Flow batteries: Engineering and performance

Why separating power from energy changes the cost of long hours

The defining engineering advantage is that power (stack size) and energy (tank size) are sized separately, which favours 6 to 12 hour storage. The penalties are lower efficiency than lithium-ion and a large footprint.

Round-trip efficiency

Flow batteries lose energy to pumps, to shunt currents through the shared electrolyte and to ions crossing the membrane. Sandia puts flow batteries at 70 to 80% round trip, against 85 to 90% for lithium-ion, while DOE's 2030 cost model assumes 65% for a complete 100 MW vanadium system at the AC connection. Field data show why the range is wide: NREL's tests on a 2 MW / 8 MWh Sumitomo vanadium battery found efficiency varied with power and state of charge, with maximum dynamic efficiency of 82% for peak shaving and 86% for base-load operation. Losses rise sharply at low power, when pumps run but little energy flows.⁴¹⁷¹⁸

Power and energy decoupling

Adding hours means adding electrolyte and tanks, not stacks, so the marginal cost per kWh falls as duration rises. In DOE's 2030 projection for a 100 MW, 10-hour vanadium plant, the storage block costs about USD 166/kWh and the balance of plant about USD 30/kWh. PNNL found a 100 MW, 10-hour vanadium system at about USD 385/kWh in 2021, close to lithium iron phosphate at USD 356/kWh, and DOE's baseline for 1,000 MW is about USD 365/kWh. Zinc-bromine is only partly decoupled because zinc plating area limits energy.¹⁴

Footprint and siting flexibility

Low energy density means more land and tank volume per MWh than lithium-ion. Tanks can be stacked or placed indoors, and the systems suit industrial or former mining land; Idemitsu plans a 108 MW, 8-hour, 864 MWh vanadium plant at the former Muswellbrook coal mine in New South Wales, with construction expected from early 2028. The Jimusar project shares a 1,870 ha site with a 1 GW solar plant.³¹³

Safety and thermal runaway risk

The electrolytes are water-based and do not burn, so thermal runaway of the lithium-ion kind is absent. Hazards are chemical: acid spills for vanadium, toxic bromine for zinc-bromine and hydrogen gas from side reactions in some chemistries; PNNL's iron-vanadium design was developed partly to avoid the hydrogen problem of iron-chromium cells. Stationary installations in the US fall under NFPA 855 like other battery systems. Germany's Prolux recalled all its Storac home vanadium systems in Dec 2025, citing service costs and some early electrolyte leaks, and switched to lithium iron phosphate.¹⁰¹⁵

Sources

1. 2022 Grid Energy Storage Technology Cost and Performance Assessment (PNNL-33283), Executive Summary p. vii Pacific Northwest National Laboratory for US Department of Energy, 2022
2. China connects world's largest vanadium flow battery project ESS News (pv magazine), 2026
3. Technology Strategy Assessment: Flow Batteries (Energy Storage Grand Challenge), Table 2 p. 5 US Department of Energy, 2023
4. Redox flow batteries for grid-scale energy storage (available technologies) Pacific Northwest National Laboratory, 2024
5. Idemitsu plans 864 MWh vanadium flow battery at Australian coal mine site (22 Jul 2026) ESS News (pv magazine), 2026
6. Prolux Solutions recalls vanadium redox flow home storage systems and switches to LFP (5 Dec 2025) ESS News (pv magazine), 2025
7. Energy Storage Overview (H. Passell, NECPUC Energy Storage Webinar Series, 19 Mar 2021) Sandia National Laboratories, 2021
8. Utility-scale vanadium redox flow battery for distribution grid support: system dynamics and efficiencies (NREL/PO-5D00-74248) National Renewable Energy Laboratory with Sumitomo Electric USA, 2019