The Space Congress® Proceedings 1984 (21st) New Opportunities In Space

Apr 1st, 8:00 AM

Magnetic : The Promise and the Problems

J. A. Barclay Project Leader, Group P-10, MS-K764, Los Alamos National Laboratory, Los Alamos, NM 87545

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Scholarly Commons Citation Barclay, J. A., "Magnetic Refrigeration: The Promise and the Problems" (1984). The Space Congress® Proceedings. 3. https://commons.erau.edu/space-congress-proceedings/proceedings-1984-21st/session-6/3

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J. A. Barclay, Project Leader Group P-10, MS-K764 Los Alamos National Laboratory Los Alamos, NM 87545

ABSTRACT tests, but several studies of efficiency as a function of cooling power of gas-based Magnetic refrigeration uses the - for cryogens have been and field-dependence of the of some published, including compilations of mass magnetic materials to accomplish cooling. and volume.2-7 The main sources of ineffi­ Because of the intrinsically high efficiency ciency are the room-temperature compressors of the magnetization and demagnetization with their associated aftercoolers and the process and because of the potential for gas expanders. Generally, the compressors excellent transfer between solids and and expanders are the least-reliable compo­ fluids, magnetic refrigerators promise to nents too. All of the efficiency studies have higher efficiency than existing gas- agree that ^35% of Carnot efficiency is the cycle refrigerators. Many ground-based and best that is now possible, but then only for space-based applications could benefit very large plants. For smaller machines, significantly from the cost savings implied the fraction of Carnot efficiency can become by higher efficiency. Other attributes of very small, e.g., 1-W refrigerators gener­ these devices are high reliability and low ally operate in the range 2-8% of Carnot. volume and mass per unit cooling power. The One study 5 on liquid hydrogen indicates that development of these refrigerators is under­ no significant advances in liquefaction way at several places around the world, efficiency are probable by the year 2000 including the Los Alamos National Laboratory. unless very large investments are made in The progress to date has been encouraging research and development. Therefore, it is but some problems have been clearly identi­ important to investigate other promising fied. The arguments for high efficiency and technologies to see if they offer higher the problems that will need to be solved to efficiency with less investment. One such achieve this goal are discussed. technology is magnetic refrigeration (MR). MR exploits the temperature and magnetic- field dependence of the magnetic entropy of INTRODUCTION some solid materials to extract heat from a low-temperature source and transfer it to a Important features of refrigerators include higher temperature sink. The temperature efficiency, reliability, lifetime, size, and change associated with the application or mass. The efficient liquefaction of cryo- removal of a to an isolated gens, such as liquid hydrogen, is important sample is called the magnetocaloric effect. because the liquefaction cost is a signifi­ This paper presents several reasons why cant fraction of their total cost. The magnetic refrigeration offers significant costs of liquid nitrogen and liquid oxygen promise and identifies some of the problems are also tightly coupled to the cost of that must be overcome before this promise is refrigeration for air separation. At realized. present, all of the refrigeration required to produce cryogens is provided by gas-cycle devices in which gas is compressed in one FUNDAMENTALS OF MAGNETIC REFRIGERATION^ part of the cycle to reject heat from the gas and expanded in another part of the Any refrigeration system requires some mecha­ cycle to cool and eventually liquefy the nism to increase or decrease the entropy of gasJ Lifetime and reliability studies of a working substance. In addition, heat refrigerators are limited to specific design transfer at appropriate times during a cycle

6-10 is necessary. Further analysis of various cycle. From reference 8, the rate of work magnetic designs produces a list of a magnetic cycle can be written as of the functions required to execute the thermodynamic cycles. A summary of functions IdT in the basic magnetic cycle over a large • • /TH \ TH)W,(T-T temperature span such as 20 to 77 K is given w = Q ( Jl. i I + —J—L±—— below: \ Tc / magnetization of ferromagnets near the hot temperature to reject heat to a sink such as liquid nitrogen, by circulation of a heat exchange fluid, such as J IRR.dT gas; (1) regeneration of the magnetic material to dT cool it to near the cold temperature p (20 K); this regeneration requires heat and the relative efficiency can be written transfer between an external regenera­ as tive material or the magnetic material itself and a moving fluid; demagnetization of the ferromagnet near n . (2) the cold temperature to absorb heat from a load, such as hydrogen by circulation of a heat exchange fluid, such as helium where Qc is the reversible cooling power, gas; and Wj is power added externally to move fluid, regeneration of the magnetic material to qj is power introduced through heat conduc­ warm it to near the hot temperature tion from the surroundings, andASiRR is the (77 K); as before, this regeneration rate of irreversible entropy production from requires heat transfer between a regener­ different mechanisms. The irreversibility ator material and a moving heat transfer due to heat transfer across a temperature fluid. difference in the regenerative parts of the The work for the magnetic cycle is provided cycle was retained in the expressions above by a motor working against the difference in and a quantitative expression was evaluated, forces during the magnetization and demagnet­ given as ization. The cycle requires either moving the material or the or some equivlent change. The rate of movement through the WTOTAL = QC | ~ " magnetic cycle is very slow compared to many c gas cycles; ranging from ^0.1 Hz to ^1 Hz.

( TH . THE PROMISE OF MAGNETIC REFRIGERATION dT K" IRR Magnetic refrigeration is an unproven techno­ (3) logy but it does have several attributes which make it worthy of development. The first and perhaps most exciting is the pros­ pect of much higher efficiency than existing gas-cycle refrigerators or liquefiers. In Following the analysis, th,e resultant effi­ general terms, this claim is based upon two ciency expressions were written as arguements. First, the magnetization and demagnetization of a magnetic solid provides •(TH -Tc) an entropy change that is reversible at low (4) frequencies, such as 1 Hz which are the normal operating frequencies of magnetic refrigerators. Secondly, the heat transfer for the temperature range where the heat in magnetic refrigerators is between a solid capacity of the magnetic solid is approxi­ and a gas, which is intrinsically better mately constant. N|U is the number of heat than gas-to-sol id-to-gas, as is required in transfer units in the regenerative-cycle step gas-cycle devices. andATc is the adiabatic temperature change at the cold end of the cycle. The hot and An analysis of the sources of inefficiency cold are T^ and Tc , respec­ in magnetic cycles^ clearly indicates that tively. When the solid is a the biggest fundamental source of irrevers- linear function of temperature, the effi­ ibility in magnetic refrigerators is that ciency expression becomes due to heat transfer across a temperature difference in the regenerative parts of the

6-11 Associated with the above problem is the (5) reduction of the adiabatic temperature change by the thermal addenda of the struc­ ture containing the refrigerent. The problem reduces to generation of clever The analysis in reference 8 shows that designs that can satisfy the structural magnetic refrigerator spanning 4 K to 300 K demands and, at the same time, have can be considered in three ranges on the extremely small thermal mass. basis of regeneration. The range 4-20 K needs little or no regeneration, the range For ferromagnets near their Curie tempera­ 20-150 K needs regeneration where the heat ture, the adiabatic temperature change is capacity of the working solid is approxi­ almost linearly proportional to the field mately linear in T, and the range 150-300 K strength. Therefore, the largest practical needs regeneration where the heat capacity magnetic field must be used to produce the is approximately constant. Projected effi­ largest adiabatic temperature change. For ciencies for several 4-20 K stages are «70% standard solenoids, made from readily avail­ of Carnot. Optimistic estimates of numbers able NbTi superconductor, 9T is an upper for a magnetic refrigerator operating above limit at 4.2 K, but fields above 10 T can be 20 K are N|U «500 andATc «15 K. Substi­ reached if the magnet is cooled to 1.8 K. tution of these values into Eqs. (5) and (4) If Nb3$n is used as the superconductor, for the 20-150 K and 150-300 K range, respec­ magnetic fields of 15 T can be obtained but tively, gives n (20-150 K) = 81% and n at the expense of significant manufacturing (150-300 K) = 98%. When the efficiencies of difficulties. Magnetic refrigerator designs the three stages are combined, the resultant have been made that use racetrack and helm- efficiency is a very impressive 62% of Carnot hoi tz or even more exotic-shaped coils. for a 4.5-to-300 K magnetic refrigerator. Because these coils have a poorer ratio of For hydrogen liquefaction applications, the field at the center of the coil to field in amount of cooling power at 4 K to maintain the windings than for a solenoid, it is more the liquid-helium for the will be difficult to achieve high fields in the very small compared to the cooling power at working space. If shapes other than sole­ and above 20 K. In this case, the potential noids are needed, a development effort will efficiency for the example above approaches be required to achieve magnets providing 80% of Carnot! Clearly, the other irreversi- working fields of 9-10 T or higher. bilities in the refrigerator will lower the efficiency but they should be small compared Equation (4) also shows that the number of to the regenerative heat transfer irreversi- heat transfer units in the regenerative bility and so the overall efficiency is parts of the cycle strongly influences the overall efficiency. The N tlj is defined as potentially extremely high compared to 35% _ * t * f\ i • . . ^^i. or less for conventional systems.

THE PROBLEMS OF MAGNETIC REFRIGERATION p are the mass flow rate and heat capacity of the fluid, respectively. Because the As with any new technology, there are some conductance between the fluid and solid is problems that must be solved in order to intrinsically limited by the thermal conduc­ achieve the final result. The equations tivity of the boundary layer of fluid on the already presented pinpoint some of the prob­ solid, the contact area has to be increased lem areas* From Eq. (4) it is clear that as much as possible to obtain the highest the efficiency is strongly dependent upon N^u . Geometries such as tubes, sheets, and the adiabatic temperature change ATC . For particles have very large surface areas per metal near its unit volume if the dimensions are carefully (290 K), the adiabatic.temperature change is chosen. However, the N^ u can not be arbi­ approximately 2 K per Tesla up to ^9 T for trarily increased by increasing the contact a upper limit of ^20 K. When 1ntermetal11c area because the pressure drop also rises coumpounds of gadolinium are used to obtain rapidly in geometries where the surface area the lower Curie temperatures, for example, is increased by making all of the flow chan­ GdNi at ^7Q K, the adiabatic temperature nels very small. The longitudinal conduc­ change decreases because of the added . tion of the geometry is also of Importance Measured values'" of the adiabatic tempera­ in the design of efficient regenerators. ture change for GdNI show about 12 K at 9 T The optimum balance of heat transfer,,, pres­ right at the Curie temperature. Average sure drop, and longitudinal conduction in AT's over a 20-30 K span about the Curie magnetic-material structures suitable for temperature are 25-40% less. Therefore, one cyclic movement to execute a regenerative problem is to find magnetic materials that cycle is definitely one of the key problems have the largest possible adiabatic tempera- in magnetic refrigeration. tyre changes.

8-12 The structural design is important because CONCLUSION of the large forces between the magnetic solid and the magnetic field of the magnet. The potential elimination of the compressor One of the problems that must be carefully and after-cooler of a conventional gas-cycle addressed during the design is how to cancel liquefier and the possibility of excellent the magnetic forces from one part of the regenerative heat transfer gives MR promise cycle against those of another part of the for much higher efficiency than gas cycle cycle. For example, in a reciprocating devices. Higher efficiency has many cost design, the cylinder of magnetic solid is benefits for large-scale ground based appli­ strongly attracted into the magnet during cations, such as liquefaction of hydrogen magnetization and slightly more strongly for shuttle flights. Space-station appli­ attracted by the magnet during demagneti­ cations, such as cryogen fuel storage and zation. The difference between these two handling, will also need efficient refriger­ large forces is the net force that provides ators to help reduce system costs. High the cycle work. If two cylinders are efficiency is one of the most exciting arranged so that the forces almost cancel, features of MR, but because of the low speed the structure separating the two cylinders and relatively few moving parts, the must be designed to withstand the large reliability and lifetime of these devices compressive stress but not thermally short also promises to be excellent. For some circuit the cylinders. space missions, this feature is even more important than the efficiency. Designs with Two further problems that must be addressed clearance seals and hermetic power couplings are illustrated by the following expression are possible and should add to the reli­ for the rate of total refrigerator work, ability. Bearing lifetime will have to be i.e., considered but balanced mechanical loads should avoid serious limitations here. "REF Two further desirable features of MR are the (6) potential for smaller mass and volume per "TOTAL n dn- ve n pump unit cooling power. These features result where WREF is the rate of work from the from using a solid working material instead refrigerator cycle, n^rive 1S the efficiency of a gas. The magnet/dewar combination is of the motor and drive providing the refrig­ more compact than the compressor system when erator work, w is the external work rate all but very small cooling powers are from the fluid pumps and np ump is the pump required. efficiency. The pump^work rate w is gener­ ally much lower than WREF so the drive-motor While many desirable features are possible and gear-mechanism efficiency is directly if magnetic refrigerators are successfully related to the overall refrigerator effi­ developed, it is apparent that a significant ciency. Unfortunately, ordinary AC synchro­ number of problems must be solved before nous motors geared down to ^1 Hz have effi­ success is attainable. The materials and ciencies near 25% of ideal which, would heat transfer problems present certain limit the refrigerator efficiency to 25% of limitations that must be carefully handled Carnot. To achieve the high efficiency in any optimized design. The pumps, drive projected earlier, drive motor/mechanisms motors, and magnets present engineering with 90-95% of ideal efficiency have to be problems that must be overcome. At this developed. Efficient and reliable low- time, the outlook for successful development temperature pumps definitely have to be of MR is excellent but a great deal of work developed before magnetic refrigerators can needs to be done. achieve their potential long life and high reliability. NOMENCLATURE AND UNITS External heat exchangers are the last major problem area that must be considered, partic­ Ac contact area ularly into a multistage refrigerator. If every stage spanning MO K requires over­ Cp fluid heat capacity (J/hgK) lapping heat exchangers, the effect on the m fluid flow rate (kg/s) overall efficiency is very serious. Ideally, new staging concepts will eliminate separate heat exchanges for each material and require N|U number of heat transfer units in only the cold source exchanger and the hot regenerator (dimension! ess) sink exchanger. qj rate of heat flow into refrigerator from external sources via conduction, radiation, etc.

6-13 power at T c (W) 7. Strobridge, T. R., "Cryogenic Refriger­ cooling ators - an Updated Survey," NBS TN-655 cold temperature (K) (1974). (K) 8. Barclay, J. A., "A Comparison of the hot temperature Efficiency of Gas and Magnetic Refriger­ power introduced into refrigerator ators," submitted to 22nd Nat 1 ! Heat system from external pumps (W) Transfer Conf., Niagra Falls, N.Y., August 1984. of work (w) W rate 9. Barclay, J. A., "An Analysis of Lique­ sources faction of Helium Using Magnetic WJOTAL tota1 rate work from a11 Refrigerators," Los Alamos National (W) Laboratory Report, LA-8991; Dec. 1981. rate of irreversible entropy production (W/K) 10. Barclay, J. A., Stewart, W. F., Overton, W. C., Chesebrough, R. McCray, M., and ATC adiabatic temperatures change at McMillan, D., "An Apparatus to Determine cold temperature (K) the Heat Capacity and Thermal Conduc­ tivity of a Material from 1 to 300 K in in n efficiency (dimensionless or %) Magnetic Fields up to 9 T," Adv. Cryog. Eng., to be published 1984. a thermal conductance (W/m^K) 11. Barclay, J. A., and Sarangi , S., "Selection of Regenerator Geometry for Magnetic Refrigerator Applications," ACKNOWLEDGEMENTS paper submitted to the 1984 ASME Winter New We are grateful for funding support of the Meeting, December 9-13, 1984, magnetic refrigeration program at Los Alamos Orleans, LA, National Laboratory from Kennedy Space Center, the Defense Advance Research Projects Agency, and the Division of Advanced Energy Projects of the Department of Energy.

REFERENCES 1. Barron, R. F., "Liquefaction Cycles for Cryogens, "Adv. in Cryog. Eng. 17, 20 (1972). 2. Parrish, W. R. and Voth, R. 0., "Cost and Availability of Hydrogen," in Selected Topics on Hydrogen Fuel, 1 J. Hord, ed., NBSIR-75-803 (1975). 3. Voth, R. 0. and Daney, D. E., "H 2 Liquefaction: Effects of Component Efficiencies," Proc. of IECEC, Newark, Delaware, pp. 1356-1362 (Aug. 18-22, 1975). 4. Baker, C. R. and S'haver, R. L., "A Study of the Efficiency of Hydrogen Lique­ faction," Int. J. Hydrogen Energy 3, 321-334 (1978). 5. Baker, C. R,, "Economics of Hydrogen Production and Liquefaction updated to 1980," NASA Contractor Report 159163 (1979); also see NASA CR-132631, NASA CR-145077, NASA CR-145282. 6. Voth, R. 0., "Maximum Practical Effi­ ciency of Helium Temperature Refriger­ ators," Cryogenics 21, 635 (1981).

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