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EETE SEPT 2013

Flow batteries going grid scale By R. Colin Johnson Cheaper and 10 times the power density of lithium-ion batteries, those are the claims MIT researchers are making about their flow battery design. The Massachusetts Institute of Technology said in a press release that the low-cost, simplified flow battery design aims to satisfy the Department of Energy’s target of less than $100 per kilowatt-hour for mass adoption of grid-scale energy storage for wind farms, solar arrays, and energy-efficient buildings. The key to the cost reduction is the elimination of the ion-exchange membrane in a flow battery. MIT says its system offers “a power density that is an order of magnitude higher than that of many lithium-ion batteries.” The battery prototype, designed by MIT professors Cullen Buie and Martin Bazant and doctoral candidate An MIT flow battery is designed to simplify the rechargeable technology by eliminating expensive membranes. The lower solid graphite electrode reduces liquid bromine to hydrobromic acid, while hydrogen is oxidized at the upper porous electrode. (Source: MIT) William Braff, handles three times as much power per unit volume as even the most advanced rival designs, MIT said. The prototype uses a laminar flow of two liquids pumped through a channel in parallel without mixing. Instead of having ions permeate a membrane to travel between electrodes, the battery uses electochemical reactions at the electrodes - located at each end of the channel - to charge and discharge as it stores and supplies energy. The battery reactants are liquid bromine and hydrogen; the chemical reaction reduces liquid bromine to hydrobromic acid at a solid graphite electrode while hydrogen is oxidized at a porous graphite electrode. The strong chemical reaction between the hydrogen and bromine enables the battery to store more energy per unit volume than other flow batteries, MIT said. Reversible hydrogen-bromine reactions have been used in other flow batteries, but corrosive properties eventually cause conventional ionexchange membranes to fail. MIT’s membraneless setup is designed to solve that problem. Large-scale flow batteries could enable solar and wind systems to store energy as its produced and then meter it out as it is needed during times of peak demand. The economies of scale are right, too, since liquid bromine and hydrogen fuel are both widely available and relatively inexpensive in large quantities, “with more than 243,000 tons produced each year in the United States.” The team aims to optimize its design using a model that has already proven itself out for the initial prototype. By modeling with slightly different architectures and chemistries as the design is scaled up to grid-scale dimensions, the team hopes to achieve its long-term goal of $100 per kilowatt-hour. Battery data without additional wiring By Christoph Hammerschmidt Within their activities towards intelligent battery management technologies, a consortium of two commercial companies and two universities is currently developing a data exchange technology which does not require additional data wires within large lithium-ion battery packs - the battery data are transmitted simply across the current paths. In order to enable more efficient management technologies for the more than 100 cells within a battery pack, the scientists involved in the project IntLion have developed a technique that resembles the known Powerline Transmission, but applies these principles to DC current paths. This would enable battery manufacturers to do away with additional data wires within the battery packs and thus save space and costs. The system makes every single battery cell accessible to the management system, enabling a more precise and efficient battery management. In addition, this approach allows a better use of the energy potential of such a battery pack: if one cell fails to work properly, it can be replaced instead of replacing a group of cells or even the entire battery pack. In the IntLion project, Robert Bosch GmbH and ProDesign Electronic GmbH collaborate with the Karlsruhe Institute of Technology (KIT) and the Hochschule Hannover. The project is funded within the scope of the electromobility research program by the German federal research ministry. 12 Electronic Engineering Times Europe September 2013 www.electronics-eetimes.com


EETE SEPT 2013
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