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LIQUID CRYOGEN PUMPS INTEGRATED WITH SUPERCONDUCTING MOTORS

D. Dew-Hughes! M.D. McCulloch! K. Jim, 1 J. Aldwinckle, 1 G. J. Barnes, 1 GJones! J. R. Gaines Jr.Z and S. Sengupta 2 10xford University, Department of Engineering Science Parks Rd, Oxford OX1 3PJ, UK 2 Superconductive Components Inc. 1145 Chesapeake Ave., Columbus, OH 43212

ABSTRACT

Bulk superconducting material has an advantage over superconducting wires in that a significantly higher engineering current density is achievable. There are many different topologies in which bulk materials can be exploited. The most commercially viable application areas will be that where there is an existing cryogenic environment, such as the gas separation industry. To this end two designs of simple liquid nitrogen pumps driven by superconducting motors have been demonstrated. Existing liquid nitrogen pumps suffer from the problems of having the motor separate from the pump. Usually the motor is kept 'warm' and is connected to the pump via a long shaft. Often problems arise with the bearings of this assembly. These prototype pumps address the problem in a novel manner, by integrating the pump mechanism and the rotor of the motor, thus alleviating the need of a long shaft and complex bearings. The absence of external mechanical connections to the pump is thought to be of particular advantage for handling liquid hydrogen, where safety considerations are paramount. INTRODUCTION

Electrical machines which use high temperature superconductors (HTS) suffer from the major drawback in that they are required to be kept cold. It is therefore envisaged that one near term application of such motors will be in systems which are inherently cold. A particular application is for pumping cryogenic liquids. This paper sets out the development of an integrated HTS pump-motor. The practical application of high temperature superconductors in electrical machines has been impeded by the low critical current densities and sensitivity to magnetic field which is typical of these materials, allied to the difficulty of producing them in the form of

Advances m Cryogenic Engmeermg, Volume 45 Ed1ted by Shu eta/, Kluwer Academic I Plenum Publishers, 2000

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flexible and robust conductor. Critical current densities approaching those typical of low temperature superconductors, i.e., in excess of 10 10 Am"2 , are achieved in HTS only in epitaxial thin films. Single crystals and bulk materials in general have critical current densities that are several orders of magnitude below this value. The one remarkable exception is the seeded, melt-processed, bulk samples of the rare-earth bismuth copper oxide (REBCO) 123 phase. These samples consist of a single domain; strong flux pinning being provided by the presence of second phases, in particular the 211 phase, which may be present as a very fine dispersion occupying about 20% of the volume. Critical current densities of 108 Am"2, at 77K and in externally applied fields up to 1 Tesla, are typical of the best samples. 1 These materials are able to trap inductions of several Tesla at 77K and may be thought of as rivals to permanent magnets. Their growth, structure, properties and applications have formed the subject of a recent conference. 2 Proposed applications for these remarkable materials include levitating systems, electromagnetic buffers for space docking, magnetic bearings for inter alia energy storage flywheels/ elements of both shielded core and resistive fault-current limiters (FCLs), and as the rotors in brushless ac motors4 and generators. 5 It is with their application in motors, and in particular motors to drive liquid cryogen pumps, that this paper is concerned. All electric motors work on the same basic principle: electric coils produce a magnetic field, which moves with respect to these coils, and the rotor adjusts its position to minimize the energy stored in the magnetic field. These coils may be found on either the stator or the rotor. The other element may be passive, i.e., contain no other source of magnetic field, or active, i.e., contain a source of magnetic field. For the passive type, the energy difference is caused by a salient magnetic structure. This structure may be singly salient, as in the synchronous reluctance machine, or doubly salient, as in the switched reluctance machine. For active machines, the field may be fixed with respect to the magnetic source, as in synchronous machines, or the field may move with respect to the magnetic source, as in the induction machine and the hysteresis machine. Sometimes real machines may exploit more than one principle, e.g., a synchronous machine may have a salient magnetic structure, as well as damper bars which act as an induction motor when starting. The materials which are commonly used to make machines can be categorized either as good conductor, e.g., copper, as a hard magnetic material, e.g., NdFe, or as a soft magnetic material, e.g., magnetic steel. The former two are used as sources of magnetomotive force, while the latter is used to define the magnetic path and any saliences, if required. Like all materials, they have their limitations. For conductors, the current density is usually limited to at most 10 A/mm2• For hard magnetic materials, the highest flux density that can be generated is about 1.2 T. Soft magnetic materials can usually carry magnetic fields up to a magnetic field density of about 1.5 T. HTS materials, which have a current density several orders of magnitude greater than copper and which can trap fields much larger than conventional permanent magnets, are an attractive choice as a material for motors. SUPERCONDUCTING MOTORS

HTS wires can be wound as the main coils but AC losses, and the requirement to operate at low temperatures, 20 K, have delayed their use. Strong flux-pinning bulk HTS materials can be employed in most of the above types. The difference in permeability of the direct and quadrature paths in synchronous reluctance machines can be enhanced by the use of ferromagnetic/superconducting composite rotors. The group at the Moscow Aviation Institute in collaboration with

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Oswald Motoren GmbH have also explored this type ofrotor. 6 Modeling indicates that the maximum output power can be increased by some thirty percent over that of a simple reluctance machine of the same size. The maximum power achieved with the superconducting reluctance motor to date is 9 kW. Superconducting versions of induction motors have been constructed at Oxford. 7 These have either a squirrel cage rotor fabricated from silver dip-coated with Bi-2212 superconductor or a centrifugal melt-spun cylinder of Bi-2212. The torque versus speed characteristics measured at 77 K show the torque falling from a maximum value to zero well below synchronous speed. This is presumed to be due to flux flow in the superconductor giving rise to resistive behaviour. Any serious development of this type of machine will call for superconducting material with a much higher critical current than that exhibited by Bi-2212 at 77 K. A type II superconductor, like a ferromagnetic material, exhibits magnetic hysteresis in a changing magnetic field. A solid superconducting rotor can act in exactly the same way as a ferromagnetic rotor. Relative motion of driving field and rotor will cause the The interaction between the superconductor to traverse its magnetization curve. magnetization of the rotor and the driving field will produce a torque. In the case of the ferromagnetic rotor, the field drags the rotor after it. Because the superconductor tries to exclude magnetic flux, and has a negative susceptibility, the superconducting rotor is pushed ahead by the stator field. A collaboration between the Moscow Aviation Institute and the IPHT-Jena has pioneered the development of superconducting hysteresis motors. The rotors are made up from blocks of melt-processed YBCO, and machines with output power up to lkW have been tested. 8 The performance characteristics of these machines confirm their mode of operation as that of hysteresis machines. Their performance has been successfully modelled analytically on the assumption of hysteretic behaviour, treating the superconducting grains as a medium with an effective permeability. If a superconducting version of either an induction or an hysteresis machine is made with a rotor of high current density and strong flux pinning, the induced currents or magnetic flux may become fixed in the rotor. On starting, the machine operates as either an hysteresis or an induction motor, but as the rotor speeds up and approaches synchronous speed the mode of operation changes as the flux becomes pinned and remains stationary with respect to the rotor. The machine then operates in a similar way as a permanent magnet motor, the rotor rotating in synchronism with the driving field. The flux, trapped in the rotor, locks on to the driving field. This type of behaviour is expected of materials with supercurrent paths whose dimensions are of the same order as the dimensions of the rotor. Experiments at Oxford have confirmed that rotors made up from seeded, melt-processed single domain YBCO can act as both hysteresis and permanent magnet machines. The output torque is constant with rotational velocity up to the synchronous speed. 9•10 Finite element methods, based on the assumption of the field independent Jc Bean model for the superconductor, have been used to model flux and current distribution within the YBCO rotors, and to predict machine performance. 11 The model predicts that torque should vary as the cube of the driving field amplitude until the magnitude of the latter is such that flux has penetrated to the centre of the rotor. Thereafter the torque should become directly proportional to the field amplitude. This is in agreement with the results of the analytical modelling of the hysteresis motors mentioned above. The maximum field amplitude available in these experiments was 0.1 Tesla, a level at which the simple Bean model is expected to yield accurate results, and also insufficient to cause full flux penetration. The experimental results are in close agreement to the predictions. The model also predicts that a possibility for improvement of performance would be to preflux the rotor, by pulsing a DC current through the field coils.

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This would strengthen the permanent magnet characteristic of the machine and could increase the stalling torque by fifty percent. It was noticed during the course of the experiments on the melt-processed YBCO cylinders that the driving field not only caused the cylinders to rotate but could also provide a levitation force. The use of magnetic/superconducting bearings is a natural extension of the application of superconductors in rotating machines. By the appropriate shaping of the stator field, levitation and reaction to thrust can be incorporated in the machine operation. BiSCCO LIQUID CRYOGEN PUMP The particular advantage of machines with superconducting rotors would appear to be that they have a much greater power density in the rotor. Such machines are therefore likely to find applications in which the power to size or mass ratio is of prime importance. These applications are to be found in aircraft and space vehicles. These machines may also find application in already existing cryogenic environments, such as the gas separation industry. To this end, two configurations of simple liquid nitrogen pumps driven by superconducting rotors have been demonstrated at Oxford. Existing liquid nitrogen pumps suffer from the problems of having the motor separate from the pump. Usually the motor is at room temperature and is connected to the pump via a long shaft down which there is a considerable heat leak. Often problems arise with the bearings of this assembly. A conceptual design, illustrated in Figure I, addresses the problem in a novel manner by integrating the pump mechanism into the rotor of the motor, thus alleviating the need of a long shaft and complex bearings. Circulation of the liquid cryogen is achieved by an Archimedean screw placed inside a hollow cylinder of superconductor that acts as the rotor. Recent modelling results indicate that a hollow cylinder should produce greater torque than a solid cylinder of the same external diameter. 12 The rotor is housed inside an insulated pipe; the stator coils are wound on the outside of this pipe. The stator coils are backed by soft iron to contain any stray field. Thrust reaction can be provided by additional coils, or by shaping the soft iron, so as to concentrate the field at either end of the stator. The absence of external mechanical connections to the pump is thought to be of particular advantage for handling liquid hydrogen, where safety considerations are paramount. To demonstrate the principle of this type of pump an Archimedean screw machined from Delrin was fitted into the central bore of a centrifugal melt-spun Bi-2212 cylinder. 13 The cylinder had an internal diameter of 15 mm, external diameter of 25 mm and was 15 mm in length. The rotor just fitted inside the stator coils, which were free-standing threephase copper coils supplied by Portescap. These coils were in tum placed inside an iron tube. It was felt that levitation of the rotor would prove an unnecessary complication at this

Iron ;;i;;;;;;iiiiiiiiiiiiiii-=~;;;;;;;;.- Coi I

- - - - - - - - Screw

Figure 1. Schematic diagram of integrated superconducting motor and Archimedian pump.

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Figure 2. BiSCCO rotor with Archimedian screw.

Figure 3. Pump assembly with delivery tube.

stage, and therefore the rotor was supported by simple needle bearings. The entire assembly is shown in Figures 2 and 3. A reinforced plastic delivery tube was attached to one end of the pump and operation is achieved by immersing the pump in a liquid nitrogen flask. The best performance of this crude prototype is a rate of delivery of liquid nitrogen of 1 1/min. with a 10 em head for a power input of about 45 W. 13 The problem of low efficiency results from the low rotational speed of the rotor which is limited by the inferior superconducting properties of the Bi-2212 at 77 K. Operation at 20 K, for example in pumping liquid hydrogen, should give much more efficient performance. An improved design, that would include the use of YBCO in the rotor and levitation of the rotor to obviate the need for needle bearings, is expected to lead to an increase in performance by several orders of magnitude. Despite its lack of performance, this project has demonstrated the feasibility of an integral superconducting liquid cryogen pump. FABRICATION OF YBCO ROTOR A subsequent project has been based on a YBCO rotor supplied by Superconductive Components, Inc.(SCI). The rotor was fabricated from seeded melt-processed single domainYBCO cylinders grown by the process developed by SCI for the levitator™ material. 14 The process involves synthesis of phase pure powder, single crystal growth, melt processing, and oxygenation. All of these steps are critical for successful fabrication of levitators™ with desired properties. Phase pure YB~Cu3 0x and Y2BaCu05 powders were prepared by using a low-pressure calcination method. Typically, in a low pressure calcination process, the required oxides and carbonates are mixed and ball milled in a proper molar ratio. The resulting powders are then calcined in a low-pressure furnace to produce the desired phase. Calcination is carried out in 2-5 torr of oxygen. A partial vacuum is used to increase the efficiency of removal of C02 • Once the reaction is completed, the vacuum is discontinued and the powders are cooled in an oxygen atmosphere at ambient pressure to fully oxygenate the superconducting powders. The low pressure calcination method is beneficial in reducing the carbon content and processing time while maintaining small particle size. Phase pure powders of YB~Cu3 0x, Y2BaCu05 (~20-25 wt %) and Pt02 (0-0.5 wt %) were mixed homogeneously by ball milling overnight(~ 8-10 hr.). The mixed powder was then pressed in to a 1.5 inch diameter pellet and seeded with a Nd1+xBa2_xCu30Y single 1481

Figure 4. Photograph of a sliced YBCO singe domain.

crystal and melt-processed in a temperature gradient. 15 The pellets are heated to~1050 °C, slowly cooled through the peritectic temperature to ~950 oc and then finally cooled to room temperature at a rate of 100 °Cihr. The pellets are then oxygenated between 450700 °C for 8-10 days in a flowing oxygen atmosphere. The pellets were cut into 0.25"slices using a low speed diamond saw (Fig 4). Seven slices were then bonded, by using epoxy suitable for cryogenic application, to form a cylinder. The cylinder was then encased in a G I0 cylinder to support the brittle ceramic. A central bore was drilled by water jet cutting. The G I 0 support was subsequently machined away and the cylinder then used as a rotor for the superconducting motor/pump. YBCO LIQUID CRYOGEN PUMP

The central bore of this rotor, 11 mm dia., was insufficient to allow of the insertion of an Archimedian screw. It was therefore decided to use this rotor in a separate motor driving a centrifugal impeller.16 The dimensions of the rotor, 25 mm o.d. and 45 mm long, were found to be almost identical to those of the ferrite rotor in a Heidolph 203.50 motor. The coils from such a motor, wound on a circular laminated iron core enclosed in a steel casing, are run from a 50 Hz, 230 V variable power supply. The YBCO rotor was mounted on a Delrin shaft with PTFE axial bearings and a needle thrust bearing and married to the Heidolph coils. The components and assembled motor and impeller are shown in Figures 5 and 6. The output torque at 77 K was found to vary as the cube of the current in, and hence the field generated by, the coils. This is exactly as predicted by theory up to fields below those sufficient to cause full penetration of the flux into the rotor. 11 The maximum efficiency, defined in terms of output power divided by input electrical power, was measured as 64.5%; this compares favourably with the rated efficiency of the Heidolph motor at 61.5%. A plastic centrifugal impeller, sourced from a commercial garden feature water pump (Laguna 7000 Powerjet) was attached to the shaft of the superconducting motor, see Figure 6. The impeller had a plastic cover, not shown in Figure 6. By immersing the motor and impeller assembly in liquid nitrogen, pumping action was demonstrated. Best performance figures were a delivery rate of 12 1/min at a head of 0.6 m, reducing to 1.2 1/min at a head of 2 m . These values were obtained with 104 volts across the coil; the maximum voltage was limited by an acceptable rate of nitrogen boil-off due to heating of the coils. Clearly superconducting coils would provide an extra advantage, both in nitrogen economy and in attainable power levels and hence efficiency.

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Figure 5. Components of YBCO motor

Figure 6. Assembled motor and impeller

It must be stressed that in neither of these demonstrators was any attempt made to optimise the fluid dynamics aspects of the pumps. The Archimedian screw was not fitted with guide vanes at either entry or exit. The centrifugal impeller was designed for water, and no alteration was made to take account of the different hydrodynamic properties of liquid nitrogen. CONCLUSIONS A near term application for HTS machines for pumping cryogenic liquids has been identified. This paper proposes that an integrated pump-motor design offers significant advantages over more conventional approaches. Two prototype designs have been built and tested to demonstrate proof of principle. In the first design, a Bi-2212 tube formed the basis of the rotor. An Archemedian screw was inserted into the middle of the tube to provide the pumping action. This protoype was capable of pumping up to 1 1/min. However this prototype suffered from two serious deficiences: the Bi-2212 operates in the flux-flow regime, and the Archemedian screw is not an efficient pump. The second prototype addressed these two issues by using a YBCO cylinder driving a centrifical impeller. This second pump was capable of delivering a flow of 12 Vmin at a head of 0.6 m. These prototypes have demonstrated the proof of principle of an integrated HTS pump-motor. ACKNOWLEDGEMENTS The authors wish to thank Portescap and Heidolf AG for their generous contribution of coils and stators which enabled the completion of this work. This work was carried out as final year undergraduate engineering projects, in the Department of Engineering Science, Oxford University, by J Aldwinckle (1997-1998) and G Jones (1998-1999), working to a limited budget. REFERENCES I. M. Murakami, S. Goth, N. Koshizuka, S. Tanaka, T. Matsushita, S. Kambe and K.

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Kitazawa, Critical currents and flux creep in melt processed high T c oxide superconductors, Cryogenics 30:390 (1990). 2. International Workshop on the Processing and Applications of Large Grain (RE)BCO High Temperature Superconductors, proceedings published in Materials Science and Engineering 53:1 (1998). 3. T. S. Luhman, M. Strasik, A. C. Day, D. F. Garrigus, T. D. Martin, K. E. MacCrary, and H. G. Ahlstrom, H. G., Superconducting bearings and flywheel batteries for power quality applications, in: "Applied Superconductivity 1995," D. Dew-Hughes, ed., lOP Publishing, Bristol (1995), p. 35. 4. M. D. McCulloch, and D. Dew-Hughes, Brushless ac machines with high temperature superconducting rotors, Materials Science and Engineering 53:211 ( 1998). 5. L. K. Kovalev et al, Alternators which use HTSC wire coils and bulk YBCO materials, in:"ICEC17," D. Dew-Hughes, R. G. Scurlock and J. H. P. Watson, eds., lOP Publishing, Bristol (1998), p. 379. 6. B. Oswald et al., Electric motors with HTS bulk material, in: "ICEC17," D. DewHughes, R. G. Scurlock and J. H. P. Watson, eds., lOP Publishing, Bristol (1998), p. 547; L. K. Kovalev et al., HTS electric motors with compound HTS-ferromagnetic rotor, in: "ICEC17," D. Dew-Hughes, R. G. Scurlock and J. H. P. Watson, eds., lOP Publishing, Bristol (1998), p. 527. 7. M.D. McCulloch, D. Dew-Hughes, K. Jim and C. Morgan, The measurement and numerical calculation of the torque-speed curve of high temperature superconducting hysteresis motor, lEE Conference Publication No 444: Institution of Electrical Engineers, London (1987), p. 268. 8. L. K. Kovalev et al., Hysteresis electrical motors with bulk melt-textured YBCO, Materials Science and Engineering B 53: 216 (1998). 9. M.D. McCulloch, K. Jim, Y. Kawai and D. Dew-Hughes, Prospects for brushless ac machines with HTS rotors, in: "Applied Superconductivity 97," H. Rogalla and D. A. H. Blank, eds., lOP Publishing, Bristol (1997), p. 1519. 10. M.D. McCulloch and D. Dew-Hughes, Brushless ac machines with high temperature superconducting rotors, Materials Science and Engineering B 53:211 (1998). 11. G. J. Barnes, M.D. McCulloch and D. Dew-Hughes, The computer modelling of type II superconductors in applications, Superconductor Science and Technology 12: 518 (1999). 12. G. J. Barnes, M.D. McCulloch and D. Dew-Hughes, Finite difference modelling of HTS hysteresis machines, presented at the 9th International Workshop on Critical Currents, Madison, Wisconsin, July 1999, and submitted for publication in Superconductor Science and Technology. 13. J. Aldwinckle, A superconducting liquid nitrogen pump, Final Year Project Report, Department ofEngineering Science, Oxford University (1998); D. Dew-Hughes, M.D. McCulloch, J Aldwinckle and K. Jim, Specification for a pump, UK Patent Application No. 9810361.7, 14th May 1998. 14. S. Sengupta, J. Corpus, M. Agarwal, and J. R. Gaines, Jr., Feasibility of manufacturing large domain YBCO levitators by using melt processing techniques, in: "Impact of Recent Advances in Processing of Ceramic Superconductors," W. Wong-Ng, U. Balachandran and A.S. Bhalla, eds., The American Ceramic Society (1998), p.13. 15. V. R. Todt, S. Sengupta, D. Shi, J. R. Hull, P.R. Sahm, P. J. McGinn, R. B. Poeppel, Processing of large YBa2Cu30x domains for levitation applications by NdBa2Cu30x seeded melt-growth technique, J. of Electronic Materials, 23:1127 (1994). 16. G. Jones, A superconducting pump for liquid nitrogen, Final Year Project Report, Department ofEngineering Science, Oxford University (1999).

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