Vanadium redox battery

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Vanadium redox battery
Specific energy 10–20 Wh/kg (36–72 J/g)
Energy density 15–25 Wh/L (54–65 kJ/L)
Charge/discharge efficiency 75-80%<.[1]
Time durability 10–20 years
Cycle durability >10,000 cycles
Nominal cell voltage 1.15–1.55 V

The vanadium redox (and redox flow) battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy. The present form (with sulfuric acid electrolytes) was patented by the University of New South Wales in Australia in 1986.[2] An earlier German Patent on a titanium chloride flow battery was registered and granted in July 1954 to Dr. Walter Kangro, but most of the development of flow batteries was carried out by NASA researchers in the 1970s. Although the use of vanadium in batteries had been suggested earlier by Pissoort,[3] by NASA researchers and by Pellegri and Spaziante in 1978,[4] the first known successful demonstration and commercial development of the all-vanadium redox flow battery employing vanadium in a solution of sulfuric acid in each half was by Maria Skyllas-Kazacos and co-workers at the University of New South Wales in the 1980s.[5]

There are currently a number of suppliers and developers of these battery systems including UniEnergy Technologies and Ashlawn Energy in the United States, Renewable Energy Dynamics (RED-T) in Ireland, Gildemeister AG (formerly Cellstrom GmbH in Austria) in Germany, Cellennium in Thailand, Prudent Energy in China,[6][7][8] Sumitomo in Japan and H2, Inc. in South Korea.[9] The vanadium redox battery (VRB) is the product of over 25 years of research, development, testing and evaluation in Australia, Europe, North America and elsewhere.

The vanadium redox battery exploits the ability of vanadium to exist in solution in four 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 are 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.

Diagram of a Vanadium Flow Battery

Operation[edit]

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.[10]

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. Sulfuric acid-based vanadium batteries only work between about 10 to 40 °C. Below that temperature range, the ion-infused sulfuric acid crystallizes.[11] Round trip efficiency in practical applications is around 65-75%.[12] Second generation vanadium redox batteries (vanadium/bromine) may approximately double the energy density and increase the temperature range in which the battery can operate.

Energy density[edit]

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).

Researchers at the Fraunhofer Institute for Chemical Technology claim to have built a prototype for an improved cell stating “We can now increase the mileage four or fivefold, to approximately that of lithium-ion batteries,”.[13] This same article makes a very optimistic claim that vanadium flow batteries can be recharged at a gas station by simply filling the vehicle tanks with fresh electrolytes.

There are at least two problems with this claim. First, the electrolytes used in flow batteries are dangerously acidic. Pouring acids is a very different exercise from pouring gasoline and is much more dangerous. Face shields and other protective gear is necessary for handling strongly acidic solutions. Second, any electrolyte still in the vehicle will need to be drained into a recycling system. This means that self-service will not be a possibility unless and until the fluid transfer processes can be fully automated. Until the processes are fully automated, it will be necessary for carefully trained attendants to do this job for the station's customers. This is something that is unlikely to be done by existing fueling facilities with the level of training given to existing fuel station personnel. There are at least as many liability issues with this plan as there would be with a gaseous fuel such as hydrogen or methane, but this technology does hold out the promise that the necessity for burning any kind of fuel in an automobile might be eliminated in the foreseeable future.[verification needed]

Applications[edit]

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.

The limited self-discharge characteristics of vanadium redox batteries make them useful in applications where the batteries must be stored for long periods of time with little maintenance while maintaining a ready state. This has led to their adoption in some military electronics, such as the sensor components of the GATOR mine system.

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[edit]

Currently installed vanadium batteries include:

  • A 1.5 MW UPS system in a semiconductor fabrication plant in Japan.
  • A 600 kW, Six hour system, installed by Prudent Energy in Oxnard, California, USA.
  • A 275 kW output balancer in use on a wind power project in the Tomari Wind Hills of Hokkaido.
  • A 200 kW, 800 kW·h (2.9 GJ) output leveler in use at the Huxley Hill Wind Farm on King Island, Tasmania.
  • A 250 kW, 2 MW·h (7.2 GJ) load leveler in use at Castle Valley, Utah.
  • Two 5-kW units installed in St. Petersburg, Florida, under the auspices of USF's Power Center for Utility Explorations.
  • A 100 kWh unit supplied with 18 kW stacks manufactured by Cellstrom (Austria) has been installed in Vierakker (Gelderland, The Netherlands) as part of an integrated energy concept called 'FotonenBoer'/'PhotonFarmer' (InnovationNetwork/Foundation Courage)
  • A 400 kW, 500 kWh output balancer in use on a solar power project in the Bilacenge Village in Sumba Island, Indonesia.
  • A 5 kW unit integrated with photovoltaic generation at University of Évora, Portugal.[14]
  • A 100 kW installation is planned for the island of Gigha, Scotland.[15]

See also[edit]

References[edit]

Additional references[edit]

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