26-12-2012, 02:12 PM
Flywheel energy and power storage systems
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Abstract
For ages flywheels have been used to achieve smooth operation of machines. The early models
where purely mechanical consisting of only a stone wheel attached to an axle. Nowadays flywheels
are complex constructions where energy is stored mechanically and transferred to and from the
flywheel by an integrated motor/generator. The stone wheel has been replaced by a steel or composite
rotor and magnetic bearings have been introduced. Today flywheels are used as supplementary UPS
storage at several industries world over. Future applications span a wide range including electric
vehicles, intermediate storage for renewable energy generation and direct grid applications from
power quality issues to offering an alternative to strengthening transmission.
One of the key issues for viable flywheel construction is a high overall efficiency, hence a reduction
of the total losses. By increasing the voltage, current losses are decreased and otherwise necessary
transformer steps become redundant. So far flywheels over 10 kV have not been constructed, mainly
due to isolation problems associated with high voltage, but also because of limitations in the power
electronics. Recent progress in semi-conductor technology enables faster switching and lower costs.
The predominant part of prior studies have been directed towards optimising mechanical issues
whereas the electro technical part now seem to show great potential for improvement. An overview of
flywheel technology and previous projects are presented and moreover a 200kW flywheel using high
voltage technology is simulated.
Introduction
Several hundred years ago pure mechanical flywheels where used solely to keep machines
running smoothly from cycle to cycle, thereby render possible the industrial revolution. During
that time several shapes and designs where implemented, but it took until the early 20th
century before flywheel rotor shapes and rotational stress were thoroughly analysed [1]. Later
in the 1970s flywheel energy storage was proposed as a primary objective for electric vehicles
and stationary power backup. At the same time fibre composite rotors where built, and in the
1980s magnetic bearings started to appear [2]. Thus the potential for using flywheels as electric
energy storage has long been established by extensive research.
Flywheel basics
Energy storage in flywheels
A flywheel stores energy in a rotating mass. Depending on the inertia and speed of the
rotating mass, a given amount of kinetic energy is stored as rotational energy. The flywheel
is placed inside a vacuum containment to eliminate friction-loss from the air and
suspended by bearings for a stabile operation. Kinetic energy is transferred in and out of
the flywheel with an electrical machine that can function either as a motor or generator
depending on the load angle (phase angle). When acting as motor, electric energy supplied
to the stator winding is converted to torque and applied to the rotor, causing it to spin
faster and gain kinetic energy. In generator mode kinetic energy stored in the rotor applies
a torque, which is converted to electric energy. Fig. 1 shows the basic layout of a flywheel
energy storage system [9]. Apart from the flywheel additional power electronics is required
to control the power in- and output, speed, frequency etc.
Magnetic bearings
Mechanical bearings used in the past cannot, due to the high friction and short life, be
adapted to modern high-speed flywheels. Instead a permanent or electro permanent
magnetic bearing system is utilized. Electro permanent magnetic bearings do not have any
contact with the shaft, has no moving parts, experience little wear and require no
lubrication. It consists of permanent magnets, which support the weight of the flywheel by
repelling forces, and electromagnets are used to stabilize the flywheel, although it requires
a complex guiding system. An easier way to stabilize is to use mechanical bearings at the
end of the flywheel axle, possible since the permanent magnet levitates the flywheel and,
thus, reduce the friction [13,14]. The best performing bearing is the high-temperature
super-conducting (HTS) magnetic bearing, which can situate the flywheel automatically
without need of electricity or positioning control system. However, HTS magnets require
cryogenic cooling by liquid nitrogen [12].
Flywheel technical considerations
For decades, most engineers have used the concept of storing kinetic energy in a
spinning mass to smooth their operation. Until recently the vast majority constituted of
steel wheels coupled with a motor/generator, where the high rotary inertia allowed long
ride-through time without significant decrease in flywheel rotational speed. Since the
change in rotational speed directly reflects the electrical frequency the power delivery of
those flywheels rarely exceeded 5% of the stored energy.
Motor/generator
Requirements for standardized electric power have made most flywheel system designers
elect variable speed AC generators (to accommodate the gradual slowing of the flywheel
during discharge) and diodes to deliver DC electricity. The two major types of machines
used are the axial-flux- and the radial-flux permanent magnet machines (AFPM and
RFPM, respectively). There are numerous alternatives for the design of an AFPM machine
such as internal rotor, internal stator, multidisc, slotted or slot-less stator, rotors with
interior or surface-mounted magnets [15–18]. Unlike radial machines, axial machines can
have two working surfaces. Either two rotors combined with one stator or one rotor
combined with two stators. The benefit of using a two surface working machine is the
increase in power output [17,19]. The axial machines seem to have more advantages over
the radial such as, a planar adjustable air gap and easy cooling arrangements, which is
important when working under low-pressure conditions [20]. Fig. 3a shows a one-rotor
two stator AFPM configuration without the cable winding in the stators. It can be seen
that the permanent magnets are an integral part of the flywheel rotor and the stators are
fixed to the housing.
High voltage
Even though different kinds of flywheels constructed today benefit from the recent
progress in technology, there is one thing all of them have in common, the inability to
directly produce high voltage ð436 kVÞ. So-called ‘high voltage’ flywheels have been
constructed. However, the highest voltage attained so far is a 10-pole permanent magnet
machine with a continuous voltage of 6.7 kV and a peak voltage of 10 kV constructed in
2001 [28]. The result is that for true high voltage applications a transformer has to be used,
introducing more unwanted losses.
Apart from the PM motor/generator used in almost all flywheels there is also the
possibility of using a Synchronous Reluctance Motor/Generator. In 1996 a 60 kW
flywheel, utilizing this motor, was developed [29]. Table 3 shows advantages/disadvantages
with PM and induction machines.
Power electronics
A brushless permanent magnet generator (in a flywheel) produces variable frequency AC
current. In most applications though, the load requires a constant frequency making it
necessary to first rectify the current and then convert it back to AC. Power converters for
energy storage systems are based on SCR, GTO or IGBT switches. In an early stage of
energy storage utility development, SCRs where the most mature and least expensive
semiconductor suitable for power conversion. SCRs can handle voltages up to 5 kV,
currents up to 3000A and switching frequencies up to 500 Hz. Due to the need of an
energized power line to provide the external on/off signal to those switches they where
replaced with GTOs, which do not depend on an energized line to function. The GTO
device can handle voltages up to 6 kV, currents up to 2000A and switching frequencies up
to 1 kHz. In the last several years IGBTs has emerged, Fig. 5. The IGBT is a solid-state
switch device with ability to handle voltages up to 6.7 kV, currents up to 1.2 kA and most
important high switching frequencies [30,31].
Conclusions
Flywheel storage systems have been used for a long time. Material and semiconductor
development are offering new possibilities and applications previously impossible for
flywheels. The fast rotation of flywheel rotors is suitable for direct generation of high
voltage. Thus for flywheel applications, the motor/generator part has a large upgrade
potential. In this article a 200kW permanent magnet air gap winding motor/generator
with axial flux has been simulated. This motor/generator setup incorporates high voltage
technology with the use of NdFeB permanent magnets and an air wound stator. The
simulation topology used was partly chosen for simulation simplicity and can most likely
be enhanced. The simulated motor/generator is intended for a flywheel storage system
situated in e.g. a bus. However, this flywheel technology is scalable and larger machines
can be constructed for the applications of e.g. stabilizing the electric grid.