But batteries bring several disadvantages with them, including high-maintenance and replacement costs, limited cycles, need for a controlled environment to handle hydrogen gas, lead, and acid, and footprint issues. And perhaps the biggest downside is that batteries are unpredictable when it comes to failures.

An alternative, flywheel-based systems from Vycon, Cerritos, Calif. (vyconenergy.com), are said to be reliable, cost effective, and need little maintenance.

One of the company's newer systems is Regen, which is built for shipyard cranes. It stores regenerated energy scavenged from the crane's ac hoist motor to reuse when the crane lifts containers, the peak power demand put on the crane. The system captures this regenerated power once a minute during two 8-hr shifts, six days a week.

The flywheel stores 2,100 kW-sec of power and has a nominal rating of 140 kW for 15 sec. At full speed, the rotor turns 36,000 rpm, and its burst speed, the speed at which the rotor flies apart, is 50,000 rpm. The output is dc, so a converter is needed to get ac.