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FlywheelsFlywheel Technologies | Advantages of Flywheels | Disadvantages of Flywheels | Further Information | ReferencesFlywheel Storage TechnologiesA flywheel is a mass rotating about an axis, which can store energy mechanically in the form of kinetic energy. Energy is required to accelerate the flywheel so it is rotating. This is usually achieved by an electric motor when being used in an electrical system. Once it is rotating, it is in effect a mechanical battery, that has a certain amount of energy that can be stored depending on its rotational velocity and its moment of inertia. The faster a flywheel rotates, the more energy it stores. This stored energy can be retrieved by slowing down the flywheel and returning the kinetic energy to the electrical motor, which is used as a generator.
The amount of power produced by renewable energy devices such as photovoltaic cells and wind turbines varies significantly on an hourly, daily and seasonal basis due to the variation in the availability of the sun, wind and other renewable resources. Even when conventional technologies are generating electricity at a constant rate, there are demand fluctuations throughout the day. This mismatch of load to electrical supply means that power is not always available when it is required and on other occasions, there is excess power. Flywheel technologies can used to provide power when there is insufficient power being generated, and to store excess production. Another important application for flywheel technologies is for power conditioning and for providing power when there are durations of total power loss as a result of electricity grid failure. 
Figure 1 A G2 Flywheel Module, courtesy of NASA
Flywheels are a developing technology that may replace conventional batteries in such applications as stand-alone power systems, vehicles and uninterruptible power systems (UPS) and are also are playing a growing role in power conditioning in conventional grid systems.
Advantages of FlywheelsSome of the advantages of flywheels are that they can store and release large amounts of power very quickly and efficiently when compared to conventional batteries. Also, the lifetime and maintenance of flywheel technologies are around 20 to 30 years and some can operate with no maintenance in that time. Batteries often need strict environmental conditions to operate correctly, such as operating temperatures below 40 degrees. Flywheels also do not suffer from the memory effect, which plagues some types of batteries. Flywheels can operate under higher temperatures and a wider range of environmental conditions. Flywheels 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. 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. However 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. (Wikipedia, 2007). 
Figure 2 A power and energy storage comparison diagram.
(courtesy of NREL)
Disadvantages of FlywheelsHowever, 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 instead of large chunks of high-velocity shrapnel, we obtain a containment vessel filled with red-hot sand. However, 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. (Wikipedia, 2007).  Figure 3 Beacon Powers Smart Energy 6 system produces a 2 kW of maximum power and stores 6 kWh
(© 2006 Beacon Power)
Figure 4 Active Powers’s CleanSource DC provides 100 – 2000 kW of power.
(copyright © 2006 Active Power, Inc) Further InformationRISE Resources - Information regarding available renewable energy resources.RISE Technologies - An extensive collection of information regarding renewable energy technologies.RISE Applications & System Design - Renewable energy application information and system designs.RISE System Displays - Case studies and information on installed renewable energy systems & performance data.
NASA’s Aerospace Flywheel Development Beacon Power
ReferencesWikipedia, 2007. “Flywheel Energy Storage” (Online) http://en.wikipedia.org/wiki/Flywheel_power_storage (Accessed 21 February 2007). |
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