346 Ë 9 HTS Maglev bearing and flywheel energy storage system
9.4.2HTS Maglev FESS
As discussed above, HTSB is very suitable for the application to FESS because of these
inherent advantages. China [58, 59], Germany [60–62], USA [17], Japan [23–27, 47, 48],
England [63], Korea [64–68], Brazil [69], Italy [70], have carried out research on HTS
Maglev FESS. The HTS Maglev FESSs from Boeing Company [16, 17], ATZ [60, 61], ISTEC
[23, 24, 26, 27, 47], and Korea Advanced Institute of Science and Technology (KAIST)
[64–68] are the most typical ones. Table 9.4 shows the parameters of the HTSBs in
these typical FESSs. The parameters of RSB from Nexans SuperConductors GmbH are
also listed for comparison.
In Boeing’s FESS, the upper PM bearing supports most of the rotor mass of 132 kg
and ASB plays the function of stability. The maximum storage energy is 5 kWh and the
FESS can supply 208 V, 3 kW AC power. The ASB is composed of a YBCO HTS stator
and a∼220-mm-diameter PM rotor. Axial and radial stiffness are 144 and 69 N/mm,
respectively, when the working gap is 3.2 mm. Boeing tested its rotational loss under
the maximum speed of 14,500 rpm.
The FESS from ATZ can store 10-kWh energies when the flywheel speeds up to
10,000 rpm and supply a power of 250 kW. The 600-kg rotor is levitated by the upper
RSB and the lower PM bearing. At 72 K, the maximum axial carrying capacity and axial
stiffness of the RSB are 10.08 kN and 4.5 kN/mm (3.3 mm) and 4.7 kN and 1.8 kN/mm
(3.2 mm), respectively. Diameter and length of the PM rotor are 200 and about 120 mm,
respectively. The inner diameter of the HTS stator is 205 mm and working gap is 2 mm.
Force density at 70 K is 13 N/cm^2 axially and 6.5 N/cm^2 radially. The HTS stator is
cooled by a cryocooler and the G-10 support structure can support the 2-ton load with
a heat loss less than 0.3 W.
ISTEC FESS can store 10 kWh at 15,860 rpm. The 425-kg rotor is suspended by the
RSB and the top axial electromagnetic bearing. Rotational stability is increased by
introducing two radial electromagnetic bearings. The RSB has an inner HTS stator and
an outer PM rotor. The PM rotor has an outer diameter of about 170 mm and length of
about 300 mm. The maximum axial force of the RSB reaches 8.7 kN.
Compared with the above large-scale FESSs, KAIST has developed a small-sized
FESS with ASB. This FESS is used for the energy storage and the attitude control of
nanosatellites. The HTS stator is made up of seven 10-mm-diameter, 1.8-mm-depth
HTS bulks. A 50-mm-diameter and 63.2-g flywheel is levitated above the HTS stator
with a gap of 1–7 mm. At 0.13 Pa, the rotor can rotate at 51,000 rpm with stored energy
of 337 J. The whole rotation lasts 3 h 20 min at 7-mm working gap.
Presently, KEPRI is developing a 35-kWh FESS with a 1588-kg rotor which is
levitated by two RSBs, a PM bearing, and a thrust electromagnetic bearing. The RSB
is composed of 12 YBCO bulks and the working gap is 2.5 mm. The radial stiffness
and damping coefficient of RSB are 346.6 N/mm and 1255 Ns/m, respectively. The
maximum design speed is 12,000 rpm, and it has achieved an experimental speed of
6500 rpm successfully [31, 73].