Metal-Free 'Rhubarb' Battery Could Store Renewable Grid Energy

sciencehabit writes "A molecule nearly identical to one in rhubarb may hold the key to the future of renewable energy. Researchers have used the compound to create a high-performance 'flow' battery, a leading contender for storing renewable power in the electric utility grid. If the battery prototype can be scaled up, it could help utilities deliver renewable energy when the wind is calm and the sun isn't shining." Abstract.

More like a reversible fuel cell

[Animats]

EETimes has a more useful article. [eetimes.com] This is more like a reversible fuel cell. The working fluid is pumped through the cell, where a chemical reaction occurs. The process is reversible. So there's a "charged" fuel tank, a "discharged" fuel tank, pumps, and plumbing. No info yet on the energy density of the "charged" fuel tank, which is the big question.

Some numbers from the paper

[Michael Woodhams]

In the galvanic direction, peak power densities were 0.246Wcm2 and 0.600W cm2 at these same SOCs, respectively (Fig. 1c). To avoid significant water splitting in the electrolytic direction, we used a cut-off voltage of 1.5V, at which point the current densities observed at 10% and 90% SOCs were 2.25 A cm2 and 0.95Acm2, respectively, with corresponding power densities of 3.342Wcm2 and 1.414Wcm2. ...

The galvanic discharge capacity retention (that is, the number of coulombs extracted in one cycle divided by the number of coulombs extracted in the previous cycle) is above 99%, indicating the battery is capable of operating with minimal capacity fade and suggesting that current efficiencies are actually closer to 99%. ...

AQDS has an aqueous solubility greater than 1M at pH 0, and the quinone solution can thus be stored at relatively high energy density—volumetric and gravimetric energy densities exceed 50Whl1 and 50Whkg1, respectively. ...

As shown in Fig. 2, current efficiency starts at about 92% and climbs to about 95% over ~15 standard cycles. Note that these measurements are done near viable operating current densities for a battery of this kind. Because of this, we believe this number places an upper bound on the irreversible losses in the cell. In any case, 95% is comparable to values seen for other battery systems.

I'm not an expert in any applicable field, but as I have institutional access to the original paper, I scanned it to find what looked to me like relevant numbers. As I interpret the above:

It generates about 0.5W cm^-2 of membrane, so you'd need 2m^2 to get 1 kW output. (But presumably this can be in some compact folded/layered configuration.)
It can charge much faster than it discharges: that 2m^2 of membrane would let you charge at about 4kW.
The storage capacity of the battery fades at less than 1% per charge/discharge cycle.
One litre of reactants lets you store 50Wh of energy (i.e. 20kg for a kilowatt hour)
I think the last paragraph is saying that, neglecting pumping costs, it returns about 95% of the energy you put into it.

Note that we can expect these numbers to improve with further research, but whether there are big improvements to come or only minor ones I couldn't say.

Also: They use a two-reactant-tank set up rather than four tanks, so each tank holds a mixture of the 'charged' and 'discharged' forms of its reactants (e.g. one tank holds a mixture of Br2 and HBr.) I'd naively expected a four tank set up.

It's not about density

[localroger]

It's about storing a large amount of energy in a very large amount of electrolyte without similarly large plates and electrical connections. For power storage they are thinking in terms of batteries the size of buildings, perhaps built like current sewerage-treatment plants, to store energy in the electrolyte and move it along, bringing it back to the electrical assembly with pumps as needed. It can be considerably less energy-dense than current batteries in pounds per erg and still be far more practical for the kind of large-scale storage the tech is aimed at.