They are in various stages of development and commercialization and offer differing advantages.
Pumped hydropower storage uses off-peak electric power to operate pumps that fill a water reservoir. At peak demand, the stored water is released through a hydroelectric generating plant. The technology is well understood and has been commercially deployed, for example, by the TVA at the Raccoon Mountain Plant which has a generating capacity of 1600 megawatts. Hydropower responds quickly to changes in demand and can generate high levels of power for long times. The difficulty with pumped hydropower is that it requires a large reservoir with attendant environmental problems, and systems are very expensive to construct. Projected improvements rely on variable speed pumps and turbines which can lead to at least a 3% increase in efficiency.
Compressed air storage uses off-peak power to pump compressed air into a storage container. On a commercial scale, the container will probably be a limestone cavity. Should CAES be used to support distributed generation, the container will a pressurized tank. There are two large CAES facilities built as demonstration plants although there are no commercial facilities as yet. CAES is less environmentally damaging than pumped hydro and as a distributed system is projected to work as a natural partner with wind generation. Large scale systems require a reservoir to store the compressed air, and small scale systems have safety problems with the possibility of exploding containers. Technical advances include development of small scale systems for distributed generation and better storage containers for the compressed air.
Batteries are a major technology for portable energy storage and find wide application in transportation and portable appliances. Here we consider only their application to the storage of electric power. In these applications which are primarily commercialized at facilities like the Fabs where power outages are disastrous, battery banks are located close to the facility that is being protected. Local power companies also use battery banks to supply emergency power in areas where power demand has rapidly growing peak demand. Batteries offer high energy storage densities, rapid response times, and they are portable. However, they are very expensive and have limited life times. The materials of which they are made pose environmental hazards. There are major research efforts underway to develop batteries that cost less and have longer life times. The research is creative but there is a long way to go before batteries will be an affordable option for electricity storage on a residential or industrial scale.
Flywheels store energy as rotational kinetic energy. They can store more energy if they operate at greater rotational velocities or if they are larger. They are limited by the properties of the materials of which they are made since large wheels tend to break apart at high angular velocities and by dynamical instabilities in rotation. Flywheels respond very quickly and can be connected in "farms" for large energy storage. At present, they are in a prototype phase and are very expensive. The obvious research needs are in materials science.
Superconducting magnetic energy storage uses high currents in superconducting coils to store electrical energy. SMES systems offer the possibility of very fast response with discharge of high power. For large scale energy storage, they can be networked, and they have long lifetimes because they have no moving parts. However, they require cryogenic systems which do wear out, and they are very expensive and currently experimental. However, it is possible that developments in materials such as high Tc superconductors could make this appealing technology a practical method of storing electrical energy.
Conventional capacitors store energy as a charge on electrodes separated by a dielectric material. Charge storage depends on the area of the electrodes. "Super" capacitors increase the electrode area by using porous electrodes and vary materials to increase operating voltages. They are potentially capable of rapid and high power discharges. Like SMES systems, they have no moving parts and potentially long lifetimes. At present, they are experimental, expensive and able to store little energy.
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