temperature of 34 K was attained, and in JWST, where the radiators will achieve 38 K). This temperature range is typically associated with high-performance IR instruments. In these cases stored cryogens or closed-cycle active cooling must be used. A recent European example of the former is the use of liquid Helium to cool the Herschel science instruments. As stored cryogens are boiled off to space this system has a limited lifetime. In addition, notable failures have occurred where the cryogen is prematurely expelled, resulting in short mission lifetimes and lower science yield [19]. Closed-cycle active coolers do not suffer from depletion of the cryogen. However, these systems can consume a considerable amount of power, export vibrations and/or introduce electromagnetic interference. In addition, the presence of moving mechanical parts can imply wear-and-tear producing failures, which must be absorbed by including a sufficient level of redundancy and/or reliability in the design. A number of coolers also impose volume and/ or geometric constraints on the spacecraft, for example pulse-tube coolers, which require a relatively short distance between cold tip and compressor. Physical sorption cooling has been developed for over a decade by the University of Twente, supported by Dutch Space for spacebased applications, and more recently also by Kryoz and Cooll. This has realized a form of closed-cycle active cooling which retains the advantages of passive cooling (low power use, low exported vibrations, low electromagnetic interference, high reliability), whilst extending the achievable temperatures down to the sub-Kelvin range.
2. Physical sorption cooling Several papers exist describing the fundamental principles of physical sorption cooling, e.g. [5,6]. The following description is taken from [5]: Sorption cooling employs a closed cycle Joule–Thomson expansion process to achieve the cooling effect (Fig. 1). In most Joule– Thomson coolers a mechanical compressor delivers the required gas flow, whereas in sorption cooling a sorption compressor is applied. A sorption compressor is a thermally driven compressor that operates with a sorber material. In case of physical sorption the sorber material is activated carbon. Due to its highly porous structure, activated carbon has a very large internal surface, which is able to adsorb large quantities of gas. By heating the sorber material, the gas is desorbed and a high pressure can be established. This high-pressure gas is expanded through a restriction, in many cases resulting in a phase transition of the gas to the liquid state. Cooling is achieved by evaporating this liquid at the evaporator or, in case of no phase transition, by heat exchange with the cold low-pressure gas. After cooling the sorber material via contact with a heat sink, the low-pressure gas is re-adsorbed and the cycle described above is started over. The nature of physical sorption implies that the process is fully reversible, i.e. no degradation occurs and the cycle can in principle be repeated infinitely. Upon this basic principle several refinements have been introduced which enable practical applications, all of which have required dedicated developments. Among these are the following: Check valves to direct the flow of gas. Stringent back-flow requirements at cryogenic temperatures have required a dedicated design effort [7]. The use of buffers to stabilize the gas flow. In particular the lowpressure buffer has been specially designed to hold a sufficient quantity of gas at low pressure, whilst minimizing volume. Use of a gas-gap heat switch to reduce heat losses to the heat sink during the heating phase of the cycle.
A wide range of temperature levels can be achieved at the evaporator depending on the choice of working fluid and low-pressure level selected (Table 1). For practical reasons, a low pressure level between 0.2 Bar and 10 Bar is typically assumed. For a given gas, the range of achievable temperatures is then determined by the gas triple point or the vapor temperature at 0.2 Bar, whichever is higher (minimum) and the vapor temperature at 10 Bar (maximum).
3. Comparison to other cooling technologies Several papers present overviews of current space missions employing cryogenic cooling in their designs [19,9,4]. Based on these sources several types of closed-cycle active cooling systems can be distinguished which provide cooling in the same range applications. Indicated between brackets are examples of representative missions using the respective type of cooling:
Reverse turbo-Brayton (e.g. NICMOS/HST; 7 W @ 77 K [10]). Stirling (e.g. SPI/INTEGRAL; 2.1 W @ 85 K [11]). Pulse-tube (e.g. AIRS/Aqua; 1.3 W @ 55 K [12]). Joule–Thomson (e.g. Planck; 19 mW @ 4.5 K [13]). Joule–Thomson with chemical sorption compressor (e.g. Planck; 1 W @ 19 K [14]).
Sub-Kelvin cooling systems like Adiabatic Demagnetization Refrigeration (ADR) or dilution refrigerators are excluded, as are thermo-electric coolers, stored cryogens and passive cooling since these technologies have unique operating parameters, which are not easily compared to the cooler types above. It is interesting to compare the above 5 types of closed-cycle active cooling systems to physical sorption cooling in various critical areas.
3.1. Exported vibrations With the advent of low-vibration attitude and orbit control (AOCS) technologies such as magnetic bearing reaction wheels and microthrusters, active cryocoolers could be expected to become the dominant source of micro-vibrations in spacecraft. One of the key advantages of sorption cooling in general over many other active cooling technologies is the absence of large moving parts, not only in the cold stage, such as in pulse-tube coolers, but also in the entire cooling system. Although theoretically some vibration mechanisms do exist, such as the low-frequency movement of the check-valves, the cyclic thermal expansion and contraction of the compressor cells and fluctuations in the flow of gas through the system, the resulting vibration level can simply not be measured in a laboratory setup due to the ambient noise levels in the laboratory. Mechanical analysis predicts the total exported vibrations to p be below 1 lN/ Hz in the frequency band 0.1–1 Hz [15]. The reverse turbo-Brayton cooler for NICMOS was deployed in 2002 on the Hubble Space Telescope as the only active cooling system existing at the time which could meet the exported vibration requirement of 1 milli-arc-second (mas) rms contribution to the Hubble pointing jitter. To meet this requirement, the low-mass turbine shafts are supported on gas bearings and operated at high rotational speeds exceeding 1200 rps, resulting in a low level of (low-) frequency vibrations [16]. However, this requires the turbines to be precision-engineered and finely balanced adding to system complexity and cost. It should also be noted that currently no space qualified reverse turbo-Brayton coolers are known to exist for cooling to temperatures below 65 K. Pulse-tube coolers are preferred over Stirling coolers in terms of exported vibrations due to the fact that no moving parts are re-
Please cite this article in press as: Benthem B et al. Present status of developments in physical sorption cooling for space applications. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.02.010
B. Benthem et al. / Cryogenics xxx (2014) xxx–xxx
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Fig. 1. Schematic representation of a Joule–Thomson cooler with a sorption compressor. Taken from [5].
quired in the cold stage. However the gas compressors still generate vibrations, as does the pressure wave inside the cold stage. Joule–Thomson (JT) sorption coolers can be expected to provide the ultimate performance in terms of exported vibrations when compared to the currently existing closed-cycle active cooling systems.
An advantage of the physical sorption technology developed by the University of Twente is the fully reversible nature of the sorption process. Unlike chemical sorption using metal hydrides as sorber material, no degradation of performance occurs. On the Planck spacecraft for example, the metal-hydride sorption cooler was designed for a mission life of 2 years [14]. For physical sorption possible remaining long-term degradation mechanisms such as restriction/filter clogging, degradation of the activated carbon and degradation of the gas gap heat switch have not been fully characterized by test yet, although continuous operation of a Helium sorption cooler for a period of 4 months did not show any degradation of performance [18]. It is expected that with proper design measures the lifetime of a physical sorption cooler can easily exceed 15 years of continuous service. The only active components in a sorption cooler are heaters, which are well-characterized in terms of reliability. Redundancy can easily be included by applying heaters in pairs for cold redundancy. Furthermore, at the compressor level the modular nature of the sorption cooler allows the use of additional compressor cells for hot redundancy. The only moving components of a sorption cooler are the (passive) check valves. These have so far been life tested by the University of Twente up to 300,000 cycles without failures or degradation, representing a typical on-orbit lifetime of around 5 years.
3.2. Reliability/lifetime
3.3. Scalability of cooling power
The reliability and lifetime have long been factors limiting the use of active cooling systems in space [17]. In recent years however, lifetimes up to 10 years have been demonstrated in-orbit for various cooler types [17,4].
The cooling power delivered at the cold tip is roughly linearly dependent on the mass-flow of gas through the JT restriction. The compressor stage of a sorption cooler is highly modular. Besides contributing positively to the reliability of the system by pro-
Table 1 Overview of theoretical achievable Joule–Thomson cooling working temperatures. Data taken from [5] and [8]. A solid fill indicates development models using these gases that have been realized by the authors.
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ducing redundancy as described above, this modular nature also allows linear scaling of the mass-flow (i.e. cooling power) simply by adding or removing compressor cells in parallel. However the JT cold stage scales in a slightly less obvious way. In general, JT cold stages scale favorably in terms of cooling power at the cold tip compared to other designs with moving components (like reverse turbo-Brayton and Stirling) [17]. The main considerations in the scaling of a sorption cooler JT cold stage are the pressure losses in the piping, the dimensioning of the heat exchangers and flow restriction in relation to the mass flows, and to lesser extent the magnitude of parasitic heat leaks to the environment. Large commercial earth-based helium liquefiers with a JT cold stage deliver cooling powers up to 1 kW at 4.5 K, whereas the lowest-power micro JT stages developed by Kryoz and University of Twente deliver cooling powers in the order of milliwatts, showing scalability over six orders of magnitude. 3.4. Accommodation on spacecraft The accommodation of a cooler on a spacecraft is in many cases not straightforward. Passive cryogenic coolers typically require complete shading from all external fluxes (sun, planetary) requiring either elaborate baffle structures or strict constraints on spacecraft orientations. Also, thermal links to the cooling targets require special measures. For stored cryogens the accommodation impact is even more significant: in many cases the spacecraft is simply designed around the cryostat as was the case for the Herschel PLM. The design was dominated by the 2400 L liquid helium cryostat vacuum vessel [29]. Less obvious are the accommodation constraints imposed by active coolers. Typically mechanical compressors operate at or around room temperature. The cooler structure therefore necessarily experiences a temperature gradient between room temperature and the temperature of the cold tip. Besides requiring dedicated design measures on the level of the cooler to reduce parasitic heat flows, this also means that the cooler structure has to ‘‘cross’’ one or several temperature zones on the spacecraft. For example in the case of the Planck 4 K and 20 K JT coolers the cold stages crossed all layers of the V-groove radiator [13], thereby complicating the final spacecraft assembly & integration. The same is valid for the JT cooler for the MIRI instrument on board the JWST [20]. On the other hand the process of physical sorption is generally more efficient at lower temperatures. The sorption compressors developed by the University of Twente for DARWIN for example are heat sunk on 50 K and 100 K temperature levels achieved on the cold side of a V-groove radiator [5]. This configuration would allow for a less complex assembly & integration at the spacecraft level. The drawback is that the heat rejection capability of radiators at these temperatures is limited for fundamental reasons. Therefore the required radiator sizes are relatively large – up to 6 m2 for the Helium stage [5]. It should be noted however that the required radiator size is strongly dependent on the working fluid and required temperature levels. A helium stage cooling at 4 K represents a ‘‘worst-case’’ configuration in terms of required radiator surface area, due to the relatively poor adsorption on activated carbon of helium compared to other working gases [21]. The feasibility of a cooling chain with sorption cells mounted on a deployable radiator (which effectively increases surface area by radiating on both sides while simultaneously allowing a large surface area to be accommodated on the spacecraft) is currently being investigated by Dutch Space B.V. An advantage of recuperative cooling in general (JT, reverse turbo-Brayton), in terms of accommodation on the spacecraft, is the flexibility offered in terms of cold stage layout. The only fundamental effects limiting the dimensions of the cold stage are pressure losses and parasitic heat loads on the gas lines. Provided
sufficient hydraulic diameter and thermal insulation of the lines is available, there is considerable flexibility in the separation of the compressors and the cold tip. For example, the JWST JT cold tip is physically separated from the compressors by a distance of over 10 m [20], while the micro JT cold stages developed by Kryoz and the University of Twente are around 5 cm long. This flexibility can in many cases obviate the need for thermal straps, heat pipes and other heat transport devices. 3.5. Power usage JT coolers are known to have a lower efficiency (especially at cooling temperatures >20 K) compared to other cooling cycles, due to the fundamentally irreversible nature of the JT process [4]. The efficiency of the sorption coolers manufactured by the University of Twente varies from 2% of Carnot for a 4.5 K Helium cooler including power consumption of the pre-cooling stage [15] to around 4% of Carnot for a 14.5 K Hydrogen cooler [22]. The 40 K Neon cooler currently under development for a ground-based application is predicted to achieve an efficiency of 1.3% of Carnot. Despite this, the electrical power required by a physical sorption cooler can be very small compared to other active coolers, due to the fact that heat rejection takes place at low temperatures as described above. For example the 4.5 mW, 4.5 K Helium cooler consumes less than 2 W of electrical input power at the level of the cooler. This power is rejected at 50 K by a large passive radiator. Calculations indicate a required radiator surface area in the order of 6 m2, which may be available e.g. on a V-groove radiator as described in the previous section. An additional 10 W of electrical power is estimated to be required by the flight electronics (rejected by a radiator at room temperature) resulting in a total power consumption of 12 W. It is interesting to compare this figure to the similar 4 K JT cooler with mechanical compressors used on Planck, which reject their heat at room temperature (300 K). This cooler provided 19 mW at 4.5 K with an input power of 120 W. Scaled to match the 19 mW heat lift requirement, the DARWIN Helium cooler can be expected to consume roughly 20 W of electrical power, a factor 6 less than the Planck design. The penalty for this low electrical power usage is the larger required radiator surface area. 3.6. Electromagnetic interference The only potential sources of electromagnetic interference (EMI) in sorption coolers, besides the drive electronics, are the heaters used in the compressor cells and the gas gap actuators. Of these, only the compressor cell heaters require a significant current. These heaters are typically of the cartridge type, where the resistance wires are suspended axially in a steel tube (‘‘cartridge’’), filled with a mineral powder. The heater’s steel casing and the compressor cell itself acts as electric shielding, while the close proximity of the two wires minimizes the exported magnetic field. The heaters are fed with a direct current. In principle the heaters switch fully on or fully off at the cycling frequency of the sorption cell, which is well below the radio frequency (RF) bands thereby limiting any interference. If proportional heater control is desired (for example to stabilize the high-pressure level or to accommodate two heaters in hot redundancy), pulse-width modulation can be used. In this case twisted pair leads can be applied to reduce EMI. 4. Present status of sorption cooling Table 2 presents an overview of the relevant development activities performed by the University of Twente and Dutch Space, and more recently Kryoz and Cooll, since the realization of the first prototype sorption cooler in 2001. Since 2001, development has progressed broadly along two parallel and complementary lines.
Please cite this article in press as: Benthem B et al. Present status of developments in physical sorption cooling for space applications. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.02.010
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B. Benthem et al. / Cryogenics xxx (2014) xxx–xxx Table 2 Overview of physical sorption/micro JT developments by University of Twente, Kryoz Technologies and Dutch Space. Gas/model
Performance
Compressor
Pre-cooling
Year
Reference
Ethylene sorption cooler
200 mW @ 169 K
Physical sorption at 293 K
Thermoelectric
2001
[24]
Neon single stage micro JT cold stage
43 mW @ 30 K
Gas bottle
2005
[25]
Helium sorption cooler
4.5 mW @ 4.5 K
Physical sorption at 50 K
Liquid nitrogen GM-cooler
2007
[15]
Nitrogen single stage micro JT cold stage
150 mW @ 90 K
Gas bottle
–
2007
Kryoz datasheet
Methane single stage micro JT cold stage
Gas bottle
–
2012
[26]
Hydrogen sorption cooler
8–150 mW @ 140 K 18.5 mW @ 14.5 K
Physical sorption at 90 K
GM-cooler
2013
In press
Nitrogen sorption compressor
–
Physical sorption at 293 K
–
2013
–
Neon Nitrogen two-stage micro JT cold stage
20 mW @ 30 K
Gas bottle
–
2013
Kryoz datasheet
4.1. Vibration-free sorption cooling The first line is being pursued primarily by University of Twente, Cooll and Dutch Space and is aimed at vibration-free sorption cooling for space and terrestrial cooling applications. The emphasis is on achieving the technology readiness level which is appropriate for space applications. This includes in particular demonstrations of performance, reliability and compatibility with a launch environment, while respecting established space product assurance requirements. A major focus of the activities during the period 2003–2007 was development, manufacturing and testing of a 4 K 5 mW Helium sorption cooler for the ESA Darwin mission, funded by ESA Technological Research Programme (TRP). During this programme a Critical Design Review (CDR) of the cooler was held and several papers were published [7,15,18,23], presenting the analysis and test results. The cooler features 4 helium compressor cells in 2 stages; each cell is 15 mm in diameter and 157 mm long. The cells are filled with monolithic carbon pills and in this test-setup are heat-sunk on a Gifford McMahon (GM) cooler, representing a radiator at 50 K. Thermal coupling of the compressor to the heat sink is controlled by a hydrogen gas-gap heat switch. Helium flow rates in the cold stage are around 0.41 mg/s with high- and low pressures of 16.5 Bar and 1.3 Bar respectively. The helium gas expands
Image
–
–
into the liquid phase after the JT restriction. Heat exchange with the load is through an evaporator. Pre-cooling is performed at 14.5 K by a GM-cooler. For the Darwin mission this pre-cooling would be performed by a hydrogen sorption cooler. Tests were performed on the cooler and also on component level during the programme including the following: Thermal performance [15]: 4.5 mW at 4.5 K. Input power [15]: 1.96 W. Redundancy [15]: operation with 3 out of 4 cells requires 10% higher input power. 1 h temperature stability [15]: 1 mK (controlled) and 12 mK (uncontrolled). Life performance [18]: no performance degradation for 4 months continuous operation. Valve accelerated life test on component-level [7]: >300,000 cycles (=± 5 years typical). Carbon monolith vibration test on component-level: No pulverization, slight particle loss observed. So far it has not been practical to measure the cold tip vibration levels in a laboratory setup due to the ambient noise levels in the laboratory. Mechanical analysis however predicts total exported p vibrations at a level <1 lN/ Hz [15] in the frequency band 0.1 to 1 Hz.
Please cite this article in press as: Benthem B et al. Present status of developments in physical sorption cooling for space applications. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.02.010
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B. Benthem et al. / Cryogenics xxx (2014) xxx–xxx
In the period 2008–2013 development continued on the hydrogen sorption cooler for the ESA Darwin mission funded by ESA TRP. During the programme a CDR of the cooler was held. In January 2013 the cooler had successfully reached a cold head temperature of 14.5 K with a net cooling power of 18.5 mW. Papers describing these tests are currently in press. This cooler features 3 hydrogen compressor cells in two stages, each with cells 10 mm in diameter and 100 mm long. As before the cells are filled with monolithic carbon pills and in this test-setup are heat-sunk on a GM-cooler, representing a radiator at 90 K. Thermal coupling of the compressor to the heat sink is controlled by an argon gas-gap heat switch. A chemical getter is included to prevent possible contamination of the gas gap with hydrogen gas leaking from the sorber container. Hydrogen flow rates in the cold stage are around 0.14 mg/s with high- and low pressures of 50 Bar and 0.1 Bar respectively. The hydrogen gas expands into the liquid phase after the JT restriction. Heat exchange with the load is through an evaporator. Pre-cooling is performed by a GM-cooler representing a radiator at 50 K. This cooler presented several unique challenges not present in the helium cooler [22]. First of all, the pressures in the system required dedicated check valves for the low- and high-pressure lines, each able to cope with the stringent leak rate requirements. Secondly, the dissociation and subsequent diffusion of hydrogen through the sorption container wall into the gas-gap required the installation of a hydrogen getter to prevent degradation of gasgap heat switch performance over time. Thirdly the cold stage design was revised to prevent hydrogen in the 50 Bar high pressure line from freezing at 14.5 K and clogging the restriction. Currently, besides JT sorption coolers, a closed cycle dilution refrigerator for cooling to 50 mK driven by a physical sorption compressor is also under study by the University of Twente and Cooll in cooperation with the Neel Institute in Grenoble, France. A paper describing these developments is currently in press.
4.2. Miniature JT cold stages A second parallel line of development concentrates on further miniaturization and optimization of the JT cold stages for terrestrial applications and is being pursued primarily by Kryoz Technologies. Besides performance and mass/volume, it emphasizes manufacturability and user friendliness. Space applications of these micro JT cold stages are also studied by Kryoz together with University of Twente and Dutch Space.
A major development relevant to space application has been the realization of a nitrogen sorption compressor in 2013 within the frame of an ESA Industrial Triangle Initiative (ITI) programme. The compressor features 3 cells roughly 10 mm Ø, 100 mm long heat sunk to the ambient air temperature (293 K) by fans. For space these would obviously be replaced by a passive radiator. The mass flow rate goal is roughly 4 mg/s at a high pressure of about 120 Bar and a low pressure of about 37 Bar. Currently a cold stage is being developed in a follow-up ITI programme (see section VI) which is compatible with this nitrogen sorption compressor. This cold stage will provide a cooling power of between 50 mW and 150 mW at roughly 135 K. 5. TRL assessment The ability to make good decisions concerning the inclusion or exclusion of new technologies and novel concepts, and to do so in early stages of development, is essential to success of many space programs. A critical step in making such decisions is the consistent assessment of maturity of various advanced technologies prior to their incorporation in new system development projects. The technology readiness levels have been defined to provide a common metric by means of which knowledge of new technology’s maturity might be communicated among program executives, system developers and technology researchers, and among individuals from different organizations. The technology readiness levels as defined by ESA are shown in Table 3. The TRL assigned to a technology depends on the context of the application, the operational environment and the configuration of the system using the technology, especially for the higher levels. A system based on a certain technology may be ‘‘flight proven’’ (TRL 9) for an earth observation mission, but be incompatible with the environment encountered in a mission to Mercury. Also the technology in question should be made precise: While (chemical) sorption cooling has been ‘‘flight proven’’ on Planck, physical sorption cooling can be considered a different technology and an equally high TRL cannot be justified. 6. Future developments Future developments for space applications will build on the already established complementary lines of vibration-free sorption cooling and further miniaturization of JT cold stages described in the previous sections. On the one hand, these developments aim
Table 3 ESA TRL definition from [27]. Readiness level
Definition
Explanation
TRL 1
Basic principles observed and reported
TRL 2
Technology concept and/or application formulated
TRL 3
Analytical and experimental critical function and/or characteristic proof of concept Component and/or breadboard validation in laboratory environment Component and/or breadboard validation in relevant environment System/subsystem model or prototype demonstration in a relevant environment System prototype demonstration in a space environment Actual system completed and ‘‘flight qualified’’ through test and demonstration Actual system ‘‘flight proven’’ through successful mission operations
Lowest level of technology readiness. Scientific research begins to be translated into applied research and development Once basic principles are observed, practical applications can be invented and R&D started. Applications are speculative and may be unproven Active research and development is initiated, including analytical/laboratory studies to validate predictions regarding the technology Basic technological components are integrated to establish that they will work together
TRL 4 TRL 5 TRL 6 TRL 7 TRL 8 TRL 9
The basic technological components are integrated with reasonably realistic supporting elements so it can be tested in a simulated environment A representative model or prototype system is tested in a relevant environment A prototype system that is near, or at, the planned operational system In an actual system, the technology has been proven to work in its final form and under expected conditions The system incorporating the new technology in its final form has been used under actual mission conditions
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B. Benthem et al. / Cryogenics xxx (2014) xxx–xxx Table 4 TRL assessment of physical sorption cooling systems.
a b
Cooling application
Intended operational environment (example mission)
System configuration
Input powera
Massb
TRL level
65 mW @ 4.5 K
L2 (DARWIN)
4
L2 (DARWIN/EChO)
2.0 W @ 50 K 3.9 W @ 90 K 3.9 W @ 90 K
17.3 kg
620 mW @ 14.5 K 6150 mW @ 30 K 6100 mW @ 100 K 6200 mW@ 169 K
Helium sorption cooler with hydrogen sorption pre-cooler Hydrogen sorption cooler with passive pre-cooler
9.0 kg
4
L2 (EChO)
Neon sorption cooler without pre-coolers
3.2 W @ 55 K
TBC
3
LEO sun synch. (Sentinel 5)
Nitrogen sorption cooler without pre-coolers
25 W @ 260 K
15.0 kg
3
Any ()
Ethylene sorption cooler with thermo-electric precooler
20 W @ 293 K
Not meas.
4
Excluding power consumption of control electronics. These can be estimated at 10 W, dissipated at room temperature. Excluding mass of passive radiators. Mass is established by weighing of the test items, which are not mass-optimized.
at increasing the TRL of the existing sorption cooler configurations described in Table 4. On the other hand, the range of applications is being expanded as well. These are summarized below: An ESA Innovation Triangle Initiative (ITI) programme has started in 2013 to bring the TRL of the nitrogen sorption cooler (in particular the micro cold stage) from 4 to 5. An ESA Core Technology Programme (CTP) contract is about to start with the aim of increasing the TRL of the hydrogen sorption cooler from 4 to 5 in preparation for the EChO mission. An ESA Technology Research Programme (TRP) is being considered to develop a sorption cooler for Earth Observation missions in the range 40–80 K with a cooling power on the order of 1 W. An ongoing study under Dutch national funding is being conducted by Dutch Space which explores the possibility of using a deployable radiator systems based on well-established heritage with rigid deployable structures to provide the required radiator surface area for sorption coolers. In parallel to these space-related developments there is also considerable interest in vibration-free sorption cooling for terrestrial applications. Although these developments represent a separate line of activities, there is considerable potential for spin-off’s and spin-in’s to space. Under national funding, and in parallel to related technology development, a study is running to plan and to outline the design, development and verification approach for sorption-based cooling of the METIS instrument for the European Extremely Large Telescope [28]. This application requires considerably higher cooling powers than envisaged for space applications. First prototyping activities have started late in 2012, consisting of developing a neon sorption cooler which will demonstrate a 1 W cooling power at 40 K. Kryoz develops and commercialises micro cryogenic cooling systems for a variety of markets including material research, life sciences, medical - and telecom systems. The Dutch National Institute for Nuclear Physics and High Energy Physics (NIKHEF) has expressed interest in exploring the possibility of using sorption cooling for 3rd generation terrestrial gravitational wave detectors. These detectors are extremely sensitive to vibrations and require cryogenic cooling to achieve acceptable signal-to-noise ratios. 7. Conclusion Since 2001 an effort has been underway by the University of Twente, Dutch Space and Kryoz Technologies in the field of physical sorption-based Joule–Thomson cooling for space applications.
To date, experimental proofs of concept have been obtained using cooling setups of various levels of complexity with helium, hydrogen, neon, nitrogen, methane and ethylene as working fluids. Cooling powers from between 5 mW to 200 mW have been achieved in a temperature range between 4.5 K and 169 K, validating the technology over a wide range of potential applications. Major developments have been the 4.5 mW 4.5 K helium sorption cooler and the 18.5 mW 14.5 K hydrogen sorption cooler (both of which have achieved TRL 4), and the compressor and cold stage for a nitrogen sorption cooler. Future developments seek to upgrade the TRL of these existing systems to 5 under ESA funding to enable application on actual missions. Simultaneously, efforts are underway (both for space and terrestrial applications) to demonstrate that the technology allows upscaling to cooling powers from several mW to the order of Watts. Acknowledgments The work described in this paper was supported by the European Space Agency (ESA), the Dutch Technology Foundation (STW) and the Netherlands Space Office (NSO). References [1] Sherman A. National aeronautics and space administration needs and trends in cryogenic cooling. Cryogenics 1983;23(7):348–52. [2] Kraus Allan D, Bar-Cohen Avram. Thermal analysis and control of electronic equipment, vol. 633. Washington, DC: Hemisphere Publishing Corp.; 1983. p. 1. [3] Donabedian Martin, Gilmore David G, editors. Spacecraft Thermal Control Handbook: Cryogenics, vol. 2. Aiaa; 2003. [4] Linder M et al. Cryogenics in space. ESA Bull 2001;107. [5] Doornink DJ, Burger JF, ter Brake HJM. Sorption cooling: a valid extension to passive cooling. Cryogenics 2008;48(5):274–9. [6] Wiegerinck GFM et al. A sorption compressor with a single sorber bed for use with a Linde-Hampson cold stage. Cryogenics 2006;46(1):9–20. [7] Veenstra TT et al. Development of a stainless steel check valve for cryogenic applications. Cryogenics 2007;47(2):121–6. [8] Wu Y, Zalewski DR, ter Brake Marcel, Optimization of the working fluid for a sorption-based Joule–Thomson cooler. AIP Conference Proceedings, vol. 1434; 2012. [9] Ross Jr Ronald G, Boyle RF. An overview of NASA space cryocooler programs— 2006; 2006. [10] Swift WL, Dolan FX, Zagarola MV. The NICMOS cooling system—5 years of successful on-orbit operation. AIP Conference Proceedings, vol. 985; 2008. [11] Gibson AS et al. Heritage overview: 20 years of commercial production of cryocoolers for space. AIP Conference Proceedings, vol. 985; 2008. [12] Ross RG et al. AIRS PFM pulse tube cooler system-level performance. Cryocoolers 2002;10:119–28. [13] Mennella Aniello et al. Planck early results. II. The thermal performance of Planck. Astron Astrophys 2011;536. A2–1. [14] Bhandari P et al. Sorption coolers using a continuous cycle to produce 20 K for the Planck flight mission. Cryogenics 2004;44(6):395–401. [15] Burger JF et al. Long-life vibration-free 4.5 K sorption cooler for space applications. Rev Sci Instrum 2007;78:065102.
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[16] Jedrich Nick et al. Cryo cooler induced micro-vibration disturbances to the hubble space telescope. In: 5th Cranfield Space Dynamics Conference; 2002. [17] Radebaugh Ray. Cryocoolers: the state of the art and recent developments. J Phys: Condens Matter 2009;21(16):164219. [18] Burger JF et al. Further developments on a vibration-free helium–hydrogen sorption cooler. Cryocoolers 2009;15:23–30. [19] DiPirro M, Johnson DL, Shirron P. Cryogenic technology for CMBPol. J Phys: Conf Ser 2009;155(1). IOP Publishing. [20] Banks Kimberly et al. James Webb Space Telescope Mid-Infrared Instrument cooler systems engineering. Astronomical telescopes and instrumentation: synergies between ground and space. International Society for Optics and Photonics; 2008. [21] Lozano-Castelló D et al. Characteristics of an activated carbon monolith for a helium adsorption compressor. Carbon 2010;48(1):123–31. [22] Burger JF et al. Development of a 15 K hydrogen-based sorption cooler. AIP Conference Proceedings, vol. 1218; 2010.
[23] Burger JF et al. Vibration-free 5 K sorption cooler for ESA’s Darwin mission. Cryogenics 2002;42(2):97–108. [24] Burger JF et al. Construction and operation of a 165 K microcooler with a sorption compressor and a micromachined cold stage. Cryocoolers 2003;12:643–9. [25] Wiegerinck GFM. Improving sorption compressors for cryogenic cooling. University of Twente; 2005. [26] Derking JH et al. Micromachined Joule–Thomson cold stages operating in the temperature range 80–250 K. Int J Refrigeration 2012;35(4):1200–7. [27] Technology Readiness Levels Handbook For Space Applications, TEC-SHS/ 5551/MG/ap issue 1 revision 6. [28] Ter Brake HJM et al. Sorption-based vibration-free cooler for the METIS instrument on E-ELT. SPIE Astronomical Telescopes+ Instrumentation. International Society for Optics and Photonics; 2012. [29] Pilbratt GL et al. Herschel space observatory: an ESA facility for far-infrared and submillimetre astronomy. Astron Astrophys 2010;518(1).
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Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Present status of developments in physical sorption cooling for space applications B. Benthem a,⇑, J. Doornink a, E. Boom a, H.J. Holland b, P.P.P.M. Lerou c, J.F. Burger d, H.J.M. ter Brake b a
Dutch Space B.V., P.O. Box 32070, 2303 DB Leiden, The Netherlands University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands c Kryoz Technologies B.V., Pantheon 18-22, 7521 PR Enschede, The Netherlands d Cooll Sustainable Energy Solutions B.V., Hengelosestraat 298A, 7521 AM Enschede, The Netherlands b
a r t i c l e
i n f o
Article history: Available online xxxx
a b s t r a c t 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. 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. This paper starts with a recapitulation of the principles of physical sorption cooling followed by an overview of the strengths and weaknesses of the technology in relation to other space cooling technologies, such as pulse-tube cooling and Stirling cooling. Next, the present status of physical sorption cooling technology is presented based on developments previously and currently being performed by the University of Twente, Dutch Space and Kryoz Technologies. A summary will be given of the various existing demonstrator- and lab-models which have been built, along with an overview of the tests which have so far been performed. The central result of this paper is an assessment of the current Technology Readiness Level (TRL) of various sorption cooler configurations, along with their application range in terms of temperatures, heat loads and mission profile. Finally, an outline is given on the way forward currently being pursued by the developers to achieve full maturity of the technology. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Since the 1960s spacecraft have been flown with components that require cooling at temperature ranges anywhere from subKelvin to slightly above room temperature. Cooling can be required for various reasons, including: 1. Improvement of signal-to-noise ratio in detectors, especially in infrared (IR) instruments. The elements of the instruments that may require cooling are radiation detectors, optical components, baffles, or in some cases, the whole instrument. Cryogenic cooling is necessary to provide the required detector response, reduce pre-amplifier noise, and/or reduce background radiation [1].
⇑ Corresponding author. Address: P.O. Box 32070, NL-2303 DB Leiden, The Netherlands. Tel.: +31 (0)71 524 5212. E-mail address: [email protected] (B. Benthem).
2. Removal of waste heat and maintain components within operational limits, e.g. for dissipating electronic equipment or momentum wheels [2]. 3. Reduction of boil-off during long-term on-orbit storage of cryogenic fluids, e.g. for propulsion systems [3]. Various methods exist for providing this cooling, including passive cooling using radiators, usage of stored cryogens and closedcycle active cooling [1,4]. Passive cooling is particularly attractive in many of the above applications because it takes advantage of the cold space environment to reject heat via radiators without using consumables or power, and without exporting vibrations or producing electromagnetic interference, while simultaneously providing high cooling reliability and long-term performance [5]. However, at temperatures below typically 40–60 K passive coolers run into fundamental limitations that prevent their practical accommodation on a spacecraft, although there are exceptions
http://dx.doi.org/10.1016/j.cryogenics.2014.02.010 0011-2275/Ó 2014 Elsevier Ltd. All rights reserved.
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(e.g. Spritzer, where a radiator temperature of 34 K was attained, and in JWST, where the radiators will achieve 38 K). This temperature range is typically associated with high-performance IR instruments. In these cases stored cryogens or closed-cycle active cooling must be used. A recent European example of the former is the use of liquid Helium to cool the Herschel science instruments. As stored cryogens are boiled off to space this system has a limited lifetime. In addition, notable failures have occurred where the cryogen is prematurely expelled, resulting in short mission lifetimes and lower science yield [19]. Closed-cycle active coolers do not suffer from depletion of the cryogen. However, these systems can consume a considerable amount of power, export vibrations and/or introduce electromagnetic interference. In addition, the presence of moving mechanical parts can imply wear-and-tear producing failures, which must be absorbed by including a sufficient level of redundancy and/or reliability in the design. A number of coolers also impose volume and/ or geometric constraints on the spacecraft, for example pulse-tube coolers, which require a relatively short distance between cold tip and compressor. Physical sorption cooling has been developed for over a decade by the University of Twente, supported by Dutch Space for spacebased applications, and more recently also by Kryoz and Cooll. This has realized a form of closed-cycle active cooling which retains the advantages of passive cooling (low power use, low exported vibrations, low electromagnetic interference, high reliability), whilst extending the achievable temperatures down to the sub-Kelvin range.
2. Physical sorption cooling Several papers exist describing the fundamental principles of physical sorption cooling, e.g. [5,6]. The following description is taken from [5]: Sorption cooling employs a closed cycle Joule–Thomson expansion process to achieve the cooling effect (Fig. 1). In most Joule– Thomson coolers a mechanical compressor delivers the required gas flow, whereas in sorption cooling a sorption compressor is applied. A sorption compressor is a thermally driven compressor that operates with a sorber material. In case of physical sorption the sorber material is activated carbon. Due to its highly porous structure, activated carbon has a very large internal surface, which is able to adsorb large quantities of gas. By heating the sorber material, the gas is desorbed and a high pressure can be established. This high-pressure gas is expanded through a restriction, in many cases resulting in a phase transition of the gas to the liquid state. Cooling is achieved by evaporating this liquid at the evaporator or, in case of no phase transition, by heat exchange with the cold low-pressure gas. After cooling the sorber material via contact with a heat sink, the low-pressure gas is re-adsorbed and the cycle described above is started over. The nature of physical sorption implies that the process is fully reversible, i.e. no degradation occurs and the cycle can in principle be repeated infinitely. Upon this basic principle several refinements have been introduced which enable practical applications, all of which have required dedicated developments. Among these are the following: Check valves to direct the flow of gas. Stringent back-flow requirements at cryogenic temperatures have required a dedicated design effort [7]. The use of buffers to stabilize the gas flow. In particular the lowpressure buffer has been specially designed to hold a sufficient quantity of gas at low pressure, whilst minimizing volume. Use of a gas-gap heat switch to reduce heat losses to the heat sink during the heating phase of the cycle.
A wide range of temperature levels can be achieved at the evaporator depending on the choice of working fluid and low-pressure level selected (Table 1). For practical reasons, a low pressure level between 0.2 Bar and 10 Bar is typically assumed. For a given gas, the range of achievable temperatures is then determined by the gas triple point or the vapor temperature at 0.2 Bar, whichever is higher (minimum) and the vapor temperature at 10 Bar (maximum).
3. Comparison to other cooling technologies Several papers present overviews of current space missions employing cryogenic cooling in their designs [19,9,4]. Based on these sources several types of closed-cycle active cooling systems can be distinguished which provide cooling in the same range applications. Indicated between brackets are examples of representative missions using the respective type of cooling:
Reverse turbo-Brayton (e.g. NICMOS/HST; 7 W @ 77 K [10]). Stirling (e.g. SPI/INTEGRAL; 2.1 W @ 85 K [11]). Pulse-tube (e.g. AIRS/Aqua; 1.3 W @ 55 K [12]). Joule–Thomson (e.g. Planck; 19 mW @ 4.5 K [13]). Joule–Thomson with chemical sorption compressor (e.g. Planck; 1 W @ 19 K [14]).
Sub-Kelvin cooling systems like Adiabatic Demagnetization Refrigeration (ADR) or dilution refrigerators are excluded, as are thermo-electric coolers, stored cryogens and passive cooling since these technologies have unique operating parameters, which are not easily compared to the cooler types above. It is interesting to compare the above 5 types of closed-cycle active cooling systems to physical sorption cooling in various critical areas.
3.1. Exported vibrations With the advent of low-vibration attitude and orbit control (AOCS) technologies such as magnetic bearing reaction wheels and microthrusters, active cryocoolers could be expected to become the dominant source of micro-vibrations in spacecraft. One of the key advantages of sorption cooling in general over many other active cooling technologies is the absence of large moving parts, not only in the cold stage, such as in pulse-tube coolers, but also in the entire cooling system. Although theoretically some vibration mechanisms do exist, such as the low-frequency movement of the check-valves, the cyclic thermal expansion and contraction of the compressor cells and fluctuations in the flow of gas through the system, the resulting vibration level can simply not be measured in a laboratory setup due to the ambient noise levels in the laboratory. Mechanical analysis predicts the total exported vibrations to p be below 1 lN/ Hz in the frequency band 0.1–1 Hz [15]. The reverse turbo-Brayton cooler for NICMOS was deployed in 2002 on the Hubble Space Telescope as the only active cooling system existing at the time which could meet the exported vibration requirement of 1 milli-arc-second (mas) rms contribution to the Hubble pointing jitter. To meet this requirement, the low-mass turbine shafts are supported on gas bearings and operated at high rotational speeds exceeding 1200 rps, resulting in a low level of (low-) frequency vibrations [16]. However, this requires the turbines to be precision-engineered and finely balanced adding to system complexity and cost. It should also be noted that currently no space qualified reverse turbo-Brayton coolers are known to exist for cooling to temperatures below 65 K. Pulse-tube coolers are preferred over Stirling coolers in terms of exported vibrations due to the fact that no moving parts are re-
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Fig. 1. Schematic representation of a Joule–Thomson cooler with a sorption compressor. Taken from [5].
quired in the cold stage. However the gas compressors still generate vibrations, as does the pressure wave inside the cold stage. Joule–Thomson (JT) sorption coolers can be expected to provide the ultimate performance in terms of exported vibrations when compared to the currently existing closed-cycle active cooling systems.
An advantage of the physical sorption technology developed by the University of Twente is the fully reversible nature of the sorption process. Unlike chemical sorption using metal hydrides as sorber material, no degradation of performance occurs. On the Planck spacecraft for example, the metal-hydride sorption cooler was designed for a mission life of 2 years [14]. For physical sorption possible remaining long-term degradation mechanisms such as restriction/filter clogging, degradation of the activated carbon and degradation of the gas gap heat switch have not been fully characterized by test yet, although continuous operation of a Helium sorption cooler for a period of 4 months did not show any degradation of performance [18]. It is expected that with proper design measures the lifetime of a physical sorption cooler can easily exceed 15 years of continuous service. The only active components in a sorption cooler are heaters, which are well-characterized in terms of reliability. Redundancy can easily be included by applying heaters in pairs for cold redundancy. Furthermore, at the compressor level the modular nature of the sorption cooler allows the use of additional compressor cells for hot redundancy. The only moving components of a sorption cooler are the (passive) check valves. These have so far been life tested by the University of Twente up to 300,000 cycles without failures or degradation, representing a typical on-orbit lifetime of around 5 years.
3.2. Reliability/lifetime
3.3. Scalability of cooling power
The reliability and lifetime have long been factors limiting the use of active cooling systems in space [17]. In recent years however, lifetimes up to 10 years have been demonstrated in-orbit for various cooler types [17,4].
The cooling power delivered at the cold tip is roughly linearly dependent on the mass-flow of gas through the JT restriction. The compressor stage of a sorption cooler is highly modular. Besides contributing positively to the reliability of the system by pro-
Table 1 Overview of theoretical achievable Joule–Thomson cooling working temperatures. Data taken from [5] and [8]. A solid fill indicates development models using these gases that have been realized by the authors.
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ducing redundancy as described above, this modular nature also allows linear scaling of the mass-flow (i.e. cooling power) simply by adding or removing compressor cells in parallel. However the JT cold stage scales in a slightly less obvious way. In general, JT cold stages scale favorably in terms of cooling power at the cold tip compared to other designs with moving components (like reverse turbo-Brayton and Stirling) [17]. The main considerations in the scaling of a sorption cooler JT cold stage are the pressure losses in the piping, the dimensioning of the heat exchangers and flow restriction in relation to the mass flows, and to lesser extent the magnitude of parasitic heat leaks to the environment. Large commercial earth-based helium liquefiers with a JT cold stage deliver cooling powers up to 1 kW at 4.5 K, whereas the lowest-power micro JT stages developed by Kryoz and University of Twente deliver cooling powers in the order of milliwatts, showing scalability over six orders of magnitude. 3.4. Accommodation on spacecraft The accommodation of a cooler on a spacecraft is in many cases not straightforward. Passive cryogenic coolers typically require complete shading from all external fluxes (sun, planetary) requiring either elaborate baffle structures or strict constraints on spacecraft orientations. Also, thermal links to the cooling targets require special measures. For stored cryogens the accommodation impact is even more significant: in many cases the spacecraft is simply designed around the cryostat as was the case for the Herschel PLM. The design was dominated by the 2400 L liquid helium cryostat vacuum vessel [29]. Less obvious are the accommodation constraints imposed by active coolers. Typically mechanical compressors operate at or around room temperature. The cooler structure therefore necessarily experiences a temperature gradient between room temperature and the temperature of the cold tip. Besides requiring dedicated design measures on the level of the cooler to reduce parasitic heat flows, this also means that the cooler structure has to ‘‘cross’’ one or several temperature zones on the spacecraft. For example in the case of the Planck 4 K and 20 K JT coolers the cold stages crossed all layers of the V-groove radiator [13], thereby complicating the final spacecraft assembly & integration. The same is valid for the JT cooler for the MIRI instrument on board the JWST [20]. On the other hand the process of physical sorption is generally more efficient at lower temperatures. The sorption compressors developed by the University of Twente for DARWIN for example are heat sunk on 50 K and 100 K temperature levels achieved on the cold side of a V-groove radiator [5]. This configuration would allow for a less complex assembly & integration at the spacecraft level. The drawback is that the heat rejection capability of radiators at these temperatures is limited for fundamental reasons. Therefore the required radiator sizes are relatively large – up to 6 m2 for the Helium stage [5]. It should be noted however that the required radiator size is strongly dependent on the working fluid and required temperature levels. A helium stage cooling at 4 K represents a ‘‘worst-case’’ configuration in terms of required radiator surface area, due to the relatively poor adsorption on activated carbon of helium compared to other working gases [21]. The feasibility of a cooling chain with sorption cells mounted on a deployable radiator (which effectively increases surface area by radiating on both sides while simultaneously allowing a large surface area to be accommodated on the spacecraft) is currently being investigated by Dutch Space B.V. An advantage of recuperative cooling in general (JT, reverse turbo-Brayton), in terms of accommodation on the spacecraft, is the flexibility offered in terms of cold stage layout. The only fundamental effects limiting the dimensions of the cold stage are pressure losses and parasitic heat loads on the gas lines. Provided
sufficient hydraulic diameter and thermal insulation of the lines is available, there is considerable flexibility in the separation of the compressors and the cold tip. For example, the JWST JT cold tip is physically separated from the compressors by a distance of over 10 m [20], while the micro JT cold stages developed by Kryoz and the University of Twente are around 5 cm long. This flexibility can in many cases obviate the need for thermal straps, heat pipes and other heat transport devices. 3.5. Power usage JT coolers are known to have a lower efficiency (especially at cooling temperatures >20 K) compared to other cooling cycles, due to the fundamentally irreversible nature of the JT process [4]. The efficiency of the sorption coolers manufactured by the University of Twente varies from 2% of Carnot for a 4.5 K Helium cooler including power consumption of the pre-cooling stage [15] to around 4% of Carnot for a 14.5 K Hydrogen cooler [22]. The 40 K Neon cooler currently under development for a ground-based application is predicted to achieve an efficiency of 1.3% of Carnot. Despite this, the electrical power required by a physical sorption cooler can be very small compared to other active coolers, due to the fact that heat rejection takes place at low temperatures as described above. For example the 4.5 mW, 4.5 K Helium cooler consumes less than 2 W of electrical input power at the level of the cooler. This power is rejected at 50 K by a large passive radiator. Calculations indicate a required radiator surface area in the order of 6 m2, which may be available e.g. on a V-groove radiator as described in the previous section. An additional 10 W of electrical power is estimated to be required by the flight electronics (rejected by a radiator at room temperature) resulting in a total power consumption of 12 W. It is interesting to compare this figure to the similar 4 K JT cooler with mechanical compressors used on Planck, which reject their heat at room temperature (300 K). This cooler provided 19 mW at 4.5 K with an input power of 120 W. Scaled to match the 19 mW heat lift requirement, the DARWIN Helium cooler can be expected to consume roughly 20 W of electrical power, a factor 6 less than the Planck design. The penalty for this low electrical power usage is the larger required radiator surface area. 3.6. Electromagnetic interference The only potential sources of electromagnetic interference (EMI) in sorption coolers, besides the drive electronics, are the heaters used in the compressor cells and the gas gap actuators. Of these, only the compressor cell heaters require a significant current. These heaters are typically of the cartridge type, where the resistance wires are suspended axially in a steel tube (‘‘cartridge’’), filled with a mineral powder. The heater’s steel casing and the compressor cell itself acts as electric shielding, while the close proximity of the two wires minimizes the exported magnetic field. The heaters are fed with a direct current. In principle the heaters switch fully on or fully off at the cycling frequency of the sorption cell, which is well below the radio frequency (RF) bands thereby limiting any interference. If proportional heater control is desired (for example to stabilize the high-pressure level or to accommodate two heaters in hot redundancy), pulse-width modulation can be used. In this case twisted pair leads can be applied to reduce EMI. 4. Present status of sorption cooling Table 2 presents an overview of the relevant development activities performed by the University of Twente and Dutch Space, and more recently Kryoz and Cooll, since the realization of the first prototype sorption cooler in 2001. Since 2001, development has progressed broadly along two parallel and complementary lines.
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B. Benthem et al. / Cryogenics xxx (2014) xxx–xxx Table 2 Overview of physical sorption/micro JT developments by University of Twente, Kryoz Technologies and Dutch Space. Gas/model
Performance
Compressor
Pre-cooling
Year
Reference
Ethylene sorption cooler
200 mW @ 169 K
Physical sorption at 293 K
Thermoelectric
2001
[24]
Neon single stage micro JT cold stage
43 mW @ 30 K
Gas bottle
2005
[25]
Helium sorption cooler
4.5 mW @ 4.5 K
Physical sorption at 50 K
Liquid nitrogen GM-cooler
2007
[15]
Nitrogen single stage micro JT cold stage
150 mW @ 90 K
Gas bottle
–
2007
Kryoz datasheet
Methane single stage micro JT cold stage
Gas bottle
–
2012
[26]
Hydrogen sorption cooler
8–150 mW @ 140 K 18.5 mW @ 14.5 K
Physical sorption at 90 K
GM-cooler
2013
In press
Nitrogen sorption compressor
–
Physical sorption at 293 K
–
2013
–
Neon Nitrogen two-stage micro JT cold stage
20 mW @ 30 K
Gas bottle
–
2013
Kryoz datasheet
4.1. Vibration-free sorption cooling The first line is being pursued primarily by University of Twente, Cooll and Dutch Space and is aimed at vibration-free sorption cooling for space and terrestrial cooling applications. The emphasis is on achieving the technology readiness level which is appropriate for space applications. This includes in particular demonstrations of performance, reliability and compatibility with a launch environment, while respecting established space product assurance requirements. A major focus of the activities during the period 2003–2007 was development, manufacturing and testing of a 4 K 5 mW Helium sorption cooler for the ESA Darwin mission, funded by ESA Technological Research Programme (TRP). During this programme a Critical Design Review (CDR) of the cooler was held and several papers were published [7,15,18,23], presenting the analysis and test results. The cooler features 4 helium compressor cells in 2 stages; each cell is 15 mm in diameter and 157 mm long. The cells are filled with monolithic carbon pills and in this test-setup are heat-sunk on a Gifford McMahon (GM) cooler, representing a radiator at 50 K. Thermal coupling of the compressor to the heat sink is controlled by a hydrogen gas-gap heat switch. Helium flow rates in the cold stage are around 0.41 mg/s with high- and low pressures of 16.5 Bar and 1.3 Bar respectively. The helium gas expands
Image
–
–
into the liquid phase after the JT restriction. Heat exchange with the load is through an evaporator. Pre-cooling is performed at 14.5 K by a GM-cooler. For the Darwin mission this pre-cooling would be performed by a hydrogen sorption cooler. Tests were performed on the cooler and also on component level during the programme including the following: Thermal performance [15]: 4.5 mW at 4.5 K. Input power [15]: 1.96 W. Redundancy [15]: operation with 3 out of 4 cells requires 10% higher input power. 1 h temperature stability [15]: 1 mK (controlled) and 12 mK (uncontrolled). Life performance [18]: no performance degradation for 4 months continuous operation. Valve accelerated life test on component-level [7]: >300,000 cycles (=± 5 years typical). Carbon monolith vibration test on component-level: No pulverization, slight particle loss observed. So far it has not been practical to measure the cold tip vibration levels in a laboratory setup due to the ambient noise levels in the laboratory. Mechanical analysis however predicts total exported p vibrations at a level <1 lN/ Hz [15] in the frequency band 0.1 to 1 Hz.
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In the period 2008–2013 development continued on the hydrogen sorption cooler for the ESA Darwin mission funded by ESA TRP. During the programme a CDR of the cooler was held. In January 2013 the cooler had successfully reached a cold head temperature of 14.5 K with a net cooling power of 18.5 mW. Papers describing these tests are currently in press. This cooler features 3 hydrogen compressor cells in two stages, each with cells 10 mm in diameter and 100 mm long. As before the cells are filled with monolithic carbon pills and in this test-setup are heat-sunk on a GM-cooler, representing a radiator at 90 K. Thermal coupling of the compressor to the heat sink is controlled by an argon gas-gap heat switch. A chemical getter is included to prevent possible contamination of the gas gap with hydrogen gas leaking from the sorber container. Hydrogen flow rates in the cold stage are around 0.14 mg/s with high- and low pressures of 50 Bar and 0.1 Bar respectively. The hydrogen gas expands into the liquid phase after the JT restriction. Heat exchange with the load is through an evaporator. Pre-cooling is performed by a GM-cooler representing a radiator at 50 K. This cooler presented several unique challenges not present in the helium cooler [22]. First of all, the pressures in the system required dedicated check valves for the low- and high-pressure lines, each able to cope with the stringent leak rate requirements. Secondly, the dissociation and subsequent diffusion of hydrogen through the sorption container wall into the gas-gap required the installation of a hydrogen getter to prevent degradation of gasgap heat switch performance over time. Thirdly the cold stage design was revised to prevent hydrogen in the 50 Bar high pressure line from freezing at 14.5 K and clogging the restriction. Currently, besides JT sorption coolers, a closed cycle dilution refrigerator for cooling to 50 mK driven by a physical sorption compressor is also under study by the University of Twente and Cooll in cooperation with the Neel Institute in Grenoble, France. A paper describing these developments is currently in press.
4.2. Miniature JT cold stages A second parallel line of development concentrates on further miniaturization and optimization of the JT cold stages for terrestrial applications and is being pursued primarily by Kryoz Technologies. Besides performance and mass/volume, it emphasizes manufacturability and user friendliness. Space applications of these micro JT cold stages are also studied by Kryoz together with University of Twente and Dutch Space.
A major development relevant to space application has been the realization of a nitrogen sorption compressor in 2013 within the frame of an ESA Industrial Triangle Initiative (ITI) programme. The compressor features 3 cells roughly 10 mm Ø, 100 mm long heat sunk to the ambient air temperature (293 K) by fans. For space these would obviously be replaced by a passive radiator. The mass flow rate goal is roughly 4 mg/s at a high pressure of about 120 Bar and a low pressure of about 37 Bar. Currently a cold stage is being developed in a follow-up ITI programme (see section VI) which is compatible with this nitrogen sorption compressor. This cold stage will provide a cooling power of between 50 mW and 150 mW at roughly 135 K. 5. TRL assessment The ability to make good decisions concerning the inclusion or exclusion of new technologies and novel concepts, and to do so in early stages of development, is essential to success of many space programs. A critical step in making such decisions is the consistent assessment of maturity of various advanced technologies prior to their incorporation in new system development projects. The technology readiness levels have been defined to provide a common metric by means of which knowledge of new technology’s maturity might be communicated among program executives, system developers and technology researchers, and among individuals from different organizations. The technology readiness levels as defined by ESA are shown in Table 3. The TRL assigned to a technology depends on the context of the application, the operational environment and the configuration of the system using the technology, especially for the higher levels. A system based on a certain technology may be ‘‘flight proven’’ (TRL 9) for an earth observation mission, but be incompatible with the environment encountered in a mission to Mercury. Also the technology in question should be made precise: While (chemical) sorption cooling has been ‘‘flight proven’’ on Planck, physical sorption cooling can be considered a different technology and an equally high TRL cannot be justified. 6. Future developments Future developments for space applications will build on the already established complementary lines of vibration-free sorption cooling and further miniaturization of JT cold stages described in the previous sections. On the one hand, these developments aim
Table 3 ESA TRL definition from [27]. Readiness level
Definition
Explanation
TRL 1
Basic principles observed and reported
TRL 2
Technology concept and/or application formulated
TRL 3
Analytical and experimental critical function and/or characteristic proof of concept Component and/or breadboard validation in laboratory environment Component and/or breadboard validation in relevant environment System/subsystem model or prototype demonstration in a relevant environment System prototype demonstration in a space environment Actual system completed and ‘‘flight qualified’’ through test and demonstration Actual system ‘‘flight proven’’ through successful mission operations
Lowest level of technology readiness. Scientific research begins to be translated into applied research and development Once basic principles are observed, practical applications can be invented and R&D started. Applications are speculative and may be unproven Active research and development is initiated, including analytical/laboratory studies to validate predictions regarding the technology Basic technological components are integrated to establish that they will work together
TRL 4 TRL 5 TRL 6 TRL 7 TRL 8 TRL 9
The basic technological components are integrated with reasonably realistic supporting elements so it can be tested in a simulated environment A representative model or prototype system is tested in a relevant environment A prototype system that is near, or at, the planned operational system In an actual system, the technology has been proven to work in its final form and under expected conditions The system incorporating the new technology in its final form has been used under actual mission conditions
Please cite this article in press as: Benthem B et al. Present status of developments in physical sorption cooling for space applications. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.02.010
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B. Benthem et al. / Cryogenics xxx (2014) xxx–xxx Table 4 TRL assessment of physical sorption cooling systems.
a b
Cooling application
Intended operational environment (example mission)
System configuration
Input powera
Massb
TRL level
65 mW @ 4.5 K
L2 (DARWIN)
4
L2 (DARWIN/EChO)
2.0 W @ 50 K 3.9 W @ 90 K 3.9 W @ 90 K
17.3 kg
620 mW @ 14.5 K 6150 mW @ 30 K 6100 mW @ 100 K 6200 mW@ 169 K
Helium sorption cooler with hydrogen sorption pre-cooler Hydrogen sorption cooler with passive pre-cooler
9.0 kg
4
L2 (EChO)
Neon sorption cooler without pre-coolers
3.2 W @ 55 K
TBC
3
LEO sun synch. (Sentinel 5)
Nitrogen sorption cooler without pre-coolers
25 W @ 260 K
15.0 kg
3
Any ()
Ethylene sorption cooler with thermo-electric precooler
20 W @ 293 K
Not meas.
4
Excluding power consumption of control electronics. These can be estimated at 10 W, dissipated at room temperature. Excluding mass of passive radiators. Mass is established by weighing of the test items, which are not mass-optimized.
at increasing the TRL of the existing sorption cooler configurations described in Table 4. On the other hand, the range of applications is being expanded as well. These are summarized below: An ESA Innovation Triangle Initiative (ITI) programme has started in 2013 to bring the TRL of the nitrogen sorption cooler (in particular the micro cold stage) from 4 to 5. An ESA Core Technology Programme (CTP) contract is about to start with the aim of increasing the TRL of the hydrogen sorption cooler from 4 to 5 in preparation for the EChO mission. An ESA Technology Research Programme (TRP) is being considered to develop a sorption cooler for Earth Observation missions in the range 40–80 K with a cooling power on the order of 1 W. An ongoing study under Dutch national funding is being conducted by Dutch Space which explores the possibility of using a deployable radiator systems based on well-established heritage with rigid deployable structures to provide the required radiator surface area for sorption coolers. In parallel to these space-related developments there is also considerable interest in vibration-free sorption cooling for terrestrial applications. Although these developments represent a separate line of activities, there is considerable potential for spin-off’s and spin-in’s to space. Under national funding, and in parallel to related technology development, a study is running to plan and to outline the design, development and verification approach for sorption-based cooling of the METIS instrument for the European Extremely Large Telescope [28]. This application requires considerably higher cooling powers than envisaged for space applications. First prototyping activities have started late in 2012, consisting of developing a neon sorption cooler which will demonstrate a 1 W cooling power at 40 K. Kryoz develops and commercialises micro cryogenic cooling systems for a variety of markets including material research, life sciences, medical - and telecom systems. The Dutch National Institute for Nuclear Physics and High Energy Physics (NIKHEF) has expressed interest in exploring the possibility of using sorption cooling for 3rd generation terrestrial gravitational wave detectors. These detectors are extremely sensitive to vibrations and require cryogenic cooling to achieve acceptable signal-to-noise ratios. 7. Conclusion Since 2001 an effort has been underway by the University of Twente, Dutch Space and Kryoz Technologies in the field of physical sorption-based Joule–Thomson cooling for space applications.
To date, experimental proofs of concept have been obtained using cooling setups of various levels of complexity with helium, hydrogen, neon, nitrogen, methane and ethylene as working fluids. Cooling powers from between 5 mW to 200 mW have been achieved in a temperature range between 4.5 K and 169 K, validating the technology over a wide range of potential applications. Major developments have been the 4.5 mW 4.5 K helium sorption cooler and the 18.5 mW 14.5 K hydrogen sorption cooler (both of which have achieved TRL 4), and the compressor and cold stage for a nitrogen sorption cooler. Future developments seek to upgrade the TRL of these existing systems to 5 under ESA funding to enable application on actual missions. Simultaneously, efforts are underway (both for space and terrestrial applications) to demonstrate that the technology allows upscaling to cooling powers from several mW to the order of Watts. Acknowledgments The work described in this paper was supported by the European Space Agency (ESA), the Dutch Technology Foundation (STW) and the Netherlands Space Office (NSO). References [1] Sherman A. National aeronautics and space administration needs and trends in cryogenic cooling. Cryogenics 1983;23(7):348–52. [2] Kraus Allan D, Bar-Cohen Avram. Thermal analysis and control of electronic equipment, vol. 633. Washington, DC: Hemisphere Publishing Corp.; 1983. p. 1. [3] Donabedian Martin, Gilmore David G, editors. Spacecraft Thermal Control Handbook: Cryogenics, vol. 2. Aiaa; 2003. [4] Linder M et al. Cryogenics in space. ESA Bull 2001;107. [5] Doornink DJ, Burger JF, ter Brake HJM. Sorption cooling: a valid extension to passive cooling. Cryogenics 2008;48(5):274–9. [6] Wiegerinck GFM et al. A sorption compressor with a single sorber bed for use with a Linde-Hampson cold stage. Cryogenics 2006;46(1):9–20. [7] Veenstra TT et al. Development of a stainless steel check valve for cryogenic applications. Cryogenics 2007;47(2):121–6. [8] Wu Y, Zalewski DR, ter Brake Marcel, Optimization of the working fluid for a sorption-based Joule–Thomson cooler. AIP Conference Proceedings, vol. 1434; 2012. [9] Ross Jr Ronald G, Boyle RF. An overview of NASA space cryocooler programs— 2006; 2006. [10] Swift WL, Dolan FX, Zagarola MV. The NICMOS cooling system—5 years of successful on-orbit operation. AIP Conference Proceedings, vol. 985; 2008. [11] Gibson AS et al. Heritage overview: 20 years of commercial production of cryocoolers for space. AIP Conference Proceedings, vol. 985; 2008. [12] Ross RG et al. AIRS PFM pulse tube cooler system-level performance. Cryocoolers 2002;10:119–28. [13] Mennella Aniello et al. Planck early results. II. The thermal performance of Planck. Astron Astrophys 2011;536. A2–1. [14] Bhandari P et al. Sorption coolers using a continuous cycle to produce 20 K for the Planck flight mission. Cryogenics 2004;44(6):395–401. [15] Burger JF et al. Long-life vibration-free 4.5 K sorption cooler for space applications. Rev Sci Instrum 2007;78:065102.
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B. Benthem et al. / Cryogenics xxx (2014) xxx–xxx
[16] Jedrich Nick et al. Cryo cooler induced micro-vibration disturbances to the hubble space telescope. In: 5th Cranfield Space Dynamics Conference; 2002. [17] Radebaugh Ray. Cryocoolers: the state of the art and recent developments. J Phys: Condens Matter 2009;21(16):164219. [18] Burger JF et al. Further developments on a vibration-free helium–hydrogen sorption cooler. Cryocoolers 2009;15:23–30. [19] DiPirro M, Johnson DL, Shirron P. Cryogenic technology for CMBPol. J Phys: Conf Ser 2009;155(1). IOP Publishing. [20] Banks Kimberly et al. James Webb Space Telescope Mid-Infrared Instrument cooler systems engineering. Astronomical telescopes and instrumentation: synergies between ground and space. International Society for Optics and Photonics; 2008. [21] Lozano-Castelló D et al. Characteristics of an activated carbon monolith for a helium adsorption compressor. Carbon 2010;48(1):123–31. [22] Burger JF et al. Development of a 15 K hydrogen-based sorption cooler. AIP Conference Proceedings, vol. 1218; 2010.
[23] Burger JF et al. Vibration-free 5 K sorption cooler for ESA’s Darwin mission. Cryogenics 2002;42(2):97–108. [24] Burger JF et al. Construction and operation of a 165 K microcooler with a sorption compressor and a micromachined cold stage. Cryocoolers 2003;12:643–9. [25] Wiegerinck GFM. Improving sorption compressors for cryogenic cooling. University of Twente; 2005. [26] Derking JH et al. Micromachined Joule–Thomson cold stages operating in the temperature range 80–250 K. Int J Refrigeration 2012;35(4):1200–7. [27] Technology Readiness Levels Handbook For Space Applications, TEC-SHS/ 5551/MG/ap issue 1 revision 6. [28] Ter Brake HJM et al. Sorption-based vibration-free cooler for the METIS instrument on E-ELT. SPIE Astronomical Telescopes+ Instrumentation. International Society for Optics and Photonics; 2012. [29] Pilbratt GL et al. Herschel space observatory: an ESA facility for far-infrared and submillimetre astronomy. Astron Astrophys 2010;518(1).
Please cite this article in press as: Benthem B et al. Present status of developments in physical sorption cooling for space applications. Cryogenics (2014), http://dx.doi.org/10.1016/j.cryogenics.2014.02.010