Grid-Scale Iron-Chromium Redox Flow Battery Connected

One of the world’s first grid-scale iron-chromium redox flow batteries was interconnected this May to the distribution grid. The EnerVault Turlock, which its developer EnerVault says is a 250-kW, 1-MWh battery grid-scale energy storage system, will be charged by a 150-kW dual-axis tracking solar photovoltaic system in an almond orchard in California’s Central Valley, will power a 260-kW irrigation pump, and will inject energy back into the grid during peak times (Figure 4).

4. Novel development. EnerVault connected its unique iron-chromium redox flow battery to the grid this May. Courtesy: Government Contractors Network

According to the Energy Storage Association (ESA), iron-chromium flow batteries, pioneered and studied extensively by NASA in the 1970s and 1980s, are essentially redox flow batteries—a class of electrochemical energy storage devices that employ reduction and oxidation reactions to store energy in liquid electrolyte solutions, which flow through a battery of electrochemical cells during charge and discharge.

That means, unlike other types of batteries, which are packaged in small modules, iron-chromium flow batteries consist of two large tanks that store electrolytes containing iron and chromium—both abundant, relatively low-cost materials that are relatively environmentally benign. During discharge, the electrolytes are pumped through an electrochemical reaction cell and power becomes available. To store energy, the process is reversed.

EnerVault, however, has developed a new electrolyte pumping system to improve the efficiency of the charge/discharge cycle. In its design, electrolytes flow through a “cascade” of cell stacks, each holding electrolyte with a progressively lower state of charge. That allows an increase in energy density compared to a conventional flow battery.

Development of the technology was backed by $5 million in Department of Energy (DOE) funding. The DOE says that flow batteries could be an especially good solution for small island grids such as Hawaii or at military bases. Or, they could be paired with renewables and used in a microgrid that can continue to operate during a power outage.

Sonal Patel, associate editor (@POWERmagazine, @sonalcpatel)