Unlike Lithium, Sodium Ion and other solid-form batteries, Redox (Reduced Oxygen) Flow batteries employ a liquid electrolyte to store the electric charge. Ions are exchanged across the membrane separator (or exchanger) whilst two liquids are pumped around their next-together doughnut shaped chambers.


The latest generation of Vanadium Redox Flow batteries might be regarded by some today as ‘state of the art’. However, the Flow battery itself has a long-established history and, importantly, a track record of safe and reliable performance. Pioneered in the 1970s at Harwell Research Laboratories, the first Flow batteries were first deployed for NASA moon space missions, although they are now embedded within larger-scale power plants and transmission grids. Today, Flow batteries are increasingly seen as the energy storage system of choice for wind power & solar PV grids; as well as for resilience projects and emergency power supply applications.

Whilst demand for this kind of battery has increased, the renaissance of the Flow battery itself was seeded in a technological breakthrough: the use of Vanadium as the electrolyte instead of other metallic-acid compounds. The discovery was being able to use exactly the same (Vanadium) compound in both chambers for the first time, only at two different states of oxidisation. Previous Flow battery designs used two chemically different (typically Bromide) compounds, with one in each chamber. This solved a long-standing cross-contamination problem associated with two chemically-different liquids, which could seep into the adjacent chamber through the separator, impairing performance and reliability. Vanadium electrolyte has been shown to resolve the problem and Vanadium Redox Flow batteries today can offer endurance and reliability virtually unmatched by any solid-form battery.

As Vanadium has a lower energy density than Lithium, a typical containerised Flow battery will occupy a larger footprint than its Lithium ion counterpart. However. as Flow batteries are not prone to overheating like Lithium ion units, their containers can be stacked on top of each other up to three decks high, in order to partially offset this disadvantage, although the final footprint is still larger.

As discussed below, Flow batteries can offer the end-user cost savings, trading and flexibility advantages. Also, they can offer improved business resilience for renewables and emergency back-up projects which need longer-duration storage, and prices in the demand side response auction and private-wire markets with utilities are already beginning to reflect this.

Flexibility Benefits

  • Flow battery cells do not degrade as fast and round-trip efficiency (the energy loss every charge or discharge) is roughly constant at circa 75%. Lithium cells on the other hand can deteriorate rapidly, reducing efficiency over time, although their starting-point efficiency is significantly higher, at circa 85% +.
  • Flow batteries are more flexible in that they can charge up and discharge to any 0% to 100% State of Charge (SoC) level, at any time and any number of times without damaging the battery. Thus, almost all 100% of the ‘boilerplate rated’ MWh storage capacity purchased can be used. This contrasts with many Lithium ion batteries, which cannot safely self-interrupt or reverse-flow mid cycle. The charging or discharging is often governed by strict SoC parameters set by the manufacturer, typically 20% to 80% lest it damage the battery cell or invalidate the warranty. Consequently, Flow batteries may be the better choice for rental or short-term leasing operators.
  • Lithium batteries will drain power and run flat if left unused over time, wheras Flow batteries will always hold their charge and can cold start at any time, even if unused for days, months, years even on end. This makes Flow batteries especially suited for business critical, isolated wind and solar facilities, as well as emergency back up facilities.
  • Flow batteries are modular so that, crucially, the storage (kWh) and power (kW) components can be separated. It is possible to increase a Flow battery’s storage capacity retrospectively, simply by adding new tanks. Likewise, power rating may be upgraded by fitting extra cell-stacks. This ‘retro-fit flexibility’ can project risk and configuration costs and in many cases avoid wastage and further cut capital expenditure at the outset. In theory, a Flow battery site can store an unlimited reserve of electricity.

Economic Benefits

  • In some instances, a Flow battery can work out significantly cheaper than a Lithium Ion battery after capital costs, operating costs, depreciation losses, and warranty and battery disposal are all taken into account. Although the initial CAPEX cost for a Flow battery will be higher.
  • For example, OPEX may be lower as Flow batteries are more robust and relatively ‘maintenance free’. They may not need such extensive monitoring, ‘hands on’ management or elaborate ‘fire protection’ systems.
  • Flow batteries are longer-lasting. Built to last 25 years with option to extend life to 45. Lithium ion batteries have a shorter or more variable lifespan, perhaps 8 – 12 years. Or possible less, subject to usage and degradation. This is one reason why Flow batteries have lower capital depreciation costs.
  • This 25 year lifespan will often match the intended project life of the micro-grid asset which the Flow battery used to optimise. This natural investment match can lead to more accurate and potentially higher NPV valuations for a project overall.

Environment, Fire Risk and Business Resilience Considerations

  • As well as Lithium itself, such batteries typically entail the mining and refining of Cobalt, Neodymium, Terbium, Lanthanum and Dysprosium – most classified as scarce ‘rare earth’ metals which are becoming ever more costly in human as well as financial terms. Vanadium, on the other hand, is a relatively cheap, produced in many different countries by large mining corporations.
  • Lithium is unstable. Although very rarely, Lithium ion batteries have a documented history of catching fire, invariably followed by an explosion. Should they over-heat (possibly due to an impurity causing a short circuit, especially if near one of the terminals) then thermal runaway can taka hold quickly. Any resulting fire can be very destructive and can burn for days. Elaborate fire prevention systems may be built into the battery container but no ‘fire prevention’ system is true to its word. Furthermore, once the system kicks in, thebattery down, cutting the power and undermining the resilience it was supposed to provide. In all cases, such risks may affect insurance premiums and underwriters will certainly need informing either way lest cover be voided after any type of battery is installed.
  • Vanadium is a safe and benign metal. Dissolved in dilute sulphuric acid, this liquid electrolyte has a PH value akin to that of a standard car battery. Although it exists the fire risk is very much smaller and the risk of runaway or an explosion is zero. Another reason why Flow batteries may be more suitable to micro-grid, emergency generation or rental companies. Especially micro-grid project developers in very hot or humid countries, military installations and solar arrays situated out in the dessert where the chance and financial consequences of a fire may be high.
  • Just like there is no such thing as a true ‘fire prevention system’, there is no such thing as a ‘recyclable lithium battery’ either. Spent Lithium remains toxic. It is also expensive to dispose of. Financial models need to consider this extra cost vs. Flow or other batteries although not all of them do. The Vanadium on the other hand can truly be recycled (to the tune of 99%): the electrolyte can be returned for use in the same battery from whence it came, over and over again losing 1% each 5 or so years the liquid needs replenishing. And once the 25 or 45 year life is up, the waste disposal cost for a Flow battery is significantly less.


Flow batteries are not optimal for every application, and are significantly more expensive to buy than most Lithium ion batteries. However, once lifetime running, servicing, warranty and fuel cell (stack) replacement costs are taken into account, Flow batteries can work out cheaper and cost almost half the levelised-lifetime cost of the equivalent Lithium ion system, where the battery is cycled twice a day or, as is often the case with wind, solar PV and especially network projects, more frequently still.

About the Author:

Prospect Law is a multi-disciplinary practice with specialist expertise in the energy and environmental sectors with particular experience in the low carbon energy sector. The firm is made up of lawyers, engineers, surveyors and finance experts.

This article remains the copyright property of Prospect Law and Prospect Advisory and neither the article nor any part of it may be published or copied without the prior written permission of the directors of Prospect Law and Prospect Advisory.

Prices quoted are indicative and may be based on approximate or readjusted prices, indices or mean levels discussed in the market. No warranty is given to the accuracy of any view, statement or price information made here which readers must verify.

Dominic Whittome is an economist with 25 years of commercial experience in oil & gas exploration, power generation, business development and supply & trading. Dominic has served as an analyst, contract negotiator and Head of Trading with four energy majors (Statoil, Mobil, ENI and EDF). As a consultant, Dominic has also advised government clients (including the UK Treasury, Met Office and Consumer Focus) and private entities on a range of energy origination, strategy and trading issues.

For more information or assistance with a particular query, please in the first instance contact Adam Mikula on 020 7947 5354 or by email on [email protected].

  1. Energy Storage

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