錆電池：鉄空気技術が100時間グリッドをどのように実現するか

Key Takeaways

• The Problem: Lithium-ion is great for 4 hours, but we need 100 hours of backup for when the wind stops blowing (the “dunkelflaute”).
• The Solution: Iron-Air Batteries (like those from Form Energy) use the process of rusting to store vast amounts of power.
• The Economics: They are heavy and slow, but 10x cheaper than Lithium-Ion, making them the perfect partner for solar and wind farms.

Introduction

If you want to build a fast electric car, you use Lithium-Ion. It’s light, power-dense, and charges quickly.

But if you want to run a city for a week when a winter storm covers the solar panels and kills turn wind turbines, Lithium-Ion is bankruptingly expensive.

Enter the “Rust Battery.”

It’s about the size of a washing machine. It’s incredibly heavy. It charges slowly. And it might be the most important energy invention of the decade.

The Chemistry: Reversible Rusting

The core technology, pioneered by companies like Form Energy (founded by former Tesla executive Mateo Jaramillo), relies on one of the most common chemical reactions on Earth: Oxidation.

The battery breathes.

Discharging (The “Rust” Phase)

When the battery needs to provide power, it takes in oxygen from the air and lets it react with the iron anode inside the battery.

• Reaction: Iron + Oxygen = Rust (Iron Oxide) + Energy.
• The electrons released during this rusting process flow out to the grid.

Charging (The “Un-Rust” Phase)

When there is excess solar or wind power on the grid, the battery uses that electricity to reverse the reaction.

• Reaction: Rust + Electricity = Iron + Oxygen.
• It literally turns the rust back into metallic iron, releasing the oxygen back into the air.

The Engineering Challenge: It’s Not Just Rust

While the chemistry is simple (grade school science), building a reliable industrial battery is incredibly hard.

The Electrolyte Issue

The battery is filled with a water-based liquid electrolyte (potassium hydroxide). This creates several engineering headaches:

1. Hydrogen Evolution: Sometimes, instead of rusting, the water splits into hydrogen gas. This is bad. It wastes energy and creates a localized explosion hazard. Form Energy had to develop specialized catalysts to suppress this side reaction.
2. Water Management: The battery “breathes” air, but it can’t lose its water. In hot climates (like Arizona), evaporation is a killer. In cold climates (Minnesota, where the first pilot is), freezing is a risk. The thermal management system has to be robust enough to handle these extremes without consuming too much parasitic power.

The “Air Breathing” Membrane

The cathode needs to let oxygen in but keep the electrolyte liquid out. It’s like a high-tech Gore-Tex. If this membrane clogs with dust or degrades over 10 years, the battery dies. Form’s breakthrough was creating a cheap, durable air electrode that doesn’t require expensive platinum catalysts.

The Economics: $20/kWh vs $130/kWh

The Economics: $20/kWh vs $130/kWh

The magic isn’t the chemistry; it’s the cost.

• Lithium-Ion: ~$130 per kWh. Needs cobalt, nickel, and lithium (expensive, supply-constrained).
• Iron-Air: ~$20 per kWh. Needs iron (dirt cheap, abundant) and air (free).

Because the materials are so cheap, you can build massive banks of these batteries without worrying about the cost of the raw metals. This unlocks Long-Duration Energy Storage (LDES).

Because the materials are so cheap, you can build massive banks of these batteries without worrying about the cost of the raw metals. This unlocks Long-Duration Energy Storage (LDES).

The 100-Hour Math: LCOS Explained

When utilities buy batteries, they don’t look at the upfront price tag. They look at LCOS (Levelized Cost of Storage)—the total cost to store and release one megawatt-hour over the battery’s life.

• Lithium-Ion: Wonderful for short bursts (frequency regulation, peak shaving). But if you try to stretch it to 100 hours, you need 25x more batteries. The cost curve is linear and painful.
• Iron-Air: The “power” components (the electronics that move electrons) and the “energy” components (the iron pellets) are decoupled. To double the duration, you just add more cheap iron pellets, not expensive electronics.

At 100 hours of duration, Form Energy’s LCOS drops to roughly 1/10th that of Lithium-Ion. It competes directly not with other batteries, but with combined-cycle natural gas plants.

Manufacturing at Scale: The West Virginia Gamble

Form Energy isn’t just a lab experiment. They are currently building “Form Factory 1” in Weirton, West Virginia.

Why Weirton? It’s a former steel town. The symbolism is intentional. The same workforce that spent a century making steel is now being hired to make batteries out of iron.

• Capacity: The factory aims to produce 500 MW / 50 GWh of batteries annually.
• Impact: These aren’t delicate microchips made in cleanrooms. They are heavy industrial equipment. Revitalizing the Rust Belt to build the “Rust Battery” is a political masterpiece that has secured them bipartisan support.

The Geopolitical Angle: Iron vs. Lithium

Energy security is national security. The current battery supply chain is a geopolitical minefield.

• Lithium: Mining is concentrated in Australia and Chile; processing is 60%+ controlled by China.
• Cobalt: The Congo (DRC) dominates supply, with massive ethical and stability concerns.
• Iron: It is everywhere. It is the most mined metal on Earth. Minnesota alone has enough iron ore to build batteries for the entire US grid.

By switching to Iron-Air for grid storage, the US decouples its critical infrastructure from foreign supply chains. If a trade war cuts off lithium imports, the lights stay on because the battery materials are dug up in the Midwest.

The Environmental Wins (and Costs)

While “rust” sounds dirty, the materials are benign.

• Recyclability: At the end of its 20-year life, the battery is essentially a pile of scrap steel. It can be tossed into an electric arc furnace and recycled with 100% efficiency.
• No Fire Risk: Unlike Lithium-ion, which creates self-sustaining chemical fires that are nearly impossible to extinguish, Iron-Air batteries are practically non-flammable. The electrolyte is water-based. You could shoot it with a gun, and it would just leak wet rust.

The Competitors

The Competitors

Form isn’t alone in the LDES race, though they are currently winning the PR war.

1. Ambri: Founded by MIT’s Donald Sadoway, they use “Liquid Metal” batteries (calcium and antimony). They operate at high temperatures (500°C), which offers high efficiency but presents containment challenges.
2. EOS Energy: Uses Zinc-Halide chemistry. They are already public and shipping, but their duration targets are shorter (3-12 hours), positioning them as a bridge between Lithium and Iron-Air.
3. Flow Batteries (ESS Inc): Use liquid electrolytes (like iron and saltwater) pumped through tanks. Great for scaling, but typically suffer from complex plumbing mechanics.

Solving the “Dunkelflaute”

German energy engineers have a word for the nightmare scenario: Dunkelflaute (dark doldrums). It’s a period of several days with no sun and no wind.

A Lithium-Ion battery setup usually provides 4 hours of backup. That covers the evening peak, but it won’t save you during a 3-day blizzard.

Form Energy’s batteries are designed for 100-hour storage. That’s four full days. This allows utilities to retire natural gas “peaker” plants that only run a few weeks a year, replacing them with a reliable, emissions-free backup that costs a fraction of the price to maintain.

What’s Next?

The first massive pilot projects are breaking ground now in places like Minnesota (Xcel Energy) and Georgia.

The Rust Battery will likely never power your phone or your car—it’s too heavy. But the next time your lights stay on during a week-long storm, you might have a pile of rusting metal to thank.