Energy and storage · Climate and environment
Storing energy for days: iron-air, heat and hydrogen
How long can each route store energy, and what public evidence shows how well it works?
Iron-air batteries, thermal storage and hydrogen are three routes discussed for holding energy over days rather than hours. The public evidence differs in kind: a developer's stated battery duration, an operator-reported heat store, a global capacity total and a funding notice for pilot projects, so the figures below are not directly comparable.
| Option | Discharge durationHours a system can deliver power at a stated rate before reaching its limit. Give the power rating and whether the figure is measured or rated. | Efficiency as statedRound-trip efficiency as the source states it, in percent, with the metric named. Heat-in to heat-out and electricity-in to electricity-out are not comparable. | Largest operating systemName, size (MW, MWh or GW) and year for the largest system with a reported operating record. Planned or capacity-total figures are marked as such. | Main limitationThe biggest constraint the cited sources describe, in one line. |
|---|---|---|---|---|
| Iron-air batteries | Developer describes a 100-hour battery. No measured duration in the July 2023 article.Reported, unverified[1] | No reliable public figure | Planned 10 MW and 1,000 MWh Minnesota demonstration, recently approved by the Minnesota Public Utilities Commission (July 2023). No operating record given.Reported, unverified[1] | Little public field performance data, and cost claims are the company's own (July 2023 article).Reported, unverified[1] |
| Thermal energy storage | Pornainen, Finland: 100 MWh at 1 MW thermal, which implies about 100 hours at full output (operator figures, June 2025). NREL pilot: sized at 10 hours at 100 kW, planned at the time of the September 2024 article.Reported, unverified[3][2] | Heat in to heat out: the article says round-trip efficiency of the stored heat exceeds 95% for multiple days, counting only heat left in the silos, with about 1% of heat lost per day in an insulated silo like the one proposed. Electricity in to electricity out: the team's model predicts 50-52%. These are design or modelled figures, not measured results (SolarPACES, 2024).Proposed[2] | Pornainen, Finland: 1 MW thermal and 100 MWh sand battery, announced as in operation on 11 June 2025 for district heating. Figures are the company's own.Reported, unverified[3] | Most figures are operator or company claims. The Tata Steel unit's CO2 and gas savings are projections (Canary Media, 2026).Reported, unverified[3][4] |
| Hydrogen from electrolysis | No reliable public figure | No reliable public figure | No single operating system in the sources. Globally, 20 GW of electrolyser capacity had reached final investment decision, as reported on 2 October 2024 (IEA). This is planned capacity, not operating capacity.Reported, unverified[5] | Developers are holding off on investment while they wait for clarity on government support (IEA, 2 October 2024).Reported, unverified[5] |
| Multi-day storage goal | A US funding notice targets long-duration systems of 10 hours or more (DOE, July 2024). Great River Energy and Form Energy describe a 1.5 MW project in Cambridge, Minnesota, capable of storing energy for up to 100 hours (August 2024).Reported, unverified[6][7] | No reliable public figure | No operating record in the sources. The 2024 announcement expected the Cambridge, Minnesota 1.5 MW project to be operational by late 2025 and reports no operating data. The sources do not show whether it has started operating.Reported, unverified[7] | The 2024 announcement reports no operating data. Great River Energy plans a multi-year study after completion to assess performance and wider use.Reported, unverified[7] |
What the table shows
- Iron-air is described by its developer as a 100-hour battery. A 10 MW, 1,000 MWh Minnesota demonstration was reported as recently approved in July 2023, with no operating record given.
- Thermal storage has a Finnish unit that the operator reports as in operation, rated at 1 MW thermal and 100 MWh, with figures supplied by the operator.
- The hydrogen row shows a global capacity total at final investment decision, not a storage duration or efficiency, so its duration and efficiency cells are empty.
- The multi-day storage goal has a US funding notice for 10 hours or more and one project described as capable of up to 100 hours, with no operating data in the sources.
Read with care
- The figures mix company and operator announcements, trade-press reports, modelling, funding notices and capacity totals. They are not like-for-like, and most are not independently measured.
- Heat-in to heat-out efficiency (above 95%, SolarPACES) and modelled electricity-in to electricity-out efficiency (50-52%, same source) measure different things and should not be compared directly.
- An empty cell means no reliable public figure was found for that measure in the cited sources, or the measure does not apply. It does not mean poor performance.
Sources
- news
Form Energy's 100-hour battery is almost here
John Engel. Power Engineering, 24 Jul 2023. Accessed 21 Oct 2026. - news
Long-duration thermal energy storage in sand begins NREL demo
Susan Kraemer. SolarPACES, 18 Sept 2024. Accessed 21 Oct 2026. - company announcement
World's Largest Sand Battery Now in Operation
Polar Night Energy, 11 Jun 2025. Accessed 21 Oct 2026. - news
Global giant Tata Steel is using a heat battery to curb emissions
Maria Gallucci. Canary Media, 27 Feb 2026. Accessed 21 Oct 2026. - other
Low-emissions hydrogen projects grow as policy support races to catch up
International Energy Agency, 2 Oct 2024. Accessed 21 Oct 2026. - government report
OCED Issues Notice of Intent for up to $100 Million for Non-Lithium Long-Duration Energy Storage Pilot Projects
US Department of Energy, Office of Clean Energy Demonstrations, 2 Jul 2024. Accessed 21 Oct 2026. - company announcement
Great River Energy, Form Energy break ground on first-of-its-kind multi-day energy storage project
Great River Energy, 15 Aug 2024. Accessed 21 Oct 2026.