Rhubarb battery stores energy for cloudy, windless days

How do you store energy from intermittent renewables for a rainy day, or any night for that matter? Harvard engineers have developed a cheap battery that just might do the trick.

prototype flow battery at Harvard SEAS.jpg
When the sun’s not shining and the wind’s not blowing, relax because rhubarb battery’s got your excess renewable energy stored away.  

Technically, it’s a new kind of “flow battery” that uses a cheap chemical -- called quinone -- that’s almost identical to that found in rhubarb, which uses these types of organic molecules to store energy.

“The intermittent renewables storage problem is the biggest barrier to getting most of our power from the sun and the wind,” says Michael Aziz of Harvard. An economical battery that regulates the output of intermittent energy sources could help us transition off fossil fuels. 

First, what’s a flow battery? Nature explains
Flow batteries work by pumping different chemical broths over two electrodes separated by a membrane. The chemicals exchange protons across the membrane and shuttle electrons around the circuit that connects the electrodes, which discharges the battery. Reversing the reaction recharges the cell.

Unlike conventional batteries -- which pack reactants inside (imagine a car battery) -- the electro-active components in flow batteries are held in fluid form outside of the battery itself. That means the system can store arbitrarily large amounts of energy. 

One big limitation is the cost of those electro-active materials. So, Aziz and colleagues developed a flow battery based on abundant, low-cost quinones -- which can also be synthesized from crude oil. 

Existing flow batteries rely on an expensive metal called vanadium. Aziz told Technology Review that quinones will cut the energy storage material costs down to just $27 per kilowatt-hour -- one-third the price of the vanadium system. 

Here’s how the "rhubarb-flow battery" (pictured) works, according to Science
  • In one tank, they place quinone dissolved in water. They place bromine liquid in another tank. (Bromine is toxic, so they’re working on replacing it in future versions.)
  • To get electricity out, they pump the two liquids past adjoining electrodes separated by a thin membrane. 
  • At one electrode, each quinone molecule gives up two electrons and two protons. 
  • The electrons zip through an outside circuit to the opposite electrode, where they meet up with the protons that passed through the membrane. 
  • Those combine with a bromine atom to make molecules of hydrogen bromide (HBr). 
  • To store energy, the pumps are run in reverse, providing energetic electrons. 
  • That coaxes the hydrogens to break away from bromine atoms and reattach themselves to the quinone at the opposite electrode.

A device the size of a home heating oil tank “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,” Harvard’s Michael Marshak says in a news release

The team received funding from U.S. Advanced Research Projects Agency-Energy (ARPA-E) and is working with Connecticut-based Sustainable Innovations to develop and deploy a prototype “about the size of a horse trailer” within three years.

The work was published in Nature earlier this month.

Image: Eliza Grinnell, Harvard SEAS Communications

This post was originally published on Smartplanet.com