30-08-2014, 10:43 AM
Flywheel storage systems
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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 analyzed.
Later in the 1970s flywheel energy storage was proposed as a primary objective for electric vehicles and stationary power backup. At the same time fiber composite rotors where built, and in the1980s magnetic bearings started to appear .Thus the potential for using flywheels as electric energy storage has long been established by extensive research.
More recent improvements in material, magnetic bearings and power electronics make flywheels a competitive choice for a number of energy storage applications. The progress in power electronics, IGBTs and FETs, makes it possible to operate flywheel at high power with a power electronics unit comparable in size to the flywheel itself or smaller. The use of composite materials enables high rotational velocity with power density greater than that of chemical batteries. Magnetic bearings offer very low friction enabling low internal losses during long-term storage. High speed is desirable since the energy stored is proportional to the square of the speed but only linearly proportional to the mass.
Flywheel
Flywheels are rotating wheels used to store kinetic energy, much like a spinning top. Electricity is used to "wind" the wheel up through a system of gears. The flywheel then delivers rotational energy to power an electric generator until friction dissipates it. The sum of the kinetic energy of the individual mass elements that comprise the flywheel equals the energy stored.
The kinetic energy of a flywheel is given by where I is the moment of inertia (the ability of an object to resist changes in its rotational velocity), and w is the rotational velocity in rpm. The moment of inertia is defined as where M is the mass, R is the radius, and k is the inertial constant. The inertial constant depends on the shape of the object
Schematic diagram of fly wheel system
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 he 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 . Apart from the flywheel additional power electronics is required to control the power in- and output, speed, frequency etc. The kinetic energy stored in a flywheel is proportional to the mass and to the square of its rotational speed according to Eq. (1).
MAGNETIC BEARING
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 flywheelalthough 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 . 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
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 designer select 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 . 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 .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 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 436 kV. 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. The result is that for true high voltage applications a transformer has to be used, introducing more unwanted losses. A part from the PM motor/generator used in almost all flywheels there is also the possibility of using a Synchronous Reluctance Motor/Generator.
NUMBER OF POLES
The choice of number of poles to be used in a machine is essential to the overall performance. Two pole motor/generators are most common in high-speed machines, mainly to keep the voltage down but it also has other good properties. Depending on axial or radial flux configuration a multi-pole rotor can experience substantial electromagnetic axial or radial forces generated by the stator winding, if there is a net attractive force between a pole-pair and the stator. In a two-pole rotor, however, the only two poles are directly opposite one another resulting in a net force on the rotor of approximately zero. Eliminating these forces reduces the load requirements on the bearings, which is particularly important if magnetic bearings are used.
WORK DONE USING FLYWHEEL
SMALL SCALE
Small-scale flywheel energy storage systems have relatively low specific energy figures once volume and weight of containment is comprised. But the high specific power possible, constrained only by the electrical machine and the power converter interface, makes this technology more suited for buffer storage applications. Development of alternative dual power source electric vehicle systems that combine a flywheel peak power buffer with a battery energy source has been undertaken.
PEAK POWER BUSES
The uses of a flywheel as power buffer in an electric vehicle can significantly reduce the peak currents drawn from the ordinary storing supply e.g. battery. Elimination of the peak currents will prolong the battery life.
ELECTRIC START
Flywheel energy storage is being investigated as a direct result of the potential use of electric starters on U.S. Navy gas turbine engines. All current gas-turbine powered ships of the U.S. Navy use compressed air to provide start-up capability to the engines. This applies to the main propulsion plants (LM2500) as well as power generation units (501-K17/34). In order to meet air flow demands to start these engines, the complex pneumatic system increases maintenance schedules and high failure rates occur that often disrupt critical ship schedules, increase manpower allocations for its service, as well as increased engine downtime intervals. Another critical aspect of the pneumatic starter system is the ship’s space committed to store pressure flasks, piping, valves, pumps, cooling devices, and other support subsystems. Traditionally, the current start systems on these engines have always been on the Navy’s TMA/TMI failure list, and the recently demonstrated Electric Start System can provide the Navy with a much needed improvement. Installation of the Electric Start System will present the fleet with a simple, more reliable start system that was not available in the past. Installation of this system will assist NAVSEA in optimizing fleet readiness while reducing maintenance and manning requirements, improving equipment reliability, and reducing total ownership cost in both current and future installations.
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 in corporate 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.