Vanadium redox battery

The vanadium redox (and redox flow) battery in its present form (with sulfuric acid electrolytes) was patented by the University of New South Wales in Australia in 1986 ref|Skyllas. It is a type of rechargeable flow battery that employs vanadium redox couples in both half-cells, thereby eliminating the problem of cross contamination by diffusion of ions across the membrane. Although the use of vanadium redox couples in flow batteries had been suggested earlier by Pissoortref|Pissoort, by NASA researchers and by Pellegri and Spaziante in 1978 ref|Pelligri, the first successful demonstration and commercial development was by Maria Skyllas-Kazacos and co-workers at the University of New South Wales in the 1980's ref|Rychik. The Vanadium redox battery exploits the ability of vanadium to exist in solution in 4 different oxidation states, and uses this property to make a battery that has just one electroactive element instead of two.

The main advantages of the vanadium redox battery is that it can offer almost unlimited capacity simply by using larger and larger storage tanks, it can be left completely discharged for long periods with no ill effects, it can be recharged simply by replacing the electrolyte if no power source is available to charge it, and if the electrolytes are accidentally mixed the battery suffers no permanent damage.

The main disadvantages with vanadium redox technology are a relatively poor energy-to-volume ratio, and the system complexity in comparison with standard storage batteries.

Operation

A Vanadium redox battery consists of an assembly of power cells in which the two electrolytes are separated by a proton exchange membrane. Both electrolytes are vanadium based, the electrolyte in the positive half-cells contains VO₂⁺ and VO²⁺ ions, the electrolyte in the negative half-cells, V³⁺ and V²⁺ ions. The electrolytes may be prepared by any of several processes, including electrolytically dissolving vanadium pentoxide (V₂O₅) in sulfuric acid (H₂SO₄). The solution remains strongly acidic in use.

In vanadium flow batteries, both half-cells are additionally connected to storage tanks and pumps so that very large volumes of the electrolytes can be circulated through the cell. This circulation of liquid electrolytes is somewhat cumbersome and does restrict the use of vanadium flow batteries in mobile applications, effectively confining them to large fixed installations, although one company has focused on electric vehicle applications, using rapid replacement of electrolyte to refuel the battery.

When the vanadium battery is charged, the VO²⁺ ions in the positive half-cell are converted to VO₂⁺ ions when electrons are removed from the positive terminal of the battery. Similarly in the negative half-cell, electrons are introduced converting the V³⁺ ions into V²⁺. During discharge this process is reversed and results in a typical open-circuit voltage of 1.41 V at 25 °C.

Other useful properties of Vanadium flow batteries are their very fast response to changing loads and their extremely large overload capacities. Studies by the University of New South Wales have shown that they can achieve a response time of under half a millisecond for a 100% load change, and allowed overloads of as much as 400% for 10 seconds. The response time is mostly limited by the electrical equipment. Round trip efficiency in practical applications is around 65-75%ref|vrbpower.

Generation 2 Vanadium redox batteries (vanadium/polyhalide) may approximately double the energy density and increase the temperature range in which the battery can operate.

Energy density

Current production Vanadium redox batteries achieve an energy density of about 25 Wh/kg of electrolyte. More recent research at UNSW indicates that the use of precipitation inhibitors can increase the density to about 35 Wh/kg, with even higher densities made possible by controlling the electrolyte temperature. This energy density is quite low as compared to other rechargeable battery types, e.g. Lead-acid (30-40 Wh/kg) and Lithium Ion (80-200 Wh/kg).

Applications

The extremely large capacities possible from vanadium redox batteries make them well suited to use in large power storage applications such as helping to average out the production of highly variable generation sources such as wind or solar power, or to help generators cope with large surges in demand.

Their extremely rapid response times also make them superbly well suited to UPS type applications, where they can be used to replace Lead-acid batteries and even diesel generators.

Installations

Currently installed vanadium batteries include:

* A 1.5MW UPS system in a semiconductor fabrication plant in Japan

* A 275 kW output balancer in use on a wind power project in the Tomari Wind Hills of Hokkaido

* A 200 kW, 800kWh output leveler in use at the Huxley Hill Wind Farm on King Island, Tasmania

* A 250 kW, 2MWh load leveler in use at Castle Valley, Utah

* A 12 MWh flow battery is also to be installed at the Sorne Hill wind farm, Donegal, Ireland [http://www.leonardo-energy.org/drupal/node/959] .

See also

*Battery (electricity)
*Flow Battery
*Lead-acid battery
*Electrochemical cell
*Fuel cell
*Energy storage

References

#Note|Skyllas M. Skyllas-Kazacos, M. Rychcik and R. Robins, in AU Patent 575247 (1986), to Unisearch Ltd.
#Note|Pissoort P. A. Pissoort, in FR Patent 754065 (1933)
#Note|Pelligri A. Pelligri and P. M. Spaziante, in GB Patent 2030349 (1978), to Oronzio de Nori Impianti Elettrochimici S.p.A.
#Note|Rychik M. Rychcik and M. Skyllas-Kazacos, J. Power Sources, 22 (1988) 59-67
#Note|vrbpower VRB Power Systems FAQ [http://www.vrbpower.com/technology/faqs.html]

Additional references

* [http://www.electricitystorage.org/pubs/2001/IEEE_PES_Summer2001/Miyake.pdf Presentation paper from the IEEE summer 2001 conference]
* [http://www.ceic.unsw.edu.au/centers/vrb/ UNSW Site on Vanadium batteries]
* [http://www.worldenergy.org/wec-geis/congress/papers/SkyllasKazacosm0904.pdf Report by World Energy]

External links

* [http://www.vrb.unsw.edu.au VRB at UNSW]
* [http://everything2.com/index.pl?node_id=1291494 VRB at everything2]
* [http://www.cellstrom.at/index.php?id=63&L=1 History of the VRB]