In an almond orchard outside Turlock in the Central Valley, two large tanks hold water, minerals - and more importantly, energy.

The tanks in Stanislaus County are part of a "flow battery" that stores energy from nearby solar panels. It's the largest battery of its kind in the world. And it could play a role in California's push to develop bigger and better ways to store large quantities of energy.

This particular flow battery, unveiled to the public Thursday during a ceremony with state and federal officials, was built by EnerVault of Sunnyvale, part of the Bay Area's fast growing energy-storage industry. Like most of its competitors, EnerVault is young, founded in 2008, with about $30 million in venture funding to date.

Some companies try to perfect the lithium-ion batteries found in laptops and electric cars. Others, including EnerVault and Primus Power of Hayward, specialize in flow batteries, which store energy in tanks of electrolytes. The fluid is then pumped through the battery's cells when power is needed. In contrast, the batteries found at a grocery store contain the electrolyte, cathode and anode all in one package.

"Flow batteries are batteries turned inside out," said Jim Pape, EnerVault's chief executive officer.

His company's flow batteries use iron and chromium, blended into the water inside its tanks. Both materials are safe to handle. (For those of you thinking "Erin Brockovich," EnerVault uses the kind of chromium found in multivitamins, not the infamous hexavalent chromium featured in the movie). Iron and chromium also have the benefit of being cheap.