I've been taking a look at materials for creating a flywheel energy storage system and for the life of me I can't understand why people don't use basalt fibers as the material for the flywheel. It has similar properties to carbon fiber but is cheaper and heavier. Since the energy storage scales exponentially with speed but linearly with weight, it should make sense to optimize for materials with a high tensile strength. Basalt fiber is pretty easy to manufacture and there are tons of it all over the world.