A POWERFUL DWARF

Pumped storage, i.e., pumping water into an elevated reservoir and letting it rush out through hydraulic turbines when needed, stores a lot of energy, but takes thousands of acre-feet of water, so that its energy density is low. But the new flywheels, as demonstrated in the early 70's, could hold 1 to 3 watt-hours per lb, which only some five years ago was considered high.

Now Rockwell International is developing a flywheel module for the US Army (for undisclosed military applications, possibly to give tanks short bursts of peak power) which packs no less than 10 watt-hours into each pound of flywheel. And when a system parameter increases by a factor of 10 in five years, there is something of a revolution underway. (Can you imagine, in the next 5 years, a car that does 400 miles to the gallon? Or chicken at a nickel a pound?)

And Rockwell's module is no laboratory toy: It stores a total energy of 30 kWh, and its two flywheels, though only about 20 inches in radius, can let go with an awesome 6.4 MW (via two 3.2 MW generators; the flywheels are spun to 15,000 rpm by a 4,000 HP gas turbine).

A sizable mass spinning at such high speeds becomes a gyroscope, whose axis wants to keep its orientation in space (with respect to the stars). That makes gyroscopes great for stabilizing compasses, ships and satellites (also for some baffling toys); but they are entirely undesirable in vehicles to be maneuarred quickly. To eliminate gyroscopic effects, the Rockwell module uses two counter-rotating shafts, so that these effects cancel each other.

The rundown time for the Rockwell module is not given, but seems to be comparatively short. To achieve a run-down time greater than 24 hours, new bearings (magnetic or other) and low-loss, high-speed vacuum seals al such high lesels ot energs and power must first be developed.

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Rockwell's module with 10 Wh/lb at high energy levels (the figure is readily achieved for very small flywheels) may not be in the limelight for very long. RCA, which produced the three 96-ton flywheels for Princeton's Plasma Physics Lab (for high-power electric pulses), is aiming for six times greater energy densities .

Also in the works at RCA: a "rim-only" flywheel, a ring spinning through the contactless supports by four electromagnets. The energy is fed into, and discharged from, the ring electromagnetically via the same four structures. Indeed, if the only purpose of the "wheel" is to store energy, who needs a hub and a shaft any more?