October 1, 2020: RWE Gas Storage West and CMBlu Energy announced on September 30 they had started a collaborative project to investigate whether organic electrolytes can be used in a flow battery project in RWE’s salt caverns.
The project will be in three phases.
The first, which has just begun, is a feasibility study to determine which organic electrolyte should be used.
The second, starting next spring. will involve the construction and commissioning of the plant.
The third will be a fine tuning of the system which, when finished by the spring of 2024, aims to result in an operational system with an output of 100kW/1,000kWh.
The choice of the organic electrolyte will be an interesting one given that the roughly dozen firms around the world developing organic electrolytes keep their technology a well guarded secret.
“Organic electrolytes are being increasingly proposed as an alternative to vanadium or zinc in flow batteries as they are not subject to the wild price swings that these metals have had historically,” says Anthony Price, principal of his consultancy firm Swanbarton. “They are also viewed as environmentally more positive.”
In the past sudden huge rises in the price of vanadium have forced many flow battery start-ups to the wall. Two years ago, for example, China insisted that all steel would need to be re-inforced by vanadium. This caused a spike in the price of the metal — roughly 80% of the metal is used in the steel industry — and the downfall of many promising flow battery firms.
The price of vanadium pentoxide moved from a low of $2.4/lb in January 2016 to $33.9/lb in November 2018. Given that the price of vanadium is the most expensive factor in these flow batteries, such a price leap is devastating.
“The perfect electrolyte for a flow battery should have around five specific qualities,” says Price. “It shouldn’t degrade over time — flow batteries are meant to be for the long haul, say in use for 20+ years, they should have good ionic conductance, a high electrochemical potential, they shouldn’t be corrosive and are well suited to the environment: safe, in other words. And last, they should be affordable and remain so.
“A good organic electrolyte should tick all these boxes.”
The second phase of the project is likely to be relatively straight forward as flow battery construction firms already exist with prepared designs that can be adapted. For example, firms such as Pinflow Energy Storage, based in Prague, the Czech Republic, can already provide customized products.
One puzzling question is whether there is a need to build flow batteries in these vast caverns.
Flow battery electrolytes typically have an energy density of between 20kWh per litre to 100kWh/l. Much higher densities — including more than 500kWh/l — might theoretically be possible but have not yet been verified in practice.
For this flow battery to achieve an output of 1,000kWh and assuming that the electrolyte has a density of 40Wh/l — this compares favourably with a typical VRFB delivering 30Wh/l —would require 25,000 litres of electrolyte. Give that 1,000 litres equates to one cubic metre, then this battery would occupy just 25m3. That is roughly two-thirds the size of a 20 teu container.
RWE Gas Storage West is part of German utility RWE Group. It operates five underground (salt cavern) natural gas storage facilities for the north-west European gas market with a working gas volume of about 1.7 billion cubic metres.
CMBLu Energy describes itself as a pioneer and market leader in sustainable organic flow batteries, It is based near Frankfurt-am-Main in Germany.
RWE is likely to have plans to scale the facility up. “These underground caverns could potentially store capacities of up to several gigawatt hours of electricity from renewable sources,” says a statement from the firm. “By comparison, Europe’s largest battery at present — based on lithium-ion technology — is located in Jardelund, Schleswig-Holstein, and has a storage capacity of about 50MWh.
Andreas Frohwein, technical managing director of RWE Gas Storage West, said: “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.”