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Large, powerful redox flow battery for renewable energy storage

17 March 2013

Powerful electric energy storage devices are necessary to level out the irregularities of renewable energy supplies. Fraunhofer's answer is a redox flow battery.

Photo courtesy of Fraunhofer UMSICHT
Photo courtesy of Fraunhofer UMSICHT

Fraunhofer scientists have developed a redox flow battery that reaches a stack power of up to 25kW with a cell size of 0.5 square metres. This is eight times larger than the previous A4-sized systems.

Redox flow batteries offer an effective way to balance out fluctuations in the supply of renewable energy and thus guarantee its constant availability. The batteries store electrical energy in liquid electrolytes, which are charged and discharged in small reaction chambers.

Several of these cells are lined up in stacks. However, the batteries that are currently available on the market, which are roughly the size of A4 paper (1/16 square meters), can only generate 2.3kW.
Scientists at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen, Germany, have succeeded in significantly increasing the size of the stack and, with it, its capacity.

Scientists will showcase this battery at the Energy Alliance’s Fraunhofer booth at Hannover Messe (Hall 13, booth C10). The prototype has an efficiency of up to 80 percent, and can take a load of up to 500A.
During the development, scientists tested new membrane materials and researched battery management and battery design. Flow simulations helped them to optimise the cell structure.

A complete redesign of the battery followed which enabled the Fraunhofer team to make their breakthrough.. “The biggest challenge we faced for producing batteries with this level of performance was the development of a completely new stack structure andthe scale-up,” explains Dr Jens Burfeind, Group Manager for Electrochemical Storage Systems at Fraunhofer UMSICHT.
How a redox flow battery works
The redox flow cell (red for reduction = electron uptake, ox for oxidation = electron release) is an accumulator. It stores energy in electrolyte solutions contained in tanks. The electrolytes circulate from these tanks through a cell, which generates electricity from the fluid in a chemical process.

The most common kind of this type of battery is the vanadium redox flow battery. The vanadium is charged and discharged in small reaction chambers. Several of these cells are lined up in stacks, which increases the battery’s power. Redox flow batteries offer several advantages: they are cost-effective, robust, durable, and can be individually customised.

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