Molecules change shape to store solar energy

"My molecule releases neither CO2, nor any other chemical compounds while working. It is ‘sunlight in-power out,’" says Anders Bo Skov. (Credit:

Anders Bo Skov and colleagues are trying to to develop molecules capable of harvesting and holding substantial amounts of solar energy, storing it for significant amounts of time, and releasing it on demand.

However, a pesky law of nature kept getting in the way. As the capacity of the molecules to hold energy seemed to improve, the capacity to store it over time dropped—and vice versa.

Skov, a graduate student at the University of Copenhagen, may have identified a partial solution to storing solar energy on demand.

He is working with molecules known as the Dihydroazulene-Vinylheptafulvene system. Put very simply, this stores energy by changing shape, but every time the group managed to design improved molecules, the molecules lost some of their ability to hold their “energy storage” shape, says Mogens Brøndsted Nielsen, a professor of chemistry.

Anders Bo Skov
Anders Bo Skov (Credit: University of Copenhagen)

“Regardless of what we did to prevent it, the molecules would change their shape back and release the stored energy after just an hour or two. Anders’ achievement was that he managed to double the energy density in a molecule that can hold its shape for a hundred years.

“Our only problem now is how we get it to release the energy again. The molecule does not seem to want to change its shape back again,” explains Brøndsted.

Just the beginning?

Regardless of method, when you store energy, there is a theoretical limit to the energy density. And then there is reality.

In theory, a kilogram of the right molecules could store a megajoule of energy if they were perfectly designed. With that amount of energy you can heat three liters of water from room temperature to boiling.

A kilo of Skov’s molecules can boil only 75 centiliters but it does that in just three minutes. This means that his molecules could bring 15 liters of water to boil per hour, and Skov as well as his supervisor are convinced that this is just the beginning.

“What Anders has achieved is an important breakthrough. Admittedly we do not have a good method to release the energy on demand, and we should increase the energy density further still. But now we know which path to take in order to succeed,” says Brøndsted.

Totally nontoxic

Skov’s molecules are sustainable on more levels than just the obvious one. Not only do they harvest sustainable solar energy. They are also nontoxic.


“When it comes to storing solar power, our biggest competition comes from lithium ion batteries, and lithium is a poisonous metal. My molecule releases neither CO2, nor any other chemical compounds while working. It is ‘sunlight in-power out.’

“And when the molecule wears out one day, it degrades to a colorant [that] is also found in chamomile flowers,” says Skov.

Details of the work were published recently in Chemistry: A European Journal.

Source: University of Copenhagen