Vanadium redox flow battery utility application installed in U.S.

One early application of flow battery technology in the United States was a project developed in a partnership between the large western utility, PacifiCorp, and VRB Power Systems, a Vancouver, British Columbia, developer of vanadium-based flow battery energy storage systems. The project, located in Castle Valley, Utah, involves a 250 kW installation with an eight-hour storage capacity. In energy terms, this is a 2 MWh system.

Installed in 2003, the Castle Valley system is charged overnight by baseload resources, and supplements the supply of power to a small community during the hottest part of the day, when the distribution feeder is overloading. This arrangement helped PacifiCorp avoid having to install a new transmission line to Castle Valley, and increased the utilization of existing infrastructure. Nearly two-dozen other flow battery systems are in place around the world, with tens of thousands of reliable charge-discharge cycles on record.

Flow batteries also appear to match up quite well with the needs of utility-scale wind farms. In Japan, where utilities are required to generate a portion of their energy from renewable sources such as wind, the utility J-Power added a 4 MW, 90-minute (6 MWh) vanadium-based flow battery to an existing 30 MW wind farm.

The wind farm charges the storage system, and the storage system serves to level the output of the wind farm to the broader distribution grid. When the wind rises or falls over the course of a few seconds, the storage system smooths the frequency variations that would normally arise. This protects energy consumers from deviations in their expected power quality.

When the wind suddenly cuts out more than that, though, the flow batteries really earn their keep. In those cases, the flow batteries can provide burst power up to 6 MW, creating the power bridge that gives utility operators the chance to bring peaking plants or other generation resources online.

The additional layer of equipment adds perhaps 15 percent to the cost of a stand-alone wind farm, but more than makes up for the additional cost by increasing the value of the electricity that's generated.

As with any relatively new technology, there are a few chasms that must be crossed before widespread development of wind-integrated flow battery systems occurs. VRB Power Systems and others are working to build the capacity to supply products in volume to the market, as well as to build market awareness of the technology and to continue to drive down the cost of manufacturing the energy storage systems. Just as importantly, utilities are beginning to understand how energy storage can increase the value of their existing system resources, and how it can help them reshape their resource base over time, if necessary.

The notion of complementary ramp rates—of flow batteries and peaking plants matching the falloff from wind farms—is beginning to be discussed and fine-tuned by utility planners. Utilities such as J-Power and PacifiCorp, by installing flow batteries on their grids, are providing invaluable real-world proving grounds and racking up years of performance records that will ultimately make or break the market.

In short, energy storage technologies are rapidly being commercialized to enable the widespread integration of intermittent renewable energy sources into the grid. Hydropower, compressed air, flow batteries, and other technologies continue to be fine-tuned as tools for the utility planner's tool belt. The increasing understanding of how storage can build a bridge from wind power to dispatchable resources creates options that were not available even a few years ago. And, as the months go by, these technologies make it more feasible for an increasingly reliable, clean, and secure electric energy system to emerge.

All of this brings us to a novel type of battery known as a flow battery. Flow batteries essentially comprise two key elements: cell stacks, where power is converted from electrical form to chemical form, and tanks of electrolytes where energy is stored.

The most popular flow battery on the market uses a vanadium redox technology, using charged vanadium in a dilute sulfuric acid solution to store energy. The appeal of flow batteries is that for grid applications they combine the strengths of both conventional batteries and fuel cells.

Like a fuel cell, a flow battery has a long life and is both energy-efficient and environmentally friendly. Also, like a fuel cell, the energy rating of the system is a separate design variable from the power rating. Increasing the volume of the electrolyte tanks increases the amount of energy that the system can store and release; increasing the number of cell stacks increases the power that the system can generate.

Like traditional batteries, but unlike fuel cells, flow batteries are an "electricity in, electricity out" system. There is no external fuel source, such as hydrogen, that is added regularly to recharge the system. Instead, electric energy is supplied to the system at one time, and the system stores that electric energy in electrochemical form until it is needed later. For grid applications, this simpler arrangement avoids the need to create new fuel or distribution systems.

In addition, unlike fuel cells, flow batteries are not based on rare or valuable materials. Fuel cells typically use platinum or other expensive catalysts to speed the oxidation of their energy carrier. Instead, the material at the heart of a flow battery cell is vanadium, a plentiful, nontoxic metal.

While a flow battery using an electrolyte solution doesn't have the same energy density as a fuel cell using hydrogen as an energy carrier, for most grid applications high energy density is not a key design factor.