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A mixed-gas miniature Joule–Thomson cooling system
Cryogenics 57 (2013) 26–30
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
A mixed-gas miniature Joule–Thomson cooling system J.H. Derking a, C.H. Vermeer a, T. Tirolien b, M.R. Crook c, H.J.M. ter Brake a,⇑ a
Energy, Materials and Systems, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands European Space Agency, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands c STFC, Rutherford Appleton Laboratory, Harwell Science and Information Campus, Didcot 0X11 0QX, United Kingdom b
a r t i c l e
i n f o
Article history: Received 13 November 2012 Received in revised form 26 April 2013 Accepted 26 April 2013 Available online 7 May 2013 Keywords: Joule–Thomson Gas cooler Mixed-refrigerants Miniaturization Optimization
a b s t r a c t A mixed-gas Joule–Thomson (JT) cooling system is investigated in which a micromachined JT cold stage of 60 10 0.7 mm3 is combined with a linear compressor. The cooling system is operated between 1.3 bar and 9.4 bar with a ternary gas mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane. It cools down to below 130 K, and at a cold-tip temperature of 150 K, a cooling power of 46 mW is obtained at a mass-flow rate of 1.35 mg s1. The background losses are experimentally determined to be 20 mW and are in good agreement with the calculated value of 21 mW. The linear compressor can be used to drive 19 of these miniature JT cold stages in parallel, e.g. for cooling optical detectors in future space missions. In this mode, the compressor pressure ratio is slightly less, resulting in a net cooling power of 23 mW per miniature JT cold stage. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Electronic devices, such as low-noise amplifiers and infrared detectors, often are cooled to reduce their noise and thereby improve their performance [19]. In many cases, these devices only dissipate a few milliwatts, making the use of miniature Joule– Thomson (JT) coolers very attractive. These coolers are small, highly reliable, have a fast cool down and are relatively cheap [13,17]. Furthermore, by using a mixed-gas refrigerant, highly efficient JT coolers can be realized [11]. For cooling small optical detectors in future space missions, we have investigated a JT cooling system in which a micromachined JT cold stage is combined with a linear compressor. Operated with a mixed-gas refrigerant, it cools down to below 130 K. In this paper, the mixed-gas JT cooling system is described. Pioneering work in producing a micromachined JT cold stage was performed by Garvey et al. [6] and Little [12]. Burger et al. [2] combined a miniature JT cold stage with a sorption compressor and in that way produced a closed-cycle cooling system. Lerou et al. [8,9] successfully developed and tested fully micromachined JT cold stages with cooling powers up to 20 mW at 100 K. Recently, Lin et al. [11] developed a miniature mixed-gas JT cooler with a cooling capacity of a few milliwatts at 77 K and Cao et al. [3] presented two-stage micromachined JT coolers that cool to 30 K. The miniature JT cold stage described in the present paper is based on the design of Lerou et al. [8] and a schematic representation is ⇑ Corresponding author. Tel.: +31 53 489 4349; fax: +31 53 489 1099. E-mail address: [email protected] (H.J.M. ter Brake). 0011-2275/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cryogenics.2013.04.004
given in Fig. 1. The present paper provides further insight in the application of mixed-gas refrigerants for driving these microcoolers, and addresses specific issues such as distributed cooling, temperature control and clogging of the microchannels. The microcooler consists of a stack of three glass wafers. The high and low-pressure lines, etched as rectangular channels with supporting pillars, are placed on top of one another. At the end of the high-pressure line, a flow restriction and an evaporator volume are etched. The latter is extended across the center wafer and connected to the low-pressure channel, in that way forming a counterflow heat exchanger (CFHX). The active flow areas for the high pressure and low-pressure channels are 0.063 mm2 and 0.22 mm2, respectively. A highly reflective gold layer (0.2 lm thick) is sputtered on the outer surface to minimize the radiative heat loss. The outer dimensions are 60 10 0.7 mm3. The pillar design and the fabrication of these miniature JT cold stages are extensively described by Lerou et al. [8]. A two-stage compressor is used to drive the JT cold stage. This compressor was developed by the Rutherford Appleton Laboratory and a variant was used as part of a 4 K closed-cycle JT cooling system for the high frequency instrument on the Planck explorer launched in 2009 [1,16]. A compression of 1 bar to 10 bar is obtained for helium at a mass-flow rate of 5 mg s1. This spacequalified oil-free compressor is based on linear motor drive with a diaphragm spring suspension system consisting of spiral arm springs with high radial stiffness enabling clearance seals to be used. The absence of rubbing parts facilitates a long life-time (10+ years). To minimize vibrations, a dual-opposed balanced pair of compressors is used that are driven in anti-phase. Drive
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J.H. Derking et al. / Cryogenics 57 (2013) 26–30
electronics monitor the out of balance of the compressor pair via force transducers and adjust the drive currents accordingly [1]. Reed valves rectify the alternating pressure pulse of the compressor to a DC flow as required by JT cold stages. Section 2 considers the theory of mixed-gas JT cooling and the selection of a gas mixture for our closed-cycle JT cooling system. The performance of the micromachined JT cold stage combined with the two-stage compressor was measured and the results are discussed in Section 3. Multiple JT cold stages can be combined with the compressor to obtain a distributed cooling system, which is specifically attractive for space applications. Such distributed cooling is discussed in Section 4. This paper ends with conclusions in Section 5. 2. Mixed-gas Joule–Thomson cooling The operating principle of a JT cooler is based on the reduction of enthalpy of the working fluid by increasing its pressure. The gross cooling power ðQ_ gross Þ is determined by [11]
_ Dhmin Q_ gross ¼ m
ð1Þ
_ is the mass-flow rate and Dhmin is the minimum isotherwhere m mal enthalpy difference between the high and low-pressure flows. For pure working fluids, this minimum occurs at the warm end of the heat exchanger. For mixed-gas refrigerants, however, this minimum can be at a colder point in the heat exchanger and, therefore, should be determined over the complete operating temperature range of the heat exchanger. Close to its phase change from gas to liquid, the enthalpy of a fluid depends strongly on pressure. Therefore, JT coolers are more efficient when the compression takes place closer to the two-phase region. Since cryogenic JT coolers have to establish a large temperature difference, it is clear that, when operating with a pure working fluid, these JT coolers have a relatively poor efficiency. During compression at the warm end, the working fluid behaves almost as an ideal gas resulting in a small enthalpy decrease and thus in a modest JT cooler efficiency even at high pressure ratios. The efficiency of a JT cooler can be increased by adding components with a higher boiling point into the pure working fluid. Maytal et al. [15] have shown that the performance of a JT cooler can be further improved as the number of components is increased. Mixed-refrigerants do not boil at a constant temperature as pure fluids do. Their operating temperature directly depends on the amount of heat supplied to the cold tip. The mixture composition can be optimized by maximizing the value of Dhmin [11]. In this procedure, components are selected that boil at different temperatures across the range of interest. A complication is that freezing of the mixture in the JT cooler should be avoided, but not much is known about the freezing point of mixed-refrigerants in JT systems. In general, the freezing point temperature of a gas mixture is taken as the mole fraction weighted average of the triple points of the various components [15]. For our cooling system, we are aiming at a gross cooling power of 60 mW at an operating temperature of 150 K, the warm end being at room temperature (294 K). Because of the relatively high operating temperature, the gas mixture will consist of hydrocarbons only. Running with hydrocarbons, the compressor cannot
3
_ mðH; PÞ ¼
wh 12l
Z
Ph Pl
qf ðH; PÞ dP lf ðH; PÞ
ð2Þ
where w, h and l are the width, height and length of the restriction, respectively, P l and Ph are the low and high pressures, respectively, qf is the density and lf is the dynamic viscosity of the fluid [5]. Operating this type of JT cold stage with methane and nitrogen, we found that, at operating conditions, the final mass-flow rate is rather similar to the value obtained at the moment that two-phase fluid starts to flow through the restriction. Assuming that this will also be the case for the selected gas mixture, Eq. (2) can be used to give a rough estimate of the required restriction dimensions for obtaining a mass-flow rate of 1 mg s1 at operating conditions. This results in 25 slits each with a width, length and height of 140 lm, 360 lm and 1.1 lm, respectively.
isothermal enthalpy difference (J kg-1)
Fig. 1. Schematic representation of a miniature JT cold stage.
establish the compression of 1 bar to 10 bar as it can with helium. Assumed that the compression is adiabatic, for a given swept volume, the compression that can be reached depends on the fluid. For monatomic fluids, the compression ratio will be higher than for diatomic or polyatomic fluids, because the ratio of the heat capacity at constant pressure to the heat capacity at constant volume is larger. Also, the input power to the compressor will be higher for monatomic fluids. In the case of hydrocarbons, the compression is experimentally determined to be from 1.3 bar to 9.4 bar. We chose to optimize the mixture for three components as a compromise between higher attainable efficiency (more components) and a less complex optimization and production process (less components). The fluid properties are taken from Refprop [7]. During an optimization study, the optimal composition of the ternary gas mixture is determined. Various combinations of the hydrocarbons methane, ethane, ethylene, propane, normal butane and isobutane were taken into account in different compositions. A gas mixture of methane, ethane and isobutane is selected. Methane is required for reaching a cold-end temperature of 150 K, while isobutane is required for a warm end at room temperature. The boiling temperature range of ethane overlaps both that of methane and isobutane and therefore is selected. On the basis of maximizing Dhmin , a mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane is selected. Fig. 2 gives the isothermal enthalpy difference between the high-pressure and low-pressure flows as a function of temperature. As shown, Dhmin is 0.58 105 J kg1 and occurs at 184 K. In order to obtain a gross cooling power of 60 mW, the massflow rate should be around 1 mg s1 (Eq. (1)). Assuming that the pressure drop across the CFHX is negligibly small, the mass-flow rate of the JT cold stage for a given pressure difference is determined by the dimensions of the restriction, which in our case is provided by rectangular slits [8]. The mass-flow rate for a gas flow through the restriction can be predicted by
3x105
2x105
1x105
Δhmin = 0.58x105 J kg-1
0 140
180
220
260
300
Temperature (K) Fig. 2. Isothermal enthalpy difference versus the temperature for a gas mixture of 39 mol% methane, 20 mol% ethane, 41 mol% isobutane between 1.3 bar and 9.4 bar.
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J.H. Derking et al. / Cryogenics 57 (2013) 26–30
3. Closed-cycle Joule–Thomson microcooling 3.1. Measurement set-up
3.2. Measurement results In the experiments, the compressor is operated at a compressor frequency of 42 Hz and a piston stroke of 7 mm. This results in low, medium, and high pressures of 1.3 bar, 3.9 bar and 9.4 bar, respec320
(a) PID control at sensor 1
240
tively. The total input power to the compressor is about 50 W. Fig. 4 gives the measured temperatures, mass-flow rate and heater power versus the time. As shown, the JT cold stage cools down to 128 K in about 14 min. The second temperature sensor reads 134 K. The difference in temperature between sensor 1 and sensor 2 is due to the locations of these sensors. Sensor 1 is located on the evaporator directly after the restriction, so this sensor measures the lowest temperature. Sensor 2 is located at the low-pressure side of the CFHX. At this position, the low-pressure fluid is already taking up heat from the high-pressure fluid and thus has a higher temperature than at the location of sensor 1. During the cool down phase, the mass-flow rate increases from 0.9 to 1.5 mg s1 and after cool down it becomes 0.8 mg s1. When sensor 1 measures below 240 K, the mass-flow rate starts to fluctuate with an amplitude of about 0.4 mg s1. This is around the saturated vapor temperature of the gas mixture at 1.3 bar. These fluctuations were also observed during the operation of this type of JT cold stages with methane and with nitrogen [5]. In that case, the fluctuations started when the cold-tip temperature was at the saturated vapor temperature of the respective fluid. So, the fluctuations indicate that two-phase fluid is formed in the evaporator. Also, the large amplitude of fluctuation was observed earlier [5]. The calculated mass-flow rate at the start of the cool down according to Eq. (2) is 0.9 mg s1 matching the measured value very well. During cool down, two-phase fluid is formed and in this case Eq. (2) can be used for first-order estimates only [5]. The net cooling power of the JT cold stage is measured at different temperatures by placing sensor 1 and the heater resistor in a PID control loop (see Fig. 4). After 0.8 h, the cold-tip temperature is PID 2.0
(b)
1.5 1.0 0.0
0
0.5
1.0
1.5
2.0
2.5
Time (h) 200
2 1
160
120
Heater power (mW)
Temperature (K)
280
Fig. 3. Photograph of the micromachined JT cold stage prepared for the experiments. The cold stage is equipped with two temperature sensors and one heater. The temperature sensors 1 and 2 are located at a distance of 2 mm and 9 mm from the cold end, respectively.
Mass-flow rate (mg s-1)
The miniature JT cold stage is combined with the compressor to obtain a closed-cycle JT cooler, and the system is filled with the selected gas mixture to a pressure of 4.6 bar. The gas mixture is supplied from a pressurized gas bottle and the composition accuracy is specified to be within a relative uncertainty of ±2%. The mixture is pressurized by the compressor and under high pressure cooled to room temperature by heat exchange with the laboratory environment. Then, it flows through a microtorrÒ getter filter from Saes Gas Inc. [18] to decrease the amount of water contamination in the gas mixture below one part-per-billion. This is done to prevent clogging of the JT cold stage due to freezing of water particles in the restriction [10]. The gas mixture flows through the JT cold stage that is placed in a glass vacuum chamber. The mass-flow rate of the outgoing gas flow is measured by a calibrated Bronkhorst massflow meter with an accuracy of ±3.0%. The high, medium and low pressures of the compressor stages are measured by calibrated Druck Ltd. pressure transducers with an accuracy of ±0.7%. The circulating gas composition is not measured during operation of the closed-cycle JT cooler. For the experiments, the JT cold stage is mounted into a vacuum flange and for additional electrical connections in the tests it is surrounded by printed-circuit-boards (PCBs). The connections from the gas tubes to the cold stage are made with indium seals. Two Pt1000 temperature sensors are glued to the cold stage (measuring accuracy ±1 K) and a standard surface-mounted-device resistor is used as a heater for supplying heat to the cold tip. To distribute the supplied heat equally over the width of the cold tip, the resistor is glued onto a small piece of silicon that subsequently is glued on top of the evaporator. This arrangement simulates a future device to be cooled by the JT cold stage. The silicon slice is covered with a thin layer of gold to reduce the radiative heat load. All gluing is done with silver-filled conducting paint. Aluminum bond wires with a diameter of 25 lm are used to make the electrical connections between the sensors and the PCBs. Fig. 3 shows the JT cold stage equipped with two temperature sensors and one heater. The temperature sensors 1 and 2 are located at a distance of 2 mm and 9 mm from the cold end, respectively.
0
0.5
1.0
1.5
Time (h)
2.0
2.5
100
(c)
50
0
0
0.5
1.0
1.5
2.0
2.5
Time (h)
Fig. 4. Measurement results of a closed-cycle JT cooler operating with a gas mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane between 1.3 bar and 9.4 bar. (a) Temperature of two sensors, (b) mass-flow rate and (c) supplied heater power versus the time.
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J.H. Derking et al. / Cryogenics 57 (2013) 26–30
1.0 0.8 0.6 0.4 0.2 0.0 -20
0
20
40
60
Supplied heater power (mW) Fig. 5. Measured warm-up rate versus the supplied heater power to determine the background losses of the JT cold stage at 150 K.
controlled at 140 K and the mass-flow rate increases to 1.25 mg s1. The net cooling power of the JT cold stage corresponds to the supplied heater power, which in this situation is 38 mW. After 1.2 h, the cold-tip set-point is increased to 150 K. The mass-flow rate increases to 1.35 mg s1 and the net cooling power to 46 mW. When the cold tip is controlled at 160 K the mass-flow rate decreases slowly, as can be observed in Fig. 4. This is caused by the deposition of ice crystals inside the restriction, reducing the height and thereby the mass-flow rate (Eq. (2)). Finally, this will result in clogging of the restriction and heating up of the cold stage [10]. A cold stage operating at 160 K is more sensitive to clogging than one operating at a lower temperature, because the cold-tip temperature is much closer to the phase-transition temperature of 180–220 K at which ice crystals deposit [5,10]. While heating, the restriction will have a temperature in the clogging temperature range, resulting in ice forming in the restriction and thereby clogging of the restriction. The background losses, consisting of radiation, and conduction through the wiring of the sensors and through the CFHX material, are measured by determining the slope of the warm-up curve at 150 K for different supplied heater powers. During warming-up, the compressor is switched off. Fig. 5 shows the warm-up rate at 150 K versus the supplied heater power. The background losses are obtained by extrapolating the warm-up rate to zero. In this way, a background loss of about 20 mW at 150 K is found. Calculations of the background losses result in a radiative heat loss of 11 mW and a conductive heat loss through the bond wires of 10 mW. The total calculated loss of 21 mW is in good agreement with the measured value. Conduction through the CFHX material adds an extra loss of a few mW. 320
(a)
Temperature (K)
280
PID control at sensor 1
240
4. Distributed cooling The linear compressor can deliver a much larger flow than the mg s1 level required by a single miniature JT cold stage. Such a larger flow can be established with only a small decrease in compression ratio. Therefore, this compressor can be used in a distributed cooling system in which multiple miniature JT cold stages are driven in parallel by a single compressor. In a distributed JT cooling system, in principle, each JT cold stage is operated between the same pressures. Therefore, Dhmin is equal and the cooling power of each JT cold stage depends on the established mass-flow rate (Eq. (2)). This means that each JT cold stage can be optimized in terms of cooling power by adjusting its restriction dimensions. The operating temperature of each JT cold stage depends on the supplied heat. The performance of the compressor operating with the selected gas mixture operating at a much higher flow rate is mapped and a compression of 2.2 bar to 8.9 bar is measured at a mass-flow rate of 25 mg s1 [4]. The performance of the JT cold stage operating between these pressures is measured to investigate whether it can cool down and the results are given in Fig. 6. As shown, the JT cold stage cools down to 138 K in 0.33 h. When the cold-tip is PID controlled at 150 K, the measured cooling power is 13 mW at a massflow rate of 1.3 mg s1. For a compression of 2.2 bar to 8.9 bar, 2.0
(b)
1.5 1.0 0.0
0
0.2
0.4
0.6
0.8
1.0
Time (h) 200
2
160
120
1 0
0.2
0.4
0.6
Time (h)
0.8
1.0
Heater power (mW)
Warm-up rate (K s-1)
1.2
The selected gas mixture operating between 1.3 bar and 9.4 bar has a Dhmin of 0.58 105 J kg1 that occurs at 184 K (Fig. 2). At a mass-flow rate of 1.35 mg s1, this results in a gross cooling power of 78 mW. Taking into account the background losses of 20 mW, the theoretical net cooling power is about 58 mW whereas a value of 46 mW was measured. The difference of roughly 12 mW between the measured and the calculated net cooling power is caused by a number of reasons. Firstly, the temperature of the gas mixture at the entrance of the CFHX could be too high. As shown in Fig. 2, the isothermal enthalpy difference decreases very steeply at the warm end of the CFHX. This means that if, at the entrance of the CFHX, the temperature of the gas-mixture is slightly higher than the anticipated value of 294 K, Dhmin occurs at the warm side, resulting in a lower cooling power. Furthermore, the background losses could be some 5 mW more (see Fig. 5) and the inaccuracy in the measured cooling power could be several mW (see Fig. 4). Finally, the composition of the gas in the cold stage may be different from the above-specified composition resulting in a different Dhmin [14]. To determine the long-life performance of the closed-cycle JT cooler, it is operated for 2 h at 150 K. During this period, no change in performance of the JT cooler is measured as for example could have been caused by clogging or demixing of the gas mixture.
Mass-flow rate (mg s-1)
1.4
60
(c)
40 20 0 0
0.2
0.4
0.6
0.8
1.0
Time (h)
Fig. 6. Measurement results of a closed-cycle JT cooler operating with a gas mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane between 2.2 bar and 8.9 bar. (a) Temperature, (b) mass-flow rate and (c) supplied heater power versus the time.
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J.H. Derking et al. / Cryogenics 57 (2013) 26–30
Dhmin is 0.41 105 J kg1 and occurs at 187 K. So, the gross cooling power is 53 mW. Taking into account the measured background losses of 20 mW at 150 K (Fig. 5), the theoretical net cooling power is 33 mW. The difference of 20 mW between the measured and calculated cooling power is again caused by an uncertainty in the temperature of the gas mixture when it enters the CFHX, the measuring inaccuracy and uncertainties in the composition of the gas mixture, as discussed in Section 3. Without thermometers and heaters, the net cooling power of the JT cold stage is about 23 mW (parasitic heat load through wiring calculated at 10 mW). As the compressor delivers a mass-flow rate of 25 mg s1, about 19 of these JT cold stages can be combined with this compressor. 5. Conclusion A closed-cycle JT cooler consisting of a micromachined JT cold stage and a linear compressor is investigated. The cooling system is operated with a gas mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane between 1.3 bar and 9.4 bar. The gas mixture is selected by optimizing the minimum isothermal enthalpy difference along the heat exchanger. The cold stage cools down to below 130 K and at a cold-tip temperature of 150 K, a net cooling power of 46 mW is obtained at a mass-flow rate of 1.35 mg s1. Operating this cooling system for 2 h at 150 K does not show a change in performance. Furthermore, the linear compressor used can deliver a mass-flow rate that is adequate to drive about 19 micromachined JT cold stages in parallel. In this mode, the compressor pressure ratio is slightly less (2.2 bar to 8.9 bar) resulting in a lower net cooling power of 23 mW per miniature JT cold stage. In that way, a distributed cooling system can be realized. Acknowledgements The authors gratefully acknowledge the support of the European Space Agency under contract 10768/NL/EM, the Dutch Technology Foundation (STW) for supporting the development of miniature JT cold stages in project 08014, P.P.P.M Lerou of Kryoz Technologies for the development of the JT cold stages and the
cryogenics department of the Rutherford Appleton laboratory for supplying the compressor. References [1] Bradshaw TW, Orlowska AH, Jewell C. Life test and performance testing of a 4 K coolers for space applications. Cryocoolers 1999;10:521–9. [2] Burger JF, Holland HJ, Seppenwoolde JH, Berenschot E, ter Brake HJM, Gardeniers JGE, et al. 165 K microcooler operating with a sorption compressor and a micromachined cold stage. Cryocoolers 2001;11:551–60. [3] Cao HS, Holland HJ, Vermeer CH, Vanapalli S, Lerou PPPM, Blom M, et al. Micromachined cryogenic cooler for cooling electronic devices down to 30 K. J Micromech Microeng 2013;23:025014. [4] Derking JH. Distributed Joule–Thomson microcooling for optical detectors in space. PhD-thesis, University of Twente, The Netherlands; 2011. [19] Walker G, Fauvel R, Reader G. Miniature refrigerators for cryogenic sensors and cold electronics. Cryogenics 1989;29:841–5.
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
A mixed-gas miniature Joule–Thomson cooling system J.H. Derking a, C.H. Vermeer a, T. Tirolien b, M.R. Crook c, H.J.M. ter Brake a,⇑ a
Energy, Materials and Systems, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands European Space Agency, ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands c STFC, Rutherford Appleton Laboratory, Harwell Science and Information Campus, Didcot 0X11 0QX, United Kingdom b
a r t i c l e
i n f o
Article history: Received 13 November 2012 Received in revised form 26 April 2013 Accepted 26 April 2013 Available online 7 May 2013 Keywords: Joule–Thomson Gas cooler Mixed-refrigerants Miniaturization Optimization
a b s t r a c t A mixed-gas Joule–Thomson (JT) cooling system is investigated in which a micromachined JT cold stage of 60 10 0.7 mm3 is combined with a linear compressor. The cooling system is operated between 1.3 bar and 9.4 bar with a ternary gas mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane. It cools down to below 130 K, and at a cold-tip temperature of 150 K, a cooling power of 46 mW is obtained at a mass-flow rate of 1.35 mg s1. The background losses are experimentally determined to be 20 mW and are in good agreement with the calculated value of 21 mW. The linear compressor can be used to drive 19 of these miniature JT cold stages in parallel, e.g. for cooling optical detectors in future space missions. In this mode, the compressor pressure ratio is slightly less, resulting in a net cooling power of 23 mW per miniature JT cold stage. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Electronic devices, such as low-noise amplifiers and infrared detectors, often are cooled to reduce their noise and thereby improve their performance [19]. In many cases, these devices only dissipate a few milliwatts, making the use of miniature Joule– Thomson (JT) coolers very attractive. These coolers are small, highly reliable, have a fast cool down and are relatively cheap [13,17]. Furthermore, by using a mixed-gas refrigerant, highly efficient JT coolers can be realized [11]. For cooling small optical detectors in future space missions, we have investigated a JT cooling system in which a micromachined JT cold stage is combined with a linear compressor. Operated with a mixed-gas refrigerant, it cools down to below 130 K. In this paper, the mixed-gas JT cooling system is described. Pioneering work in producing a micromachined JT cold stage was performed by Garvey et al. [6] and Little [12]. Burger et al. [2] combined a miniature JT cold stage with a sorption compressor and in that way produced a closed-cycle cooling system. Lerou et al. [8,9] successfully developed and tested fully micromachined JT cold stages with cooling powers up to 20 mW at 100 K. Recently, Lin et al. [11] developed a miniature mixed-gas JT cooler with a cooling capacity of a few milliwatts at 77 K and Cao et al. [3] presented two-stage micromachined JT coolers that cool to 30 K. The miniature JT cold stage described in the present paper is based on the design of Lerou et al. [8] and a schematic representation is ⇑ Corresponding author. Tel.: +31 53 489 4349; fax: +31 53 489 1099. E-mail address: [email protected] (H.J.M. ter Brake). 0011-2275/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cryogenics.2013.04.004
given in Fig. 1. The present paper provides further insight in the application of mixed-gas refrigerants for driving these microcoolers, and addresses specific issues such as distributed cooling, temperature control and clogging of the microchannels. The microcooler consists of a stack of three glass wafers. The high and low-pressure lines, etched as rectangular channels with supporting pillars, are placed on top of one another. At the end of the high-pressure line, a flow restriction and an evaporator volume are etched. The latter is extended across the center wafer and connected to the low-pressure channel, in that way forming a counterflow heat exchanger (CFHX). The active flow areas for the high pressure and low-pressure channels are 0.063 mm2 and 0.22 mm2, respectively. A highly reflective gold layer (0.2 lm thick) is sputtered on the outer surface to minimize the radiative heat loss. The outer dimensions are 60 10 0.7 mm3. The pillar design and the fabrication of these miniature JT cold stages are extensively described by Lerou et al. [8]. A two-stage compressor is used to drive the JT cold stage. This compressor was developed by the Rutherford Appleton Laboratory and a variant was used as part of a 4 K closed-cycle JT cooling system for the high frequency instrument on the Planck explorer launched in 2009 [1,16]. A compression of 1 bar to 10 bar is obtained for helium at a mass-flow rate of 5 mg s1. This spacequalified oil-free compressor is based on linear motor drive with a diaphragm spring suspension system consisting of spiral arm springs with high radial stiffness enabling clearance seals to be used. The absence of rubbing parts facilitates a long life-time (10+ years). To minimize vibrations, a dual-opposed balanced pair of compressors is used that are driven in anti-phase. Drive
27
J.H. Derking et al. / Cryogenics 57 (2013) 26–30
electronics monitor the out of balance of the compressor pair via force transducers and adjust the drive currents accordingly [1]. Reed valves rectify the alternating pressure pulse of the compressor to a DC flow as required by JT cold stages. Section 2 considers the theory of mixed-gas JT cooling and the selection of a gas mixture for our closed-cycle JT cooling system. The performance of the micromachined JT cold stage combined with the two-stage compressor was measured and the results are discussed in Section 3. Multiple JT cold stages can be combined with the compressor to obtain a distributed cooling system, which is specifically attractive for space applications. Such distributed cooling is discussed in Section 4. This paper ends with conclusions in Section 5. 2. Mixed-gas Joule–Thomson cooling The operating principle of a JT cooler is based on the reduction of enthalpy of the working fluid by increasing its pressure. The gross cooling power ðQ_ gross Þ is determined by [11]
_ Dhmin Q_ gross ¼ m
ð1Þ
_ is the mass-flow rate and Dhmin is the minimum isotherwhere m mal enthalpy difference between the high and low-pressure flows. For pure working fluids, this minimum occurs at the warm end of the heat exchanger. For mixed-gas refrigerants, however, this minimum can be at a colder point in the heat exchanger and, therefore, should be determined over the complete operating temperature range of the heat exchanger. Close to its phase change from gas to liquid, the enthalpy of a fluid depends strongly on pressure. Therefore, JT coolers are more efficient when the compression takes place closer to the two-phase region. Since cryogenic JT coolers have to establish a large temperature difference, it is clear that, when operating with a pure working fluid, these JT coolers have a relatively poor efficiency. During compression at the warm end, the working fluid behaves almost as an ideal gas resulting in a small enthalpy decrease and thus in a modest JT cooler efficiency even at high pressure ratios. The efficiency of a JT cooler can be increased by adding components with a higher boiling point into the pure working fluid. Maytal et al. [15] have shown that the performance of a JT cooler can be further improved as the number of components is increased. Mixed-refrigerants do not boil at a constant temperature as pure fluids do. Their operating temperature directly depends on the amount of heat supplied to the cold tip. The mixture composition can be optimized by maximizing the value of Dhmin [11]. In this procedure, components are selected that boil at different temperatures across the range of interest. A complication is that freezing of the mixture in the JT cooler should be avoided, but not much is known about the freezing point of mixed-refrigerants in JT systems. In general, the freezing point temperature of a gas mixture is taken as the mole fraction weighted average of the triple points of the various components [15]. For our cooling system, we are aiming at a gross cooling power of 60 mW at an operating temperature of 150 K, the warm end being at room temperature (294 K). Because of the relatively high operating temperature, the gas mixture will consist of hydrocarbons only. Running with hydrocarbons, the compressor cannot
3
_ mðH; PÞ ¼
wh 12l
Z
Ph Pl
qf ðH; PÞ dP lf ðH; PÞ
ð2Þ
where w, h and l are the width, height and length of the restriction, respectively, P l and Ph are the low and high pressures, respectively, qf is the density and lf is the dynamic viscosity of the fluid [5]. Operating this type of JT cold stage with methane and nitrogen, we found that, at operating conditions, the final mass-flow rate is rather similar to the value obtained at the moment that two-phase fluid starts to flow through the restriction. Assuming that this will also be the case for the selected gas mixture, Eq. (2) can be used to give a rough estimate of the required restriction dimensions for obtaining a mass-flow rate of 1 mg s1 at operating conditions. This results in 25 slits each with a width, length and height of 140 lm, 360 lm and 1.1 lm, respectively.
isothermal enthalpy difference (J kg-1)
Fig. 1. Schematic representation of a miniature JT cold stage.
establish the compression of 1 bar to 10 bar as it can with helium. Assumed that the compression is adiabatic, for a given swept volume, the compression that can be reached depends on the fluid. For monatomic fluids, the compression ratio will be higher than for diatomic or polyatomic fluids, because the ratio of the heat capacity at constant pressure to the heat capacity at constant volume is larger. Also, the input power to the compressor will be higher for monatomic fluids. In the case of hydrocarbons, the compression is experimentally determined to be from 1.3 bar to 9.4 bar. We chose to optimize the mixture for three components as a compromise between higher attainable efficiency (more components) and a less complex optimization and production process (less components). The fluid properties are taken from Refprop [7]. During an optimization study, the optimal composition of the ternary gas mixture is determined. Various combinations of the hydrocarbons methane, ethane, ethylene, propane, normal butane and isobutane were taken into account in different compositions. A gas mixture of methane, ethane and isobutane is selected. Methane is required for reaching a cold-end temperature of 150 K, while isobutane is required for a warm end at room temperature. The boiling temperature range of ethane overlaps both that of methane and isobutane and therefore is selected. On the basis of maximizing Dhmin , a mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane is selected. Fig. 2 gives the isothermal enthalpy difference between the high-pressure and low-pressure flows as a function of temperature. As shown, Dhmin is 0.58 105 J kg1 and occurs at 184 K. In order to obtain a gross cooling power of 60 mW, the massflow rate should be around 1 mg s1 (Eq. (1)). Assuming that the pressure drop across the CFHX is negligibly small, the mass-flow rate of the JT cold stage for a given pressure difference is determined by the dimensions of the restriction, which in our case is provided by rectangular slits [8]. The mass-flow rate for a gas flow through the restriction can be predicted by
3x105
2x105
1x105
Δhmin = 0.58x105 J kg-1
0 140
180
220
260
300
Temperature (K) Fig. 2. Isothermal enthalpy difference versus the temperature for a gas mixture of 39 mol% methane, 20 mol% ethane, 41 mol% isobutane between 1.3 bar and 9.4 bar.
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J.H. Derking et al. / Cryogenics 57 (2013) 26–30
3. Closed-cycle Joule–Thomson microcooling 3.1. Measurement set-up
3.2. Measurement results In the experiments, the compressor is operated at a compressor frequency of 42 Hz and a piston stroke of 7 mm. This results in low, medium, and high pressures of 1.3 bar, 3.9 bar and 9.4 bar, respec320
(a) PID control at sensor 1
240
tively. The total input power to the compressor is about 50 W. Fig. 4 gives the measured temperatures, mass-flow rate and heater power versus the time. As shown, the JT cold stage cools down to 128 K in about 14 min. The second temperature sensor reads 134 K. The difference in temperature between sensor 1 and sensor 2 is due to the locations of these sensors. Sensor 1 is located on the evaporator directly after the restriction, so this sensor measures the lowest temperature. Sensor 2 is located at the low-pressure side of the CFHX. At this position, the low-pressure fluid is already taking up heat from the high-pressure fluid and thus has a higher temperature than at the location of sensor 1. During the cool down phase, the mass-flow rate increases from 0.9 to 1.5 mg s1 and after cool down it becomes 0.8 mg s1. When sensor 1 measures below 240 K, the mass-flow rate starts to fluctuate with an amplitude of about 0.4 mg s1. This is around the saturated vapor temperature of the gas mixture at 1.3 bar. These fluctuations were also observed during the operation of this type of JT cold stages with methane and with nitrogen [5]. In that case, the fluctuations started when the cold-tip temperature was at the saturated vapor temperature of the respective fluid. So, the fluctuations indicate that two-phase fluid is formed in the evaporator. Also, the large amplitude of fluctuation was observed earlier [5]. The calculated mass-flow rate at the start of the cool down according to Eq. (2) is 0.9 mg s1 matching the measured value very well. During cool down, two-phase fluid is formed and in this case Eq. (2) can be used for first-order estimates only [5]. The net cooling power of the JT cold stage is measured at different temperatures by placing sensor 1 and the heater resistor in a PID control loop (see Fig. 4). After 0.8 h, the cold-tip temperature is PID 2.0
(b)
1.5 1.0 0.0
0
0.5
1.0
1.5
2.0
2.5
Time (h) 200
2 1
160
120
Heater power (mW)
Temperature (K)
280
Fig. 3. Photograph of the micromachined JT cold stage prepared for the experiments. The cold stage is equipped with two temperature sensors and one heater. The temperature sensors 1 and 2 are located at a distance of 2 mm and 9 mm from the cold end, respectively.
Mass-flow rate (mg s-1)
The miniature JT cold stage is combined with the compressor to obtain a closed-cycle JT cooler, and the system is filled with the selected gas mixture to a pressure of 4.6 bar. The gas mixture is supplied from a pressurized gas bottle and the composition accuracy is specified to be within a relative uncertainty of ±2%. The mixture is pressurized by the compressor and under high pressure cooled to room temperature by heat exchange with the laboratory environment. Then, it flows through a microtorrÒ getter filter from Saes Gas Inc. [18] to decrease the amount of water contamination in the gas mixture below one part-per-billion. This is done to prevent clogging of the JT cold stage due to freezing of water particles in the restriction [10]. The gas mixture flows through the JT cold stage that is placed in a glass vacuum chamber. The mass-flow rate of the outgoing gas flow is measured by a calibrated Bronkhorst massflow meter with an accuracy of ±3.0%. The high, medium and low pressures of the compressor stages are measured by calibrated Druck Ltd. pressure transducers with an accuracy of ±0.7%. The circulating gas composition is not measured during operation of the closed-cycle JT cooler. For the experiments, the JT cold stage is mounted into a vacuum flange and for additional electrical connections in the tests it is surrounded by printed-circuit-boards (PCBs). The connections from the gas tubes to the cold stage are made with indium seals. Two Pt1000 temperature sensors are glued to the cold stage (measuring accuracy ±1 K) and a standard surface-mounted-device resistor is used as a heater for supplying heat to the cold tip. To distribute the supplied heat equally over the width of the cold tip, the resistor is glued onto a small piece of silicon that subsequently is glued on top of the evaporator. This arrangement simulates a future device to be cooled by the JT cold stage. The silicon slice is covered with a thin layer of gold to reduce the radiative heat load. All gluing is done with silver-filled conducting paint. Aluminum bond wires with a diameter of 25 lm are used to make the electrical connections between the sensors and the PCBs. Fig. 3 shows the JT cold stage equipped with two temperature sensors and one heater. The temperature sensors 1 and 2 are located at a distance of 2 mm and 9 mm from the cold end, respectively.
0
0.5
1.0
1.5
Time (h)
2.0
2.5
100
(c)
50
0
0
0.5
1.0
1.5
2.0
2.5
Time (h)
Fig. 4. Measurement results of a closed-cycle JT cooler operating with a gas mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane between 1.3 bar and 9.4 bar. (a) Temperature of two sensors, (b) mass-flow rate and (c) supplied heater power versus the time.
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J.H. Derking et al. / Cryogenics 57 (2013) 26–30
1.0 0.8 0.6 0.4 0.2 0.0 -20
0
20
40
60
Supplied heater power (mW) Fig. 5. Measured warm-up rate versus the supplied heater power to determine the background losses of the JT cold stage at 150 K.
controlled at 140 K and the mass-flow rate increases to 1.25 mg s1. The net cooling power of the JT cold stage corresponds to the supplied heater power, which in this situation is 38 mW. After 1.2 h, the cold-tip set-point is increased to 150 K. The mass-flow rate increases to 1.35 mg s1 and the net cooling power to 46 mW. When the cold tip is controlled at 160 K the mass-flow rate decreases slowly, as can be observed in Fig. 4. This is caused by the deposition of ice crystals inside the restriction, reducing the height and thereby the mass-flow rate (Eq. (2)). Finally, this will result in clogging of the restriction and heating up of the cold stage [10]. A cold stage operating at 160 K is more sensitive to clogging than one operating at a lower temperature, because the cold-tip temperature is much closer to the phase-transition temperature of 180–220 K at which ice crystals deposit [5,10]. While heating, the restriction will have a temperature in the clogging temperature range, resulting in ice forming in the restriction and thereby clogging of the restriction. The background losses, consisting of radiation, and conduction through the wiring of the sensors and through the CFHX material, are measured by determining the slope of the warm-up curve at 150 K for different supplied heater powers. During warming-up, the compressor is switched off. Fig. 5 shows the warm-up rate at 150 K versus the supplied heater power. The background losses are obtained by extrapolating the warm-up rate to zero. In this way, a background loss of about 20 mW at 150 K is found. Calculations of the background losses result in a radiative heat loss of 11 mW and a conductive heat loss through the bond wires of 10 mW. The total calculated loss of 21 mW is in good agreement with the measured value. Conduction through the CFHX material adds an extra loss of a few mW. 320
(a)
Temperature (K)
280
PID control at sensor 1
240
4. Distributed cooling The linear compressor can deliver a much larger flow than the mg s1 level required by a single miniature JT cold stage. Such a larger flow can be established with only a small decrease in compression ratio. Therefore, this compressor can be used in a distributed cooling system in which multiple miniature JT cold stages are driven in parallel by a single compressor. In a distributed JT cooling system, in principle, each JT cold stage is operated between the same pressures. Therefore, Dhmin is equal and the cooling power of each JT cold stage depends on the established mass-flow rate (Eq. (2)). This means that each JT cold stage can be optimized in terms of cooling power by adjusting its restriction dimensions. The operating temperature of each JT cold stage depends on the supplied heat. The performance of the compressor operating with the selected gas mixture operating at a much higher flow rate is mapped and a compression of 2.2 bar to 8.9 bar is measured at a mass-flow rate of 25 mg s1 [4]. The performance of the JT cold stage operating between these pressures is measured to investigate whether it can cool down and the results are given in Fig. 6. As shown, the JT cold stage cools down to 138 K in 0.33 h. When the cold-tip is PID controlled at 150 K, the measured cooling power is 13 mW at a massflow rate of 1.3 mg s1. For a compression of 2.2 bar to 8.9 bar, 2.0
(b)
1.5 1.0 0.0
0
0.2
0.4
0.6
0.8
1.0
Time (h) 200
2
160
120
1 0
0.2
0.4
0.6
Time (h)
0.8
1.0
Heater power (mW)
Warm-up rate (K s-1)
1.2
The selected gas mixture operating between 1.3 bar and 9.4 bar has a Dhmin of 0.58 105 J kg1 that occurs at 184 K (Fig. 2). At a mass-flow rate of 1.35 mg s1, this results in a gross cooling power of 78 mW. Taking into account the background losses of 20 mW, the theoretical net cooling power is about 58 mW whereas a value of 46 mW was measured. The difference of roughly 12 mW between the measured and the calculated net cooling power is caused by a number of reasons. Firstly, the temperature of the gas mixture at the entrance of the CFHX could be too high. As shown in Fig. 2, the isothermal enthalpy difference decreases very steeply at the warm end of the CFHX. This means that if, at the entrance of the CFHX, the temperature of the gas-mixture is slightly higher than the anticipated value of 294 K, Dhmin occurs at the warm side, resulting in a lower cooling power. Furthermore, the background losses could be some 5 mW more (see Fig. 5) and the inaccuracy in the measured cooling power could be several mW (see Fig. 4). Finally, the composition of the gas in the cold stage may be different from the above-specified composition resulting in a different Dhmin [14]. To determine the long-life performance of the closed-cycle JT cooler, it is operated for 2 h at 150 K. During this period, no change in performance of the JT cooler is measured as for example could have been caused by clogging or demixing of the gas mixture.
Mass-flow rate (mg s-1)
1.4
60
(c)
40 20 0 0
0.2
0.4
0.6
0.8
1.0
Time (h)
Fig. 6. Measurement results of a closed-cycle JT cooler operating with a gas mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane between 2.2 bar and 8.9 bar. (a) Temperature, (b) mass-flow rate and (c) supplied heater power versus the time.
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J.H. Derking et al. / Cryogenics 57 (2013) 26–30
Dhmin is 0.41 105 J kg1 and occurs at 187 K. So, the gross cooling power is 53 mW. Taking into account the measured background losses of 20 mW at 150 K (Fig. 5), the theoretical net cooling power is 33 mW. The difference of 20 mW between the measured and calculated cooling power is again caused by an uncertainty in the temperature of the gas mixture when it enters the CFHX, the measuring inaccuracy and uncertainties in the composition of the gas mixture, as discussed in Section 3. Without thermometers and heaters, the net cooling power of the JT cold stage is about 23 mW (parasitic heat load through wiring calculated at 10 mW). As the compressor delivers a mass-flow rate of 25 mg s1, about 19 of these JT cold stages can be combined with this compressor. 5. Conclusion A closed-cycle JT cooler consisting of a micromachined JT cold stage and a linear compressor is investigated. The cooling system is operated with a gas mixture of 39 mol% methane, 20 mol% ethane and 41 mol% isobutane between 1.3 bar and 9.4 bar. The gas mixture is selected by optimizing the minimum isothermal enthalpy difference along the heat exchanger. The cold stage cools down to below 130 K and at a cold-tip temperature of 150 K, a net cooling power of 46 mW is obtained at a mass-flow rate of 1.35 mg s1. Operating this cooling system for 2 h at 150 K does not show a change in performance. Furthermore, the linear compressor used can deliver a mass-flow rate that is adequate to drive about 19 micromachined JT cold stages in parallel. In this mode, the compressor pressure ratio is slightly less (2.2 bar to 8.9 bar) resulting in a lower net cooling power of 23 mW per miniature JT cold stage. In that way, a distributed cooling system can be realized. Acknowledgements The authors gratefully acknowledge the support of the European Space Agency under contract 10768/NL/EM, the Dutch Technology Foundation (STW) for supporting the development of miniature JT cold stages in project 08014, P.P.P.M Lerou of Kryoz Technologies for the development of the JT cold stages and the
cryogenics department of the Rutherford Appleton laboratory for supplying the compressor. References [1] Bradshaw TW, Orlowska AH, Jewell C. Life test and performance testing of a 4 K coolers for space applications. Cryocoolers 1999;10:521–9. [2] Burger JF, Holland HJ, Seppenwoolde JH, Berenschot E, ter Brake HJM, Gardeniers JGE, et al. 165 K microcooler operating with a sorption compressor and a micromachined cold stage. Cryocoolers 2001;11:551–60. [3] Cao HS, Holland HJ, Vermeer CH, Vanapalli S, Lerou PPPM, Blom M, et al. Micromachined cryogenic cooler for cooling electronic devices down to 30 K. J Micromech Microeng 2013;23:025014. [4] Derking JH. Distributed Joule–Thomson microcooling for optical detectors in space. PhD-thesis, University of Twente, The Netherlands; 2011