April 6, 2011 -- Several Fraunhofer Institutes are working jointly on "redox flow" fluid batteries of the future. The researchers are presenting their large battery installation at the Hannover Messe (April 4-8, 2011, Hall 13, Booth C41).
For the intermittent power output of wind and solar photovoltaic installations, large-scale stationary storage facilities will be needed, substations centrally located in the grid and capable of buffering energy in megawatt quantities for low-current periods.
A Fraunhofer consortium is currently driving the development of large-scale energy-storage systems known as redox flow batteries. The experts' long-term goal is to build a handball-court-sized battery installation with a capacity of 20 MWh -- enough energy to provide power to roughly 2000 households through a long winter night or a cloudy day. The results have not advanced quite so far: At the moment, the largest laboratory facilities at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT have an output of several kW.
At the Hanover Fair (Hannover Messe), the researchers will demonstrate the operation of the redox flow battery using a 2kW plant. Three Fraunhofer institutes are involved in the consortium working to expedite the development of these storage batteries. "The process already works reliably," notes Dr. Christian Dötsch, business unit manager for Energy Efficiency Technologies at UMSICHT, one of the participating institutes. "The challenge lies in the upscale version, the enlargement of these plants."
Redox flow batteries are large-scale vanadium-based liquid batteries in which chemical vanadium bonds alternately pick up and emit electrons along membranes. Because these batteries use only vanadium bonds and not two different fluids at the same time as found in other systems, impurities are eliminated. Dr. Tom Smolinka, in charge of coordinating the work at the Fraunhofer Institute for Solar Energy Systems ISE, states that this exchange makes the batteries more robust.
The vanadium charges and discharges in tiny reaction chambers. Several of these chambers are arrayed in stacks to increase a battery installation's output. Currently, the membranes -- and hence the individual cells -- have a surface area roughly equal to that of a DIN A4 sheet of office paper. "To achieve megawatt values, we need to reach a size of at least DIN A0 (ca. 85 × 120 cm)," estimates Dr. Jens Tübke, division director at the third project partner, the Fraunhofer Institute for Chemical Technology ICT. One of the challenges is to insure that the vanadium fluid flows smoothly through these large membranes and past the felt-like carbon electrodes in the cells themselves. To accomplish this, Fraunhofer researchers are using flow simulations to improve cell design.
Since last year, the Fraunhofer consortium has also been working on new membrane materials and battery designs in a cooperation project funded by the German federal ministry for the environment. Another project is scheduled to begin this year and will involve industry participation. On principle, batteries with up to 80kW of storage capacity can be built in the new Fraunhofer redox flow laboratory and a 20-kW plant is scheduled to go into operation at the end of next year. The researchers hope to cross the megawatt threshold in roughly five years.