Advancements in Flow Batteries for Long Duration Energy Storage
Authors: Guarnieri Massimo 
Published: December 2024

The full decarbonization of electric grids, as planned by the European Union and other Administrations for 2050, calls for energy storage (ES) systems capable of discharging at full power over periods longer than 4-5 hours, which is the typical duration of internal storage batteries such as Li-ion and Sa-X. Such long discharge periods are already in the capability of some energy storage technologies, notably pumped-hydro (PH) ES, which was introduced at the beginning of the 20th century and today accounts worldwide for 165 GW of power capacity and 1.6 TWh of energy storage, corresponding to 96% and 99% of the global storage figures, respectively. However, a major increase in ES demand is expected in the coming decades heading to 1.25 TW and 5 TWh by 2050, which cannot be covered by PH, due to geomorphological, environmental, and technical constraints. While conventional batteries (e.g. Li-ion, Na-ion) will continue to expand to face the growing demand for fast energy storage, the increasing request for Long Duration Energy Storage will rely on other technologies and Flow Batteries are emerging as strong candidates for taking over an important share.
They are presently investigated and developed in over 50 different chemistries which range from low-medium to high Technological Readiness Level (TRL). The former include still technologically immature systems typically studied at laboratory level in small single cells (1–101 W) for characterizing materials, not systems. Research is focused on some chemistries based on non-critical materials, among which: Copper-Copper, Polysulfide-Bromine, Iron-Chromium, Zinc-Cerium, Organic compounds: Quinone, Viologen, TEMPO, Polymers of various nature, and Lithium-ion flow and some larger pilot systems have been made (102 kW) but related data is often classified or not accessible. The high TRL types are often already produced and marketed, even in very large sizes (10 kW–102 MW and 10 kWh–102 MWh) using chemistries such as: Vanadium-Vanadium, Zinc-Bromine, Iron-Iron, Hydrogen-Bromine. Although the all-vanadium type exhibits the best performance, it raises geopolitical issues due to the strong localization of ore reserves which make this metal a critical raw material (CRM), e.g. in the European Union. In this scenario the research on some iron-complex-based FBs is taking momentum due to wide accessibly and low cost of the metal, despite performance still call for major improvements. Techno-economic analyses and forecasts using tools such as the levelized cost of storage (LCOS) and the net present value (NPV) can provide important insight in addressing strategically the developing research.

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