FLYWHEEL STORAGE

Pumped storage is virtually pollutionless, and it does not even significantly heat the used water; it also provides recreational facilities and new habitats for wildlife, so that it leaves environmentalists hard pressed for arguments with which to mask their technophobia (which has not prevented them filing suits against the Ludington plant and other pumped storage plants).

However, pumped storage has other drawbacks. It is expensive in capital costs, it requires large areas of land, and its efficiency is not very high. The Ludington reservoir, for example, with a capacity of 27 billion gallons, covers a site of 1,900 acres; its construction involved moving 52 million cubic yards of soil and building six miles of earthen dikes, at a cost of $340 million. Also, pumped storage is only 50% efficient -- twice as much energy is needed to fill the reservoir as can be obtained by emptying it, most of the balance is sacrificed to friction in transporting the water. This is not bad as electric power conversion goes (30% efficiency for nuclear plants, up to 40% for fossil burning plants), but it is still very wasteful.

These disadvantages may be overcome in the not too distant future by flywheel storage similar to that described for automotive power in last month's AtE. During periods of low demand (night), electric power is used to impart a high velocity to "superflywheels" made of composite layers of plastic fiber materials, designed to store large amounts of kinetic energy. During the peak demand period, a spinning flywheel would then drive a generator to supply the additionally needed power, converting kinetic energy into electrical energy. Several such units could be used to provide the required storage.

A study of the feasibility of flywheel storage for electric utilities has been made by Richard and Stephen Post (father and son, father a professor at the University of California at Davis and a research group leader at LLL, son a student of mechanical engineering at California State, San Obispo).

They suggest flywheels 15 to 20 feet in diameter, weighing 100 to 200 tons, spinning in a hydrogen or helium atmosphere below atmospheric pressure (to reduce friction). At full charge, such a unit would spin at 3,500 rpm and store an energy of 10,000 to 20,000 kilowatt hours.

Flywheel storage would be superior to pumped storage in every way. First, the efficiency of the system is not 50%, but 93 to 95%. Second. only 20 by 20 feet of base area are needed for a flywheel unit storing as much energy (10 MWh) as 2 to 4 acres of pumped storage. Third, the flywheel storing the energy need not be run at a central location, but several flywheel units can be distributed over the substations of the utility system (which, among other advantages, reduces transmission losses). Finally, the capital costs of flywheel storage facilities work out to about $110 per kilowatt lower than for pumped storage ($180/kW for the Ludington plant).