@VDCDriver - The flywheel technology for storing energy ran into a lot of problems. It’s being looked at again. The problem was the wheel had to get up to about 100k rpms in order for it to be usable. At that speed…it would sometimes break apart. And when this happened…it would send pieces of metal flying up to several hundred feet away. If a battery explodes…it won’t kill you (unless your face is right over it when it explodes). This thing could kill you if you were 100’ away.
Last I read…they had some new materials that were more stable…and if there was a problem…it would basically break into very small lightweight pieces that wouldn’t go far and not hurt anyone. But not too many companies are working on this.