Flywheel Energy Storage

 
Overview:
            A flywheel, in essence is a mechanical battery - simply a mass rotating about an axis. Flywheels store energy mechanically in the form of kinetic energy. They take an electrical input to accelerate the rotor up to speed by using the built-in motor, and return the electrical energy by using this same motor as a generator. Flywheels are one of the oldest and most common mechanical devises in existence. They may still prove to serve us as an important component on tomorrow's vehicles and future energy needs. Flywheels are one of the most promising technologies for replacing conventional lead acid batteries as energy storage systems for a variety of applications, including automobiles, economical rural electrification systems, and stand-alone, remote power units commonly used in the telecommunications industry. Recent advances in the mechanical properties of composites has rekindled interest in using the inertia of a spinning wheel to store energy.

            In addition to energy density, flywheel energy storage systems (FES) also offer several important advantages over chemical energy storage. The rate at which energy can be exchanged into or out of the battery is limited only by the motor--generator design. Therefore, it is possible to withdraw large amounts of energy in a far shorter time than with traditional chemical batteries. Indeed, research into exploiting this property of FES systems to get short, intense bursts of energy is ongoing with the most notable projects being a magnetic tank gun and a fusion ignition system. Of course it is also possible to quickly charge FES batteries making them desirable for application in electric cars where the charge time could be dropped from a matter of hours to a matter of minutes.
 

Advantages: Flywheels store energy very efficiently (high turn-around efficiency) and have the potential for very high specific power compared with batteries. Flywheels have very high output potential and relatively long life. Flywheels are relatively unaffected by ambient temperature extremes.

 Disadvantages: Current flywheels have low specific energy. There are safety concerns associated with flywheels due to their high speed rotor and the possibility of it breaking loose and releasing all of it's energy in an uncontrolled manner. Flywheels are a less mature technology than chemical batteries, and the current cost is too high to make them competitive in the market.
 
Physics:
        Stored energy = sum of kinetic energy of individual mass elements that comprise the flywheel
        Kinetic Energy = 1/2*I*w2 , where
        I = moment of inertia (ability of an obeject to resist changes in its rotational velocity)
        w = rotational velocity (rpm)
        I = k*M*R2  (M=mass; R=radius); k = inertial constant (depends on shape)
Inertial constants for different shapes:
        Wheel loaded at rim (bike tire); k = 1
        solid disk of uniform thickness; k = 1/2
        solid sphere; k = 2/5
        spherical shell; k = 2/3
        thin rectangular rod; k = 1/2

                In order to optimize the energy-to-mass ratio, the flywheel needs to spin at the maximum possible speed. This is because kinetic energy only increases linearly with Mass but goes as the square of the rotational speed. Rapidly rotating objects are subject to centrifugal forces that can rip them apart. Centrifugal force for a rotating object goes as M*R*w2 . Thus while dense material can store more energy it is also subject to higher centrifugal force and thus fails at lower rotational speeds than low density material. Therefore the tensile strenght is more important than the density of the material.
 


Development of a Flywheel Energy Storage System
    This section describes the doctoral research being performed by Scott Thielman and David Stienmier.

Flywheel Energy Storage
    Technology awarded task by advanced research projects agency.

Flywheels and Energy Storage
    Description: A wheel winds up through some system of gears and then delivers rotational energy until friction dissipates it. This site porvides some of the basic mathematical formulae for calculating the energy transfer and storage of kinetic energy flywheels.

Carbon Fiber Flywheel for Power
    Generation and attitude control. This is a very interesting site pertaining to flywheels and energy storage in general.

Flywheels Offer Energy Storage Potential
    The spinners: A flywheel, in essence, is a mechanical battery - simply a mass rotating about and axis and is one of the odest and most common mechanical devices in existence.

Hybrid Electric Vehicle Program - HEV Storage
    Hybrid electric vehicle components. Electric energy storage. The devices most likely to be used for storage of electrical energy in hybrid vehicles are batteries, ultracapacitors, and flywheels.

Flywheel Energy Storage Using HTS Magnetic Bearings
    Recent advances in the development of very low friction bearings and high-strength composite rotor material has made flywheel technology more pluasible.

Flywheel Motor/Alternator Design for Hybrid Electric Vehicles
    Heath Hofmann. An important issue in hybrid electric vehicle design is available instantaneous power.

Flywheel Energy Storage
    System stores kinetic eergy in the form of a rotating flywheel. The flywheel must rotate at high velocities and must be made of very strong material so the the flywheel does not rip apart.

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