Solar chemical

The other, other solar power

News from MIT this week is that researchers might have found a solution to the challenge that is thermo-chemical solar power. Described as the method of capturing the sun's energy in the configuration of certain molecules which can then release it on demand to produce usable heat, thermo-chemical has always lagged behind photovoltaics and solar thermal as a viable solar power source. Around, in theory, since the 1970s, it has a distinct advantage to conventional solar-thermal systems in that the heat stored can remain stable for years, explains MIT News. But its development was eventually sidelined because nobody could find a chemical that could reliably switch back and forth between the two states. And when, in 1996, such a compound was discovered, it included a rare and expensive element called ruthenium, making it impractical for commercialisation. And as no one quite understood how the compound worked, efforts to find a cheaper option were hampered.

But it seems MIT researchers have overcome that obstacle, discovering exactly how the molecule, called fulvalene diruthenium, achieves its energy storage and release. "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," says MIT News. "Then, triggered by a catalyst, it snaps back to its original shape, releasing heat in the process." But the research team also discovered a surprise – an intermediate step when the molecule forms a semi-stable configuration partway between the two previously known states.

In effect, explains Jeffrey Grossman – associate professor of power engineering at MIT – this makes it possible to produce a “rechargeable heat battery” that can repeatedly store and release heat gathered from sunlight or other sources. "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.”

Despite this breakthrough, ruthenium’s rarity and cost is still "a dealbreaker," says Grossman. But now that they have revealed the fundamental mechanism of how the molecule works, it might be possible to find similar chemicals based on more abundant, less expensive materials. 

By Sophie Vorrath