Technique promises indefinitely storable solar energy

Posted by Emma Woollacott

MIT scientists are developing an alternative to photovoltaic and solar-thermal systems for capturing the sun's energy.

Discovered decades ago, but largely undeveloped, the thermo-chemical approach captures solar energy in the configuration of certain molecules which can then release it on demand to produce usable heat.

Unlike conventional solar-thermal systems, which even with effective insulation let the heat gradually leak away, the heat-storing chemicals can remain stable for years.

But researchers couldn't 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.

One such compound was discovered in 1996, but it included ruthenium, a rare and expensive element - and nobody understood how it worked, making it harder to find a cheaper variant.

Now, researchers at MIT have worked out exactly how the molecule, called fulvalene diruthenium, accomplishes its energy storage and release. This, they say, should make it possible to find similar chemicals based on more abundant, less expensive material.

Essentially, the molecule undergoes a structural transformation when it absorbs sunlight, putting it into a higher-energy state where it can remain stable indefinitely. Then, triggered by the addition of heat or a catalyst, it snaps back to its original shape, releasing heat in the process.

However, it turns out there’s an intermediate step that plays a major role, says Jeffrey Grossman. In this step, the molecule forms a semi-stable configuration partway between the two previously known states.

"That was unexpected," he says. "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."

The discovery 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, "can get as hot as 200 degrees C, plenty hot enough to heat your home, or even to run an engine to produce electricity."

The system, he says, has 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," he says. "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 only problem now is to find a cheaper material that works in the same way.