Both of my ideas in the original OP are dead. A simple glance at my physics textbook makes it impractical for the utilization of strain energy for energy storage. The ideas in the above post are excellent. The basic concept of the B-G Project is water storage (gravitational potential energy using a fluid medium). Beacon's flywheels are storage in the form of rotational kinetic energy. Andasol uses random kinetic energy (heat) to store energy. All of them are excellent. I would like to propose an additional one. This is somewhat difficult to achieve, but possibly more efficient and better controlled than water storage. Why not pour a large concrete block (or a block made from some heavier material), set it on two railroad trucks, and haul them up a plateau (via electric rail) for storage? This is gravitational energy using dense solids, and without fluid friction one can greatly maximize efficiency. Furthermore, the number of cars that descend can be controlled at a constant rate, which leads to better power output control. Maybe a large railroad carrier can afford to build something like this.