Cryocoolers For Aircraft Superconducting Generators And .
CRYOCOOLERS FOR AIRCRAFT SUPERCONDUCTINGGENERATORS AND MOTORS*Ray RadebaughNational Institute of Standards and TechnologyBoulder, Colorado, 80305, USAABSTRACTThe proposal by NASA to use high-temperature superconducting (HTS) generatorsand motors on future ( 2035) aircraft for turboelectric propulsion imposes difficultrequirements for cryocoolers. Net refrigeration powers of about 5 kW to 10 kW at 50 K to65 K are needed for this application. A 2010 survey by Ladner of published work between1999 and 2009 on existing Stirling and Stirling-type pulse tube cryocoolers showedefficiencies in the range of 10 to 20 % of Carnot at 50 K, much less than the 30 % ofCarnot needed to make the concept feasible. A cryocooler specific mass less than about 3kg/kW of input power is required to keep the cryocooler mass somewhat less than the massof the superconducting machinery. Current cryocoolers have specific masses about 3 to 10times this desired value, even for those designed for airborne or space use. We discuss lossand mass sources and make suggestions where improvements can be made. For Stirlingand Stirling-type pulse tube cryocoolers, most of the mass is concentrated in thecompressor. We show that higher frequency and pressure can have a major influence onreducing the compressor mass. Frequencies up to about 120 Hz and average pressures upto about 5 MPa may significantly reduce the overall cryocooler size and mass whilemaintaining high efficiency. Other suggestions for reducing the mass are also given.KEYWORDS: Aircraft, cryocoolers, efficiency, machines, pulse tubes, regenerators,Stirling, superconductors, specific massINTRODUCTIONNASA has set the following performance goals for future transport aircraft: (1) reducedairport noise, (2) reduced emissions (both pollutants and greenhouse gasses), and (3)reduced fuel burn [1]. With reduced noise, aircraft could fly into and out of small centrallylocated metropolitan airports. To do so also requires a capability for short take-off and*Contribution of NIST, not subject to copyright in the US. The use of company names does not implyendorsement by NIST, but they are mentioned only to identify unique equipment relevant to this study.Advances in Cryogenic EngineeringAIP Conf. Proc. 1434, 171-182 (2012); doi: 10.1063/1.47069182012 American Institute of Physics 978-0-7354-1020-6/ 0.00171
TABLE 1. Dates and technology development for future aircraft [1].GenerationN 1N 2N 3Approximate date201520202030TechnologyConventional tube & wingUnconventional hybrid wing-bodyAdvanced aircraft conceptslanding (STOL). Future generations of aircraft are designated as N 1, N 2, etc., with Nbeing today’s technology. TABLE 1 gives the approximate time frame for new aircrafttechnology development to meet the NASA performance goals. The performance goals forN 3 aircraft set by NASA are very stringent, such as a 55 dB noise reduction, 75 %emission reduction, and a 70 % fuel burn reduction. One concept being investigated byNASA to meet the N 3 goals is a distributed turboelectric propulsion concept as illustratedin FIGURE 1 [2]. With this concept superconducting motors are used to drive propulsorfans distributed over much of the upper surface of the trailing edge of the wings. Withcurrent high-bypass jet engine technology, the fans connected to the engine provide about85 % of the engine thrust with the trailing exhaust providing only 15 % of the thrust. Thefan speed is limited to a few thousand RPM in order to prevent supersonic speeds at theblade tips. Electric power to the motors can be provided by two superconductinggenerators driven by high-speed gas turbines. The generators and turbines can be madevery efficient and compact by running them at much higher speed than that of the motors.The generators and motors would be connected through an “electrical gearbox.” Thisrevolutionary concept, which decouples speed and torque, provides many controladvantages in addition to the potential of meeting the N 3 performance goals.The electrically driven propulsion system is feasible only if the electrical motors can beabout the same size as, or smaller than, existing gas turbines. FIGURE 2 shows acomparison of power densities for electric motors and aircraft engines taken from data byMasson, et al. [3]. The values shown in this figure represent general trends, but values forvarious motors and engines can vary significantly. The specific power of gas turbine cores(without propulsor) for use on the Boeing 787 is about 16 kW/kg, whereas the specificpower of most conventional electrical motors is limited to about 0.5 kW/kg, although verylarge industrial motors can have power densities up to about 3 kW/kg. Some electricmotors developed for use on automobiles have power densities as high as 4.8 kW/kg [4].Therefore, because of their low power density, conventional electric motors cannot be usedfor aircraft propulsion. High power density electric motors are a part of NASA and DoDresearch and development plans. Higher power densities require higher current densities inthe conductors and/or higher magnetic energy densities in permanent magnets. The outputSuperconducting motor-drivenfans in a continuous nacelleWing-tip mountedsuperconductingturbogeneratorsFIGURE 1. Artist’s concept of future turboelectric aircraft utilizing superconducting generators andmotors [2].172
FIGURE 2. Power density comparison for gas turbines and electric machines.power of a single generator for aircraft use may be as large as 30 MW [1].Superconducting motors and generators offer the possibility of very high powerdensities and efficiencies because of the greatly reduced Joule heating. Current densities insuperconducting BSCCO coils can be in the range of 10,000 to 20,000 A/cm2 in a 2 Tmagnetic field. The newer YBCO superconducting coils can have even higher currentdensities. As a result, power densities are much higher, and at this time the highest powerdensity of a superconducting machine is 7.8 kW/kg, achieved with a 16,000 rpm HTSgenerator developed for the Air Force [5]. At present, most HTS machines usesuperconductors only in the rotor where DC currents are used to produce a DC magneticfield, which interacts with an AC excitation in the copper stator windings. Designs for allsuperconducting machines show typical power densities as high as 40 kW/kg for rotationalspeeds of about 10,000 rpm [3]. The use of HTS wire in the stator armature winding leadsto significant AC losses in present YBCO materials. However, research on low AC lossHTS materials is quite active, so future motors and generators could become allsuperconducting at some time well before 2035. Superconducting machines requirecooling with a cryocooler or with a cryogenic fuel, such as liquid hydrogen. This paperdiscusses cryocoolers for this application. Radebaugh and Ladner give a more detailedreport [6]. Potential candidate cryocooler types are the Brayton, Stirling, and Stirling-typepulse tube. This paper focuses mostly on the latter two types of cryocoolers, although anapproach to reduce the specific mass of Brayton cryocoolers is briefly mentioned here.REFRIGERATION REQUIREMENTSRefrigeration temperature and powerThe use of superconducting machines onboard aircraft requires the development ofreliable cryocoolers that also have high power densities. Many very small and very reliablecryocoolers have been developed for space applications, but their cooling powers are ordersof magnitude less than those required for these aircraft applications. Large industrialcryocoolers or refrigerators have been developed that can provide the required refrigerationpower, but no effort has been made to make those systems lightweight. Thus, it is difficultto determine whether existing cryocooler technology can meet the requirements for aircraft173
use. First, we must establish the requirements for the cryocooler, such as the operatingtemperature, the net refrigeration power, the maximum input power, and the maximumweight.A superconductor normally operates within a three-dimensional space limited bymagnetic field, temperature and transport current. The superconductor goes normalwhenever any one of these limits is exceeded. To remain superconducting while carryinghigh currents, the superconducting wire must operate at temperatures well below thecritical field value to allow for an adequate margin. For Bi-2223, or first generation HTSwire, the operating temperature must be around 30 K to 40 K. Second generation HTSwires, made with YBCO, can operate at about 50 K to 60 K. For N 3 aircraft operatingaround 2035, YBCO and possibly even better HTS wire would be available. For this study,operating temperatures of 50 K to 70 K are used as the goal to be provided by cryocoolersin future aircraft [7].The heat load on the cryocooler for any HTS motor or generator consists mainly of (1)the active heat dissipation from the HTS winding due to AC losses, (2) heat conduction inthe current leads, and (3) background heat leaks due to radiation and due to conductionthrough shafts and supports. One quality measure of any superconducting machine is theratio of required refrigeration power to the electrical (generator) or mechanical (motor)power output of the superconducting machine. This quality measure can be defined as thecryogenic inefficiency of the superconducting machine, given byQ λsc 1 ε sc net ,(1)Pswhere λsc is the cryogenic inefficiency, εsc is the cryogenic efficiency, Q is the net heatnetload on the cryocooler, and Ps is the power rating of the superconducting machine.The cryogenic efficiency is different from the overall efficiency of the motor orgenerator, which is given by the ratio of the output power to the input power. Thedifference between the input and output powers of an electrical machine is the total loss,which is in the form of heat. However, some of the losses in a superconducting machinewill occur in the ambient-temperature components, such as the copper windings of thestator or hysteresis losses in iron. Also, the background thermal loss in a cryogenic systemis not part of the overall loss associated directly with the superconducting machine.Published numbers for λsc in discussions of HTS machines are seldom reported, but suchnumbers would be very helpful in planning for future refrigeration requirements and sizingof cryocoolers. In 2009 Sivasubramaniam reported measurements with a 1.3 MWgenerator designed for airborne applications [8] that had a design power density of 8.8kW/kg at full power. It was a HTS homopolar inductor alternator that utilized a stationaryHTS field excitation coil, a solid rotor forging, and an advanced stator. The HTS coil forthis configuration of generator operates with DC current, so AC losses are not important.The cooling requirement for this machine was 40 W at about 30 K. The cryogenicinefficiency for this machine is then λsc 3 x 10-5. A 4 MVA generator developed bySiemens that used HTS for the rotating field coil required a refrigeration power of about 50W at 30 K [5]. For this machine λsc 1.3 x 10-5.To achieve the very high power densities discussed earlier (as high as 40 kW/kg), allsuperconducting machines (rotor and stator) would be necessary. In such machines, thecryogenic losses are dominated by the AC losses in the superconductor. AC losses incurrently available YBCO tapes are much too high to consider their use in the armature,where large AC currents are required and high frequencies are needed for high rotationalspeeds. However, much research is underway to reduce AC losses in YBCO. When lowAC loss YBCO becomes commercially available, such conductors could be used in future174
high-power density HTS machines for aircraft use. Barnes et al. [9] estimated total lossesfor such advanced machines. For a 5 MW machine, they estimated that the total loss wouldbe about 500 W at 65 K, which results in a cryogenic inefficiency λsc of 1 x 10-4.For a 30 MW machine with a cryogenic inefficiency of 1 x 10-4 the heat load to 65 Kwould be 3 kW. A somewhat more conservative projection for AC losses in YBCO wouldhave λsc 5 x 10-4, in which case the refrigeration power is 15 kW at 65 K. Fordemonstrations within the next ten years we may need up to about 10 kW of refrigeration ata temperature of 50 K. However, as advances are made in HTS, the required refrigerationpower may decrease somewhat and the operating temperature may increase to about 65 K.For an aircraft with the size and power of the Boeing 737-200, the expected electricalpower for thrusters would be about 10 MW, which requires 1 kW of refrigeration when thecryogenic inefficiency is 1 x 10-4. For purposes of this report we focus on refrigerationpowers in the range of 1 kW to 10 kW for temperatures in the range of 50 K to 65 K.Input power and massSuperconducting generators and motors offer many advantages for use on aircraft, butthe power input and mass of cryocoolers required to maintain the necessary coldtemperature results in a disadvantage for such systems. The overall system becomessuccessful only if the cryocooler input power and mass are small compared with the poweroutput and mass of the superconducting machinery. We define these ratios asP(3)XP cPsandM(4)XM c ,Mswhere Pc is the input power to the cryocooler, Ps is the power rating of the superconductinggenerator or motor, Mc is the mass of the cryocooler, and Ms is the mass of thesuperconducting machine. The power fraction of the cryocooler can be expressed in termsof the machine cryogenic inefficiency λsc and the second-law efficiency ηc of thecryocooler asXp λsc (Th Tc ),ηcTc(5)where Tc is the cold temperature and Th is the hot or ambient temperature of the cryocooler.FIGURE 3 shows how the power fraction varies with λsc and ηc for the case of Tc 50 Kand Th 300 K. (Th could be reduced in flight.) This figure shows that cryocoolerefficiencies of only 10 % to 20 % of Carnot yield power fractions less than 0.1 for λsc 0.001. However, we shall see in the following paragraph that to achieve low massfractions, the cryocooler efficiency must be closer to 30 % of Carnot. For a netrefrigeration power of 10 kW at 50 K, the input power becomes 167 kW when theefficiency is 30 % of Carnot.The mass of the cryocooler can be expressed asM c mc Pc mc X P Ps ,(6)where mc is the specific mass of the cryocooler (mass per unit of input power). When bothsides of equation (6) are divided by the mass of the superconducting machine, we haveX M mc X P ps ,175(7)
FIGURE 4. Ratio of cryocooler mass to HTSmachine mass for various cryocooler specificmasses vs. power fraction. HTS machine powerdensity is 20 kW/kg for this figure.FIGURE 3. Ratio of cryocooler power to HTS machinepower for various cryocooler efficiencies vs. cryogenicinefficiency of the HTS machine.where ps is the power density of the superconducting machine. FIGURE 4 shows how themass fraction varies with power fraction and the cryocooler specific mass for the case of asuperconducting machine power density of 20 kW/kg. We note that with an expectedpower fraction of only 0.01, the specific mass mc of the cryocooler must be less than about3 kg/kW if the mass fraction is to be kept below about 0.6. Reducing the power fraction to0.005 would allow the specific mass to be increased to 6 kg/kW for the same mass fraction,but the efficiency would have to be doubled to 60 % of Carnot for the same λsc and ps. Fora cryocooler specific mass of 3 kg/kW, the power density is 0.33 kW/kg. This powerdensity is much less than that of conventional high-power electric motors, as shown inFIGURE 2. An alternate expression for equation (7) is XM mc*λscps, where mc* is thecryocooler mass per unit of net refrigeration power. However, most of the cryocooler massis that of the compressor, so mc* becomes a strong function of the cryocooler efficiency.SURVEYS OF EXISTING CRYOCOOLERSIn order to compare the efficiency and specific mass of existing cryocoolers to therequirements discussed in the previous section, we turn to the results of cryocooler surveys.A survey by Strobridge [10] in 1974 is often used as a comparison for any newly developedcryocooler. The extensive survey of 235 cryocoolers by ter Brake and Wiegerinck [11] in2002 provides a valuable update of the Strobridge survey. However, many of the surveyslump cryocooler cold-end temperatures together, which result in 80 K cryocoolersdominating the reported performance. The recent survey by Ladner [12] shows howefficiency and mass vary with temperature. Ladner’s survey included most new Stirlingand pulse tube cryocoolers reported between 1999 and 2009.EfficiencyFIGURE 5 shows how the efficiency of cryocoolers varies with refrigeration power.The lower curve in this figure is from the 1974 survey of Strobridge, and the upper curve isthe upper limit of efficiency from the ter Brake survey [11], as given by Kittel [13]. Thisfigure shows that in the range of 1 to 10 kW of refrigeration power, the maximumefficiency is about 20 to 25 % of Carnot and has not increased much in 35 years. FIGURE6 shows data from Ladner [12] for the highest efficiencies of Stirling and Stirling-typepulse tube cryocoolers and how they vary with temperature. We note that maximum176
FIGURE 5. Cryocooler efficiencies from Strobridge [10] and ter Brake [11] with the upper solid linefrom Kittel [13] showing the maximum values. Capacity is the same as net refrigeration power.efficiencies of about 25 % of Carnot are reported for Stirling cryocoolers in the temperaturerange of 50 K to 65 K, whereas pulse tube cryocoolers show maximum efficiencies in therange of 10 % to 15 % of Carnot in that temperature range. Most of these cryocoolers haverefrigeration powers of only a few watts, although one of the pulse tube cryocoolersconsidered in the survey had a refrigeration power of about 1 kW, and two largecommercial Stirling crocoolers of 1 kW and 4 kW at 77 K have efficiencies of about 25 %of Carnot in this temperature range. The largest Stirling cryocooler ever made provided 20kW of refrigeration at 70 K with an efficiency of 41 % of Carnot [14]. Although thisindustrial cryocooler was much too heavy (49 kg/kW) for aircraft application, it shows thatan efficiency goal of about 30 % of Carnot in large Stirling cryocoolers for temperatures of50 K to 65 K should be realistic. Further research on large pulse tube cryocoolers isnecessary to overcome flow nonuniformities if such high efficiencies are to be achieved.Specific MassFIGURE 7 shows how cryocooler mass varies with input power from the CILTECsurvey of ter Brake [11] and the Ladner survey [12] compared with the goal for aircraft use.The graph also shows mass data for five relevant cryocoolers with power inputs greaterFIGURE 6a. Efficiencies of high-performance pulse FIGURE 6b. Efficiencies of high-performanceStirling cryocoolers from the Ladner survey [12].tube cryocoolers from the Ladner survey [12].177
FIGURE 7. Comparison of cryocooler mass from surveys with mass goal for aircraft applications.than 10 kW. The GM cryocooler is a large single-stage commercial cryocooler. Theaircraft turbo-Brayton cryocooler was design for air liquefaction [15]. The 1 kWcommercial pulse tube cryocooler is based on the work of Potratz, et al. [16] that uses aflexure bearing compressor for long life. The 4-cylinder Stirling cryocooler is acommercial kinematic version driven with a rotary motor and designed for industrial use[17]. The MCC Stirling cryocooler indicated in the figure is one under development for theDept. of Energy, where the input and refrigeration powers are design values [18]. It is aunique design that uses three alpha-version Stirling cryocoolers that operate 120 out ofphase in a three-phase arrangement, so three-phase power can be used directly to drive thecooler. In addition, the pistons are double acting, so both sides of the pistons are active.The backside of each piston is the pressure oscillator, and the front side with
N 3 2030 Advanced aircraft concepts FIGURE 1. Artist’s concept of future turboelectric aircraft utilizing superconducting generators and motors [2]. landing (STOL). Future generations of aircraft are designated as N 1, N 2, etc., with N being today’s technology. TABLE 1 gives the approximate time frame for new aircraft
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Micro-miniature Stirling cryocoolers for high temperature applications have bee ndeveloped . The unavoidable friction from bearings, piston and other parts in a rotary cooler wastes the partial drive power. The proportion of the friction consumption to total power is higher than those cooler
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