20-11-2012, 05:57 PM
Flywheel energy storage
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Main components
The main components of a typical flywheel.
A typical system consists of a rotor suspended by bearings inside a vacuum chamber to reduce friction, connected to a combination electric motor and electric generator.
First generation flywheel energy storage systems use a large steel flywheel rotating on mechanical bearings. Newer systems use carbon-fiber composite rotors that have a higher tensile strength than steel but are an order of magnitude less heavy.[3]
Magnetic bearings are sometimes used instead of mechanical bearings, to reduce friction.
The expense of refrigeration led to the early dismissal of low temperature superconductors for use in magnetic bearings. However, high-temperature superconductor (HTSC) bearings may be economical and could possibly extend the time energy could be stored economically. Hybrid bearing systems are most likely to see use first. High-temperature superconductor bearings have historically had problems providing the lifting forces necessary for the larger designs, but can easily provide a stabilizing force. Therefore, in hybrid bearings, permanent magnets support the load and high-temperature superconductors are used to stabilize it. The reason superconductors can work well stabilizing the load is because they are perfect diamagnets. If the rotor tries to drift off center, a restoring force due to flux pinning restores it. This is known as the magnetic stiffness of the bearing. Rotational axis vibration can occur due to low stiffness and damping, which are inherent problems of superconducting magnets, preventing the use of completely superconducting magnetic bearings for flywheel applications.
Since flux pinning is the important factor for providing the stabilizing and lifting force, the HTSC can be made much more easily for FES than for other uses. HTSC powders can be formed into arbitrary shapes so long as flux pinning is strong. An ongoing challenge that has to be overcome before superconductors can provide the full lifting force for an FES system is finding a way to suppress the decrease of levitation force and the gradual fall of rotor during operation caused by the flux creep of SC material.
[edit] Physical characteristics
See also: Flywheel#Physics
General
Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance;[2] full-cycle lifetimes quoted for flywheels range from in excess of 105, up to 107, cycles of use),[4] high energy density (100-130 W•h/kg, or 360-500 kJ/kg),[4][5] and large maximum power output. The energy efficiency (ratio of energy out per energy in) of flywheels can be as high as 90%. Typical capacities range from 3 kWh to 133 kWh.[2] Rapid charging of a system occurs in less than 15 minutes.[6] The high energy densities often cited with flywheels can be a little misleading as commercial systems built have much lower energy density, for example 11 W•h/kg, or 40 kJ/kg.[7]
[edit] Energy density
The maximum energy density of a flywheel rotor is mainly dependent on two factors, the first being the rotor's geometry, and the second being the properties of the material being used. For single-material, isotropic rotors this relationship can be expressed as[8]
,
where the variables are defined as follows:
- kinetic energy of the rotor [J]
- the rotor's mass [kg]
- the rotor's geometric shape factor [dimensionless]
- the tensile strength of the material [Pa]
- the material's density [kg/m^3]
[edit] Geometry (shape factor)
The highest possible value for the shape factor of a flywheel rotor, is , which can only be achieved by the theoretical constant-stress disc geometry.[9] A constant-thickness disc geometry has a shape factor of , while for a rod of constant thickness the value is . A thin cylinder has a shape factor of .
[edit] Material properties
For energy storage purposes, materials with high strength, and low density are desirable. For this reason, composite materials are frequently being used, in advanced flywheels. The strength-to-density ratio of a material can be expressed in the units [Wh/kg], and values greater that 400 Wh/kg can be achieved by certain composite materials.
Composite rotors
Several modern flywheel rotors are made from composite materials. Examples include the Smart Energy 25 flywheel from Beacon Power Corporation,[10] and the PowerThru flywheel from Phillips Service Industries.[11]
For these rotors, the relationship between material properties, geometry and energy density can be expressed by using a weighed-average approach.[12]
Applications
Transportation
Road
In the 1950s, flywheel-powered buses, known as gyrobuses, were used in Yverdon, Switzerland and there is ongoing research to make flywheel systems that are smaller, lighter, cheaper and have a greater capacity. It is hoped that flywheel systems can replace conventional chemical batteries for mobile applications, such as for electric vehicles. Proposed flywheel systems would eliminate many of the disadvantages of existing battery power systems, such as low capacity, long charge times, heavy weight and short usable lifetimes. Flywheels may have been used in the experimental Chrysler Patriot, though that has been disputed.[13]