Flywheel Energy Storage - Advantages and Disadvantages

Advantages and Disadvantages

Flywheels are not as adversely affected by temperature changes, can operate at a much wider temperature range, and are not subject to many of the common failures of chemical rechargeable batteries. Unlike lithium ion polymer batteries which operate for a finite period of roughly 36 months, a flywheel can potentially have an indefinite working lifespan. Flywheels built as part of James Watt steam engines have been continuously working for more than two hundred years. Working examples of ancient flywheels used mainly in milling and pottery can be found in many locations in Africa, Asia, and Europe. They are also less potentially damaging to the environment, being largely made of 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 to red-hot powder once broken, instead of large chunks of high-velocity shrapnel. Still, many customers of modern flywheel energy-storage systems prefer to have them embedded in the ground to halt any material that might escape the containment vessel.

An additional limitation for some flywheel types is energy storage time. Flywheel energy storage systems using mechanical bearings can lose 20% to 50% of their energy in 2 hours. Much of the friction responsible for this energy loss results from the flywheel changing orientation due to the rotation of the earth (a concept similar to a Foucault pendulum). This change in orientation is resisted by the gyroscopic forces exerted by the flywheel's angular momentum, thus exerting a force against the mechanical bearings. This force increases friction. This can be avoided by aligning the flywheel's axis of rotation parallel to that of the earth's axis of rotation.

Conversely, flywheels with Magnetic bearings and high vacuum can maintain 97% mechanical efficiency, and 85% round trip efficiency.

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 utilized to keep the car balanced so as to keep it from rolling over during sharp turns. Conversely, the effect can be almost completely removed by mounting the flywheel within an appropriately applied set of gimbals, where the angular momentum is conserved without affecting the vehicle (see Properties of a gyroscope). This doesn't avoid the complication of gimbal lock, and so a compromise between the number of gimbals and the angular freedom is needed. A single gimbal, for instance, could free a car for the 360 degrees necessary for regular driving. However, for instance driving up-hill would require a new gimbal mechanism with a new degree of freedom. Two gimbals would theoretically solve this problem and never lock unless the car rolls.

An alternative solution to the problem is to have two joined flywheels spinning synchronously in opposite directions. They would have a total angular momentum of zero and no gyroscopic effect. A problem with this solution is that when the difference between the momentum of each flywheel is anything other than zero the housing of the two flywheels would exhibit torque. Both wheels must be maintained at the same speed to keep the angular velocity at zero. Strictly speaking, the two flywheels would exert a huge torqueing moment at the central point, trying to bend the axle. However, if the axle were sufficiently strong, no gyroscopic forces would have a net effect on the sealed container, so no torque would be noticed.