Rhubarb Batteries

“The intermittent renewables storage problem is the biggest barrier to getting most of our power from the sun and the wind. A safe and economical flow battery could play a huge role in our transition off fossil fuels to renewable electricity. I'm excited that we have a good shot at it.”

Michael J. Aziz of the Harvard School of Engineering and Applied Sciences is talking about a new type of inexpensive battery used to store energy, which he and his team are developing and testing.

Flow batteries aren’t a brand new idea. They store energy in chemical fluids contained in external tanks—as with fuel cells—instead of within the battery container itself. In fact, several are in operation around the world, with more orders—specifically for storing solar power—on the way. (Nature News offers a nice illustration of how a flow battery works.)

But most of these flow batteries use vanadium as the active component. Vanadium’s cost sets a rather high minimum price for this solution—around $81 per kilowatt-hour for just the metal. Other flow batteries contain precious metal electrocatalysts such as the platinum used in fuel cells. Also pricy.

So Aziz and his team looked to cheaper solutions. His team discovered a metal-free option that relies on the electrochemistry of naturally abundant, inexpensive, small organic (carbon-based) molecules called quinones, which are similar to molecules that store energy in plants and animals.

Quinones are abundant in crude oil as well as in green plants, putting the price tag at around $27 per kilowatt-hour. The molecule that the Harvard team used in its first quinone-based flow battery is almost identical to one found in rhubarb. The quinones are dissolved in water, which prevents them from catching fire.

Then it’s just a matter of the external storage tanks. To back up a commercial wind turbine, a large storage tank is needed, possibly located in a below-grade basement, says Michael Marshak, also at Harvard School of Engineering and Applied Sciences. Or if you had a whole field of turbines or a large solar farm, you could imagine a few very large storage tanks.

“Imagine a device the size of a home heating oil tank sitting in your basement,” he continues. “It would store a day’s worth of sunshine from the solar panels on the roof of your house, potentially providing enough to power your household from late afternoon, through the night, into the next morning, without burning any fossil fuels.”

This technology could also provide very useful backup for off-grid rooftop solar panels—an important advantage considering some 20 percent of the world’s population does not have access to a power distribution network.

Aziz says the next steps in the project will be to further test and optimize the system demonstrated in the lab and bring it toward a commercial scale. “So far, we’ve seen no sign of degradation after more than 100 cycles, but commercial applications require thousands of cycles,” he says. He also expects to achieve significant improvements in the underlying chemistry of the battery system. “I think the chemistry we have right now might be the best that’s out there for stationary storage and quite possibly cheap enough to make it in the marketplace. But we have ideas that could lead to huge improvements.”

Their current research was published last week in Nature.

Image: Eliza Grinnell, Harvard