Pilot plant demonstrates feasibility of iron-based hydrogen storage

By 2050, photovoltaics are expected to cover more than 40% of Switzerland’s electricity needs. However, solar energy is not always available at the right time: it is too abundant in summer and too scarce in winter, when the sun shines less often and heat pumps are running at full capacity. According to the Confederation’s energy strategy, Switzerland wants to close the winter electricity deficit by combining imports, wind and hydropower as well as alpine solar power plants and gas-fired power plants.

One way to reduce the need for imports and gas-fired power plants in winter is to produce hydrogen from cheap solar power in summer, which could then be converted into electricity in winter. However, hydrogen is highly flammable, extremely volatile and weakens many materials.

Storing gas from summer to winter requires special pressurized tanks and cooling technology. These require a lot of energy and the many safety precautions that must be taken into account make the construction of such storage facilities very expensive. In addition, hydrogen tanks are never completely leak-proof, which harms the environment and increases costs.

Researchers at ETH Zurich, led by Wendelin Stark, Professor of Functional Materials in the Department of Chemistry and Applied Biosciences, have developed a new technology for seasonal hydrogen storage that is much safer and less expensive than existing solutions. The researchers use a well-known technology and the fourth most abundant element on Earth: iron.

The results are published in the journal Sustainable energy and fuels.

Storage of chemicals

To better store hydrogen, Stark and his team are relying on the steam-iron process, which has been known since the 19th century. If there is excess solar energy during the summer months, it can be used to split water and produce hydrogen. This hydrogen is then fed into a stainless steel reactor filled with natural iron ore at 400 degrees Celsius. There, the hydrogen extracts oxygen from the iron ore – which in chemical terms is just iron oxide – to produce elemental iron and water.

“This chemical process is similar to charging a battery. This means that the energy contained in hydrogen can be stored in the form of iron and water for long periods of time with almost no loss,” Stark explains.

In winter, when energy is needed again, the researchers reverse the process: They feed hot steam into the reactor to convert the iron and water into iron oxide and hydrogen. The hydrogen can then be converted into electricity or heat in a gas turbine or fuel cell. To minimize the energy needed for the discharge process, the steam is produced from the waste heat of the discharge reaction.

From cheap iron ore to expensive hydrogen

“The great advantage of this technology is that the raw material, iron ore, is easy to obtain in large quantities. Moreover, it does not even need to be processed before being placed in the reactor,” Stark explains. In addition, the researchers assume that large iron ore storage facilities could be built worldwide without significantly influencing the price of iron on the world market.

The reactor in which the reaction takes place does not have to meet any special safety requirements either. It consists of stainless steel walls that are only 6 millimetres thick. The reaction takes place at normal pressure and the storage capacity increases with each cycle.

Once filled with iron oxide, the reactor can be reused for an unlimited number of storage cycles without having to replace its contents. Another advantage of this technology is that researchers can easily expand the storage capacity. All they have to do is build larger reactors and fill them with more iron ore. All these advantages make this storage technology estimated to be ten times cheaper than existing methods.

Hydrogen does have one drawback, however: its production and conversion are inefficient compared to other energy sources, with up to 60% of its energy lost in the process. As a storage medium, hydrogen is therefore particularly interesting when wind or solar energy is available in sufficient quantities and other options are excluded. This is particularly the case for industrial processes that cannot be electrified.

Pilot installation on the Hönggerberg campus

The researchers demonstrated the technical feasibility of their storage technology in a pilot plant on the Hönggerberg campus. This consists of three stainless steel reactors with a capacity of 1.4 m³, which the researchers each filled with 2 to 3 tonnes of commercially available raw iron ore.

“The pilot plant can store around 10 megawatt hours of hydrogen over a long period of time. Depending on how the hydrogen is converted into electricity, between 4 and 6 megawatt hours of electricity are obtained,” explains Samuel Heiniger, a doctoral student in Stark’s research group. This corresponds to the electricity consumption of three to five Swiss single-family homes during the winter months. For the time being, the plant is still running on grid power and not on solar energy generated on the Hönggerberg campus.

Things are set to change soon: The researchers want to expand the system so that by 2026, the ETH Hönggerberg campus can cover a fifth of its winter electricity needs with its own summer solar energy. This would require reactors with a volume of 2,000 cubic meters, which could store around 4 gigawatt hours (GWh) of green hydrogen.

Once converted into electricity, the stored hydrogen would provide about 2 GWh of electricity. “This facility could replace a small lake in the Alps as a seasonal energy storage facility. For comparison, that’s about one-tenth the capacity of the Nate de Drance pumped storage plant,” Stark says. In addition, the unloading process would generate 2 GWh of heat, which the researchers want to integrate into the campus heating system.

Good scalability

But could this technology be used to ensure seasonal energy storage throughout Switzerland? The researchers have made some initial calculations: to supply Switzerland with electricity in the future from seasonal hydrogen storage systems, around 10 terawatt hours (TWh) per year – which would be a lot, admittedly – ​​between 15 and 20 TWh of green hydrogen and around 10,000,000 cubic metres of iron ore would be needed.

“This represents about 2% of the annual iron ore production of Australia, the largest producer,” says Stark. For comparison, the Swiss Federal Office of Energy (SFOE) predicts in its Energy Outlook 2050+ a total electricity consumption of around 84 TWh in 2050.

If reactors were built that could each store around 1 GWh of electricity, their volume would be around 1,000 cubic metres. This would require around 100 square metres of building land. Switzerland would have to build around 10,000 of these storage systems to obtain 10 TWh of electricity in winter, which corresponds to an area of ​​around 1 square metre per inhabitant.