...AND CHEAPER...:

"The battery is made of iron salt and water," said Hossfeld. "Unlike lithium-ion batteries, iron flow batteries are really cheap to manufacture."

Every battery has four components: two electrodes between which charged particles shuffle as the battery is charged and discharged, electrolyte that allows the particles to flow smoothly and a separator that prevents the two electrodes from forming a short circuit.

Flow batteries, however, look nothing like the battery inside smartphones or electric cars. That's because the electrolyte needs to be physically moved using pumps as the battery charges or discharges. That makes these batteries large, with ESS's main product sold inside a shipping container.

What they take up in space, they can make up in cost. Lithium-ion batteries for grid-scale storage can cost as much as $350 per kilowatt-hour. But ESS says its battery could cost $200 per kWh or less by 2025.

Crucially, adding storage capacity to cover longer interruptions at a solar or wind plant may not require purchasing an entirely new battery. Flow batteries require only extra electrolyte, which in ESS's case can cost as little as $20 per kilowatt hour.

"This is a big, big deal," said Eric Toone, science lead at Breakthrough Energy Ventures, which has invested in ESS. "We've been talking about flow batteries forever and ever and now it's actually happening."

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