Energy

Hydrogen stored in iron: A cheap, scalable grid battery for the winter

Iron oxide – or rust – turns out to be a very affordable and promising way to store hydrogen as a seasonal renewable grid-firming battery
Iron oxide – or rust – turns out to be a very affordable and promising way to store hydrogen as a seasonal renewable grid-firming battery

While hydrogen's high energy per mass makes it an excellent fuel, it's awfully hard and expensive to store long-term. That could change, thanks to the work of researchers at Switzerland's ETH Zurich. They've worked out a way to store hydrogen in ordinary steel-walled containers for months without losing it into the atmosphere – using iron.

Discovering solutions in the past

The research team led by Wendelin Stark, Professor of Functional Materials at ETH Zurich, hit upon this method by drawing from the steam-iron process of producing hydrogen, first invented in 1784.

Researchers Samuel Heiniger (left, holding a jar of iron ore) and Professor Wendelin Stark (right) in front of the three iron reactors containing hydrogen
ETH Zurich

The group's storage solution is especially suitable in places like Switzerland, where solar power is abundant in the summer, and scarce in the winter.

Surplus solar power is used to split water to produce hydrogen in the summer; it's then streamed into stainless steel reactors filled with iron ore at 752 °F (400 °C). The hydrogen extracts oxygen from the iron oxide, so you're left with iron and water in the reactor, ready to store without expending a lot of energy.

ETH researchers' simple technology for storing iron based on the steam-iron process

Steam is fed into the reactor to retrieve the stored hydrogen when needed; it can then be converted into electricity or heat easily enough.

There are also several other advantages of using this method:

  • The iron ore used in the reactors is cheap, plentiful, and doesn't require processing.
  • The reactors themselves are simply made of stainless steel.
  • The charging process occurs at ambient pressure – negating the need for high pressure tanks (350-700 bar) typically necessary to store hydrogen gas.

Testing a centuries-old idea

The research team piloted its tech on ETH's Hönggerberg campus, using three stainless steel reactors. Each of them have a capacity of 1.4 cubic meters, and are filled with 2-3 tons of iron ore. The test plant can store about 10 megawatt hours of hydrogen for extended periods, and that'll yield between 4-6 megawatt hours of electrical energy. That's enough to run three to five Swiss homes in the winter. The pilot project is set to grow by 2026, with the team looking to meet one-fifth of the winter electricity needs of the campus using solar power from summer months.

A stainless steel reactor containing iron ore to store hydrogen at the ETH Honnenburg campus

According to the team's research paper published last November, utilizing this system for a single home is currently more expensive than powering it with electricity from the grid. Scaling it up to 100 homes brings the energy cost nearly in line with that of the grid, and it's estimated that it'll only get cheaper as the system expands.

Powering Switzerland and beyond

The researchers note that expanding storage capacity just means adding more reactors, with the processing material serving its charging-discharging cycle duties for years before it needs to be replaced.

In order to provide all of Switzerland with power through the winter months, the team estimates that you'll need about 15–20 TWh of green hydrogen a year, and roughly 10,000,000 cubic meters of iron ore (or 2% of what Australia's iron mines produce).

You'll also need about 10,000 reactor systems, each capable of storing 1GWh. That works out to an area of land equivalent to about 1 square meter per inhabitant of Switzerland.

It's hard to arrive at a clear levelized cost of storage from this small pilot project. And while Switzerland plans to go solar for more than 40% of its electricity needs by 2050, we don't know if it'll invest in hydrogen storage at a national scale. Still, this clever technology from hundreds of years ago seems promising for our seasonal energy requirements in the future.

Source: ETH Zurich

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6 comments
TechGazer
They don't mention the round-trip efficiency. A nickel-iron battery has a watt-hour efficiency of ~60%. What's the watt-hour efficiency of this "steam iron".
Chase
My first question was also what's the round trip efficiency?
Techutante
"The test plant can store about 10 megawatt hours of hydrogen for extended periods, and that'll yield between 4-6 megawatt hours of electrical energy."

Sounds like between 40-60% efficiency. But that's also without the challenge of storing hydrogen which sort of just escapes containment, and without industrial battery acid or other byproducts considering it's just converting iron into rusty iron and back again?
Daveb
Did you explain the reactions? It sounds like dry ore FeO becomes wet iron H2O + Fe when combined with H2. The hydrogen is now locked up in water. You started with water. You used electricity to get the H2 out of the H2O, and then used the H2 to get the O out of the FeO. But now it's just water and iron. Sounds like it's the production of non-oxidized iron that is the energy storage. The H2 is gone, turned back into water. Are you saying that now you somehow rust the iron, which releases H2 from water? What does steam have to do with the H2 recovery? You had to heat the ore for the first reaction, and now you're heating the water for the second reaction?
I don't want to be skeptical, but without an explanation I just don't know what to think. And yeah what *do* they say the round trip efficiency is? @TechGazer @Chase
BanisterJH
So a lot of elements in their pure form like the oxygen in water better than hydrogen does. You may have heard of this in relation to powerpaste or pure silicon. What they're suggesting is that with iron, the attraction is mild enough that it can make sense to reverse it. I believe heating the water into steam just speeds up the rusting of the iron, and allows the water to penetrate into the middle of the pile of iron powder. It's probably worth pointing out that if you have the real estate, using solar power to heat stuff directly, without making electricity first can be really inexpensive (like a plywood parabolic cone with some mylar on the inside of the curve) (or just sheet metal spray painted black with copper tubing stuck to it to heat water). I've heard about ionizing hydrogen and sending the nuclei into iron powder (making iron hydrides) and being able to store it more densly than you could by compressing it since the early 80s at least. Presumably, they think that using the hydrogen to get the oxygen out of the oxide is more economical than ionizing it.
Karmudjun
While this is yet another incremental win for the Swiss, the several articles referenced did point out a 40-60% efficiency of the system. And analyzing the efficiency in relation to other forms of storage is ridiculous - there are costs beyond mere efficiency and this is a low cost, easily fabricated system that can be scaled up today using green hydrogen and minimally treated iron oxide. Once in place, it merely needs maintenance as the container loss of hydrogen is rendered a moot point! Great article! Better links!