Even if we do manage to set up a shiny futuristic renewable smart grid made of glitter and staffed by zebra unicorns, there are still going to be times when it poops out. Maybe the wind isn’t blowing; maybe a mean old cloud got in the way. And when that happens, there aren't enough boat batteries in the world to store all the electricity we're going to need to keep everything running.
That's why utilities are investing in giant batteries — and the giantest of them all are called "flow batteries." Composed of large tanks of liquid electrolyte, they're easy to scale: just strap on bigger tanks of electrolyte! They're also made from cheap materials (unlike those fancy lithium ion batteries that are more compact but harder to source) and should last a really long time.
Silicon Valley startup EnerVault "fills one tank of electrolyte with iron (the energy storing material) and another electrolyte tank with chromium," says Ucilia Wang at GigaOm. The two materials don't mix, but they are pumped past one another on either side of a membrane that allows ions to pass through.
EnerVault's innovation is an "engineered cascade design" that uses a staggered set of membranes with different permeability to get all the ions across in the most efficient and cost-effective way possible. A prototype of their system is set to go live in an almond orchard in California in late 2012. By the time they’ve got the kinks worked out, maybe we’ll even have a widespread network of renewable energy sources for them to store!