9.6 Application of HTS FESS Ë 3599.6 Application of HTS FESS
9.6.1Subway regenerative braking
The huge electric energy consumption of a subway system is a problem chroni-
cally troubling the modern metropolis. Regenerative braking technology is a good
candidate to solve this problem. Nearly 30%–40% of the kinetic energy of a subway
vehicle can be fed back to the supply network of the subway system through rege-
nerative braking technology [81]. Unfortunately, the feedback energy from a braking
subway vehicle cannot be utilized by other subway vehicles or electric equipment
synchronously since it is mostly consumed by the brake resistor in the form of heat. It
is better to introduce an energy storage unit to receive the feedback energy and then
supply the feedback energy to other subway vehicles or electrical equipment.
Compared with other energy storage models, the FESS has higher power density,
long service life, and many more charge cycles, so it is very suitable for the application
in a regenerative subway braking system. In fact, several FESS units have been
successfully applied in a subway or light railway system. They have given excellent
performances in energy savings and power quality improvements.[82, 83] However,
the bearings used in those FESS units are conventional mechanical type, which give
frictional loss and inconvenience in maintenance for the subway application.
The HTS Maglev bearing has low rotating friction and requires no active control,
which means higher rotate speed, larger energy storage capacity, smaller size, and
lower standby loss. Furthermore, the HTS Maglev bearing has good capacity of resis-
ting disturbances and excellent self-recovery ability, which come from the flux pinning
characteristics of HTS materials.
Besides the ESC and the I/O power, the installation and operation model is also
important for application of HTS FESS in subway systems. If the HTS FESS operates
on-board the subway vehicle, there is a strict limitation in volume and weight. Then,
the HTS FESS must work at a very high speed, usually more than 30,000 rpm. However,
if the HTS FESS is installed and operates on the station of subway system, it has no
volume and weight limitation, which is beneficial to the design and test, especially for
the dynamic balance characteristics of the flywheel. Furthermore, the storage energy
of FESS in the station model can be used conveniently by other auxiliary equipment
of the subway system, such as air-conditioning and lighting facilities of the subway
stations.
The array of three HTS FESS prototypes with an available ESC of 5.75 kWh and
maximal I/O power of 400 kW can satisfy the basic operation requirement of a subway
system. Before the practical application, this HTS FESS prototype system will be tested
to simulate a subway system in the next 1–2 years. When used in the subway system,
the HTS FESS units can bring obvious energy savings. For example, under the average
departure frequency of the Beijing subway system, the operation interval of a HTS
FESS unit is about 3 minutes. Assuming 50% of the 5.75-kWh ESC can be utilized ever