German utility RWE may use salt caverns as renewable flow batteries

A subsidiary of German electric utility RWE is to investigate the possibility of utilising vast salt caverns currently used for gas storage as potential large-scale organic flow batteries for electricity generated from renewable energy sources.

RWE Gas Storage West, an RWE subsidiary and natural gas supplier, announced on Wednesday a collaboration with German large-scale storage developer CMBlu Energy AG to investigate the use of salt caverns as large-scale flow batteries.

The company already operates and markets five underground natural gas storages which hold a total volume of 1.7 billion cubic metres of natural gas.

However, these underground caverns could also be theoretically converted to store several gigawatt-hours of electricity from renewable sources by filling them with an organic electrolyte solution, which can then serve as the primary energy source. Organic flow batteries utilise the organic electrolyte solutions to store energy which can then be released through a chemical reaction in an electrochemical cell.

The comparison behind RWE’s focus on such large-scale stationary storage sites is clear: Europe’s largest battery at the moment is a lithium-ion battery with just 50MWh of storage.

And yes, over time and with increased development and cost reductions, the case for larger battery storage will improve. But considering that organic flow batteries can yield gigawatt-hours of storage capacity, these large-scale stationary storage solutions are a vital step towards greater renewable energy integration.

“The future belongs to renewables,” said Andreas Frohwein, Technical Managing Director of RWE Gas Storage West. “In order to make optimal use of green electricity, we need large stationary electricity storage systems. At RWE, we are exploring different storage solutions. In the future, we may be able to use our salt caverns as batteries for storing enormous quantities of electricity.

“Using existing technical infrastructure, they could also be connected to the electricity grid quickly.”

“Organic flow batteries are based on carbon, which is available globally in almost unlimited volumes,” added Peter Geigle, CEO of CMBlu Energy AG.

“The components are easy to recycle, and water is the largest component by volume. That means the battery isn’t flammable, so it’s safe to use. In addition, organic storage systems use no metal, unlike most other batteries.”

RWE and CMBlu Energy have already determined a potentially suitable electrolyte through a theoretical feasibility study and will now look to determine their suitability for use in salt caverns. Once a suitable electrolyte has been identified, the two companies expect to be able to begin work in the first half of 2021 to construct and operate a test system.

The planned generating capacity for their test system is 100kW with a storage capacity of up to 1,000kWh. The two companies expect this test phase to be completed by the first half of 2024.