They also have a greater storage capacity than their chemical cousins and could easily last the lifetime of the space station itself. Flywheel batteries are a cleaner and more economical source of battery power than chemical models. The flywheel is a simple technology, as old as the potter's wheel, though the models under scrutiny for the ISS are slightly more advanced. They are made of a composite material, mounted on a titanium shaft and levitated on magnetic bearings. The spinning wheel stores the energy coming in from solar cells by revolving faster and faster.
Energy being withdrawn at any time has the opposite effect, slowing the madly-turning spindle.
The flywheel that would be used in the battery on the space station is about 1.5 feet in diameter and nearly three feet long. Since flywheel battery systems are manufactured and sold commercially for power quality applications in industry, there's no need for a lengthy prototype process. A current UT Austin focus is to make the technology smaller and lighter and more space-worthy than commercial systems by exploiting the latest advances in material research and controls. The engineers hope to complete the project by the time the current space station batteries need to be replaced five years from now. Whoever does the changeover work will have the pleasure of breathing fresh air whilst the repairs are made.