Flow batteries

Monday, September 27, 2021
Veli Markovic

Go with the flow!

Lithium batteries are quite commonplace now but what about flow batteries? In this presentation we look at different chemistries, pros and cons of this method of energy storage and some exciting future developments.  

What is a flow battery?  

A flow battery is an electrical storage device that is a cross between a conventional battery and a fuel cell.

Liquid electrolyte of metallic salts is pumped through a core that consists of a positive and negative electrode, separated by a membrane.

The ion exchange that occurs between the cathode and anode generates electricity.

The liquid electrolyte is stored in external tanks.

Flow battery cycle life  

Flow batteries can: 

  • Cycle more often
  • To a greater depth of discharge at 100%
  • With no negative effects on the batteries performance life
  • Flow batteries can store power for 4 hours or more
  • Last for decades ( up 25 years before needing replacement) 

What is the electrolyte made of?  

Most large commercial flow batteries use:

  • Acid sulfur with vanadium salt as electrolyte
  • The electrodes are made of graphite bipolar plates. 
  • Vanadium is one of few available active materials that keeps corrosion under control. 
  • Flow batteries have been tried that contain precious metal, such as platinum, which is also used in fuels cells. 
  • Research is continuing to find materials that are low cost and readily available.

How well do they perform?  

Flow batteries perform best when:

  • When they are above 20 kWh (Vanadium)
  • Can deliver more than 10,000 full cycles
  • Are good for about 20 years. 

Cell voltages and energy density  

Each cell produces 1.15–1.55 volts; 

  • They are connected in series to achieve the desired voltage levels
  • The battery has a specific energy of about 40Wh/kg, which resembles lead acid
  • This makes the battery best suited for bulk energy storage

Precious element

Vanadium is the 23rd element on the periodic table and is used in the manufacturing of flow batteries:

  • Currently mined in China, Russia and South Africa
  • Nevada may soon become a contributor in the form of heavily oxidized crumbled rock
  • Currently, 90 percent of lower grade vanadium is used as an additive to strengthen steel
  • Battery scientists, mining companies and politicians are excited about vanadium becoming a strategic metal for “green energy"

Flow battery chemistries  

Manufacturers offer a variety of flow battery chemistry with different cells been developed such:

  • As redox, hybrid and membraneless 
  • Redox battery store energy in the liquid at all times 
  • Hybrid flow battery store at least some energy in solid metal during charge 
  • In a membraneless flow battery, liquid self separates in one tank 

As a rule and relying upon the science, flow batteries will in general be less reactive, simple to dispose of and are not inclined to overheating.


Brisbane based company redflow has developed the world's smallest zinc-bromine flow battery Zcell in commercial production with:

  • Operating temperature of between 10 degrees - 50 degrees
  • If temp falls outside this temperature range the system shuts off
  • Need to have thermal system to maintain electrolyte at above 10 degrees
  • Can be discharged 100 %
  • Can stay in a partial state of discharge for ever

How it works


Each battery contains about 100 litres of water-based zinc bromide salt solution that: 

  • Circulates in two separate hydraulic circuits. 
  • When charging, zinc is extracted from the zinc bromide solution and stored on a plastic membrane. 
  • During discharge, zinc is released back to the solution. 
  • Benefit of this process, causes no degradation to the battery when zinc is removed at end of each cycle,
  • Giving it a sustained, long-life energy storage capacity

Advantages of a zinc bromine 


With these batteries:

  • There is no fire risk
  • The bromide-based electrolyte is non-flammable
  • Mechanical damage does not risk explosion or ‘thermal runaway’
  • Batteries offer almost unlimited shelf life, 
  • Made from components that are easy to recycle or reuse 

Flow batteries: Comparison with Vanadium   

Vanadium batteries:

  • Vanadium systems are much larger and provide a lower energy density
  • This means have a much larger physical size,
  • Potential issues with  floor loading, weight
  • Logistics
  • Installation issues

What’s on the horizon   

Harvard researchers in 2017 developed a new flow battery:

  • It stores energy in organic molecules dissolved in neutral pH water
  • The molecules have been restructured, making them more water soluble
  • Has resulted in a battery that loses only 1% of its capacity every 1000 Cycles



As the energy storage wars become more heated the installer, designer and consumer must understand the various pros and cons of the different battery chemistries out there/

In the case of flow batteries the advantages included:

  • Flow batteries use a liquid electrolyte
  • They don’t mind being in a partial state of discharge
  • Use non flammable materials
  • Effectively the systems are totally recyclable

The only real disadvantage is the fact that these systems are less energy dense than their lithium counterparts and are also comparatively expensive but if safety is top of the list in regards to your selection criteria for an energy storage system flow batteries may be the answer.

If you’d like to see what Greenwood Solutions get up to in the real world of renewable energy, solar, battery storage and grid protection check out our industry and commercial pages:





About the author

Veli Markovic

CEC Designer
Veli has nearly two decades of experience in the renewable industry. He is passionate about providing people with valuable education and is highly regarded throughout the industry as an educator and operator.
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