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Redox Flow what it is and what it is not!

July 13, 2017




Vanadium Redox looks more like a Chemical Plant than like a battery but does that matter? Chemical storage tanks, pumps, pipes, exhausts, heating, cooling and Fuel Cell reactors sounds more like a Chemical plant to the traditional battery technologist. However a more careful look at the fact that there are electrodes, electrolytes,  lectro-active materials, ionic transport, and  most importantly chemical energy being converted into electrical energy on discharge, and vice versa on charge, shouts loud and clear that this is a battery despite its appearance.  The Bigger and more important question: Is Vanadium Redox going to make it as an Energy Storage System and that question is the real focus of this article. A topic that will as ever be presented both inimitably and candidly. Please enjoy!


A Beautiful Electrochemical System


The picture that introduces this article is of Vanadium Pentoxide crystals. Are they not truly beautiful? As are the colors of the four Vanadium oxidation states that appear during the Vanadium Redox Flow battery cycling. I did some work on Vanadium Redox back in 2009 and at the time I remember being impressed with the beauty of its operation as an electrochemical storage device. Electrochemically the V(2)/V(3) - V(4)/V(5) couple has a simplicity that many other battery chemistries would love to emulate. However while the Chemistry may be simple, the balance of plant that is required to operate the system at all, let alone to run it for over 20 years, is very complex. With a claim of 10,000 cycles, at 1 cycle/day this stated benefit can only be appreciated if the battery runs for 27 years. Operating for twenty years may not be a problem for the Vanadium but it will for sure be a challenge for the Balance of Plant. Pumps, valves, seals, pipes, system control electronics, heat exchangers etc will be challenged to last 20 years, especially in such a corrosive environment. When considering the total cost of ownership this has to be the number a major consideration in terms of ongoing costs and warranties & maintenance contracts should allow for this. 


I have been impressed with the packaging of Vanadium Redox into ISO containers, for example as described by​ UET in Seattle and RedT in the UK. Accommodation in a 20' ISO container of between 300 and 450kWh is more than I would have expected and puts Vanadium Redox ahead of many other battery options although not mainstream Lithium Ion. The modular approach also makes sense and the separation of Power versus Energy, a key selling feature of any redox system, is presented very well by this kind of approach. 



Vanadium Redox is still relatively new but there are now dozens of demonstrations of the system in operation around the World and the true beauty of this system is beginning to be appreciated. High cycle life and fast response time have been known since the University of New South Wales announced their work in the 1980's. Moreover, improvements to Energy Density, parasitic losses,  BOP reliability and of course the cost will continue to increase this system's attractiveness. However this said the system has a few inherent if not fundamental weaknesses and these need to be fully understood before charging ahead with too much confidence. For just like any system Vanadium Redox has some negatives that have to be considered within the overall trade off situation when deciding to go with it versus alternatives. 


Vanadium Supply Chain


First of all consider the supply chain. Vanadium, a transition metal, is at best a minor metal and although not rare only 76,000 metric tons are produced each year. To give this some context this is 3 x the amount of Silver produced, 2/3 the amount of Cobalt, 1/30th the amount of Nickel and 1/150 the amount of Zinc. Also consider that each kWh of Vanadium Redox Energy Storage needs 1.6Kg of Vanadium and if Vanadium Redox was to grow to 7,600 metric tones or 1/10 of todays's production output that is only enough Vanadium for 4.75GWh of Energy Storage. Today 85% of Vanadium is used in Ferro-alloys but it is not difficult to see that if Vanadium for redox flow started taking noticeable amount of the WW supply that price increases would have to follow. This has been the case for all battery commodities as values have increased to significant levels and Vanadium would be no different. At today's price of $23/Kg the effective price for the Vanadium commodity in each Vanadium redox battery is $37/kWh but if prices were to increase again to 2008 levels this would be more like $175/kWh.


As with all materials the country source must be considered,and  both, political stability and confidence in the trade agreements between producer and user countries must be factored into the consideration. With 75% of the World's Vanadium coming from China and Russia this might not be the best situation. 


Round Trip Efficiency 


The second factor to consider when it comes to Vanadium redox is its overall round trip efficiency. This is  often not included as a factor, at least up front, in a decision to procure Energy Storage capacity or not honestly included in the overall cost analyses.  Despite claims that suggest that a Vanadium Rexox system has an RTE greater than 80%, it is actually difficult to seriously expect a Vanadium Redox flow battery to have a Wh RTE better than 70 or 75%. This is a product of Coulombic efficiency at 80-90% and Voltaic efficiency at 80-85% and while there is opportunity for small improvements getting to 80% RTE will be a significant challenge. But what does this mean? Well, to compare Vanadium Redox against other battery systems, like Lithium Ion that boasts a Wh RTE of 90 to 95%, either more power generating capital is required for the Vanadium Redox, and/or there will be a higher cost of the electricity needed per charge cycle.


Consider as an example the following: 


Possible Capital cost penalty when choosing Vanadium Redox


If 100kWh of Lithium Ion Energy storage needs 25kW of solar then at $2/W for everything, excluding the battery≤ the capital cost is $50,000.


But for the same Energy output Vanadium Redox is going to need 31kW of solar that is going to cost $12,000 more or add effectively $120/kWh to the cost of the Energy Storage. 




Possible Operating cost penalty when choosing Vanadium Redox.


Electricity costs vary significantly by country or state, i.e. from $0.05/kWh to $0.45/kWh. Clearly then the RTE efficiency hit and more importantly the receptiveness to Vanadium Redox is going to be strongly influenced by location. Taking a base electricity cost of $0.10/kWh and the same size of Energy Storage system as in the Capital Cost example the analysis unfolds as follows:


The effective price of the electricity for LiIon with an RTE of 95% and inverter efficiency of 95% is, $0.118/kWh 

The effective price of the electricity for Vanadium Redox with an RTE of 75% and an inverter efficiency of 95% is $0.140 or 18.6% higher.

For a 100kWh. system cycling every day for 20 years the Lithium Ion Electricity cost is $86,140

For a 100kWh system cycling every day for 20 years the Vanadium Redox flow Electricity cost is $102,000 or $16,000 higher. 




In addition to the cost of the electricity in this analysis there will need to be increases in inverters costs because to get out 100kWh, 120kWh is going to have to be put in. This probably though is only a few thousand dollars.


Other Flow Batteries

As with everything strict definitions can get fuzzy very quickly. The A123 spin-off 24M, was looking at a Lithium Ion Flow battery. Flow but not redox. Zinc Bromine is flow but only the Bromine part of the reaction is Redox. Several Redox couple have been investigated including Iron-Chromium, Zinc/Chorine Hydrate and the always too expensive Cerium III/Cerium IV. None to date have come close to where Vanadium has arrived. One has to believe that both Zinc bromine and Iron Chromium have a chance in the future as neither has the Supply Chain challenges inherent in the Vanadium System. Solving for the stability issues in these other potential solutions will produce the necessary improvements with time and financial investment and by that time the Vanadium Redox flow Energy Storage may have set an attractive stage for Flow batteries to succeed. 



  • Vanadium Redox flow is a battery but can look like a Chemical Plant. 

  • Vanadium redox is a beautiful system with outstanding cycle life and power response time. Energy Density is better than people may believe!

  • Vanadium Supply Chain analysis indicates that there are some significant risks and these should receive every attention by anyone wanting to be involved in this technology.

  • Round Trip efficiency, oft ignored, should not be passed over when considering Vanadium Redox as an Energy Storage resource. Additional capital and/or operating expense will be required for Vanadium Redox flow versus many other systems.

  • Lithium Ion and Lead Acid are the incumbents in the Energy Storage market and there is no obvious 3rd player. Vanadium Redox clearly has the potential to own this third spot and if volumes grow as forecast the baton could be passed onto another Flow Battery system once developed.



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