Flywheel energy storage | Working| Tech Blog | Mechanical Engineering

Flywheel energy storage

INTRODUCTION

Flywheel energy storage systems store kinetic energy (i.e. energy produced by motion) by constantly spinning a compact rotor in a low-friction environment. When short-term back-up power is required (i.e. when utility power fluctuates or is lost), the rotor's inertia allows it to continue spinning and the resulting kinetic energy is converted to electricity.

Active Power's CleanSource® Flywheel Technology, as shown below, integrates the function of a motor, flywheel rotor and generator into a single integrated system. The motor, which uses electric current from the utility grid to provide energy to rotate the flywheel, spins constantly to maintain a ready source of kinetic energy. The generator then converts the kinetic energy of the flywheel into electricity. This integration of functionality reduces the cost and increases product efficiency.

The flywheel rotor spins in a near frictionless environment, created by Active Power's patented magnetic bearing technology. This innovative technology unloads a majority of the flywheel's weight from the field-replaceable mechanical bearing cartridge. The efficiency in the chamber is further enhanced by the creation of a rough vacuum, which reduces drag on the spinning flywheel. As power is transferred to the load, the flywheel's speed decreases. Additional current is then supplied to the field coil to ensure that the voltage output remains constant throughout discharge. This enables the flywheel system to provide ride-through power during power disturbances.

Active Power's flywheel-based systems provide ride-through power for the majority of power disturbances, such as voltage sags and surges, and bridges the gap between a power outage and the time required to switch to generator power.

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.

REFERENCES

1.     Processes and Materials of Manufacture by R.A. LINDBERG

2.     www.pcmag.com/encyclopedia

3.     www.me.sc.edu