Operating characteristics of a single-stage Stirling cryocooler capable of providing 700 W cooling power at 77 K

High cooling capacity Stirling cryocooler generally has hundreds to thousands watts of cooling power at liquid nitrogen temperature. It is promising in boil-off gas (BOG) recondensation and high temperature superconducting (HTS) applications. A high cooling capacity Stirling cryocooler driven by a crank-rod mechanism was developed and studied systematically. The pressure and frequency characteristics of the cryocooler, the heat rejection from the ambient heat exchanger, and the cooling performance are studied under different charging pressure. Energy conversion and distribution in the cryocooler are analyzed theoretically. With an electric input power of 10.9 kW and a rotating speed of 1450 r/min of the motor, a cooling power of 700 W at 77 K and a relative Carnot efficiency of 18.2% of the cryocooler have been achieved in the present study, and the corresponding pressure ratio in the compression space reaches 2.46.     

Study on cold head structure of a 300 Hz thermoacoustically driven pulse tube cryocooler

High reliability, compact size and potentially high thermal efficiency make the high frequency thermoacoustically-driven pulse tube cryocooler quite promising for space use. With continuous efforts, the lowest temperature and the thermal efficiency of the coupled system have been greatly improved. So far, a cold head temperature below 60 K has been achieved on such kind of cryocooler with the operation frequency of around 300 Hz. To further improve the thermal efficiency and expedite its practical application, this work focuses on studying the influence of cold head structure on the system performance. Substantial numerical simulations were firstly carried out, which revealed that the cold head structure would greatly influence the cooling power and the thermal efficiency. To validate the predictions, a lot of experiments have been done. The experiments and calculations are in reasonable agreement. With 500 W heating power input into the engine, a no-load temperature of 63 K and a cooling power of 1.16 W at 80 K have been obtained with parallel-plate cold head, indicating encouraging improvement of the thermal efficiency.     

10 W/90 K single-stage pulse tube cryocoolers

A single-stage 10 W/90 K coaxial pulse tube cryocooler has been developed for space-borne optics cooling. The design considerations are described, and the optimizations on the double-segmented inertance tubes are presented. The preliminary engineering model (EM) of the cooler has been worked out, which typically provides the cooling of 10 W at 90 K with the input power of 175.6 W at 310 K reject temperature, and achieves around 14% of Carnot efficiency at 90 K. The reject temperature dependence experiments on the EM show a smaller slope of 10.2 W/10 K and indicate a good adaptability to the reject temperature range from 290 K to 333 K.     

High-capacity 60 K single-stage coaxial pulse tube cryocoolers

A high-capacity single-stage coaxial pulse tube cryocooler operating at around 60 K has been developed to provide the appropriate cooling for the next-generation very-large-scale long wave infrared focal plane arrays under development. The application background and cooler design process are described, and the performance characteristics are presented. At present, the cooler typically provides 4.06 W at 60 K with the input power of 180 W at 300 K reject temperature. 4.72 W can also be achieved when the input power increases to 200 W, and over 9.4% of Carnot efficiency at 60 K has been realized. The larger pulse tube diameter of 14.2 mm is used and the evident orientation sensitivity is observed in the range of 55–65 Hz. The experiments also observe the obvious reject temperature dependence.     

A closed-cycle 1 K refrigeration cryostat

A 1 K closed-cycle cryostat has been developed to provide continuous cooling to a photon detector below 2 K. A two-stage 4 K pulse tube cryocooler is used to liquefy evacuated vapor from a 1 K pumping port to form a closed-cycle refrigeration loop. A 1 K instrumentation chamber, attached to the 1 K cooling station, is designed to operate with helium inside and provide more uniform cooling. The design of the cryostat has no direct mechanical contact between the pulse tube cryocooler heat exchangers and the 1 K cooling station resulting in almost no vibration transfer to instrumentation chamber. The cryostat can reach a no-load temperature of 1.62 K and provide 250 mW cooling power at 1.84 K.     

Operating characteristics of a three-stage Stirling pulse tube cryocooler operating around 5 K

A Stirling pulse tube cryocooler (SPTC) operating at the liquid-helium temperatures represents an excellent prospect for satisfying the requirements of space applications because of its compactness, high efficiency and reliability. However, the working mechanism of a 4 K SPTC is more complicated than that of the Gifford McMahon (GM) PTC that operates at the relatively low frequency of 1–2 Hz, and has not yet been well understood. In this study, the primary operating parameters, including frequency, charge pressure, input power and precooling temperature, are systematically investigated in a home-developed separate three-stage SPTC. The investigation demonstrates that the frequency and precooling temperature are closely coupled via phase shift. In order to improve the cooling capacity it is important to lower the frequency and the precooling temperature simultaneously. In contrast to the behavior predicted by previous studies, the pressure dependence of the gas properties results in an optimized pressure that decreases significantly as the temperature is lowered. The third stage reaches a lowest temperature of 4.97 K at 29.9 Hz and 0.91 MPa. A cooling power of 25 mW is measured at 6.0 K. The precooling temperature is 23.7 K and the input power is 100 W.     

18.6 K single-stage high frequency multi-bypass coaxial pulse tube cryocooler

A single-stage high frequency multi-bypass coaxial pulse tube cryocooler (PTC) has been developed for physical experiments. The performance characteristics are presented. At present, the cooler has reached the lowest temperature of 18.6 K with an electric input power of 268 W, which is the reported lowest temperature for single-stage high frequency PTC. The cooler typically provides 0.2 W at 20.6 K and 0.5 W at 24.1 K with the input power of 260 W at 300 K ambient temperature. The cooperation phase adjustment method of multi-bypass and double-inlet shows its advantages in experiments, they might be the best way to get temperature below 20 K for single-stage high frequency PTC. The temperature stability of the developed PTC is also observed.     

Cascade pulse-tube cryocooler using a displacer for efficient work recovery

Expansion work is generally wasted as heat in a pulse-tube cryocooler and thus represents an obstacle to obtaining higher Carnot efficiency. Recovery of this dissipated power is crucial to improvement of these cooling systems, particularly when the cooling temperature is not very low. In this paper, an efficient cascade cryocooler that is capable of recovering acoustic power is introduced. The cryocooler is composed of two coolers and a displacer unit. The displacer, which fulfills both phase modulation and power transmission roles, is sandwiched in the structure by the two coolers. This means that the expansion work from the first stage cooler can then be used by the second stage cooler. The expansion work of the second stage cooler is much lower than the total input work and it is thus not necessary to recover it. Analyses and experiments were conducted to verify the proposed configuration. At an input power of 1249 W, the cascade cryocooler achieved its highest overall relative Carnot efficiency of 37.2% and a cooling power of 371 W at 130 K. When compared with the performance of a traditional pulse-tube cryocooler, the cooling efficiency was improved by 32%.     

Experimental investigation and optimization of small-scale helium liquefaction with multi-cryocoolers

Small-scale helium liquefiers using regenerative cryocoolers with cooling power up to 1.5 W at 4.2 K could be used to re-liquefy evaporated helium gas of small- and medium-sized cryogenic devices such as MEG and PPMS. A serial–parallel-path helium liquefier with a liquefaction rate of 83 Litres per day (L/d) using five 4 K G-M cryocoolers is developed, and has been applied to the Wuhan National High Magnetic Field Center (WHMFC) in China. Different from parallel-path helium liquefier, the helium gas is effectively, stepwise precooled by heat exchangers on multi-cold flanges, and thus the additional purifier and precooling coil heat exchangers on the thinner part of the cold head cylinder containing the 2nd stage displacer could be removed to simplify the construction. Through theoretical calculation and conclusive analysis, an optimum configuration is proposed and makes a reference to the design of serial–parallel-path helium liquefier with multi-cryocoolers.     

Study on a 10 W/90 K in-line pulse tube cryocooler

A Stirling-type in-line pulse tube cryocooler (PTC) has been designed, built and tested at Shanghai Institute of Technical Physics (SITP), Chinese Academy of Sciences. This PTC prototype can obtain a low-noise cooling capacity of more than 10 W at around 90 K cold head temperature and is used for cooling a space-borne infrared photo detector. In order to achieve a highly efficient PTC, a simplified numerical simulation model has been established for design and optimization. The simulation results of the regenerator, pulse tube and inertance tube are analyzed in detail. Besides, some key parameters of the PTC are listed in the paper. The PTC’s performances are tested at different operating frequencies from 42 Hz to 55 Hz and its reject temperature dependence is observed in the range of 290 K to 320 K. Furthermore, the map of the PTC’s performance characteristics is presented.     

Opposed piston linear compressor driven two-stage Stirling Cryocooler for cooling of IR sensors in space application

A two-stage Stirling Cryocooler has been developed and tested for cooling IR sensors in space application. The concept uses an opposed piston linear compressor to drive the two-stage Stirling expander. The configuration used a moving coil linear motor for the compressor as well as for the expander unit. Electrical phase difference of 80 degrees was maintained between the voltage waveforms supplied to the compressor motor and expander motor. The piston and displacer surface were coated with Rulon an anti-friction material to ensure oil less operation of the unit. The present article discusses analysis results, features of the cryocooler and experimental tests conducted on the developed unit. The two-stages of Cryo-cylinder and the expander units were manufactured from a single piece to ensure precise alignment between the two-stages. Flexure bearings were used to suspend the piston and displacer about its mean position. The objective of the work was to develop a two-stage Stirling cryocooler with 2 W at 120 K and 0.5 W at 60 K cooling capacity for the two-stages and input power of less than 120 W. The Cryocooler achieved a minimum temperature of 40.7 K at stage 2.     

Theoretical and experimental investigations on the cooling capacity distributions at the stages in the thermally-coupled two-stage Stirling-type pulse tube cryocooler without external precooling

The two-stage Stirling-type pulse tube cryocooler (SPTC) has advantages in simultaneously providing the cooling powers at two different temperatures, and the capacity in distributing these cooling capacities between the stages is significant to its practical applications. In this paper, a theoretical model of the thermally-coupled two-stage SPTC without external precooling is established based on the electric circuit analogy with considering real gas effects, and the simulations of both the cooling performances and PV power distribution between stages are conducted. The results indicate that the PV power is inversely proportional to the acoustic impedance of each stage, and the cooling capacity distribution is determined by the cold finger cooling efficiency and the PV power into each stage together. The design methods of the cold fingers to achieve both the desired PV power and the cooling capacity distribution between the stages are summarized. The two-stage SPTC is developed and tested based on the above theoretical investigations, and the experimental results show that it can simultaneously achieve 0.69 W at 30 K and 3.1 W at 85 K with an electric input power of 330 W and a reject temperature of 300 K. The consistency between the simulated and the experimental results is observed and the theoretical investigations are experimentally verified.     

CFD modeling and experimental verification of a single-stage coaxial Stirling-type pulse tube cryocooler without either double-inlet or multi-bypass operating at 30–35 K using mixed stainless steel mesh regenerator matrices

This paper presents the CFD modeling and experimental verifications of a single-stage inertance tube coaxial Stirling-type pulse tube cryocooler operating at 30–35 K using mixed stainless steel mesh regenerator matrices without either double-inlet or multi-bypass. A two-dimensional axis-symmetric CFD model with the thermal non-equilibrium mode is developed to simulate the internal process, and the underlying mechanism of significantly reducing the regenerator losses with mixed matrices is discussed in detail based on the given six cases. The modeling also indicates that the combination of the given different mesh segments can be optimized to achieve the highest cooling efficiency or the largest exergy ratio, and then the verification experiments are conducted in which the satisfactory agreements between simulated and tested results are observed. The experiments achieve a no-load temperature of 27.2 K and the cooling power of 0.78 W at 35 K, or 0.29 W at 30 K, with an input electric power of 220 W and a reject temperature of 300 K.     

Intermediate cooling from pulse tube and regenerator in a 4 K pulse tube cryocooler

This paper introduces intermediate cooling by thermally attaching heat exchangers on the second stage pulse tube and regenerator in a commercial 4 K pulse tube cryocooler. Due to the large enthalpy flow in the 2nd stage pulse tube and regenerator, both intermediate heat exchangers on the pulse tube and regenerator can provide cooling capacities in the temperature range of 5–15 K without or with minor effect on the performance of the 4 K stage. Extracting cooling capacity from the pulse tube or regenerator reduces the 1st stage cooling performance in the present study. The joint intermediate heat exchanger on the pulse tube and regenerator has demonstrated promising results for applications.     

A two-stage Stirling-type pulse tube cryocooler with a cold inertance tube

A thermally coupled two-stage Stirling-type pulse tube cryocooler (PTC) with inertance tubes as phase shifters has been designed, manufactured and tested. In order to obtain a larger phase shift at the low acoustic power of about 2.0 W, a cold inertance tube as well as a cold reservoir for the second stage, precooled by the cold end of the first stage, was introduced into the system. The transmission line model was used to calculate the phase shift produced by the cold inertance tube. Effect of regenerator material, geometry and charging pressure on the performance of the second stage of the two-stage PTC was investigated based on the well known regenerator model REGEN. Experimental results of the two-stage PTC were carried out with an emphasis on the performance of the second stage. A lowest cooling temperature of 23.7 K and 0.50 W at 33.9 K were obtained with an input electric power of 150.0 W and an operating frequency of 40 Hz.     

Numerical investigation on a 300 Hz pulse tube cryocooler driven by a three-stage traveling-wave thermoacoustic heat engine

A 300 Hz pulse tube cryocooler (PTC) driven by a three-stage traveling-wave thermoacoustic heat engine (TSTHE) has been proposed and studied in this paper. In the configuration, three identical thermoacoustic heat engine units are evenly incorporated in a closed traveling-wave loop, in which three pulse tube cryocoolers are connected to the branch of each thermoacoustic heat engine. Compared with the conventional thermoacoustic heat engine which involves a traveling-wave loop and a long resonator, it has advantages of compact size and potentially high thermal efficiency. A TSTHE–PTC system was designed, optimized and studied in detail based on the thermoacoustic theory. Firstly, numerical simulation was conducted to design the system thus the optimum structure parameters of the system were obtained. With the operating condition of 4 MPa mean pressure and high working frequency, a cooling power of 7.75 W at 77 K and an overall relative Carnot efficiency of 11.78% were achieved. In order to better understand the energy conversion characteristics of the system, distributions of key parameters such as acoustic work, phase difference, dynamic pressure, volume flow rate and exergy loss were presented and discussed. Then, the coupling mechanism of the system was investigated. In addition, influence of coupling position on the system performance was further studied.     

40 K single-stage coaxial pulse tube cryocoolers

Several 40 K single-stage coaxial high frequency pulse tube cryocoolers (PTCs) have been developed to provide reliable and low-noise cooling for GaAs/AlGaAs Quantum-Well infrared photodetectors (QWIPs). The inertance tubes together with the gas reservoir become the only phase shifter to guarantee the required long-term stability. The mixed regenerator consisting of three segments has been developed to enhance the overall regenerator performance. At present, the cooler prototype has achieved a no-load temperature of 29.7 K and can typically provide 860 mW cooling at 40 K with 200 W electric input power rejecting at 300 K. The performance characteristics such as the temperature stability and ambient temperature adaptability are also presented.     

Raytheon long life cryocoolers for future space missions

Over the last several years, Raytheon has made significant advances on two long-life cryocoolers designed for efficient operation on space platforms. The first is the Low-Temperature Raytheon Stirling/Pulse Tube 2-stage (LT-RSP2) hybrid cryocooler, which is capable of providing simultaneous cooling at 55 K and 10 K nominal first and second stage temperatures. The LT-RSP2 design was finalized in mid-2009, with fabrication of the prototype unit taking place in late 2009 and early 2010 and execution of the production program in 2011–2015. During this period the LT-RSP2 has undergone extensive characterization testing and has successfully been integrated with an optical bench. The second cryocooler is the Raytheon Advanced Miniature (RAM) cryocooler, a flight packaged single stage pulse tube cooler with an integrated surge volume and inertance tube. It has been designed for high frequency operation and has been fully optimized to make use of the Raytheon Advanced Regenerator, resulting in improved efficiency relative to previous Raytheon pulse tube coolers. In this paper, aspects of both the LT-RSP2 and RAM mechanical and thermodynamic designs will be presented as well as information regarding their capabilities and performance.     

High-capacity turbo-Brayton cryocoolers for space applications

Long-life, high-capacity cryocoolers may be needed for future space systems utilizing stored cryogens. The cooling requirements for planetary and extraterrestrial exploration missions, extended-life orbital transfer vehicles, and space depots may range from 10 W to 50 W at temperatures between 20 K and 120 K. Turbo-Brayton cryocoolers are ideal for these systems because they are lightweight, compact and very efficient at high cooling loads due to the high power density of rotary machines. These benefits are in addition to their inherent attributes of high reliability; negligible vibration; long, maintenance-free lifetimes; flexibility in integrating with spacecraft systems; and ability to directly cool remote and distributed loads. To date, space-borne turbo-Brayton technology has been developed for low cooling loads. The first space implementation of a turbo-Brayton cryocooler was in the NICMOS Cooling System (NCS). The NCS has been operational on the Hubble Space Telescope for over 3.5 years without any degradation. It provides 7 W of cooling at 70 K. The scaling of the technology to higher capacities is the subject of this paper.     

A lightweight low-current 10 K magnet for space-flight ADRs

Future space missions will include detectors and other components cooled to cryogenic temperatures by adiabatic demagnetization refrigerators (ADRs) coupled with mechanical cryocoolers. In such systems the ADRs require lightweight, low-current superconducting magnets. At least one of an ADR’s magnets must operate at the cryocooler’s coldest stage temperature. This temperature should be as high as possible in order to improve operating efficiency and design flexibility. Until now all space-flight compatible magnets have been made with NbTi wire, which has limited their operating temperatures to below about 5 K. We have developed a lightweight (1 Kg) low-current (8 A) Nb₃Sn magnet which produces a 3 T central field at 10 K. We explain the choice of this magnet’s specifications and describe its performance testing.