Flywheel energy storage

Flywheel Energy Storage (FES) works by accelerating a rotor to a very high speed and maintaining the energy in the system as inertial energy. Commercially available FES systems are used for small uninterruptible power systems. The rotors normally operate at 4000 RPM or less and are made of metal. Advanced flywheels are made of high strength carbon-composite filaments that spin at speeds from 20,000-100,000 RPM in a vacuum enclosure. Magnetic bearings are necessary since friction in conventional mechanical bearings are directly proportional to speed and at such speeds, it would be high enough to cause such a great energy loss as to defeat its purpose of energy storage. Quick charging is done in less than 15 minutes. Long lifetimes of most flywheels, plus high energy densities (~ 130 Wh/kg) and large maximum power outputs are positive attributes. The energy efficiency (ratio of energy out per energy in) of flywheels can be as high as 90%. Since FES can store and release power quickly, they have found a niche providing pulsed power.

Main components

A typical system consists of rotor suspended by bearings inside a vacuum chamber to reduce friction, connected to a combination electric motor/electric generator. On larger systems, the bearings are magnetic. The rotors are generally made of steel on smaller systems but large systems use high-tensile-strength fibers (such as carbon fibers) embedded in epoxy resins, or some other high-strength composite material. Energy is stored by using an electric motor to increase the speed of the spinning flywheel. The system releases its energy by using the momentum of the flywheel to power the motor/generator.

Physical characteristics

Energy is stored in the rotor in proportion to its momentum, but the square of the angular momentum. The kinetic energy stored in a rotating flywheel is:

E_{k}={\frac {1}{2}}\cdot I\cdot \omega ^{2}

where

\omega

is the angular velocity, and

I

is the moment of inertia of the mass about the center of rotation.

The moment of inertia for a solid-cylinder is $I_{z}={\frac {1}{2}}mr^{2}$ ,
for a thin-walled cylinder is $I=mr^{2}\,$ ,
and for a thick-walled cylinder is $I={\frac {1}{2}}m({r_{1}}^{2}+{r_{2}}^{2})$ .

where m denotes mass, and r denotes a radius. More information can be found at list of moments of inertia

The amount of energy that can safely be stored in the rotor depends on the point at which the rotor will warp or shatter. The Hoop stress on the rotor is a major consideration in the design of a flywheel energy storage system.

\sigma _{t}=\rho r^{2}\omega ^{2}\

where

\sigma _{t}

is the tensile stress on the rim of the cylinder

\rho

is the density of the cylinder

r

is the radius of the cylinder, and

\omega

is the angular velocity of the cylinder.

Applications

In the 1950s flywheel-powered buses, known as gyrobuses, were used in Yverdon, Switzerland, and there is ongoing research to make flywheel systems that are smaller, lighter, cheaper, and have a greater capacity. It is hoped that flywheel systems can replace conventional chemical batteries for mobile applications, such as for electric vehicles. Proposed flywheel systems would eliminate many of the disadvantages of existing battery power systems, such as low capacity, long charge times, heavy weight, and short usable lifetimes. Flywheel systems have also been used experimentally in small electric locomotives for shunting or switching.

In the 1980s Soviet engineer Nourbey Gulia had been working on flywheel energy storage. His work resulted in many original solutions for wheel suspension, sealing the vacuum chamber, rotation rate decline compensator and hydraulic transmission. However, the primary advance was the composite flywheel capable of rotation rates exceeding 40,000 rpm, running for up to a week when not loaded, and resistant to explosive destruction. Gulia's "super flywheels" were tightly wound of metal or plastic tape. These had tensile strength higher than that of molded steel, and in the case of failure simply unwound inside the chamber, filling it and grinding to a stop. Gulia's first wheels were made of steel tape, but the latest models used Kevlar filament, wound not unlike a bobbin of thread.

Flywheel power storage systems in current production (2001) have storage capacities comparable to batteries and faster discharge rates. They are mainly used to provide load leveling for large battery systems, such as an uninterruptible power supply. Developers of such flywheel energy storage systems include Hitec Power Protection, Active Power, AFS Trinity and Beacon Power.

A long-standing niche market for flywheel power systems is facilities where circuit-breakers and similar devices are tested: even a small household circuit-breaker may be rated to interrupt a current of 10,000 or more amps, and larger units may be have interrupting ratings of 100,000 or 1,000,000 amps. Obviously the enormous transient loads produced by deliberately forcing such devices to demonstrate their ability to interrupt simulated short circuits would have unacceptable effects on the local grid if these tests were done directly off building power. So typically such a laboratory will have several large motor-generator sets, which can be spun-up to speed over some minutes; then the motor is disconnected before a circuit-breaker is tested.

Advantages and disadvantages

Flywheels are not affected by temperature changes as are chemical batteries, nor do they suffer from memory effect. Moreover, they are not as limited in the amount of energy they can hold. They are also less potentially damaging to the environment, being made of largely inert or benign materials. Another advantage of flywheels is that by a simple measurement of the rotation speed it is possible to know the exact amount of energy stored. However, use of flywheel accumulators is currently hampered by the danger of explosive shattering of the massive wheel due to overload.

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 flywheel is exceeded the flywheel will shatter, 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. Consequently, traditional flywheel systems require strong containment vessels as a safety precaution, which increases the total mass of the device. Fortunately, composite materials tend to disintegrate quickly once broken, and so instead of large chunks of high-velocity shrapnel one simply gets a containment vessel filled with red-hot sand (still, many customers of modern flywheel power storage systems prefer to have them embedded in the ground to halt any material that might escape the containment vessel). Gulia's tape flywheels did not require a heavy container and reportedly could be rewound and reused after a tape fracture.

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. This property may be detrimental to the vehicle's handling characteristics while turning. On the other hand, this property could be utilised to improve stability in curves. Two externally joined flywheels spinning synchronously in opposite directions would have a total angular momentum of zero and no gyroscopic effect.

Bearings

The expense of refrigeration led to the early dismissal of low temperature superconductors for use in magnetic bearings. High-temperature superconductor (HTSC) bearings however may be economic and could possibly extend the time energy could be stored economically. Hybrid bearing systems are most likely to see use first. HTSC 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 HTSC are used to stabilize it. The reason superconductors can work well stabilizing the load is because they are good diamagnets. In hybrid-bearing systems, a conventional magnet levitates the rotor, but the high temperature superconductor keeps it stable. 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 easier 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 can provide the full lifting force for a 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.

Parasitic losses such as friction, hysteresis and eddy current losses of both magnetic and conventional bearings in addition to refrigerant costs can limit the economical energy storage time for flywheels. However, further improvements in superconductors may help eliminate eddy current losses in existing magnetic bearing designs as well as raise overall operating temperatures. Even without such improvements, however, modern flywheels can have a zero-load rundown time measurable in years.