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Flywheel energy storage - Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Flywheel_energy_storage Flywheel energy storage From Wikipedia, the free encyclopedia Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy; adding energy to the system correspondingly results in an increase in the speed of the flywheel. Most FES systems use electricity to accelerate and decelerate the flywheel, but devices that directly use mechanical energy are being developed.[1] Since FES can be used to absorb or release electrical energy such devices may sometimes be incorrectly and confusingly described as either mechanical or inertia batteries. [2][3] Advanced FES systems have rotors made of high strength carbon-fiber composites, suspended by magnetic bearings, and NASA G2 flywheel spinning at speeds from 20,000 to over 50,000 rpm in a vacuum enclosure.[4] Such flywheels can come up to speed in a matter of minutes – reaching their energy capacity much more quickly than some other forms of storage.[4] Contents 1 Main components 1.1 Possible future use of superconducting bearings 2 Physical characteristics 2.1 General 2.2 Energy density 2.3 Tensile strength and failure modes 2.4 Energy storage efficiency 2.5 Effects of angular momentum in vehicles 3 Applications 3.1 Transportation 3.2 Uninterruptible power supplies 3.3 Laboratories 3.4 Aircraft launchers systems 3.5 NASA G2 flywheel for spacecraft energy storage 3.6 Amusement rides 3.7 Pulse power 1 of 13 10/28/15, 10:32 AM Flywheel energy storage - Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Flywheel_energy_storage 3.8 Motor sports 3.9 Grid energy storage 3.10 Wind turbines 3.11 Toys 3.12 Toggle action presses 4 Comparison to batteries 5 See also 6 References 7 Further reading 8 External links Main components 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 and are an order of magnitude less heavy.[5] Magnetic bearings are sometimes used instead of mechanical bearings, to reduce friction. The main components of a typical flywheel. Other components are hub and shaft. Possible future use of superconducting bearings 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 2 of 13 10/28/15, 10:32 AM Flywheel energy storage - Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Flywheel_energy_storage 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. 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;[4] full-cycle lifetimes quoted for flywheels range from in excess of 105, up to 107, cycles of use),[6] high energy density (100–130 W·h/kg, or 360–500 kJ/kg),[6][7] 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.[4] Rapid charging of a system occurs in less than 15 minutes.[8] 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.[9] 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[10] , 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/m3] 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.[11] 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 . Material properties 3 of 13 10/28/15, 10:32 AM Flywheel energy storage - Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Flywheel_energy_storage 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 than 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,[12] and the PowerThru flywheel from Phillips Service Industries.[13] For these rotors, the relationship between material properties, geometry and energy density can be expressed by using a weighed-average approach.[14] Tensile strength and failure modes One of the primary limits to flywheel design is the tensile strength of the material used for the rotor. Generally speaking, the stronger the disc, the faster it may be spun, and the more energy the system can store. When the tensile strength of a composite flywheel's outer binding cover is exceeded, the binding cover will fracture, followed by the wheel shattering as the outer wheel compression is lost around the entire circumference, releasing all of its stored energy at once; this is commonly referred to as "flywheel explosion" since wheel fragments can reach kinetic energy comparable to that of a bullet. Composite materials that are wound and glued in layers tend to disintegrate quickly, first into small-diameter filaments that entangle and slow each other, and then into red-hot powder, instead of large chunks of high-velocity shrapnel as can occur with a cast metal flywheel. For a cast metal flywheel, the failure limit is the binding strength of the grain boundaries of the polycrystalline molded metal. Aluminum in particular suffers from fatigue and can develop microfractures due to repeated low-energy stretching. Angular forces may cause portions of a metal flywheel to bend outward and begin dragging on the outer containment vessel, or to separate completely and bounce randomly around the interior. The rest of the flywheel is now severely unbalanced, which may lead to rapid bearing failure from vibration, and sudden shock fracturing of large segments of the flywheel. Traditional flywheel systems require strong containment vessels as a safety precaution, which increases the total mass of the device. The energy release from failure can be dampened with a gelatinous or encapsulated liquid inner housing lining, which will boil and absorb the energy of destruction. Still, many customers of large-scale flywheel energy-storage systems prefer to have them embedded in the ground to halt any material that might escape the containment vessel. Energy storage efficiency Flywheel energy storage systems using mechanical bearings can lose 20% to 50% of their energy in two 4 of 13 10/28/15, 10:32 AM Flywheel energy storage - Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Flywheel_energy_storage hours.[15] Much of the friction responsible for this energy loss results from the flywheel changing orientation due to the rotation of the earth (an effect similar to that shown by a Foucault pendulum). This change in orientation is resisted by the gyroscopic forces exerted by the flywheel's angular momentum, thus exerting a force against the mechanical bearings. This force increases friction. This can be avoided by aligning the flywheel's axis of rotation parallel to that of the earth's axis of rotation. Conversely, flywheels with magnetic bearings and high vacuum can maintain 97% mechanical efficiency, and 85% round trip efficiency.[16] Effects of angular momentum in vehicles 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.

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