326 Ë 9 HTS Maglev bearing and flywheel energy storage system
and the stiffness is large (50 N/cm^2 ) [12]. The feedback control of electromagnetic
bearings is complicated and requires continuous power input, which implies danger
in the case of sudden power failures.
HTSBs have the lowest rotational loss among all kinds of bearings (coefficient
of friction (COF) is as low as 10−^7 ) [13], and they can reach very high speed (up to
520,000 rpm) [14]. Compared with mechanical bearings, HTSBs have low noise, long
service life (∼20 years), no abrasion, and no need of lubricant. Unlike PM bearing,
HTSBs are stable in all directions. Furthermore, HTSBs do not need any active control
system like that necessary in electromagnetic bearings. Even in the case of power
failures, HTSBs have enough time to stop the rotor safely before the superconducting
state disappears. Recognized as a green technology, HTSB technology has good envi-
ronmental adaptation and can be used in a vacuum environment, which is impossible
for gas-suspending bearings. Table 9.1 shows the performance comparison of HTSB,
electromagnetic bearings, and mechanical bearings.
Because of these outstanding characteristics, HTSBs attract much research inte-
rest all over the world. Both Moon from Cornell University [15] and Hull from Argonne
National Laboratory [13] had conducted comprehensive and systematic fundamental
work on the interaction forces between HTSC and PM, relaxation and drift of forces,
low-temperature performance, vibration performance, stiffness, damping, dynamic
stability, magnetic resistance, rotational loss, long-term operation performance, struc-
ture optimization, etc. In 1990, the rotational speed of a HTSB prototype reached
120,000 rpm, and the maximum rotational speed of 520,000 rpm was reported 2
years later [14]. With the development of HTSC material (trapped field,Btrap≈3.7 T at
77 K) and PM material (NdFeB with magnetic energy product of 52 MGOe), the HTSB
technology has a gradually mature. The research has entered the competitive phase
of the full-scale prototype since the end of the last century. USA, Germany, Japan, and
Korea have already developed full-scale prototypes of HTSB for different applications.
Among these applications, FESS is one of the most promising applications. HTS
Maglev FESS is the perfect technology match of HTSB and FESS because the high
rotational speed and low rotational loss of HTSB significantly improve energy density
and prolong storage time of FESS, as well as give a longer working life. As mentioned
Tab. 9.1:Performance comparison of HTSB, electromagnetic bearing, and mechanical bearing.
HTSB Electromagnetic bearing Mechanical bearingCOF (minimum) 10 −^710 −^410 −^3
Abrasion No No Yes
Control system No Yes No
Accessory Cooling system Sensor and electronic component No
Speed limit No No Yes
Carrying capacity Low High Very high
Stiffness Low High Very high