Renewable energy has many advantages, as we all , environmentalists , know for sure. There's no need to pay for fuel since the wind and the sun are free, and that saves utilities from the price spikes seen in coal, natural gas, oil and nuclear. But wind and solar face one major problem: intermittency. When the wind doesn't blow and the sun doesn't shine, turbines and silicon panels aren't producing electricity, and there's no way to store the electricity they do produce during peak times if it's not being used. That's a serious obstacles since utilities, often by law, need to provide enough electricity to meet demand at all times.

But utility-scale solar companies are working on ways to store the energy they produce during the brightest days. One option: molten salt. It can be used in solar thermal, which employs powerful mirrors to focus the sun's heat to create steam, driving an electric turbine. The surplus heat produced during the day can be used to warm up massive amounts of salt, which can absorb significant amounts of heat. When the sun goes down — or when it's simply cloudy — that heat can be used to generate steam and run an electric turbine. It's not perfect, but it's the best battery that's been developed yet for utility-scale solar.

Source : http://www.time.com/time/specials/packages/article/0,28804,2030137_2030135_2021676,00.html