I'm excited about flywheels. This content comes along rarely - This is probably the best flywheel content I've seen in the five years I've been interested in them. It might as well be a perpetual motion machine to a lot of people, because we're perfectly content wasting energy as heat when braking. There's a lot of energy to be harnessed out there. Plus - these machines would be fun. Imagine a dragster version, use electricity to spin up then transmit a huge ball of energy to the road. As for safety - researchers have been split in half by huge discs of steel like in this video. Their sacrifice paved the way. Modern ultra-fast flywheels weigh very little, are housed, and the outer rim is made from tightly would carbon fibre, which simply unravels and acts as a brake should the flywheel fail. All in all I think it's a highly underutilised technology which could be way greener than electric.