up Power through
Navin kumar Kohli
Presentation at JJTU in July 2012
• A flywheel, in essence is a mechanical battery -
simply a mass rotating about an axis.
• Flywheels store energy mechanically in the form
of kinetic energy. They take an electrical input to
accelerate the rotor up to speed by using the
built-in motor, and return the electrical energy by
using this same motor as a generator.
• They may still prove to serve us as an important
component on tomorrow's vehicles and future
• Provide continuous energy when the energy
source is not continuous.
• Deliver energy at rates beyond the ability of
an energy source.
• Control the orientation of a mechanical
• Flywheel energy storage systems store kinetic
energy (i.e. energy produced by motion) by
constantly spinning a compact rotor in a low-
• When short-term back-up power is required
(i.e. when utility power fluctuates or is lost),
the rotor's inertia allows it to continue
spinning and the resulting kinetic energy is
converted to electricity.
• Integrates the function of a motor, flywheel
rotor and generator into a single integrated
• The motor, which uses electric current from
the utility grid to provide energy to rotate the
flywheel, spins constantly to maintain a ready
source of kinetic energy.
Flywheel Technology contd
The generator then converts the kinetic energy of
the flywheel into electricity. This integration of
functionality reduces the cost and increases
The flywheel rotor spins in a near frictionless
environment, created by Active Power's patented
magnetic bearing technology.
• The efficiency in the chamber is further enhanced
by the creation of a rough vacuum, which reduces
drag on the spinning flywheel.
• As power is transferred to the load, the flywheel's
speed decreases. Additional current is then
supplied to the field coil to ensure that the
voltage output remains constant throughout
• This enables the flywheel system to provide ride
through power during power disturbances.
• Stored energy = sum of kinetic energy of
individual mass elements that comprise the
Kinetic Energy = 1/2*I*w2 , where
I = moment of inertia (ability of an obeject to
resist changes in its rotational velocity)
w = rotational velocity (rpm)
I = k*M*R2 (M=mass; R=radius);
k = inertial constant (depends on shape)
Inertial constants for different
• Wheel loaded at rim (bike tire); k = 1
solid disk of uniform thickness; k = 1/2
solid sphere; k = 2/5
spherical shell; k = 2/3
thin rectangular rod; k = ½
• In order to optimize the energy to mass ratio, the
flywheel needs to spin at the maximum possible
speed. Because kinetic energy only increases
linearly with Mass but goes as the square of the
• Rapidly rotating objects are subject to centrifugal
forces that can rip them apart. Centrifugal force
for a rotating object goes as M*R*w2 .
• Thus while dense material can store more energy
it is also subject to higher centrifugal force and
thus fails at lower rotational speeds than low
density material. Therefore the tensile strenghth
is more important than the density of the
Flywheel Energy Storage System (FES)
• In addition to energy density, flywheel energy
storage systems (FES) also offer several
important advantages over chemical energy
storage. The rate at which energy can be
exchanged into or out of the battery is limited
only by the motor-generator design.
• Therefore, it is possible to withdraw large
amounts of energy in a far shorter time than
with traditional chemical batteries.
Act as Gyroscope
• When used in vehicles, flywheels also act as
gyroscopes, since their angular momentum is
typically of a similar order of magnitude as the
forces acting on the moving vehicle.
• This property may be detrimental to the vehicle's
handling characteristics while turning. On the
other hand, this property could be utilized to
keep the car balanced so as to keep it from rolling
over during sharp turns.
• Flywheels store energy very efficiently (high
turn-around efficiency) and have the potential
for very high specific power compared with
• Flywheels have very high output potential and
relatively long life. Flywheels are relatively
unaffected by ambient temperature extremes.
• Current flywheels have low specific energy.
There are safety concerns associated with
flywheels due to their high speed rotor and
the possibility of it breaking loose & releasing
all of it's energy in an uncontrolled manner.
• Flywheels are a less mature technology than
chemical batteries, and the current cost is too
high to make them competitive in the market.
• Flywheels are one of the most promising
technologies for replacing conventional lead acid
batteries as energy storage systems for a variety
of applications, including
automobiles, economical rural electrification
systems, and stand-alone, remote power units
commonly used in the telecommunications
• Recent advances in the mechanical properties of
composites has rekindled interest in using the
inertia of a spinning wheel to store energy.
• Processes and Materials of Manufacture by
Navin Kumar Kohli