[BusinessWeek] The future is taking shape under the windswept corn and soybean fields outside Dallas Center, Iowa. At the Iowa Stored Energy Park, a coalition of local utilities is grappling with one of the thorniest challenges in the field of renewable power: how to store the excess energy windmills create when demand is low so it can be used later, when the need is greater. The group is building a system that will steer surplus electricity generated by a nearby wind farm to a big air compressor. Connected to a deep well, the compressor pumps air into layers of sandstone. Some 3,000 feet down and sealed from above by dense shale, the porous sandstone acts like a giant balloon. Later, when demand for power rises, this flow is reversed. As the chamber empties, a whoosh of air flows back up the pipe into a natural-gas-fired turbine, boosting its efficiency by upwards of 60%.