Fraunhofer develops battery technology at 25kW with a cell size of 0.5m2

March 20, 2013 // By Julien Happich
As more and more electricity is being generated from intermittent sources of power, electric energy storage devices are necessary to level out corresponding irregularities in the power supply.

Fraunhofer scientists have developed a redox flow battery that reaches stack power up to 25kW, with a cell size of 0.5 square meters. 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 chemical compounds, the liquid electrolytes. The electrolytes 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 of power.

The prototype developed at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT has an efficiency of up to 80 percent, and can take a load of up to 500A of current. Scientists began by testing new membrane materials and researching battery management and battery design. Flow simulations helped them to optimize 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 and the scale-up," explains Dr. Jens Burfeind, Group Manager for Electrochemical Storage Systems at Fraunhofer UMSICHT.


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 electrolyte circulates 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,