Flow Batteries
Augment Wind Power


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Flow Battery

A flow battery (shown here in a three-dimensional rendering) stores energy electrochemically in the form of vanadium ions suspended in a solution.

Utility planners get headaches when contemplating clean energy sources, such as wind and solar. How can they balance instantaneous supply and load when perhaps 20 percent of the supply might be intermittent with tens or hundreds of megawatts coming and going in an unpredictable fashion.

Fast rises in wind or solar power can be accommodated through power shedding—basically, the utility throwing away energy while it gets the system into balance. More challenging, though, is when an intermittent energy source crashes—suddenly the wind dies or a cloud blocks the sun. In this case, the utility must have an energy source ready to pick up the slack; the alternative is customers losing power.

A number of energy storage technologies have been proposed to make intermittent energy sources more dispatchable, i.e., can be scheduled at the planner’s convenience, or at least used to bridge the gap between the fall-off rate of renewables and the ramp-up rate of traditional peaking plants. Peaking plants are fast-reacting facilities, such as natural gas burning plants that are operated to smooth variations in the electrical load.

The technology that promises to bridge the gap between fast crashes and peaking plant ramp up is grid-scale energy storage systems, in particular the flow battery.

Vandium

Pipes and pumps carry the vanadium solution to stacks of proton exchange membranes, which transfer electrical charge and create a current.

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 recruits vanadium redox technology, using charged vanadium in a dilute sulfuric acid solution to store energy. The appeal of flow batteries for grid applications is that 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 can be stored and released. 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.

Unlike fuel cells, flow batteries do not depend on rare or valuable catalysts, such as platinum to speed the oxidation of their energy carrier. Instead, 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.

Wind Farm

This 30 MW wind farm in Japan is augmented by a battery system that can supply 4 MW of power for up to 90 minutes.

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 winds rise or fall 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, 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.

[Adapted from “Renewable. Rechargeable. Remarkable.” by Mark T. Kuntz and Justin Dawe, for Mechanical Engineering, October 2005.]

As it turns out, the best winds are found offshore, away from topographical obstructions such as hills and forests.

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March 2011

by Mark T. Kuntz and Justin Dawe