Redox flow batteries for EV's offer near-instant recharging

[MitchJi]

Hi,

Flow batteries are normally relatively cheap, big and heavy. If they have the big and heavy under control...

No BMS. The same charged fluid gets pumped through all the cells.

Redox flow battery electric test model

http://www.greencarcongress.com/2009/10/redox.html

Fraunhofer ICT Working on New Redox Flow Batteries with Improved Capacity for EV Applications
15 October 2009
Redoxflow

Schematic representation of the processes in a redox-flow system

Researchers from the Fraunhofer Institute for Chemical Technology ICT in Pfinztal near Karlsruhe are developing improved redox flow batteries for automotive applications in an attempt to address storage capacity and charging time limitations of current Li-ion battery solutions for electric vehicles.

Redox flow batteries (RFB) are chemical energy storage devices, and use dissolved redox couples (an oxidizing agent and a reducing agent in an electron transfer process) held in separate external tanks; electricity is converted in a separate power module. The 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.

During discharge the electrodes are continually supplied with the dissolved substances from the tanks; once they are converted the resulting product is removed to the same or another tank. The devices could theoretically be recharged at a station in a few minutes. “The discharged electrolyte is simply pumped out and replaced with recharged fluid. The pumped-off electrolyte can be recharged at the gas station, for example, using a wind turbine or solar plant,” says engineer Jens Noack from ICT.

A number of different redox couples are possible; Fraunhofer has been doing some detailed work with conventional vanadium redox flow batteries for a number of years. For the new system, Noack says, “We are using different redox couples than in well-known redox flow batteries, but at this moment we can’t say what it is in detail.” Noack presented Fraunhofer’s work on conventional vanadium redox flow batteries at the just-completed 216th meeting of the Electrochemical Society in Vienna, Austria.

Redox flow batteries theoretically offer a number of advantages:
* high energy efficiency >75 % (> 95% found on lab scale)
* long calendar life, excellent cycle ability (> 10,000)
* flexible design
* fast response time
* overcharge and over discharge tolerant
* low maintenance costs
* low self discharge or no discharge depending on pumping of electrolyte

Up to 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 distance—around 25 kilometers (15 miles)—which means the driver would have to recharge the batteries four times as often.

The Fraunhofer team has increased the capacity four or fivefold, to approximately that of lithium-ion batteries, according to Noack. The team has already produced the prototype of a cell, and is in the process of assembling several cells into a larger unit and optimizing them.

This work is being carried out with colleagues from the University of Applied Sciences, Ostphalia, in Wolfenbüttel and Braunschweig. They are testing electric drives and energy storage units on model vehicles that are only a tenth of the size of normal vehicles. The research team has already built a traditional redox flow battery into a model vehicle. A vehicle on a scale of 1:5 is on display on a test rig set up at the eCarTech in Munich (Hall C3, Stand 424) from 13 to 15 October.

In the coming year the researchers also want to integrate the new battery, with four times greater mileage, into a model vehicle.

More information on Flow Batteries here:
http://en.wikipedia.org/wiki/Flow_battery

Flow battery
A flow battery is a form of rechargeable battery in which electrolyte containing one or more dissolved electroactive species flows through an electrochemical cell that converts chemical energy directly to electricity. Additional electrolyte is stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the reactor, although gravity feed systems are also known.[1] Flow batteries can be rapidly "recharged" by replacing the electrolyte liquid (in a similar way to refilling fuel tanks for internal combustion engines) while simultaneously recovering the spent material for re-energization...

Classes of flow batteries

Various classes of flow batteries exist including the redox (reduction-oxidation) flow battery, in which all electroactive components are dissolved in the electrolyte. If one or more electroactive component is deposited as a solid layer the system is known as a hybrid flow battery.[2] The main difference between these two types of flow battery is that the energy of the redox flow battery can be determined fully independently of the battery power, because the energy is related to the electrolyte volume (tank size) and the power to the reactor size.

Advantages and disadvantages

Redox flow batteries, and to a lesser extent hybrid flow batteries, have the advantages of flexible layout (due to separation of the power and energy components), long cycle life (because there are no solid-solid phase changes), quick response times (in common with nearly all batteries), no need for "equalisation" charging and no harmful emissions (in common with nearly all batteries). Some types also offer easy state-of-charge determination (through voltage dependence on charge), low maintenance and tolerance to overcharge/ overdischarge.

On the negative side, flow batteries are rather complicated in comparison with standard batteries as they may require pumps, sensors, control units and secondary containment vessels. The energy densities vary considerably but are, in general, rather low compared to portable batteries, such as the Li-ion.

 

[spinningmagnets]

University of New South Wales (Australia) pioneered a lot of work with these.

"...The golf-cart can be fully charged in around 20 minutes by simply connecting a battery charger to the vehicle. However, one of the main advantages of the vanadium battery is that it can be instantly recharged by simply replacing the discharged solutions with recharged solutions. The solutions are never wasted since they have an indefinite life. They are simply recharged and used again for an indefinite number of cycles..."

http://www.ceic.unsw.edu.au/centers/vrb/index.htm

Also heres a link with some cost info. Apparently its about $600 per one-kWH in a small home-sized system, but since scaling up only means increasing the size of the fluid storage tanks, some of the very large kWH systems are closer to $100/kWH.

http://www.seattleeva.org/smf/index.php?topic=212.0;msg=691