An emerging flow battery technology got a major boost earlier this year when Gills Onions, one of the largest fresh-cut onion processing plants in the world—began operating a 3.6-MWh vanadium redox battery (VRB) energy storage system (ESS). The 14-acre processing facility in Oxnard, Calif., was already converting 100% of its daily onion waste—up to 300,000 pounds—into a combination of renewable energy and cattle feed using an advanced energy recovery system (AERS) that extracts juice from onion peels and treats it in a high-rate anaerobic reactor to produce biogas that powers two 300-kW fuel cells. But Gills Onions thought it necessary to add energy storage to its AERS to improve the efficiency of the system and further reduce electric costs, citing their main motivation as wanting to shift electricity generation from off-peak to on-peak periods while warding off pricey demand spikes.

Encouraged by state cash rebates for certain types of energy storage, Gills Onions says it opted for the VRB-ESS that consists of three 200-kW modules with enough electrolyte to provide six hours of storage capacity (Figure 1). Not only can the VRB change from fully charging to fully discharging in seconds so that the full 600 kW are available 24 hours a day, Gills Onions says the system also can pulse an additional 50%—to 750 kW—for 10 minutes each hour, providing additional capacity for motor starts or other spikes.

1. Storing onion juice. One of the world’s largest vanadium flow battery energy storage systems began operation earlier this year at Gills Onions, an Oxnard, Calif., fresh-cut onions processing plant. The tennis court–sized system consists of cell stack modules that are connected in parallel configurations. It stores electricity during lower-cost nighttime hours and allows the Gills facility to rely on the flow battery to generate 600 kW for as long as six hours during higher-cost peak hours. Courtesy: Gills Onions

The project was built and is owned and operated by Prudent Energy Corp. subsidiary Prudent Energy Services. The Bethesda, Md.–based company gets an undisclosed share of the energy savings resulting from the project in return, calculated as avoided charges, costs, and fees that would otherwise be paid by Gills to the local utility.

As Prudent Energy describes it, the VRB-ESS consists of cell stacks, allowing modular systems to be assembled in large parallel and series configurations—with no special site requirements other than a substantial footprint. The cell stacks themselves are 10-kW sealed devices that consist of many cells, each of which contains two half-cells separated by a membrane. Electrochemical reactions occur in the half cells on inert carbon felt electrodes, creating a current used to charge or discharge the battery. “When charged electrolyte fluids [from separate storage tanks] pass through the cell stack, different ionic forms of vanadium react, resulting in a balancing electron flow into an external DC circuit—thus completing an electrochemical path for discharge,” the company says. “Forcing current into the stack from an external source recharges electrolyte in the stack, with fluids then pumped back into the reservoirs.” This “re-dox” process is reversible, allowing the battery to be charged and discharged repeatedly, the company explains.

In its most basic sense, the system stores energy chemically in different forms of a single element—vanadium, which the company has dubbed a “miracle metal”—in a proprietary electrolytic mixture. The mineral’s marvels lie in its unique attributes, particularly as a chemical catalyst for electrolytes, the company says. “It is a transition metal, which means it has the typical properties of metals, but in addition, high melting and boiling points, and high density,” it explains. And, as Prudent Energy highlights: “Vanadium forms stable, concentrated electrolytic solutions in four neighboring oxidations states.”

A selling point extolled by Gills Onions is the life of the system. The technology reportedly can produce more than 10,000 full-depth charge/discharge cycles with no degradation in performance. Meanwhile, the electrolyte never degrades and can be fully recycled.

VFB solutions have long been recognized for their edge over lithium-ion battery storage, primarily because VFBs have a practical advantage with their storage duration limit of 10 hours—compared to a 1- to 2-hour range for lithium-ion storage systems (though lithium-ion wins in terms of round-trip efficiency).

Prudent’s energy storage system isn’t unique. Several companies—including U.S.-based Ashlawn Energy, Austria-based Cellstrom GmbH, and Thailand-based Cellenium—have entered the fray, competing for global VFB market share. VFBs are already being used in the European Union, China, Japan, South Korea, and Australia—though most are in the 100-kWh range. Billed as the largest of its kind, the 3.6-MWh Gills Onions installation compares only with another Prudent Energy project completed in March 2011 involving a 1-MWh VFB at the China Electric Power Research Institute in Zhangbei, Hebei Province.

Meanwhile, industry analysts foresee tremendous growth of VFBs in the short term. A March 2012 study by Lux Research predicts that VFBs could capture about 17% of the energy grid storage market by 2017—assuming that developers can reach a target of $750/kWh for a fully installed system by then. The most significant factor that could determine the technology’s success is availability and pricing of vanadium, which is already seeing heightened demand for its traditional use: strengthening steel.

“Vanadium is not scarce,” says Prudent Energy, claiming that it not only can be mined naturally as an ore but can also be recovered as a by-product of steel manufacturing, coal-fired power generation, and oil shale production. But analysts predict a supply shortage, driven by increasing consumption by China, which produces 40% of the world’s vanadium supply (otherwise dominated by Russia and South Africa), as a result of a new rule that only vanadium-containing reinforced steel bars can be used for infrastructure and other construction projects. Increased demand for the mineral pegged to its use in flow batteries will strain global supplies.

Compounding pricing concerns is the quirk that, unlike many commodities, vanadium does not trade on the open market; deals are negotiated privately and tend to surge when steel demand is high and plunge during economic downturns. This price volatility is a major deterrent for battery producers, which require large amounts of high-purity (greater than 98.4%) vanadium, analysts point out.

Sonal Patel is POWER’s senior writer.