Environmental Engineering Reference
In-Depth Information
where I
m
is the moment of inertia and x is the rotational speed of the disk in
radians per second. Taking into account the above formula, it is clear that the
energy storage capability for a flywheel depends on its size and rotating speed, and
also on the resistance of the material of which the flywheel is made. For this
reason, many efforts have been made to increase the intrinsic low specific energy
of flywheels by using ultra-strong materials, magnetic bearings and running the
flywheel in inert gas or vacuum to reduce friction losses.
However, the use of flywheels needs to take into consideration also their safety
due to the consequence of a breaking of the flywheel, which can be made more
likely by the gyroscopic effect of the rotating disk [
39
], or of a crush of the vehicle
currying a rotating flywheel on board. In this case, without the proper precautions,
the
flywheel
energy
would
be
released
almost
instantly
with
dangerous
consequences.
5.4.2 Super Capacitors
Capacitors are devices composed by two conducting plates which are separated by
an insulating material. A schematization of a capacitor is shown in Fig.
5.16
, where
a DC voltage is applied to the capacitor, obtaining as a consequence one plate
positively charged and the other one negatively, with the result that the device can
store a charge and hence energy, since opposite charges attract each other.
The charge Q
c
stored by a capacitor of capacitance, C (F), at the voltage of
V volt is expressed by the equation:
Q
c
¼
C
V
ð
5
:
31
Þ
Generally capacitors are used in small size as components in electronic circuits,
but they can provide large energy storage in the same way as flywheels. The
capacitors with large plate areas have large energy storage capability and they are
called super capacitors. The energy in Joules that a capacitor can store is expressed
by the following equation:
Fig. 5.16
Schematization of
Positive charge
Negative charge
a capacitor
Insulating
material
DC voltage
-
+
