Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 684

contained within a vacuum to reduce aerodynamic drag losses. Flywheels store energy
by accelerating the rotor/fl ywheel to a very high speed and maintaining the energy in
the system as kinetic energy. Flywheels release energy by reversing the charging pro-
cess so that the motor is then used as a generator. As the fl ywheel discharges, the rotor/
fl ywheel slows down until eventually coming to a complete stop.
The rotor dictates the amount of energy that the fl ywheel is capable of storing.
Flywheels store power in direct relation to the mass of the rotor, but to the square
of its surface speed. Consequently, the most effi cient way to store energy in a fl y-
wheel is to make it spin faster, not by making it heavier. The energy density within
a fl ywheel is defi ned as the energy per unit mass:

2 22

I w mr w

(3)

f

E = =

f

2 2

where E f is the total kinetic energy in J, I the moment of inertia in kg/m 2 , w the
angular velocity of the fl ywheel in rad/s, m f the mass of the fl ywheel in kg and
r is the radius in m.
The power and energy capacities are decoupled in fl ywheels. In order to obtain the
required power capacity, you must optimise the motor/generator and the power elec-
tronics. These systems, referred to as 'low speed fl ywheels', usually have relatively
low rotational speeds, approximately 10,000 rpm and a heavy rotor made form steel.
They can provide up to 1650 kW, but for a very short time, up to 120 s.
To optimise the storage capacities of a fl ywheel, the rotor speed must be
increased. These systems, referred to as 'high speed fl ywheels', spin on a lighter
rotor at much higher speeds, with some prototype composite fl ywheels claiming to
reach speeds in excess of 100,000 rpm. However, the fastest fl ywheels commer-
cially available spin at about 80,000 rpm. They can provide energy up to an hour,
but with a maximum power of 750 kW.
Over the past number of years, the effi ciency of fl ywheels has improved up to
80% [3], although some sources claim that it can be as high as 90% [1]. As it is a
mechanical device, the charge-to-discharge ratio is 1:1.

4.6.1 Applications of FES
Flywheels have an extremely fast dynamic response, a long life, require little main-
tenance, and are environmentally friendly. They have a predicted lifetime of approx-
imately 20 years or tens of thousands of cycles. As the storage medium used in
fl ywheels is mechanical, the unit can be discharged repeatedly and fully without any
damage to the device. Consequently, fl ywheels are used for power quality enhance-
ments such as uninterruptible power supply (UPS), capturing waste energy that is
very useful in EV applications and fi nally, to dampen frequency variation, making
FES very useful to smooth the irregular electrical output from wind turbines.

4.6.2 Cost of FES
At present, FES systems cost between $200/kWh to $300/kWh for low speed
fl ywheels, and $25,000/kWh for high speed fl ywheels [2]. The large cost for high
speed fl ywheels is typical for a technology in the early stages of development.