Flywheels are interesting but dangerous if they malfunction because the energy is concentrated in the vicinity of the rotor. My question to you ME's is this- could you engineer a practical and/or efficient energy storage system that relied on gravity? I envision an enourmous mass on a rope. The rope is connected to the shaft of an electric motor-generator. You "charge" the device by running power to the motor/generator and lifting the mass. You get energy back out in the obvious way. It's safe because in the worst case the mass "falls" but probably doesn't kill anyone. Is this a feasible alternative to lead-acid batteries and flywheels?