MIT researchers say they have discovered a way to use solar energy cheaply even after the sun goes down, which could make it a mainstream source of power within the next decade. Solar energy has been expensive and inefficient to use after dark, said Daniel Nocera, 51, the Henry Dreyfus professor of energy and professor of chemistry at MIT. But in an article published in the July 31 issue of the journal Science, Nocera and other Massachusetts Institute of Technology researchers say they have found a simple, inexpensive process for storing solar energy. How the heck are you going to build an economy or a business only if the sun is shining?" said Nocera, the senior author. "What you really need to do is when the sun is shining, figure out how to store some of that energy so you can unleash it when the sun isn't shining." Nocera and the other researchers based their work on a compound made from cobalt and phosphate, both readily available. When the sun is out, electricity from solar panels can be fed to the compound in water, causing the water to split into hydrogen and oxygen. The elements create a chemical fuel that can be recombined to create energy later, when the sun is not shining. The discovery breaks "the connection between energy and fossil fuels because my energy is coming from water," said Nocera, "unleashing the solar energy, not in real time, but when you want to." The researchers said the findings open the door for large-scale use of solar energy around the clock - not right away but within 10 years.