9.4 HTS FESS principle model Ë 343of the RSB at lower temperature is needed for the application of the RSB to liquid
hydrogen and helium pumps.
9.3.3Future development of HTSB
What is the future development of HTSB? It is certain that researchers will improve
its performance, i.e. larger load capacity and force density. Because force density is
proportional to the square of magnetic field [45], three kinds of new HTSB designs
have been developed to increase the magnetic field. Patel [46] from University of
Cambridge put forward a RSB concept that uses superconductors for both rotor and
stator, so that the magnetic field of the superconducting rotor was greatly enlarged.
Central Japan Railway Company designed superconducting magnetic bearings with a
superconducting coil and an iron core in the rotor [47, 48]. Japan Railway Technical
Research Institute developed a superconducting bearing with a HTS bulk rotor and
a superconducting coil stator that finally obtained a stable 20 kN axial force [49].
ATZ company also simulated this type of HTSB, and found that the force density was
5 times the value (10–15 N/cm^2 ) obtained with a PM rotor [45].
Second, rotational loss must be reduced for practical applications of the HTSB. In
a small prototype, very low rotational loss is easy to realize [13]. However, for large-
scale HTSBs whose stator and rotor are made up of numerous HTS bulks and PM rings,
respectively, homogeneity of the trapped field in the HTS stator and the magnetic
field of the PM rotor are hard to guarantee. Rotational loss also comes from gas
friction, eddy current loss, etc. More focus should be systematically made to eliminate
rotational loss in practical applications of HTSB.
The last direction is to broaden the application fields of HTSB, i.e. the liquid nitro-
gen pump [44], the centrifuge [37], the superconducting non-contact mixer [42], etc.
9.4 HTS FESS principle model
9.4.1Introduction of FESS
FESS stores kinetic energy which is proportional to the rotational inertia and the
square of the rotational speed of its flywheel. A complete FESS system is composed of
a flywheel, bearing, motor/generator, vacuum chamber and safety protection device,
control system and electronic convertor. The three working stages for FESS are (1) the
energy storage stage, when the electric energy is converted into kinetic energy by
the motor while the speed of flywheel increases; (2) the standing stage, when the
motor/generator stops working (if standing losses from motor/generator, bearing, and
air drag are low enough, the energy storage time can be greatly extended); (3) the
electricity generation stage, when the kinetic energy is converted into electric energy