The batteries' unique design could smooth out the power from renewables and help usher in the rise of more resilient microgrids

No longer needed? (Image: Ed Freeman/Getty)

THE future of energy storage has taken root on an onion farm in southern California.

Seeking to offset its electricity bills, Gills Onions in Oxnard has installed a flow battery. When electricity prices from the grid peak, the farm can tap stores of energy created by processing agricultural waste. The battery can supply 600 kilowatts of electricity over six hours to run farm machinery for a fraction of the usual cost.

Flow batteries are centred around two aqueous electrolytes, which are held in separate tanks when the battery is idle. To get electricity from it, the liquids are pumped into a chamber separated by a membrane, sparking an electron-producing chemical reaction across the membrane. To store energy, an external current is applied across the membrane and the process works in reverse.


The batteries’ size – they can be as big as shipping containers – and ability to store large amounts of energy make them well suited to smoothing out the variable supply of wind, solar and other renewable energies. But they are expensive, and their pumps and tubes make them difficult to maintain.

Several firms are now coming to market with designs that they say address those concerns, opening the door to the possibility that battery backups for renewables could one day form a constellation of self-sufficient microgrids far more resilient than the present electrical infrastructure.

The batteries can deliver large amounts of energy, making them suited to smoothing renewables

Primus Power, based in Hayward, California, has designed a zinc-bromine flow battery that does away with the membrane in favour of a porous metal electrode onto which zinc is plated when the battery discharges. EnerVault, in Sunnyvale, has cut costs by improving the electrolyte pumping system and using iron chromium rather than the more expensive vanadium present in older designs, says Bret Adams of the company.

Flow batteries are also considered to be very safe, because unlike some lithium-ion designs, they are not prone to thermal runaway, which can cause battery fires. There have been growing pains, though, including anode failures, short membrane life and electrolyte leakage, says Steve Minnihan, an analyst at technology market research firm Lux Research. “It indicates to the market that flow batteries need a few more years in the lab before they can rival lithium-ion or lead-acid in high-volume deployment,” he says.

Nevertheless, they are finding their way into the field. In 2011 the Marine Corps Air Station in Miramar, California, learned the hard way how brittle today’s centralised grid is. When a massive blackout took out much of San Diego’s power, the base ground to a halt. There is a 230-kilowatt solar array on site, but it wasn’t designed to provide backup power. So they had to rely on fossil fuel generators that took hours to bring online. A microgrid like the one at Gills Onions is now being built on the base that will use a 250-kilowatt battery to store spare power generated by the solar panels.

Flow batteries could back up neighbourhoods, too. The Modesto irrigation district in central California is planning to put flow batteries at 45 electrical substations throughout the municipality so they provide power even if a transmission line is knocked out. “Power reliability – and microgrids are one example of that – will really be one of our killer apps,” says Primus Power CEO Tom Stepien.

This article appeared in print under the headline “Going with the flow”