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Will EWE’s Salt Caverns Become Home to Another “World’s Largest Battery”?

EWE AG announces a new type of flow battery that will be large enough to power all of Berlin for one hour.

There are several different companies with competing claims to the "world’s largest battery." In a recent announcement, EWE AG is suggesting that its planned 120-MW battery will be the world’s largest battery. 

But, you may ask, didn’t Vandium Corporation earlier this year say the same sort of thing?  (See Flow Battery Developer to Build World's Largest Battery Storage System). 

And what about the lithium-ion based 100-MW AES Alamitos Energy Storage Array’s next phase, which will involve 300 MW of interconnected and 600 MW of flexible battery energy storage?  (See the AES fact sheet at this link: The Alamitos Battery Energy Storage System.) 

You might also remind us to not forget the 200-MW/800-MWh energy storage station, to be built with Rongke Power’s vanadium flow battery, whose 2016 announcement stated that the China National Energy Administration “has approved the world’s largest energy storage station to be built in Dalian, China.”

While the announcement from EWE AG described below may indeed turn out to be an energy storage game-changer, it is important to look at battery technologies and unpack what various claims about “world’s largest” battery-based energy storage facilities are actually saying.

Two Definitions of Battery

Basically, an electro-chemical energy storage system can either be composed of a large number of small energy storage cells or a small number of large cells.  

While it is obvious to us when we think of our day-to-day usage of the term, let’s recall “battery” can mean “array” or it can mean “a single cell or container.” In the second sense, the massive single-unit cells, which EWE will be building, may very well warrant their claim of being the world’s largest battery. 

From the “array” point of view, or a facility size perspective rather than a single battery cell perspective, the proposed 120-MW facility, if completed as planned in 2023, is likely to be surpassed by a bunch of other battery storage facilities, including lithium-ion based ones, even if it ends up at or near the #1 position capacity-wise compared to other flow battery facilities.

Differences in EWE’s Flow Battery Design

EWE, which is among the largest electric and gas utilities in Europe, seeks to employ a well-known type of battery design in a new way. The design involves the well-known redox flow battery principle, whereby electrical energy is stored in a liquid, but will do so by means of new, environmentally friendly components that will be installed in underground salt caverns.

In comparison, existing megawatt-scale facilities using flow battery technology include a 25-MW array (consisting of a battery of 1,000 zinc-bromide flow batteries, with each sub-component battery sized 25 kW) at a Modesto Irrigation District windfarm, supplied by Primus Power, as well as facilities using vanadium redox flow batteries, including an Ontario, Canada-based SunEdison facility utilizing 5MW, 20MWh  of vanadium redox flow batteries from Imergy.

EWE currently operates eight underground gas storage facilities in its northern Germany service territory, which use caverns within salt domes. Two of the caverns, each with a volume of 100,000 m³, will be used for [what EWE says will be] the world’s largest battery. They will be flushed with water in salt domes, which will generate the required brine on site. 

Ralf Riekenberg is head of the Brine for Power project, which has been dubbed b4p. If the project proceeds according to plan, the last of its three phases will be fully operational by about the end of 2023.

“If everything works, this may fundamentally change the storage market,” Riekenberg stated.

Flow Batteries as Part of Electro-Chemical Storage in General

Flow batteries are another form of electro-chemical energy storage, a category that includes the familiar lithium-ion designs and other chemistries. The U.S. dominates in electro-chemical energy storage installations, while Germany, Japan and China are bunched up closely in rank.

EWE Flow Battery Details

By the project’s final phase, a salt cavern in Germany will house the massive 120-MW, 700-MWh flow battery. University researchers in Jena, Germany, developed the innovative components, which employ salt water and recyclable plastic.


The project’s phases are to be staged as follows:

Phase  Construction Planned operating figures
 1  Test system 1 
(on the surface)
 10 - 20 kW 
10 - 40 kWh
 2  Test system 2 
(on the surface)
 100 - 500 kW 
500 - 2,500 kWh
 3  Pilot plant 
(underground cavern)
 up to 120 MW 
up to 700 MWh

The redux flow battery stores electrical energy in two different liquids (called electrolytes) in which certain chemicals are dissolved. These electrolytes are distributed across two separate containers and store and supply power during charge and discharge cycles.

Electrolytes previously used in other types of flow batteries included environmentally polluting salts of heavy metals such as vanadium dissolved in sulphuric acid. In contrast, this redox flow battery uses recyclable polymers (plastics) dissolved in salt water as an electrolyte.

For the two initial test phases in the 10 kW to 500 kW range, instead of using actual caverns, enormous plastic containers will be set up at the EWE gas storage facility. These earlier phases are expected to start in the fourth quarter of this year.

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