The idea has its origins in the 1970s: Two fluids - electrolytes with metal ions - flow from tanks through a cell; power is generated from this flow in a chemical process. This principle of the redox-flow battery can also be reversed: If there is surplus energy from wind or solar power, the battery converts the electrical energy back into chemical energy, and stores it in the tanks. The process can be repeated up to 10,000 times. Those batteries with capacities of up to a few megawatts should be developed within the next few years, thereby making green electricity as predictable as the power from fossil fuel power plants is today. At this year's Hannover Messe, Fraunhofer's Institutes for Chemical Technology ICT, Environment, Safety and Energy Technology UMSICHT and Solar Energy Systems ISE will be displaying the results of their joint development efforts, and prototypes of redox-flow batteries. The biggest advantage of the new batteries: Their energy density is comparable to lead storage batteries - but their lifespan is almost ten times as long. 

The ICT in Pfinztal, near Karlsruhe, engineered electrolytes and new materials specifically for the membrane through which the ion exchange occurs. At the Hannover Messe, ICT demonstrates a fully operational redox-flow battery with two small tanks and a cell stack - all fitting on the surface of two sheets of letter paper. Indeed, the power capacity equals just a few watts, but this allows for a variety of materials to be sampled in the test preparation. 

The ISE in Freiburg is advancing its progress with stack development for lower capacities, as well as battery management. At the Hannover Messe, ISE researchers will showcase an example of the stack concept.  Designs for larger systems of up to 2 megawatts are currently in the development phase.

At UMSICHT in Oberhausen, the 35 kW large stacks needed for this are under development and will also be tested in the future. These stacks form the basis of the 2 megawatt systems. At the trade show, researchers will display a tiered, sliced deck with a stack made of cells. The Institute built a redox-flow battery test laboratory in Oberhausen (more information is available as of April 19 at: battery-lab.umsicht.fraunhofer.de). In the laboratory, scientists can study batteries of up to 80 kilowatts, and stacks weighing up to one ton. They are currently testing the first experimental models for individual cells.  In Hall 13, Booth E44 at the 2010 Hannover Messe, scientists present the latest research findings of the Fraunhofer Energy Alliance and the Fraunhofer networks "Energy Storage Systems and Networks" and "Battery Networks."