BU-210b: How does the Flow Battery Work?

Discover what’s new in immerging storage batteries

A flow battery is an electrical storage device that is a cross between a conventional battery and a fuel cell. (See BU-210: How does the Fuel Cell Work?) 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.

Most 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.

Activated by pumps, flow batteries perform best at a size above 20kWh. They are said to deliver more than 10,000 full cycles and are good for about 20 years. 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. Similar to the fuel cell, the power density and ramp-up speed is moderate. This makes the battery best suited for bulk energy storage; less for electric powertrains and load leveling that requires quick action.

The electrolyte is stored in tanks. To increase the energy density, the tank sizes can be doubled using ready-made storage tanks at an estimated cost increase of only 50 percent compared to a new system. When replacing the battery, the electrolyte can be reused, further saving cost. Problem areas are the membranes that tend to corrode and are expensive; additives are said to solve this issue. Figure 1 illustrates the flow battery concept.

Figure 1: Flow Battery.
Electrolyte is stored in tanks and pumped through the core to generate electricity; charging is the process in reverse. The volume of electrolyte governs battery capacity.

Vanadium is the 23rd element on the periodic table and is mined in China, Russia and South Africa.  Sun-backed central 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.” According to RWTH, Aachen, Germany (2018), the cost of the flow battery is about $350 per kWh.

For a more precise cost estimation, the flow battery is divided into power cost and energy cost. The power cost can go above $1,500/kW and consists of stacks, pumps, pipes and power electronics. The energy cost consisting of tanks and electrolyte comes in at a bit more than $300/kWh.

Large scale flow batteries exceeding 100kWh have been in use in Japan since 1996. Some of the biggest current installations boast a capability of several megawatts and a flow battery for frequency regulation is being installed in Japan that will deliver a whooping 60MWh.

There is a move towards cost and size reduction. Rather than building a monster battery resembling a chemical plant, newer systems come in container-sizes of typically 250kWh that can be stacked. Modern flow batteries are also becoming common in Europe.

The first patent for a titanium chloride flow battery was granted in July 1954. The present day vanadium redox battery was patented in 1986 by the University of New South Wales in Australia. The term “redox” comes from “electron transfer” of reduction and oxidation.

Last updated 2018-05-31

*** Please Read Regarding Comments ***

Comments are intended for "commenting," an open discussion amongst site visitors. Battery University monitors the comments and understands the importance of expressing perspectives and opinions in a shared forum. However, all communication must be done with the use of appropriate language and the avoidance of spam and discrimination.

If you have a suggestion or would like to report an error, please use the "contact us" form or email us at: BatteryU@cadex.com.  We like to hear from you but we cannot answer all inquiries. We recommend posting your question in the comment sections for the Battery University Group (BUG) to share.

Or Jump To A Different Article

Basics You Should Know
The Battery and You
Batteries as Power Source

Comments (12)

On April 29, 2015 at 2:43am
Harry Winter wrote:

Another type of flow battery is the Zinc Bromine Battery. The company who developed it to commercial avaliability, Redflow, has them currently for sale. 5kW, 11kWh, with 22MWh total energy throughput. And two of them fit on a standard pallet!!

On July 13, 2015 at 11:17pm
Peter wrote:

In the 2nd paragraph you say “Most commercial flow batteries use vanadium as electrodes”. Correct would be: Most commercial flow batteries use acid sulfur with vanadium salt as electrolyte. The elctrodes are made of graphit bipolar plates. The surface of the bipolar plates is increased by adding a graphit felt layer which is perfused by the fluent electrolyte.

On July 13, 2015 at 11:25pm
Peter wrote:

...Whats more:

- the vanadium electrolyte is very corrosive, eroding is still a problem (you say it’s not in the 2nd paragraph)
- flow batteries do have a low power density, but fuel cells have a pretty high power density. Fuel cells are also suited well for power trains, flow batteries not (i don’t trust nano flows concept)

On April 4, 2016 at 8:59am
David wrote:

Good catches, Peter. I almost quit reading for all the errors.

“Acid sulfur”: Sulfuric acid? Polysulfuric acid? Peroxymonosulfuric acid?? Others?

“Erosion can be kept under control, but erosion still exists” chemical deterioration of metals is referred to as corrosion.

Various grammatical errors but informative, none the less.

On June 12, 2016 at 4:45am
Peter wrote:

careful about costs, please (5th paragraph):
for flow batteries, costs have to be divided into power and energy costs. energy costs is the costs for tanks and electrolyte and equals the kWh of the system. Power costs are costs for the stacks, the power electronics, the pumps, pipes and peripherie.
Energy costs are about 300 EUR/kWh, but only with that you won’t have a battery. Power costs are about 1500 EUR/kW and is the more expensive part, making the system usuable for high energy systems that need many cycles.

On August 8, 2016 at 7:07pm
Harry C wrote:

You make great points however I am in need of a self substaining power generation plant. Please help me.

On August 10, 2016 at 5:08am
Peter wrote:

then, tell me a contact

On August 14, 2016 at 3:25am
Brian wrote:

I am very interested in making a flow battery is there any good diagrams on exactly how to do this

On July 4, 2017 at 6:19am
Arthur Kovalev wrote:

This is the most promising type of energy store if you use Ionic liquid

On July 4, 2017 at 6:31am
Arthur Kovalev wrote:


On January 23, 2018 at 5:43pm
Gordon wrote:

Hello knowledgeable people in the RFB world,

I am in my second year of the Northern Alberta’s Institute of Technology (NAIT) alternative energy technology two-year certificate program.  A class mate and I have been tasked with exploring the possibilities of having a redox flow battery in ATCO’s salt caverns at their salt cavern facility as our final project (capstone project). 
We are getting to the point where we require some good information from any research that has already been done in this field.  It is my understanding that the German company EWE had collaborated with Friedrich Schiller University, in Jena, Germany, and has built a bench scale model of such a battery with intentions to scale to salt cavern size in the future.
We would be excited to start a conversation with anyone to help us understand what is needed to support a battery of this magnitude infrastructurally, chemically, and mechanically.

Any other connections or potential sources to further the success of this project would be greatly appreciated. 

On February 27, 2018 at 2:10pm
derek visser wrote:

Hi Gordon!
i read about EWE, very interesting, did you know that Aquion Energy is re introducing their Mk2 salt water batteries in the near future!
Their biz was bought out by Chinese industry that is selling the technology to large clean energy energy producers like wind and solar, and they offer a high capacity energy conversion head unit where you supply your own reservoir, or a complete battery pack with smaller output.
more here, http://aquionenergy.com/homeowners/solar-battery/
pretty remarkable qualities, exerpt; YOU DISCHARGE AQUION BATTERIES TO ZERO & THEY WON’T DIE
Due to its design, the battery has different capabilities than a lead acid battery. It can be discharged all the way down to 0% depth of discharge without damaging the battery. Simply charge it back up and it will continue on same as usual.

It is also perfectly fine with always being in a partial state of charge (PSOC), which means not being recharged to 100% each cycle. Lead acid batteries need to be fully charged to prevent early death by sulphating, where sulphate crystals form on the plates when they are not charged to 100%.

Since more of the Aquion battery capacity can be used than a lead acid battery bank, a smaller Aquion battery bank is needed for the same size solar system.

Aquion batteries importantly offer more discharging cycles than lead acid batteries do for the same level of discharge. See the graphs below for the comparison.