ABSTRACT
A flywheel stores kinetic energy in rotating inertia. The stored energy can be converted into electricity using an electromagnetic machine in the generating mode. Recharging — spinning the wheel — is done using the same machine in the motoring mode. The round trip energy efficiency of the flywheel system is high — 85 to 90% compared to 70 to 75% with the electrochemical battery. Flywheel energy storage is an old concept that has now been developed for a variety of commercial applications. Automobiles, utility load leveling, and uninterrupted power supplies (UPS) are a few examples. At present, commercial UPS systems using flywheel-battery cost $300 to $500 per kWh. The space-qualified NiH2 cells cost $80,000 to $100,000 per kWh. The flywheel system is being developed by NASA and industry partners for replacing the battery in the spacecraft power system.1-4 One is scheduled to fly for flight tests on the International Space Station (ISS) in 2006. It is estimated that flywheel energy storage can improve the depth of discharge (DOD) and life by a factor of 2 compared to those of the battery. An integrated energy and momentum storage flywheel system can be even more advantageous. The following developments have made flywheel technology more
attractive now than ever before:
Advances made in high-strength lightweight fibers. For example, fibers with ultimate tensile strength of one million pounds per square inch are commercially available now.
