STIRLING CRYOCOOLER

Stirling Cryocooler

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INDEX:

ACKNOWLEDGEMENT………….………………………………………………………..4

ABSTRACT……………………………………………………………………………………..5

INTRODUCTION……………………………………………………………………………..6

HISTORY…………………………………………………………………………………………7

THEORY……………………………………………………………………………………….11

CLASSIFICATION……………………………………………………………………………13

WORKING………………………………………………………………………………………15

DESIGN APPROACH……………………………………………………………………….18

APPLICATIONS………………………………………………………………………………20

REFERENCES…………..……………………………………………………………………21

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ABSTRACT:

A Stirling engine is a machine which converts heat energy into mechanical work. It can be used for pumping water, generating electricity or turning industrial machinery. It does not need high quality refined fuels such as petrol or diesel to make it run, but can work on any source of heat. This makes it suitable for applications in Developing Countries, where these fuels are difficult or impossible to obtain, and where other types of combustible materials are locally more readily available. Stirling engines can also work on solar or geothermal energy or waste heat from industrial processes.

The modern uses of Stirling engines are invisible to almost everyone. There have been many research engines built in recent years but there are only three areas where Stirling engines have made a dramatic impact. There are Stirling engines in Submarines, stirling machines used as cryocoolers, and Stirling engines in classrooms. Cryogenics is the science of things that are exceedingly cold and Stirling engines are one tool that can be used to make things exceedingly cold. It's not obvious but a Stirling engine is a reversible device. If you heat one end and cool the other, you get mechanical work out, but if you put mechanical work in, by connecting an electric motor, one end will get hot and the other end will get cold. If you

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design the machine correctly, the cold end will get extremely cold. In fact, Stirling coolers have been made that will cool below 10 degrees Kelvin. Micro Stirling coolers have been produced in large numbers for cooling infrared chips down to 80 degrees Kelvin for use in night vision devices.

INTRODUCTION:

Development of small, efficient cryogenic coolers has been fostered by the requirements of infra-red detection systems. Infra-red system cryocooler requirements of infra-red include high efficiency, compactness , low weight, low vibrations and long, maintenance-free operation in the 20 to 80o k temperature range. Stirling cycle coolers have proven to be well suited to these requirements. More recently, stricter requirements have arisen from increasing interest in superconducting quantum interference devices (SQUID) and magnetic gradiometers. For these devices, it is desirable to have coolers which operate at temperatures below 10o k. in addition to the efficiency, compactness, weight, vibration and reliability considerations, SQUID devices require that the associated coolers produce extremely low magnetic noise and low temperature fluctuations.

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(Models of the stirling cryocoolers)

HISTORY:

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"...These imperfections have been in great measure removed by time and especially by the genius of the distinguished Bessemer. If

Bessemer iron or steel had been known thirty five or forty years ago there is scarce a doubt that the air engine would have been a great success... It remains for some skilled and ambitious mechanist in a

future age to repeat it under favorable circumstances and with complete success..."

              Rev'd Dr. Robert Stirling, 1876              from "Stirling Engines" by G. Walker

The Rev'd Robert Stirling applied for the first of his patents for this engine and the 'Economizer' in 1816, a few months after being appointed as a minister in the Church of Scotland at age 25. Others such as Sir. George Caley had devised air engines previous to this time (c. 1807) and other devices called air engines were known as early as 1699. The 'Economizer', or regenerator, has come to be recognized as a most important portion of the patent of 1816. These innovations were even more remarkable in light of the fact that they preceded the birth of thermodynamics and the writing of M. Sadi Carnot by some 40 years!

       Some historians have indicated that the reason for Rev'd Stirling's efforts at such a device were driven by his concern for the working people of his parishes as steam engines were being used extensively in

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that area and time period. Due to the lack of strength in the materials available to construct boilers ('Bessemer Iron', or Steel, was not yet available), they would frequently

explode with devastating results on the people working nearby. The effects of high pressure steam on the human body are quite awful as anyone who has experienced a steam burn in the kitchen can attest.        So Rev'd Stirling invented a safer (and more efficient) replacement for the steam engine, in order to save lives and improve the conditions of his parish life. Stirling's engine would not explode because the pressures were not elevated to that level... The machine simply stopped if the heater section failed.

       The best recorded implementation of these efforts was at the Dundee Foundry Company where Robert's brother, James Stirling, was employed. James was a very good engineer and a driving force in the implementation and perfection of the Stirling invention. A very large double-acting-piston machine with not one but two heater/displacer sections was built at the foundry under his direction (and we presume design). This engine powered the foundry for some years before material failures at inopportune times caused it to be replaced by a steam engine.        With Bessimer's discovery of a process to mass produce quality steel, steam engines became more powerful and much safer to operate, and so the Stirling engine nearly faded into obscurity.

(The early days stirling engine)

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Since then, much kind of Stirling engines were manufactured with a view to increasing the output and the efficiency. But the engine had been made a poor production, in the back of Otto engine and Diesel engine. Because Otto engine invented by N. Otto on 1877 and Diesel engine invented by R. Diesel on 1893 were higher capacity than Stirling engine in those days.

Stirling Engine was taken notice again, when PHILIPS Corporation started to research on Stirling engine as the power source of portable generator in about 1940. A 200 W Class Stirling engine was completed in about 1950. But the engine was not spread, because the transistor that can work at low power was invented.

(early stirling cryocooler of Philips made) (2kw cryocooler S.R., Japan)

Since then, Stirling engine had been studied in Europe and America. The engine had been started to develop rapidly with effect of oil-shock in 1973. In Japan, the engine was started to research later than Europe and America about 20 years.

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Then started the applications of the wide applicationed Stirling engine in the various fields of the technology in science and research.

Now, Stirling engine is studying lively in the world. For examples, Solar Stirling engine, hot spring Stirling engine, underwater Stirling engine, air-conditioning Stirling system, Stirling refrigerator and .....

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THEORY:

The whole lot story of the stirling cryocooler moves round the theory of the stirling cycle. So the question arises, what is stirling cycle?

The diagram shown below describes the process of the stirling cycle,

(P-V diagram) (T-S diagram)

1->2 constant-temperature compression at TL 2->3 constant- volume heat absorption 3->4 constant- temperature expansion at TH 4->1 constant-volume heat rejection

1 : Isothermal compression process

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In this process, both power pistons move upward, and the working gas is compressed. The engine is worked from the outside and discharges heat to the outside.

2 : Constant volume heating process With this process, the power piston on the side of a compression is upper, of expansion moves under. Working gas in the engine becomes a high temperature and flows to expansion space, as passing through a heater. A gas pressure in the engine increases.

3 : Isothermal expansion process In this process, both power pistons move to the lower side, and the working gas expands. The engine works to the outside.

4 : Constant volume cooling process In this process, the power piston on the side of a compression is lower, and of expansion moves upward. Working gas in the engine becomes a low temperature and flows to the compression space, as passing through a cooler. The gas pressure in the engine drops.

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CLASSIFICATION:

There are basically many a type of cryocoolers but yet these cryocoolers are classified as follows:

Joule-Thomson coolers.Stirling Coolers.Gifford & MacMohan coolers.Pulse tube coolers.

Joule Thomson Coolers:

In the early 1850s, two British scientists J.P. Joule and William Thomson (later Lord Kelvin) performed experiments on the expansion of gases. They expanded gases from a high pressure through a porous plug under adiabatic conditions. Under these conditions, the expansion is also isenthalpic and the temperature of the gas increases or decreases depending on the sign of the Joule-Thomson coefficient = (T/P)H . Below the so-called inversion temperature, is positive and cooling can be obtained as the result of an expansion. This inversion temperature depends on the pressure, and the maximum value (i.e., at P = 0) for helium is 40 K, for hydrogen 205 K, for neon 250 K, and for nitrogen 621 K. A cooler utilizing this effect, consists of a compressor, a control and filter unit, a counter-flow heat exchanger (CFHX), and a Joule-Thomson expansion stage including a porous plug or another type of flow restriction (the JT-valve). For an ideal CFHX, the cooling power of a JT cooler is given by the product of the mass-flow rate and the difference in specific enthalpy between the high and low-pressure

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sides at the warm end of the CFHX . Lowering the warm-end temperature, i.e., by pre-cooling the fluid before it enters the CFHX can increase this enthalpy difference, and thus the cooling power. Very large enthalpy changes can be obtained by pre-cooling to a level such that the fluid is liquefied before the expansion. In that case, the expansion valve is usually referred to as a throttling valve rather than a JT-valve . Another way to attain large enthalpy differences is to mix hydrocarbons with higher boiling points into the working fluid . The much higher enthalpy difference allows for a one order of magnitude lower pressure ratio and, as a result, straightforward air-conditioning-type compressors can be used. For example, APD cryogenics developed a commercial cooler known as the “Cryotiger” shown in Figure . A refrigerant mixture is used here in a single flow stream. The advantage of these JT-type coolers with regard to SQUID cooling is the absence of cold moving parts, which helps attaining a low level of interference. Furthermore, the cold stage can be constructed in a very simple and straightforward way and is well suitable for miniaturization . The biggest problem with these JT-type coolers is clogging of the cold stage caused by moisture and contamination. The lower the operating temperature, the bigger this problem is. In the case of 4 K cooling, a further disadvantage is that a helium-based JT cooler has to be pre-cooled to typically 20 K, because of the inversion temperature. As a result, a 3-stage cooler is configured using a 2-stage Gifford-McMahon pre-cooler. Three-stage coolers consisting of helium JT loops pre-cooled by two-stage GM coolers were used to cool low-Tc SQUIDs by Biomagnetic Technologies Inc and by Daikin . For cooling of high-Tc SQUIDs (from 300 K to below 80 K) nitrogen can be applied in a single stage, but this is very inefficient . As an alternative, gas-mixture coolers can be used in spite of problems caused by de-mixing and clogging. Cooling of high-Tc SQUIDs by a

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“Cryotiger” was investigated at the Research Center Jülich and at the University of Twente.

Gifford-MacMohan coolers:

One disadvantage of the Stirling cooler was the inability to separate by a large distance the compression and expansion sections, thus limiting the flexibility of the cooler. At the end of the 1950’s, Gifford and McMahon developed a concept separating the compressor and expander . In their approach, the compressor is just a high-pressure generator that is connected to the expansion unit by flexible gas lines that can be up to several meters long, and an active-valve unit that generates the pressure wave. Besides the advantage of separation, the compressor can be a standard oil-lubricated air-conditioning type compressor, implying much lower cost. Of course, provisions are included for filtering the compressor oil out of the working fluid. The cold head is similar to that of a Stirling cooler. The phasing of the displacer movement with respect to the pressure wave is set and controlled by means of the mechanics that drive the valve unit and the displacer/regenerator. In contrast, the Stirling cooler is a mechanically resonant system in which the proper phasing has to be tuned in the design. GM-coolers with helium gas as the working fluid were primarily developed for cryopumping and are now available from a large number of suppliers among who are the biggest suppliers of vacuum equipment. Cryopumps are the largest commercial application of cryocoolers (roughly 20,000 units per year ). In addition, GM-coolers are installed in MRI systems for reducing the boil-off rate of the liquid helium bath. These coolers are commonly manufactured in single-stage and double-stage versions. The latter type has cooling powers of several watts at the coldest stage (typically at 20 K) and several tens of watts at the other stage (at 80 K) with an input of a few kilowatts . Also, two-stage GM coolers are available that can cool to 4 K by using rare-earth regenerator materials. These materials have magnetic phase transitions in the temperature range of 4 K to 10 K, and thus a high latent heat in that temperature range . The operating frequency of the displacer and the valves is relatively low, in the range of a few Hz.

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A two-stage GM cooler equipped with an Er3Ni regenerator was used to cool two low-Tc SQUIDs by researchers from SSL and ETL in Japan . At the University of Twente, a helium-gas circulation was used to cool a high-Tc SQUID by means of a “normal” two-stage GM cooler.

Pulse Tube Coolers:

An important drawback of both Stirling and GM coolers is the presence of a moving displacer in the cold part of the cooler. This displacer limits the lifetime (due to wear) and, because of its movement, it can generate noise (vibrations and EMI ). In 1963, Gifford and Longsworth invented a regenerative refrigeration technique that did not need a displacer . They called their invention a pulse-tube refrigerator. An improved concept of it was introduced by Mikulin et al. in 1984 and later refined by Radebaugh and co-workers . In this concept, an orifice was inserted at the warm end of the pulse tube to allow some gas to flow into a relatively large reservoir volume. Here, both ends are at environmental (high) temperature TH, whereas the cold spot is in the middle at low temperature TL. The dense vertical lines in the figure symbolize heat exchangers (HX). Similar to the Stirling cycle, gas is compressed in step a  b with rejection of heat QH1 to the environment followed by a regenerative step b c. In this step, the gas is forced to flow through the regenerator and is thus cooled from TH to TL. The expansion of the working gas (c d) is not against a piston as is the case in a Stirling cooler but against a compressible gas volume in the pulse tube. This gas volume can be considered as a freely moving piston. The heat of expansion QL is absorbed at the cold heat exchanger. By means of the gas piston the mechanical work of expansion is transferred to the warm end of the tube. The associated heat QH2 (which equals QL) is rejected to the warm environment. The

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cycle is closed with the regenerative step d a. The orifice and the reservoir are needed to establish the proper phasing between the gas movement in the tube and the pressure wave imposed by the compressor. In an electronic circuit analogy, the pressure and gas flow can be seen as the voltage and current respectively, whereas the orifice plus reservoir act as an RC load. The schematic diagram of Figure 7.3 depicts a so-called in-line arrangement, in which regenerator and pulse tube are in line. Because this implies two warm ends and the cold spot in the middle, this arrangement is difficult to integrate with a device. A more compact design is possible by reversing the gas flow at the cold heat exchanger in a U-shape (regenerator and pulse tube side by side connected by the cold heat exchanger) or in a coaxial arrangement (regenerator coaxially outside the pulse tube). The advantage of these arrangements is a more compact design with a single warm end and a cold end, similar to other coolers. The advantage is a slightly lower power efficiency compared to the in-line configuration, because of losses in the flow reversal. In the last few years, pulse-tube coolers have been improved to such a level that their efficiencies at cryogenic temperatures are comparable to those of Stirling coolers. The most important improvement was a so-called double inlet that was introduced by Zhu et al. in 1990 . They added a by-pass from the warm end of the pulse tube to the inlet of the regenerator. In that by-pass a second orifice was placed. The benefit of this by-pass plus second orifice is two-fold: first, it provides an extra parameter in optimizing the phasing between mass flow in the tube and pressure wave at the inlet; second, part of the gas flow that is required for expansion and compression at the warm end of the pulse tube is taken directly from the inlet, instead of passing through the regenerator and the pulse tube. A reduction in mass flow through the regenerator means lower regenerator losses, and thus a higher efficiency, especially at higher operating frequencies. Pulse-tube coolers may exhibit temperature stability problems because of unwanted gas flows. This may be a dc flow through the double inlet or gravity-induced secondary streaming in the tube . Apart from these problems, it is clear that the pulse-tube cooler has great advantages over the Stirling or the GM cooler: the absence of moving parts in the cold results in longer life, lower interference, and lower cost. Moreover, a pulse-tube cooler is less

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sensitive to side loads (i.e. forces in the radial direction) and less sensitive to contamination coming from the compressor. Two types of pulse-tube cryocoolers can be distinguished: A Stirling-type pulse tube is directly connected to the pressure-wave generating compressor, whereas a GM-type pulse tube is connected to an active-valve unit connected to a compressor. In terms of cooling power, input power, size, operating temperature and frequency, these pulse-tube coolers resemble their Stirling and GM-type counterparts. GM-type pulse-tube coolers are also available for the 4 K range . The application of pulse-tube coolers for cooling SQUIDs was so far restricted to high-Tc SQUIDs. These investigations were especially carried out in Germany at the Universities of Giessen and Jena. Research was also conducted by a collaborative effort of CNRS and Air Liquid in France

WORKING:

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The Stirling Engine was named by Dr. Rolf. J. Meijer who at that time was a project manager with Philips of Holland.  Philips was struggling with creating a new name to call the 'Air' engine when there was no air inside the engine.  This is because in an Air engine, the air inside the engine is called the 'working gas'.  If you change the 'working gas' to a gas like Helium or Hydrogen, then it can no longer be called an 'Air' engine.  The name Stirling Engine was chosen in honor of the inventor of the regenerator (economizer) and the engine that demonstrated its use.

The Stirling Engine's most basic configuration consists of two pistons each in its own cylinder.  (Sometimes it is easier to envision these two cylinders as one long tube with the piston heads facing each other inside the tube (see the figure below)). Note that between these two pistons heads are the heater, cooler and regenerator.  The regenerator (usually a block of woven wire) is in the center of this tube and the heater is between the regenerator and one piston (in red) while the cooler is between the regenerator and the other piston (in Blue). The volume attached to the 'heater' is the 'expansion space'

Stirling Cryocooler

Cooler

Regenerator

DisplacerPiston

Displacer Flexures

PowerPiston

Power Piston Flexures

Linear AlternatorStationary Magnets Moving Iron

Heat Supplied by acetonepipes

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Where the hot gas pushes against the 'expansion piston'.  The volume attached to the 'cooler' is the 'compression space'.  

The regenerator is where the excess heat of the gas is stored in the regenerator  matrix on the way to the compression space from the expansion space and then the heat is recovered on the way back from the compression space to the expansion space.

Graphic courtesy of Dr. Israel Urieli of Ohio University.

Stirling Engine operation can be explained in a somewhat non technical way that applies to many but not to all engines that may be called Stirling Engines. 

We begin with the heater space and cooler space at their appropriate temperatures. The working gas trapped between the two piston heads is pushed by the Compression Piston through the regenerator where it is heated by the energy stored in the regenerator to the maximum temperature present in the regenerator  into the Heater volume where the gas expands due to increased temperature into the Expansion Space.  This results in an increased pressure pushing on the Expansion Piston so that it moves away from the regenerator pushing on a mechanism which changes the linear movement of the piston to a rotary motion.  This continues until all the gas that will expand has been pushed into the heater area and expanded.  The mechanism continues to push the Compression Piston

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further toward the Regenerator pushing all the gas out of the Compression Space into the gas circuit (heater, cooler, regenerator).

Which both pistons are connected (but 90 rotary degrees apart), now begins to move the Expansion piston back the other way pushing the hot gas back through the Heater and then to the Regenerator and finally into the Cooler where it begins to Cool and contract (the pressure starts to drop).  The Compression Piston is also moving away from the regenerator while the Expansion piston comes toward the regenerator moving the gas through the regenerator into the compression space without compressing the gas.

While doing this compression process in the expansion area the heat is provided to the cold tip of the cold finger through the acetone tubes which extract the heat from the controlled volume. Hence the controlled volume experiences the cold temperature. The power of the suction of temperature is so high that the temperature of the volume is taken to around of the absolute zero. Though the ambient conditions are the normal temperature and pressure.

While working in the Stirling engine the heat is provided by the radioisotopes of the devices located at the cold finger’s tip. And hence the heat energy is made to be converted to the mechanical work done.

The efficiency of this working makes it large about 50% which is extremely high comparatively to the rest of such race devices.

The linkage continues to move the pistons until the Compression Piston is at its extreme and the Expansion piston is all the way forward.  At this point the mechanical arrangement moves the pistons together but because of the way the piston moves up and down in the cylinder but the mechanism is moving in a circle, the Expansion Piston  does not move very far but the Compression Piston moves toward the regenerator actually compressing the gas and beginning to push the gas through the regenerator. (That is why it is called the Compression Piston.)

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(The following is the test section geometry of the cold finger)

(Test section of the coldfinger)

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Regenerator

One of the regenerator screens

Heater

Flow distributor

Plenum

Cylinder

Isolation duct

Piston

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Some of the results which were being carried out by the BEI Company located at Oregon, are shown below on the basis of the respective graph plotting:

(Effect of regenerator length)

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A 73K high-performance cooler has been analyzed, designed, fabricated and tested at BEI, which delivers 150mW of cooling with a mere input power of10.5 W at 60C ambient, and 7.1W at 23C. The cooldown time to 73K is approximately 4 minutes and 20 seconds. At 78K, the input power is 6.3W for a heat load of 150W at 23C ambient.

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DESIGN APPROACH:

THERMODYNAMIC CONSIDERATIONS:

The displacer moves within a thin walled titanium cold finger.

Metal regenerators are frequently used but if not to be used then the theory of extended gap or a non-metallic regenerator are to be used. The extended gap approach typically suffers the disadvantage of an unusually long dispatcher ,low cooling capacity potential electromagnetic interference(EMI) from static electric charge accumulation on the moving displacer, and short duration of continuous operation due to possible cryopumping of organic vapors toward the cold tip.

The Magnetic interference produced by eddy currents induced in the moving metal displacer by the earth’s magnetic field where estimated to be extremely low. This is due to the relatively high electrical and thin construction of the titanium displacer shell as well as the low displacer amplitude and frequency. Thus it was decided to proceed with metal regenerators.

The most frequently used regenerators are the phosphor-bronze in the metallic type and lead in the non-metallic type.

DYNAMIC CONSIDERATIONS:

Motion can be produced by a mechanical linkage connected to a crank drive or by separated displacer linear motor, or by resonance. The mechanical linkage requires extra bearings and seals, and a separate driver requires more space and power. The resonant approach was chosen.

PISTON/MOTOR:

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Eddy current losses are reduced by laminating the stator with radial cuts. To minimize the side loads on the piston seals and bearings and to meet the mass requirement for the piston assembly a moving coil rather than moving magnet motor design was chosen.

The piston seal and bearing consist of thin layer of rulon (a reinforced Teflon) bonded to the contacting surfaces of the piston assembly. The ruler surfaces sliding along the cylindrical wall of the compression space acts as the bearing as well as the seal. A second bearing at the opposite end of the piston assembly also consist of the rulon surface sliding on a guide which prevent rotation of the piston assembly.

The piston assembly is made of titanium to minimize armature weight

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And to match the thermal expansion of the permendure motor stator. Shock pads located at both extremes of the stroke prevent damage during transit or

in the event of over stroke during operation. The remainder of the motor housing is made of aluminum for low weight, low cost, and ready availability of material in the required dimensions.

DISPLACER/REGENERATOR:

Annulus type heat exchangers are used to remove thermodynamic waste heat, and to transfer heat from the device to be cooled to the working fluid. A linear variable differential transformer (LVDT) monitors the displacer position.

COLD FINGER:

For low heat leak and high mechanical strength, the cold finger is made of titanium. At the cold tip there is a lead cap for damping temperature variations. The lead provides a large heat capacity for thermal damping.

Stainless-steel coaxial signal leads (Uniform Tubes UT -85SS) are provided for connection to the cryogenically cooled device which is attached to the end of the cold finger. The leads are cooled continuously along their length by coiling them about the cold finger in contact with the titanium wall.

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APPLICATIONS:

In the satellites to preserve the testing materials like the urine, saliva, or other materials so that can be experimented later.

Precious things (unearthly bodies)

in the experimental area are cryogenically cooled to provide the specimen the local environment of its own planet. This would help in experimentation process.

Provide a safe working and storage environment to the delicate things

like sperms, which need liquefied nitrogen for the storage. This liquefied nitrogen is maintained at low temperature only.

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The infra-red systems for the night vision specialty purpose are being nourished by

the cryocooler environment.

REFERENCE:

Dr. Terry W. Simon.

W. Taylor, “ Design guidelines for stirling cryocooler: Cryogenics”.

W. Martini, “Stirling engine design manual”.

Cleveland State University.

Gedeon Associates.

NASA Glenn Research Center.

Stirling Technology Company.

Sunpower Inc., Athens.

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