Broadly speaking, 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. But there is another approach whose potential was seen decades ago, but which was sidelined because nobody found a way to harness it in a practical and economical way.
This 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 the Massachusetts Institute of Technology (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 semi-stable 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.”
In addition to Grossman, the work was carried out by Yosuke Kanai of Lawrence Livermore National Laboratory, Varadharajan Srinivasan of MIT’s Department of Materials Science and Engineering, and Steven Meier and Peter Vollhardt of the University of California, Berkeley. Their report on the work, which was funded in part by the National Science Foundation and by an MIT Energy Initiative seed grant, was published on 20 October in the journal Angewandte Chemie.
The problem of ruthenium’s rarity and cost still remains as “a dealbreaker”, Grossman said, but now that the fundamental mechanism of how the molecule works is understood, it should be easier to find other materials that exhibit the same behaviour. This molecule “is the wrong material, but it shows it can be done,” he said.
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 behaviour. “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.
Roman Boulatov, assistant professor of chemistry at the University of Illinois at Urbana-Champaign, said of this research that “its greatest accomplishment is to overcome significant challenges in quantum-chemical modelling of the reaction,” thus enabling the design of new types of molecules that could be used for energy storage. But he adds that other challenges remain: “Two other critical questions would have to be solved by other means, however. One, how easy is it to synthesise the best candidates? Second, what is a possible catalyst to trigger the release of the stored energy?”
Grossman plans to collaborate with Daniel Nocera, the Henry Dreyfus Professor of Energy and Professor of Chemistry, to tackle such questions, applying the principles learned from this analysis in order to design new, inexpensive materials that exhibit this same reversible process. The tight coupling between computational materials design and experimental synthesis and validation, he said, should further accelerate the discovery of promising new candidate solar thermal fuels.