In a major leap that could transform solar power from a marginal energy source into a mainstream energy source, MIT researchers have overcome a major barrier to large-scale solar power: storing energy for use when the sun doesn't shine. Until now, solar power has been a daytime-only energy source, because storing extra solar energy for later use is prohibitively expensive and grossly inefficient. MIT researchers have hit upon a simple, inexpensive, highly efficient process for storing solar energy.