Flywheels
A flywheel is a device that stores kinetic energy in a rotating mass. Flywheels have been used for many centuries, but modern materials and other innovations are spurring research on making flywheel systems that are smaller, lighter and cheaper, with greater capacity and lower losses. Flywheels are used in applications including energy storage uninterruptable power supplies, grid conditioners for high-cost manufacturing, and even vehicle powering. (The gyrobuses used in Sweden in the 1950s were powered by flywheels.)
Flywheels have storage characteristics comparable to batteries in that their energy storage capacity is fixed but the capacity can be drawn down quickly or slowly depending on need. Flywheels are used in remote applications to smooth out power fluctuations and allow the starting of a dispatchable generator when necessary. As compared to batteries, typical storage times at rated power in the order of minutes are expected. Flywheels, however, have several advantages. They have long cycle life, require minimum maintenance, and have fast response time. Although the limit of strength of the material used for the spinning rotor places upper limits on speed and thus capacity, most commercial flywheels are modular, so both capacity and rated power can be increased by using multiple units.
There are several R&D issues with the technology, including catastrophic failures from overloaded flywheels (these are typically in research settings), and attempts to reduce frictional losses. Although recent developments such as magnetic bearings have reduced losses, flywheels have a parasitic energy loss to keep the unit spinning and a relatively quick self discharge if additional energy is not provided.
PowerCorp (Australia) has successfully integrated its flywheel system with wind systems and wind-diesel hybrid projects, and for grid stability typically in mining applications. The use of flywheels has allowed wind-diesel systems to operate at reportedly high penetrations (as high as 90%) of wind power to offset diesel generation. In this scenario the flywheel, which ranges in size from 250 kW to 1 MW, acts as a spinning reserve and can provide frequency and voltage control.
Capacitors
A capacitor is a passive electrical component that can store energy. Capacitors are commonly used in personal electronic devices to store energy to maintain the power supply while batteries are being changed. Capacitors have no moving parts, thus a very high cycle life, fast and consistent response, but low power density. This means that large capacitor arrays are required to store meaningful amounts of energy. Conventional foil-wrapped capacitors are used extensively on electric grids today to provide voltage support. While maintaining a very high cycle life (>100,000 cycles), electrochemical capacitors, also known as ultracapacitors and supercapacitors, are able to store significantly more energy than conventional ones, but at lower energy densities than pure batteries. Capacitors have a higher voltage capability than batteries, but they store much less energy. Both capacitors and batteries are systems with multiple components and high capital costs, and can be distributed throughout the system. Capacitors are expensive, and currently there are no commercial manufacturers of large-scale capacitor storage systems.
