Also as long as you don't care about energy leakage -- no matter how good your bearings are, they're imperfect. Flywheels store energy for minutes or hours, bat

Also as long as you don't care about energy leakage -- no matter how good your bearings are, they're imperfect. Flywheels store energy for minutes or hours, batteries for days or weeks, non-volatile hydrocarbons for millions of years.