For storing wind energy or solar energy, flywheels have been suggested. The amount of energy that can be stored in a fly- wheel depends on the density and tensile strength of the material making up the flywheel and for a given weight one wants the lowest density strong material available. Can you make this plausible? (See Flywheels, by R. F. Post and S. F. Post, Scientific American, December 1973, p. 17.)

The density of the flywheel highlights the mass contained in a given volume. A denser material will be heavier than the less dense one, and will be harder to accelerate. Thus, the amount of energy needed to accelerate the flywheel made up of a denser material will also be significantly large, and may decrease the efficiency of the energy plant.
One would also want the material to have larger tensile strength so that the flywheel does not deform under the action of wind or sun. Permanent or temporary deformation, both reduces the efficiency of the system.
Therefore, one can see that a highly efficient flywheel should be made out of a low density stronger material.