Storing Sunlight

A thermochemical battery could hold and release energy as needed

The biggest problem with solar power has always been its intermittency: what do you do when the sun goes down? But an unconventional approach could offer the best way yet to store the sun’s heat and release it when it’s needed.

A thermochemical battery would capture solar energy by using sunlight to change the shape of certain molecules, which would then release the energy on demand when they snapped back to the original shape. One benefit of such a system is that heat-storing chemicals can remain stable for years. Solar-thermal systems, a conventional storage technology in which the sun’s energy is concentrated to heat water, molten salt, or other materials, gradually lose heat even with the most effective insulation available.

In 1996 researchers at Berkeley discovered that a chemical called fulvalene diruthenium can reliably and reversibly switch between two states. When it absorbs sunlight, they found, the molecule undergoes a structural transformation, assuming a higher-energy state where it can remain stable indefinitely. A catalyst can be used to return it to its original shape, releasing heat in the process. In principle, when fulvalene diruthenium releases its stored heat, it “can get as hot as 200 °C, plenty hot enough to heat your home, or even to run an engine to produce electricity,” says ­Jeffrey ­Grossman, an associate professor of power engineering. “You could put the fuel out in the sun, charge it up, then use the heat and place the same fuel back in the sun to recharge.”

But this compound includes ruthenium, a rare and expensive element. Moreover, no one understood how it worked, which hindered efforts to find a cheaper equivalent. Now Grossman and his MIT colleagues, in collaboration with the Berkeley researchers, have found out exactly how the molecule stores and releases energy, which should facilitate the search for similar but less expensive chemicals.

“It turns out there’s an intermediate step that plays a major role,” says Grossman: the molecule forms a semistable configuration partway between the two previously known states. “That was unexpected,” he says. The two-step process helps account for why the molecule is so stable and why the process is easily reversible. And now that the researchers know to look for a two-step process, it should be easier to find materials that duplicate fulvalene’s behavior.

The next step, Grossman says, is to use simulation, intuition, and databases of tens of millions of known molecules to look for such a substitute. He’s certain that as researchers come to “understand what makes this material tick,” they’ll find other materials that work the same way.

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