There have been two approaches to capturing the sun’s energy: photovoltaics, which turn the sunlight into electricity, or solar-thermal systems, which concentrate the sun’s heat and use it to boil water to turn a turbine, or use the heat directly for hot water or home heating
The third solar approach is the thermo-chemical approach, in which solar energy is captured in the configuration of certain molecules which can then release the energy on demand to produce usable heat. And unlike conventional solar-thermal systems, which require very effective insulation and even then gradually let the heat leak away, the heat-storing chemicals can remain stable for years.
Researchers explored this type of solar thermal fuel in the 1970s, but there were big challenges: Nobody could find a chemical that could reliably and reversibly switch between two states, absorbing sunlight to go into one state and then releasing heat when it reverted to the first state. Such a compound was discovered in 1996, but it included ruthenium, a rare and expensive element, so it was impractical for widespread energy storage. Moreover, no one understood how the compound worked, which hindered efforts to find a cheaper variant.
Now researchers at MIT have overcome that obstacle, with a combination of theoretical and experimental work that has revealed exactly how the molecule, called fulvalene diruthenium, accomplishes its energy storage and release. And this understanding, they said, should make it possible to find similar chemicals based on more abundant, less expensive materials than ruthenium.
Essentially, the molecule undergoes a structural transformation when it absorbs sunlight, putting the molecule into a higher-energy state where it can remain stable indefinitely. Then, triggered by a small addition of heat or a catalyst, it snaps back to its original shape, releasing heat in the process. But the team found that the process is a bit more complicated than that.
“It turns out there’s an intermediate step that plays a major role,” said Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering in the Department of Materials Science and Engineering. In this intermediate step, the molecule forms a semistable configuration partway between the two previously known states. “That was unexpected,” he said. The two-step process helps explain why the molecule is so stable, why the process is easily reversible and also why substituting other elements for ruthenium has not worked so far..
In effect, explained Grossman, this makes it possible to produce a “rechargeable heat battery” that can repeatedly store and release heat gathered from sunlight or other sources. In principle, Grossman said, a fuel made from fulvalene diruthenium, when its stored heat is released, “can get as hot as 200 degrees C, plenty hot enough to heat your home, or even to run an engine to produce electricity.”
Compared to other approaches to solar energy, he said, “it takes many of the advantages of solar-thermal energy, but stores the heat in the form of a fuel. It’s reversible, and it’s stable over a long term. You can use it where you want, on demand. You could put the fuel in the sun, charge it up, then use the heat, and place the same fuel back in the sun to recharge.”
The next step, he said, is to use a combination of simulation, chemical intuition, and databases of tens of millions of known molecules to look for other candidates that have structural similarities and might exhibit the same behavior. “It’s my firm belief that as we understand what makes this material tick, we’ll find that there will be other materials” that will work the same way, Grossman said.