352 Ë 9 HTS Maglev bearing and flywheel energy storage system
9.5 Development of a 5-kWh HTS FESS prototype
9.5.1Design and constitution
A HTS FESS prototype has been developed by the authors with the primary goal of
energy savings in a subway system. Total energy storage capacity (ESC) is the key
parameter for a FESS. For tests and application in subway system, the ESC of the HTS
FESS prototype is designed for 5 kWh. Figure 9.18 shows the feedback kinetic energy
of a subway train with different weights and speeds. For the metropolis like Beijing or
Shanghai, the weight of a subway train under full load is usually more than 320 tons
with an average operation speed of more than 60 km/h. From Fig. 9.18, it is clear that
the ESC of 20 MJ (5.56 kWh) can satisfy this requirement. Furthermore, the depth of dis-
charge (DOD) should be considered because not all of the storage energy can be used.
Withtheconsiderationofvoltageattenuation,thelowestrevolvingspeedofaHTSFESS
during operation is usually half of the highest speed, which implies a DOD of 75%.
The input/output (I/O) power of HTS FESS is also important to the application in
a subway system, which is determined by the power requirement of the subway train
during starting, acceleration, and braking. A conventional subway train has about 16
onboard drive motors and the peak power demand is more than 2 MW. The I/O power
of HTS FESS depends on the combined electrical machine and it is difficult to design
and manufacture a single FESS unit with a 2-MW power level. It is better to have the
FESS array consisting of several uniform FESS units with lower power level to satisfy
the large power demand.
To simplify the design, manufacture, and fabrication of the FESS unit, we planned
to install at least three HTS FESS units each with ESC of 5 kWh and I/O power of
Fig. 9.18:Energy storage capacity for a subway train with different weight and speed. The feedback
energy equals 40% of the total kinetic energy.