Executive Summary

Flywheel energy storage systems (FESS) use electric energy input which is stored in the form of kinetic energy.  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 power 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 electronics.  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 discharge 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 load 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 duration.

Discussion

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 density, 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 charge 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 reduction if used extensively (i.e., they can undergo many partial and full charge-discharge cycles with trivial wear per cycle.)

Conclusions and Observations

FESS are especially well-suited to several applications including electric service power quality and reliability, ride-through while gen-sets start-up for longer term backup, area regulation, fast area regulation and frequency response.  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.