344 Ë 9 HTS Maglev bearing and flywheel energy storage system
by the generator while the speed of the flywheel decreases. Speed of flywheel is
controlled over the range between the maximum speed and the minimum speed. At
the maximum speed, the stored energy of FESS reaches the peak. The maximum speed
is mainly limited by the ratio of strength and density of the material of flywheel as
well as its rotational stability. The minimum speed depends on the rotational stability
of flywheel and the efficiency of electricity generation. To reduce standing losses,
optimal design of the motor/generator, suitable selection of the bearings and high
vacuum in the chamber (lower than 10−^2 Pa) are required. The higher ratio of strength
and density the materials are, the higher limit of the energy storage density will be as
the basic technology demands. For instance, the ratio of strength and density for an
aluminum alloy is 2.1× 105 m^2 /s^2 , and its energy density limit is 23.8 Wh/kg. Ratio of
strength and density for T700 carbon fiber is 2.6× 106 m^2 /s^2 with the corresponding
energy density limit of 218.7 Wh/kg [50]. In comparison, the carbon fiber material
is a good candidate for large-energy storage flywheels. In addition, stability can be
improved by optimal design of the dynamic structure of flywheel and the speed range
should be kept away from resonance speeds. Safety can be guaranteed by many
methods, such as introducing the carbon fiber flywheel whose broken fragments
are less dangerous than those from a metal flywheel and further strengthening the
vacuum chamber. Response time depends on the control system and the electronic
convertor.
FESS has high energy efficiency of the range of 90%–95% during short periods.
It allows a very large number of charge/discharge cycles and has a long lifetime.
Monitoring the state of charge is simple and reliable because it is easy to measure
the rotational speed. It is an environmentally friendly technology because all of the
components can be recycled [51]. FESS can also be applied to vehicles, spaceflights,
and uninterrupted power supplies (UPSs). Twelve units of FESS for an American
Flywheel Systems Inc. with a total mass of 237 kg have been used in the IMPACT car
of GM Company, which reached a cruising range of 480 km [52]. Hongkong Citybus
uses the UPT tr100 FESS to successfully save 11% of the electric energy of an elec-
trically driven trolley bus [53]. Research shows that 30% of fuel can be saved and
75% of automobile exhaust can be reduced by employing FESS [54]. For spaceflight,
NASA, Fare Company, and University of Maryland developed a 50-Wh FESS with an
open core composite flywheel for satellites. The FESS replaced the original chemical
battery [55] and also had the function of attitude control [56]. FESS can be also
applied as UPS which prevent important systems from shutting down in the case
of power failure, i.e. for the computer networks of banks, cloud computing centers,
and communication base stations. Active Power Company launched FESS UPS 150i,
which can supply AC power at 380 V, 228 A, and has a standard ride-through time
of 24 s [57].
Table 9.3 summarizes the technical, economic, and environmental state of all the
energy storage technologies including FESS [51].