The four round structures pictured above may look like grain silos but they're actually giant flow batteries. They're part of a demonstration plant going online this week, and proponents say it could represent the future of long-duration energy storage on the electric grid. Startup EnerVault will unveil tomorrow what it says is the largest iron-chromium flow battery ever made. Installed in Turlock, Calif., the four-hour, 250-kilowatt battery will be charged by a solar array and power an irrigation system. The project was funded by about US $5 million from Department of Energy through the stimulus program and the California Energy Commission.