Materials Science and Engineering B 151 (2008) 195–198
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Materials Science and Engineering B
journal homepage: www.elsevier.com/locate/mseb
Performance of a conduction-cooled high-temperature
superconducting bearing夽
M. Strasik ∗ , J.R. Hull, P.E. Johnson, J. Mittleider,
K.E. McCrary, C.R. McIver, A.C. Day
The Boeing Company, PO Box 3707, MC 2T-50, Seattle, WA 98124-2207, USA
a r t i c l e
i n f o
Article history:
Received 29 October 2007
Received in revised form 17 March 2008
Accepted 20 March 2008
Keywords:
High-temperature superconductors
HTS bearings
Flywheel energy storage
a b s t r a c t
We report rotational loss measurements for a high-temperature superconducting (HTS) bearing whose
cooling consists of a thermal conduction path to the cold head of a cryocooler. Losses have been measured
for rotational rates up to 14,500 rpm at different HTS temperatures. The rotational losses decrease with
decreasing HTS temperature. For temperatures that can be obtained in a liquid-nitrogen thermosiphon
system, at a given speed and gap, the loss of the conduction-cooled HTS bearing is not significantly higher
than the loss of a nearly identical HTS bearing cooled by flowing nitrogen from the thermosiphon.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
High-temperature superconducting (HTS) bearings offer the
potential for extremely low rotational loss and have been studied
for a number of applications [1]. HTS bearings require cryogenic cooling for their operation, and closed-cycle cooling can be
accomplished with the use of commercially available cryocoolers.
Previously, the Boeing team has successfully operated flywheel
energy storage systems (FESSs) with HTS bearings that have been
cooled in a closed-cycle system in which the HTSs were immersed
in a liquid-nitrogen bath [2]. A thermosiphon that was connected to
the cold head of a cryocooler re-supplied any nitrogen that boiled
off. While the thermosiphon cooling system employed in the tests
to date has worked well, to enhance the applicability of the HTS
bearing, it is desirable to provide the cooling without the use of
cryogenic fluid outside of the cryocooler. Toward this end, the Boeing team has designed, fabricated, and tested an HTS bearing in
which the HTS components are conduction-cooled from the cold
head of a cryocooler. Conduction cooling of HTS bearings has been
investigated at least since 1999 [3], but to our knowledge, rotational
losses for such systems have only been reported for extremely low
rotational rates [4].
夽 This item was originally due to appear in the following special issue: Proceedings
of the Sixth International Workshop on the Processing and Applications of Bulk
Superconductors (PASREG), held 13th - 15th September 2007 at the University of
Cambridge, U.K. Materials Science and Engineering B volume 151, issue 1 (2008).
∗ Corresponding author. Tel.: +1 206 544 5389.
E-mail address: michael.strasik@boeing.com (M. Strasik).
0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.mseb.2008.03.019
A potential disadvantage of conduction cooling the HTSs is the
possibility that the rotating permanent magnets (PMs) of the bearing will induce eddy currents in the thermally conducting member.
These eddy currents will produce additional rotational loss in the
bearing that increases with bearing speed and could be significant
at maximum speed. Because the HTS is between the rotor PMs and
the thermal conductor, a reasonable hypothesis is that the HTSs will
magnetically shield the thermal conductor. However, such shielding in a complicated bearing structure is difficult to analyze and
some experiments have indicated the phenomena is complex [5].
Thus, it is desirable to experimentally measure that the conductioncooled HTS bearing has losses not significantly higher than an HTS
bearing cooled in liquid nitrogen in a non-conducting cryochamber.
We report here on such measurements.
2. Apparatus
The HTS bearing used in the present experiments is nearly identical to one used on the Boeing 5-kWh/100-kW FESS [2,6]. The
rotating component, shown in Fig. 1, consists of three rings of radially polarized PMs separated by ferromagnetic steel pole pieces.
The steel rings both “turn” the flux in the axial direction so that it
will have a high gradient within the adjacent superconductor, and
serve as support surfaces for the mechanically weak PMs. The magnet support structure includes a fiber-reinforced hoop on the outer
diameter of the magnet assembly, and a fiberglass laminate hub
structure to transfer loads between the magnets and a central shaft.
At the center of the assembly is shown a touch-down bearing that
is used to hold the rotor when the superconductors are warm, and
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3. Procedure
Fig. 1. Rotor part of HTS bearing, showing the surface that faces the HTSs.
to limit any positional excursions when it is rotating. The assembly
is oriented facedown and positioned several mm above a cryostat
containing YBCO crystals.
Each PM ring consists of a number of arc segments. The magnets are segmented to reduce the tendency to fracture under
centrifugal loading. Each arc segment is magnetized uniformly
in the horizontal direction, with the direction of magnetization
corresponding to the middle radius of the arc. Thus, there is a discontinuity of magnetization at the boundary between segments.
The discontinuity is partially mitigated by the pole pieces. Nevertheless, there is a measurable circumferential inhomogeneity of
the magnetic field that is associated with the segment gaps. The
mass of the bearing test rotor is 9 kg and its moment of inertia is
0.061 kg m2 .
The stator component of the HTS bearing is located immediately below the rotating component and consists of a set of HTS
crystals in a cryostat. Each crystal is a melt-textured single-domain
YBCO pellet. The YBCO tiles are typically hexagons with a tip-to-tip
dimension of 36 mm and a thickness of 4.5 mm.
Two cooling systems were used. In the first system, the HTS array
is housed in a G-10 cryostat and bathed in pool-boiling liquid nitrogen. When this paper refers to working gaps for the bearing it does
not include the thickness of the cryostat lid. Cooling of the HTS
bearing is maintained by a closed-loop, liter-sized liquid-nitrogen
reservoir, utilizing a passive thermal-siphon delivery system and
regulated by an “off-the-shelf” Gifford–McMahon cycle type cryocooler driven by an air-cooled compressor.
The second (conduction-cooled) cooling system is shown in
Fig. 2. The cold head (protruding tube at the left in Fig. 2) of a cryocooler is located outward from the HTS array and is connected
to it by means of a thick copper cold finger that connects to a
copper plate. The copper plate is mechanically supported by a G10 structure. The HTS crystals (not shown) sit immediately above
the copper plate and are attached to it with cryogenic epoxy. A
small heater on the cold head is used to control the temperature.
Thermocouples and silicon-diode temperature sensors monitor
the temperature of the cold head at several places in the HTS
array.
The HTS bearing is housed in a vacuum chamber. Low chamber
pressure is obtained from the combined action of a turbomolecular
pump and the cryopumping of the coldhead.
For operation, the rotor is held mechanically in place while the
cryogenic system is started. The period to bring the cryogenic chamber from ambient temperature to the operating temperature of 77 K
or less is about 2 h. The cooling system is stable, as evident by the
mostly constant temperatures after the initial cooldown. After the
cooldown period, the rotor is released and is levitated for the rest of
the test time. When testing is over, the rotor is again mechanically
held while the cryostat warms up.
A bi-directional eddy-current clutch accelerates and decelerates
the rotor part of the bearing. The clutch consists of a high-speed
electric motor whose vertically oriented shaft holds a steel cylinder above an aluminum disk. The aluminum disk contains 8 FeBNd
permanent magnets (PMs), and the steel cylinder is a flux return
path for the PMs. The aluminum disk is located outside the vacuum
chamber, immediately above a G-10 window. A second aluminum
disk at the top of the HTS bearing is located immediately below the
G-10 window. The rotating magnetic field of the 8 PMs induces eddy
currents in the aluminum disk of the bearing, resulting in torque. If
the rotational speed of the motor is higher (lower) than that of the
bearing, the bearing accelerates (decelerates). During coast periods, the motor and clutch are raised far enough away that its effect
on rotational drag of the rotor becomes insignificant. During coast
periods, the rpm versus time is recorded and the rotational loss rate
is calculated for data averaged over a 5 min period.
The residual gas pressure in the vacuum chamber exerts a small
rotational drag on the rotor. For the experiments reported here, this
pressure was less than 2 × 10−4 Torr, which is small enough to not
significantly affect the results.
4. Results
The rotational loss of an HTS bearing typically consists of a hysteretic loss and an eddy-current loss. If only hysteretic loss in the
HTS were present, the loss rate would be independent of speed. If
eddy-current loss is present, the loss rate would be proportional
to speed, i.e., it would increase linearly with speed. We are interested in comparing the velocity dependence of the rotational loss
for the two cooling systems. Therefore, we examine the data trends
for rotational speeds above the bearing critical. The rotor will spin
about its magnetic center below the critical frequency and about its
center of mass above the critical. Above the critical, the hysteretic
Fig. 2. Conduction-cooling apparatus for HTS bearing.
M. Strasik et al. / Materials Science and Engineering B 151 (2008) 195–198
197
both temperatures. This is consistent with our notion of the material properties. The temperature of the rotor is nearly independent
of the HTS temperature. The HTSs must magnetize to support the
rotor weight. While the magnetization within each individual HTS
may have a small temperature dependence, the general magnetization pattern seen by the bearing depends mainly on the gap upon
cooldown. Thus, the eddy currents in the rotor should not change
significantly with HTS temperature. The hysteretic loss is inversely
proportional to the HTS critical current density Jc , and Jc increases
significantly in dropping from 77 to 67 K. Thus, we expect the hysteretic loss to decrease with decreasing temperature, as seen in
Fig. 3.
4.2. Conduction-cooled results
Fig. 3. Rotational loss data at 4-mm gap at 77 and 67 K with linear fit to the 67-K
data above the bearing critical. HTSs were cooled in boiling nitrogen.
loss is found by extrapolating the loss rate to zero speed. This value
is typically larger than the actual loss at zero speed due to the rotor
whirling about its center of mass at speeds above the critical. The
eddy-current loss is characterized by the slope of the loss curve.
When the rotor drops after mechanical release, each individual
HTS crystal will magnetize. The induced current within an individual HTS crystal is nearly constant and to a first approximation
runs in a plane perpendicular to the axis of the HTS bearing. This
produces a magnetization in each crystal that is near zero at its
periphery and maximizes near its center. This pattern of magnetization produces an inhomogeneous magnetic field as seen by the
rotor part of the bearing upon rotation. The degree to which the
HTSs magnetize increases with the distance the rotor drops when
it is mechanically released. When the HTSs are bathed in liquid
nitrogen, only these eddy currents are expected to be present in
the rotating HTS bearing.
For the conduction-cooled HTSs, the eddy currents have an additional contribution from the thermal conductor connecting the
HTSs with the cryocooler. In this case, the inhomogeneity of the
magnetic field from the rotor, either intrinsic or produced by rotor
whirl, causes the eddy currents. In terms of efficiency, the eddy
currents in the thermal conductor are more significant, because
in addition to contributing directly to rotational drag, the heat
deposited must be removed by the cryogenic system.
4.1. HTS in boiling nitrogen
A number measurements of rotor losses during coast periods
were performed in late 2001 and early 2002 for an earlier design
of the FESS that used the same HTS bearing design. During these
experiments, the HTS stator was cooled in a G-10 cryostat filled
with boiling nitrogen. The pressure within the cryostat was varied
to obtain some temperature dependence.
For the comparison of this paper, we examine the rotational loss
data, shown in Fig. 3, at a gap of 4-mm and HTSs cooled by boiling
nitrogen at temperatures of 77 and 67 K. There are velocity dependent losses exhibited by the data. These losses are attributed to
magnetization of the HTS causing eddy currents in the rotor part of
the HTS bearing. Such losses are also likely to occur in the present
experiments on the conduction-cooled bearing. The purpose of
this analysis is to try and subtract these losses from the recently
acquired loss data when the HTS crystals were conduction cooled.
In Fig. 3, we note that change in temperature from 77 to 67 K
lowers the zero-speed intercept, whereas the shape of the velocity
dependence of the loss curves remains approximately the same for
In Fig. 4, we show rotational loss data from April 2007 for some of
the experiments in which the HTSs were conduction cooled. These
experiments had an HTS temperature of 50 K and gaps of 3.9 and
2.1 mm. In Fig. 4, a linear fit is shown for each data set above the
bearing critical. The gap for the April 4 experiment is only approximate, however, the difference between the extrapolated intercept
values and rotational loss slopes is probably representative of the
spread in data in these types of experiments between runs that are
nominally the same.
There are several observations with respect to the results. First,
there is a linear component to all the data, suggesting that eddycurrent losses are present. Second, if one extrapolates the linear fit
of data above the critical to below the critical, the 0 rpm intercept is
higher than the data point for a speed below the critical. This is consistent with a center of mass offset and rotation about it for speeds
above the critical, resulting in increased hysteretic loss above the
critical.
We expect some further increase in Jc of the HTS in dropping
from 67 to 50 K, but this should be considerably smaller than the
difference from 77 to 67 K. This is consistent with the extrapolated
intercept in the 3.9-mm-gap curve in Fig. 4 being about the same
as the extrapolated intercept in Fig. 3 for the 67-K data.
In Fig. 4, the slope of the linear dependence becomes less with
an increase in gap. In addition, the intercept extrapolated to zero
speed, which would be due to the hysteretic loss decreases with
increasing gap. These two trends are consistent with a smaller value
of field inhomogeneity as gap increases. The intercept falls off faster
with gap than the slope. This is consistent with the eddy current loss
being proportional to the square of the field inhomogeneity and the
hysteretic loss proportional to the cube of the inhomogeneity. How-
Fig. 4. Rotational loss versus rotational velocity at 50 K in a conduction-cooled HTS
bearing.
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The result is a more uniform pattern of magnetization among the
HTS array and a reduction in the eddy currents induced in the rotor.
A major result of the data presented in Fig. 5 is that the linear
behavior seen in Fig. 4 appears to hold at high speed. A second
important result is that the eddy-current contribution to the loss
is not significantly higher than the hysteretic component at high
speed.
5. Conclusions
Fig. 5. Rotational loss versus rotational velocity at 50 K for a gap of 2.1 mm in a
conduction-cooled HTS bearing.
ever, the analysis here is complicated by the geometry of the system,
in which the upper surface of the copper thermal bus is below the
HTS array. One would need to know how the inhomogeneities vary
with distance to better compare the losses at different distances.
We hypothesize that the slope of the curve in Fig. 3 represents
the contribution of the eddy currents deposited in the rotor to
the eddy currents seen in Fig. 4 for the 3.9-mm gap. This means
that the contribution to the eddy currents due to the conduction
cooling is 0.000350 − 0.000273 = 0.000077 (rpm/min per rpm). At
20,000 rpm, this would amount to 1.54 rpm/min – about equal to
the hysteretic loss in the bearing at this gap.
Over the temperature range of interest, the electrical resistivity
of copper can be approximated by T2.3 (see Fig. 1 of NBS 87, Cryogenic Properties of Copper and Copper Alloys – Electromagnetic
Properties), where T is the temperature in K. Thus, the ratio of resistivities from 67 to 50 K is 1.96, and the contribution to eddy-current
loss from the conduction-cooling apparatus would be expected to
reduce to half at 67 K.
In Fig. 5, we show measurements of rotational loss versus speed
for the conduction-cooled bearing at 50 K and a gap of approximately 2.1 mm, taken in August 2007. The maximum speed of
14,510 rpm was limited by the eddy current clutch, which was
unable to attain a higher speed than this under load. The extrapolated intercept is roughly equivalent to the two curves at 2.1-mm
gap in Fig. 3, indicating that the hysteresis loss is approximately
the same. The slope of the rotational loss is significantly lower than
those in Fig. 3. At the time of this writing, the reason for the discrepancy is not clear to the authors. However, we hypothesize that
the decrease is associated with improvements made in the rotor
release method, which oriented the rotor for a more uniform gap.
A conduction-cooled HTS bearing has been spun to rotational
speeds that the Boeing team typically uses for the HTS bearing in
flywheel energy-storage systems, and the rotational losses of the
bearing have been measured over this speed range at different HTS
temperatures. The rotational losses decrease with decreasing HTS
temperature. For temperatures that can be obtained in a liquidnitrogen thermosiphon system, at a given speed, the loss of the
conduction-cooled HTS bearing is not significantly higher than the
loss of a nearly identical HTS bearing cooled by flowing nitrogen
from the thermosiphon.
Based on the data to date, the contribution of the conductioncooling apparatus to the eddy current loss in the bearing seems to
be lower than the eddy current loss in the rotor part of the bearing
due to magnetization of the HTS. At 50 K, the expected contribution
at a rotational rate of 20,000 rpm would be roughly the same as the
hysteretic loss in the bearing.
Acknowledgements
Boeing’s efforts in flywheels have been partially supported
by the U.S. Department of Energy, Sandia National Laboratories
Energy Storage Program Contract 24412. The authors would like
to acknowledge the help and program guidance of Dr. Imre Gyuk
of the Department of Energy through the office of Energy Storage.
Authors also wish to thank Nancy Clark and John Boyes of Sandia
National Laboratories, Albuquerque, NM for timely technical and
program management advice.
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