David Waltham, Katherine Holt, Stefanie Kuenzel, Anirban Basu, Nora Lecoeur

Storage of flow-battery electrolytes in aquifers is a novel concept for storing electrical energy in the subsurface. Flow-batteries operate by electrochemical transformations of electrolytes, rather than of electrodes, and their energy capacity can therefore be increased indefinitely by using larger electrolyte tanks. Saline aquifers may be the cheapest way to provide large-scale storage for this purpose. Storage would be within high-porosity, high-permeability reservoirs sealed by impermeable layers but—in contrast to hydrocarbon, H2 or CO2 storage—electrolytes would be trapped in lows (rather than highs) of such formations as a consequence of their high density compared to natural brines.
We investigate a range of electrochemical, geochemical, microbiological and engineering hurdles which must be overcome if subsurface flow-batteries are to become a practical technology. No insurmountable problems were found but further laboratory studies are needed. Our economic assessment suggests that subsurface flow batteries should be more cost effective than hydrogen-based power-to-gas approaches for discharge/charge timescales of around a day but that hydrogen will be cheaper for longer-term storage. Hence, meeting future energy-storage needs may involve a combination of both approaches.