As the grid integrates more intermittent renewables, the "duration gap" grows. Lithium-ion handles 4 hours perfectly, but fails economically at 100 hours. Enter Aqueous Iron-Air: a technology that effectively "rusts" iron to discharge energy. With Form Energy's first gigafactory now shipping, we analyze the 2026 economics of the $20/kWh storage holy grail.
What You'll Learn
Quick Answer: What are Iron-Air Batteries?
Iron-Air batteries work by "reversible rusting." Discharging converts iron to rust (breathing in oxygen); charging converts rust back to iron (breathing out oxygen). They are too heavy for EVs but ideal for 100+ hour grid storage to replace coal plants.
2026 Key Metrics (Form Energy Targets)
Head-to-Head: Iron-Air vs. Lithium-Ion (LFP)
| Feature | Iron-Air (Form Energy) | Lithium-Ion (LFP) |
|---|---|---|
| Cost Target | <$20/kWh | $130-$150/kWh |
| Duration | 100 Hours (4 Days) | 4-6 Hours |
| Efficiency | ~40-50% (Low) | 85-90% (High) |
| Safety | Non-flammable (Water) | Thermal Runaway Risk |
1. Technology Benchmarks
Iron-air batteries use a simple redox reaction: iron turning to rust. The challenge isn't the chemistry (which is known) but the air electrode durability and round-trip efficiency.
| Parameter | Iron-air (Target) | Li-ion LFP (Grid) | Vanadium Flow |
|---|---|---|---|
| Target Duration | 50–150 hours | 2–8 hours | 8–20 hours |
| Round-trip Efficiency | 30–50% | 85–92% | 65–80% |
| Cycle Life | 500–3,000 (Event based) | 3,000–7,000 (Daily) | >10,000 (Unlimited) |
Efficiency Trade-off: The low efficiency (40%) means for every 10 MWh you put in, you only get 4 MWh back. This is acceptable only if the charging energy is extremely cheap (e.g., curtailed wind/solar) and the capital cost of the battery is low enough to sit idle for long periods.
Round-Trip Efficiency Comparison
2. Deep Dive: The "Air Electrode" Challenge
While the "rusting" concept is simple, the engineering reality is incredibly complex. Historically, the air electrode (cathode) has been the Achilles' heel of metal-air batteries.
Why is it so hard?
- Sluggish Kinetics: Breathing in oxygen (discharging) and breathing it out (charging) involves complex multi-step reactions that are naturally slow, leading to energy loss (overpotential).
- Carbonation: CO2 from the air can react with the alkaline electrolyte to form solid carbonates, which clog the electrode pores—literally "suffocating" the battery over time.
- Durability: The electrode must survive thousands of cycles of wetting (flooding) and drying without degrading its delicate porous structure.
Form Energy's Breakthrough: While proprietary, their success likely hinges on a specialized PTFE-based (Teflon) breathable barrier that blocks water and CO2 while letting oxygen pass, combined with non-precious metal catalysts (likely Manganese or organics) that are cheap yet durable enough for 100-hour cycles.
2. The Economics: Can it Hit $20/kWh?
Lithium-ion cells cost ~$80/kWh but require expensive packaging, cooling, and fire suppression, bringing system costs to $130-$150/kWh. Iron-Air changes the math:
Cost Stack Comparison (Projected 2026)
| Component | Lithium-Ion (LFP) | Iron-Air (Form Energy) | Why? |
|---|---|---|---|
| Cathode | $25/kWh (LFP) | <$5 /kWh (Carbon/Air) | Air is free; carbon is cheap. No nickel/cobalt. |
| Anode | $10/kWh (Graphite) | $1/kWh (Iron Sinter) | Iron pellets cost ~$100/ton vs Graphite ~$800/ton. |
| Electrolyte | $10/kWh (Li-Salts) | $1/kWh (KOH Water) | Potassium Hydroxide is basically drain cleaner (cheap). |
| System Cost | ~$140/kWh | <$20 /kWh (Target) | Total bill of materials is fundamentally lower. |
LCOS Sensitivity: When does Iron-Air win?
*Chart shows Levelized Cost of Storage across different discharge durations (4h vs 100h).
3. 2026 Project Pipeline: Xcel, Georgia Power & Dominion
2026 marks the "put up or shut up" year for Iron-Air. Form Energy is deploying its first GWh-scale systems.
Major Utility Deployments
Xcel Energy (MN)
10 MW / 1,000 MWh
Replacing the retiring Sherco Coal Plant. This is the flagship commercial pilot.
Georgia Power
15 MW / 1,500 MWh
Approved in IRP to bolster grid resilience against extreme heat events.
Dominion (VA)
5 MW / 500 MWh
Testing efficacy for offshore wind smoothing in PJM market.
4. Grid Value: Why We Need 100-Hour Storage
It's not about daily arbitrage. It's about the "Dunkelflaute" (Dark Doldrums).
In a net-zero grid, 4-hour Li-ion batteries die after sunset. What happens when a winter storm hits and solar output drops near zero for 5 days? Gas peakers usually fill this gap. Iron-Air is the only scalable non-fossil/non-nuclear asset that can ride out a 4-day lull economically.
5. Technical Challenges: Efficiency & Hydrogen Evolution
It's not all perfect. Two main technical hurdles exist:
- Low RTE (Round-Trip Efficiency): You only get ~40-50% of the energy back (vs 90% for Lithium). This means charging electricity must be extremely cheap (i.e., curtailed wind/solar at $0-$10/MWh) for the economics to work. Iron-Air is not for daily cycling—it's for rare multi-day events.
- Hydrogen Evolution: Charging iron in water can accidentally split water into Hydrogen gas (H₂). This is a safety risk and efficiency loss. Form Energy claims to have solved this with catalyst additives that suppress hydrogen generation, but long-term field data is still being collected.
- Air Electrode Degradation: The air-breathing cathode must survive thousands of cycles of wetting (flooding) and drying without degrading its delicate porous structure. CO₂ from ambient air can also "carbonate" the electrode over time.
6. Real-World Deployments: Verified Case Studies
2024-2025 marks the critical transition from pilot to commercial deployment for iron-air technology. Form Energy is leading with multiple utility partnerships now entering construction and commissioning phases.
Case Study 1: Xcel Energy Sherco — Coal Replacement with Iron-Air
CONSTRUCTION START
Q2 2024
EXPECTED COMMISSIONING
End of 2025
CAPACITY
10 MW / 1,000 MWh (1 GWh)
LOCATION
Becker, Minnesota (Sherco Site)
This is Form Energy's flagship commercial demonstration. The iron-air system is being installed at the site of the retiring Sherburne County Generating Station (Sherco), one of the largest coal plants in the Upper Midwest. It will be paired with the Sherco Solar development (up to 710 MW) to provide multi-day backup during extended low-generation periods.
With 100 hours of duration, this single installation can ride out a typical Midwest "Dunkelflaute" (extended cloudy, calm weather) that would otherwise require gas peaker plants. It represents the first GWh-scale iron-air system in the world.
Sources: Form Energy Press Release, Utility Dive, PV Magazine USA (2024).
Case Study 2: Georgia Power — Southern Company Grid Resilience
EXPECTED OPERATIONS
Early 2026
UTILITY PARTNER
Georgia Power (Southern Company)
CAPACITY
15 MW / 1,500 MWh (1.5 GWh)
DURATION
100 Hours
Georgia Power, a subsidiary of Southern Company, is deploying a 15 MW / 1.5 GWh iron-air battery system to enhance grid resilience against extreme weather events (hurricanes, heat waves). This is the largest announced iron-air project globally by energy capacity.
The system will be manufactured at Form Energy's Form Factory 1 in Weirton, West Virginia, which began trial production in early 2024 and aims for full commercial production by late 2024. This project demonstrates the scalability of iron-air manufacturing and utility-scale deployment.
Sources: Form Energy, Georgia Power IRP, Energy Storage News, Renewable Energy World (2024).
Case Study 3: Great River Energy Cambridge — First Commercial Deployment
GROUNDBREAKING
August 2024
EXPECTED OPERATIONAL
Late 2025
CAPACITY
1.5 MW / 150 MWh
LOCATION
Cambridge, Minnesota
Form Energy and Great River Energy broke ground in August 2024 on the Cambridge Energy Storage Project, marking Form Energy's first commercial deployment of its iron-air battery technology. Despite the smaller capacity (150 MWh vs the 1 GWh Sherco project), this is technically the "first" commercial iron-air system to begin construction.
Following commissioning in late 2025, Great River Energy plans to conduct a multi-year performance study to evaluate cycle efficiency, degradation rates, and operational costs under real-world conditions. This data will be critical for de-risking larger deployments.
Sources: Form Energy, Great River Energy Press Release, Energy Storage News, Utility Dive (August 2024).
Case Study 4: Dominion Energy Darbytown — PJM Market Pilot
CONSTRUCTION START
Late 2024
EXPECTED OPERATIONAL
Late 2026
CAPACITY
4.9 MW / 494 MWh
LOCATION
Henrico County, Virginia (Darbytown)
Dominion Energy Virginia is testing Form Energy's iron-air technology at the existing Darbytown Power Station in the PJM Interconnection market (the largest US power market). The project was filed with the Virginia State Corporation Commission in September 2023 and is testing alternatives to lithium-ion for grid-scale storage.
Notably, the Darbytown project will also test zinc-hybrid batteries from Eos Energy (4 MW/16 MWh) alongside the iron-air system, providing direct comparative performance data. The total project cost is approximately $70.6 million.
Sources: Dominion Energy SCC Filing, Form Energy, Virginia Business, Power Technology (2023-2024).
Global Iron-Air Project Pipeline (2024-2027)
The following table summarizes all announced Form Energy deployments. Note that Form Energy is currently the only company with commercial iron-air products.
| Project | Utility Partner | Location | Power (MW) | Energy (MWh) | Status |
|---|---|---|---|---|---|
| Cambridge Energy Storage | Great River Energy | Cambridge, MN | 1.5 | 150 | Under Construction Groundbreaking Aug 2024 |
| Sherco Demonstration | Xcel Energy | Becker, MN | 10 | 1,000 | Under Construction Online late 2025 |
| Georgia Power LDES | Georgia Power (Southern Co.) | Georgia | 15 | 1,500 | Approved Online early 2026 |
| Darbytown Pilot | Dominion Energy | Henrico, VA | 4.9 | 494 | Approved Online late 2026 |
| NYSERDA Demo | NYSERDA | New York | 10 | 1,000 | Planned Target 2026-2027 |
| TOTAL ANNOUNCED | 41.4 MW | 4,144 MWh | ~4.1 GWh Pipeline | ||
Technology Selection by Duration: When to Use What?
This decision matrix helps grid planners select the right storage technology based on use case and discharge duration requirements.
| Discharge Duration | Recommended Technology | Typical Use Case | Approx. LCOS ($/MWh) |
|---|---|---|---|
| 0 - 4 Hours | Lithium-Ion (LFP/NMC) | Peak shaving, Frequency regulation, Solar shifting | $100 - $180 |
| 4 - 12 Hours | Lithium-Ion / Flow Battery (Vanadium) | Overnight solar storage, Load balancing | $150 - $250 |
| 12 - 24 Hours | Flow Battery / Iron-Air (Emerging) | Daily cycling, Wind smoothing | $180 - $300 |
| 24 - 100+ Hours | Iron-Air (Form Energy) | Multi-day backup, Dunkelflaute, Peaker replacement | $80 - $150 |
| 100+ Hours (Seasonal) | Hydrogen / Compressed Air / Pumped Hydro | Seasonal arbitrage, Strategic reserve | $200 - $400+ |
7. Supply Chain: Iron vs. Lithium Geopolitics
One of the strongest arguments for iron-air is supply chain security. Unlike lithium-ion, which relies on a complex global web of critical minerals, iron-air batteries can be built almost entirely from domestic materials in many regions.
| Feature | Lithium-Ion (NMC/LFP) | Iron-Air |
|---|---|---|
| Primary Metals | Lithium, Nickel, Cobalt, Manganese | Iron, Manganese (trace) |
| Geographic Risk | High (China, DRC, Chile concentration) | Low (US, EU, India self-sufficient) |
| Local Manufacturing | Requires cell imports or complex new mines | Leverages existing steel/chemical supply chains |
| IRA Domestic Content Eligible | Partial (complex sourcing) | Yes (100% domestic possible) |
8. Outlook to 2035: Realistic Role for Iron-Air Storage
By 2035, iron-air could either become a mainstream option for multi-day balancing or remain a niche technology, depending on how pilot projects perform and how costs evolve. In most scenarios, it will complement rather than replace lithium-ion batteries, pumped hydro, or hydrogen.
The "Goldilocks Zone": Iron-air is optimized for 24-150 hour storage where lithium is too expensive and hydrogen is too inefficient. It fills a specific "duration gap" in the grid portfolio.
Frequently Asked Questions
How much do Iron-Air batteries cost?
Form Energy targets an installed system cost of <$20 /kWh at scale. This is roughly 7-10x cheaper than lithium-ion grid storage ($130-$150/kWh). The low cost comes from using iron (one of the cheapest metals on Earth) and a water-based electrolyte.
What is the efficiency of Iron-Air batteries?
Round-trip efficiency (RTE) is approximately 40-50%, compared to 85-90% for lithium-ion. This means for every 10 MWh you put in, you only get 4-5 MWh back. This low efficiency is acceptable because the capital cost is so low that it still achieves the lowest LCOS for long-duration applications, especially when charging with near-zero-cost curtailed renewables.
Can Iron-Air batteries power EVs or homes?
No. Iron-air batteries are extremely heavy and have low power density. They are designed only for stationary grid-scale storage where weight is irrelevant. They are not suitable for electric vehicles, consumer electronics, or residential powerwall applications.
How long do Iron-Air batteries last (cycle life)?
Form Energy targets a 20+ year lifespan with periodic maintenance. Unlike lithium batteries which cycle daily, iron-air is designed for "event-based" cycling (perhaps 20-50 full cycles per year during extended renewable droughts). The iron anode can be refurbished, and the electrolyte is cheap to replace.
Are Iron-Air batteries safe?
Yes, inherently safer than lithium-ion. The electrolyte is water-based (non-flammable), and the chemistry operates at ambient temperature. There is no thermal runaway risk. The main safety consideration is hydrogen off-gassing during charging, which requires proper ventilation.
Who is Form Energy?
Form Energy is a US-based energy storage company founded in 2017, headquartered in Somerville, Massachusetts. They raised over $800 million in funding, including from Breakthrough Energy Ventures (Bill Gates' climate fund), ArcelorMittal, and TPG Rise Climate. Their first factory (Form Factory 1) is in Weirton, West Virginia.
Are there competitors to Form Energy?
In the iron-air space specifically, Form Energy is the clear leader with the most advanced commercial deployments. Other LDES competitors include ESS Inc (iron flow), Invinity (vanadium flow), and Malta (pumped thermal). Hydrogen storage is also a competitor for ultra-long duration applications.