Executive Summary

Flywheel energy storage systems (FESS) use electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation ... input which is stored in the form of kinetic energyThe energy of motion. Any mass that is in motion has kinetic energy. Some forms of kinetic energy include vibrational, rotational and translational (i.e. due to motion from one location to another)..  Kinetic energy can be described as “energy of motion,” in this case the motion of a spinning mass, called a rotor.  The rotor spins in a nearly frictionless enclosure.  When short-term backup powerThe rate at which energy is generated, converted, transmitted, distributed or delivered. is required because utility power fluctuates or is lost, the inertia allows the rotor to continue spinning and the resulting kinetic energy is converted to electricity.  Most modern high-speed flywheel energy storage systems consist of a massive rotating cylinder (a rim attached to a shaft) that is supported on a stator by magnetically levitated bearings.  To maintain efficiency, the flywheel system is operated in a vacuum to reduce drag.  The flywheel is connected to a motor-generator that interacts with the utility grid through advanced power electronicsElectronic devices, methods and systems used to convert power with a specific form to power with a desired form. For example, power electronics is used to convert direct current electric power to alternating current electric power and vice versa..  Some of the key advantages of flywheel energy storage are low maintenance, long life (some flywheels are capable of well over 100,000 full depth of dischargeThe process of extracting stored energyThe energy available in the storage system to perform physical work through the conversion of its chemical or mechanical energy, stated in kWh or MWh. from the storage system.The portion of energy discharged from a storage system relative to the amount extractable stored energy. cycles and the newest configurations are capable of even more than that, greater than 175,000 full depth of discharge cycles), and negligible environmental impact.  Flywheels can bridge the gap between short-term ride-through power and long-term energy storage with excellent cyclic and loadAn end-use device or an end-use customer receiving electric power and using electric energy from the electrical system (grid). Note: The term load is sometimes treated as a synonym for demand, which is the measure of power that a load receives or... following characteristics.  Typically, users of high-speed flywheels must choose between two types of rims: solid steel or carbon composite.  The choice of rim material will determine the system cost, weight, size, and performance.  Composite rims  are both lighter and stronger than steel, which means that they can achieve much higher rotational speeds.  The amount of energy that can be stored in a flywheel is a function of the square of the RPM making higher rotational speeds desirable.  Currently, high-power flywheels are used in many aerospace and UPS applications.  Today 2 kW/6 kWh systems are being used in telecommunications applications.  For utility-scale storage a ‘flywheel farm’ approach can be used to store megawatts of electricity for applications needing minutes of discharge durationThe amount of time that a storage device can be discharged at the nominal power ratingThe operational limits of a transmissionAn interconnected group of lines and associated equipment for the movement or transfer of electric energy between points of supply and points at which it is transformed for delivery to customers or is delivered to other electric systems. system element under a set of specified conditions...  


Flywheel energy storage systems (FESS) employ kinetic energy stored in a rotating mass with very low frictional losses.  Electric energy input accelerates the mass to speed via an integrated motor-generator.  The energy is discharged by drawing down the kinetic energy using the same motor-generator.  The amount of energy that can be stored is proportional to the object’s moment of inertia times the square of its angular velocity.  To optimize the energy-to-mass ratio, the flywheel must spin at the maximum possible speed. Rapidly rotating objects are subject to significant centrifugal forces however, while dense materials can store more energy, they are also subject to higher centrifugal force and thus may be more prone to failure at lower rotational speeds than low-density materials.  Therefore, tensile strength is more important than the density of the material.  Low-speed flywheels are built with steel and rotate at rates up to 10,000 PRM.  More advanced FESS achieve attractive energy densityThe amount of energy that a storage system can store per unit volume occupied by the system., high efficiency and low standby losses (over periods of many minutes to several hours) by employing four key features: 1) rotating mass made of fiber glass resins or polymer materials with a high strength-to-weight ratio, 2) a mass that operates in a vacuum to minimize aerodynamic drag, 3) mass that rotates at high frequency, and 4) air or magnetic suppression bearing technology to accommodate high rotational speed. Advanced FESS operate at a rotational frequency in excess of 100,000 RPM with tip speeds in excess of 1000 m/s. FESS are best used for high power, low energy applications that require many cycles.  Additionally, they have several advantages over chemical energy storage.  They have high energy density and substantial durability which allows them to be cycled frequently with no impact to performance. They also have very fast response and ramp rates.  In fact, they can go from full discharge to full chargeThe process of injecting energy to be stored into the storage system. within a few seconds or less.  Flywheel energy storage systems (FESS) are increasingly important to high power, relatively low energy applications.  They are especially attractive for applications requiring frequent cycling given that they incur limited life reductionA chemical process that results in the acceptance of electrons by an electrode’s active material. The opposite of oxidation. if used extensively (i.e., they can undergo many partial and full charge-discharge cycles with trivial wear per cycleOne sequence of storage charging and discharging. Also known as charge-discharge cycle..)

Conclusions and Observations

FESS are especially well-suited to several applications including electric service power qualityA measure of the level of voltage and/or frequency disturbances. and reliabilityThe degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired. May be measured by the frequency, duration and magnitude of adverse..., ride-through while gen-sets start-up for longer term backup, area regulation, fast area regulation and frequency response(Equipment) The ability of a system or elements of the system to react or respond to a change in system frequency. (System) The sum of the change in demand, plus the change in generationThe manner in which electricity is generated. The electricity that flows through California, divided by the change in frequency, expressed in megawatts per 0....  FESS may also be valuable as a subsystem in hybrid vehicles that stop and start frequently as a component of track-side or on-board regenerative braking systems.