October 13, 2009
Redox Flow Batteries To Enabel Fast Liquid Refill
Some German researchers think they can make easily recharged liquid batteries as energy dense as lithium ion batteries. Imagine having the liquid in your batteries quickly pumped out and replaced with recharged electrolyte liquid at a refueling station.
Lithium-ion batteries offer a possible solution, but it takes hours to charge them – time that an automobile driver doesn’t have when on the road. Researchers from the Fraunhofer Institute for Chemical Technology ICT in Pfinztal near Karlsruhe see an alternative in redox flow batteries. “These batteries are based on fluid electrolytes. They can therefore be recharged at the gas station in a few minutes – the discharged electrolyte is simply pumped out and replaced with recharged fluid,” says engineer Jens Noack from ICT. “The pumped-off electrolyte can be recharged at the gas station, for example, using a wind turbine or solar plant.”
The principle of redox flow batteries is not new – two fluid electrolytes containing metal ions flow through porous graphite felt electrodes, separated by a membrane which allows protons to pass through it. During this exchange of charge a current flows over the electrodes, which can be used by a battery powered device.
Until now, however, redox flow batteries have had the disadvantage of storing significantly less energy than lithium-ion batteries. The vehicles would only be able to cover about a quarter of the normal distance – around 25 kilometers – which means the driver would have to recharge the batteries four times as often. “We can now increase the mileage four or fivefold, to approximately that of lithium-ion batteries,” Noack enthuses. The researchers have already produced the prototype of a cell. Now they must assemble several cells into a battery and optimize them.
Fast refill would make electric cars a lot more practical for longer distance travelers and for those who can't easily plug in a car at home.
One problem with redox cells has been permeation of the membrane. If the two electrolytes have different metals, this can progressively degrade the system, requiring the species to be separated again. Vandium redox cells use vanadium species on both sides, so they are tolerant of some crossover, but their energy density has been too low.
They have produced the prototype of a cell. They haven't even made a battery yet. But they claim that they can "increase the mileage four or fivefold..."
Before these big talking fellows start making such big claims, maybe they should prove that their cells can make a battery. Maybe they should prove that their batteries can hold up to the tough conditions of a hard-driven highway vehicle.
The redox flow cell might eventually be ideal for backing up large wind generator farms, or for extending power from a solar farm further into peak load times, beyond what sunlit hours alone could provide. For large, in-place installations. Talking about flow cells for automobiles at this stage in the technology is the ultimate in surreal trolling for attention. By the time that ever becomes realistic, much cheaper and more reliable technologies will be here.
"Breakthough in Flow Batteries?" by Robert McLeod on 14 OCT 2009:
"The previous king of the various redox flow battery chemistries is the Vanadium redox battery. It can, in general, obtain a 75 % round-trip efficiency which is fairly decent, being roughly in-between Li-ion batteries and Nickel-metal hydride batteries. The Achilles heel has always been the chicken and egg problem of the cost of Vanadium. Vanadium is not a particularly rare element, but it isn't mined in large quantities due to lack of demand and hence it is quite expensive. A single utility scale redox battery would consume a significant portion of the world's annual Vanadium production. Thus the conundrum, if no one can afford to buy a Vanadium redox battery, you'll never generate enough demand for Vanadium to open up new mines and drive the price down."