Hydrogen storage getting ironed out in Switzerland

Researchers at ETH Zurich are using iron to safely store hydrogen for long periods. This technology could be used for seasonal energy storage in the future.

Photovoltaics are set to meet over 40 per cent of Switzerland’s electricity needs by 2050, but solar power isn’t always available when needed: there’s too much of it in summer and too little in winter, when the sun shines less often and heat pumps run at full tilt. According to the Swiss  Government’s Energy Strategy, Switzerland wants to close the winter electricity gap with a combination of imports, wind and hydropower, alpine solar plants and gas-fired power plants.

One way to minimise 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 and extremely volatile, making many materials brittle. Storing the gas from summer until winter requires special pressurised containers and cooling technology. These require a lot of energy, and the many safety precautions make building such storage facilities very expensive. Moreover, hydrogen tanks are never completely leak-proof, which harms the environment and adds to the costs.

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

Chemical storage

To store hydrogen better, Professor Stark and his team are relying on the steam-iron process, which has been understood since the 19th century. If a surplus of solar power is available in the summer, it can be used to split water to produce hydrogen. This hydrogen is then fed into a stainless steel reactor filled with natural iron ore at 400° Celsius. There, the hydrogen extracts the oxygen from the iron ore – which in chemical terms is simply iron oxide – resulting in elemental iron and water.

“This chemical process is like charging a battery. It means that the energy in the hydrogen can be stored as iron and water for long periods with almost no losses,” Professor Stark aid.

When the energy is needed again in winter, the researchers reverse the process: they feed hot steam into the reactor to turn the iron and water back into iron oxide and hydrogen. The hydrogen can then be converted into electricity or heat in a gas turbine or fuel cell. To keep the energy required for the discharging process to a minimum, the steam is generated using waste heat from the discharging reaction.

Cheap iron ore meets expensive hydrogen

“The big advantage of this technology is that the raw material, iron ore, is easy to procure in large quantities. Plus, it doesn’t need processing before we put it in the reactor,” Professor Stark said.

Moreover, the researchers assume that large iron ore storage facilities could be built worldwide without substantially influencing the global market price of iron.

The reactor where the reaction occurs doesn’t have to fulfil any special safety requirements either. It consists of stainless steel walls just 6mm thick. The reaction occurs at normal pressure, and the storage capacity increases with each cycle. Once filled with iron oxide, the reactor can be reused for any number of storage cycles without replacing its contents. Another advantage of the technology is that the researchers can easily expand the storage capacity: it’s a case of building bigger reactors and filling them with more iron ore. All these advantages make this storage technology an estimated ten times cheaper than existing methods.

However, there’s also a downside to using hydrogen: its production and conversion are inefficient compared to other energy sources, as up to 60 per cent of its energy is lost. This means hydrogen is most attractive as a storage medium when sufficient wind or solar power is available, and other options are off the table. That is especially the case with industrial processes that can’t be electrified.

Pilot plant on the Hönggerberg campus

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

“The pilot plant can store around ten megawatt hours of hydrogen over long periods. Depending on how you convert the hydrogen into electricity, that’ll give you somewhere between four and six megawatt hours of power,” a doctoral student in Stark’s research group, Samuel Heiniger, said.

This corresponds to the electricity demand from three to five Swiss single-family homes in winter. Currently, the system is still running on electricity from the grid and not on the solar power generated on the Hönggerberg campus.

This is soon set to change: the researchers want to expand the system so that by 2026, the ETH Hönggerberg campus can meet one-fifth of its winter electricity requirements using its solar power from the summer. This would require reactors with a volume of 2,000 m³ , which could store around 4GWh of green hydrogen. Once converted into electricity, the stored hydrogen would supply around 2GWh of power.

“This plant could replace a small reservoir in the Alps as a seasonal energy storage facility. To put that in perspective, it equates to around one-tenth of the capacity of the Nate de Drance pumped storage power plant,” Professor Stark said. In addition, the discharging process would generate 2GWh of heat, which the researchers want to integrate into the campus’s heating system.

Good scalability

But could this technology be harnessed to provide seasonal energy storage for Switzerland? The researchers have made some initial calculations: providing Switzerland with around ten terawatt hours (TWh) of electricity from seasonal hydrogen storage systems every year in the future – which would admittedly be a lot – would require some 15–20TWh of green hydrogen and roughly 10,000,000 m³ of iron ore.

“That’s about two per cent of what Australia, the largest producer of iron ore, mines annually,” Professor Stark said. By comparison, in its Energy Perspectives 2050+ plan, the Swiss Federal Office of Energy anticipates total electricity consumption of around 84TWh by 2050.

If reactors were built to store around 1GWh of electricity each, they would have a volume of roughly 1,000 m³. This calls for around 100m² of building land. Switzerland would have to build some 10,000 of these storage systems to obtain 10TWh of electricity in winter, corresponding to an area of around 1m² per inhabitant.

Image: AddMeshCube/stock.adobe.com

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