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Organic 'mega flow' battery promises breakthrough for renewable energy

09 January 2014

A new type of battery could fundamentally transform the way electricity is stored on the grid, making power from renewable energy sources more economical and reliable.

Professor Aziz and others at Harvard University have developed a metal-free flow battery that relies on the electrochemistry of naturally abundant, small organic molecules (photo: Eliza Grinnell, SEAS Communications)
Professor Aziz and others at Harvard University have developed a metal-free flow battery that relies on the electrochemistry of naturally abundant, small organic molecules (photo: Eliza Grinnell, SEAS Communications)

The novel metal-free flow battery (reported in a paper published in Nature on January 9) 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.

It was designed, built, and tested in the laboratory of Professor Michael Aziz at the Harvard School of Engineering and Applied Sciences (SEAS). Pioneering high-throughput molecular screening methods were used to calculate the properties of more than 10,000 quinone molecules in the search for the best candidates.

Flow batteries store energy in chemical fluids contained in external tanks, the amount of energy that can be stored limited only by the size of the tanks. The design permits larger amounts of energy to be stored at lower cost than with traditional batteries.

While a growing number of engineers have focused their attention on flow batteries for grid energy storage, the technology has hitherto relied on chemicals that are expensive or difficult to maintain, driving up the energy storage costs.

The active components of electrolytes in most flow batteries have been metals. Vanadium is used in the most commercially advanced flow battery technology now in development, but its cost sets a rather high floor on the cost per kilowatt-hour at any scale.

Other flow batteries contain precious metal electrocatalysts such as platinum, which is used in fuel cells.

The new flow battery developed by the Harvard team already performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious metal electrocatalyst.

Quinones are abundant in crude oil as well as in green plants. 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.

To back up a commercial wind turbine, a large storage tank would be needed, possibly located in a below-grade basement. For a wind or solar farm, a few very large storage tanks may be required.

The same technology could also have applications at the consumer level, where a device the size of a home heating oil tank might store a day’s worth of sunshine from solar panels, potentially providing enough energy to power a household through the darker hours without burning any fossil fuels.

Professor Aziz says the next steps in the project will be to further test and optimise the system that has been demonstrated on the bench top 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,” he said. “But we have ideas that could lead to huge improvements.”

Connecticut-based Sustainable Innovations, LLC, a collaborator on the project, expects to deploy demonstration versions of the organic flow battery contained in a unit the size of a horse trailer.

The portable, scaled-up storage system could be connected to solar panels on the roof of a commercial building, and electricity from these could either directly supply the needs of the building or go into storage and come out of storage when there’s a need.

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

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