Researchers from ETH Zürich – a public research university in Switzerland – have been exploring using iron to store hydrogen safely and for long periods.
With Switzerland set to meet more than 40% of its electricity needs with solar by 2050, the research team led by Professor of Functional Materials, Wendelin Stark, have been looking into using surplus solar power, which will likely be available in the summer months, to make hydrogen.
Using the solar to split water, this hydrogen is then fed into a stainless-steel reactor filled with natural iron ore at 400°C. In here, the hydrogen extracts oxygen from the iron ore, resulting in elemental iron and water.
Fast forward to winter when the energy is needed again, the process is reversed. Hot steam is fed into the reaction which turns the iron and water back into iron oxide and hydrogen. This hydrogen can then be converted into electricity or heat in a gas turbine or fuel cell. Furthermore, to ensure that the energy required for the discharging process is minimal, the steam used is generated through waste heat from the discharging reaction.
Iron ore is easy to produce which means the researchers anticipate large iron ore storage facilities could be built worldwide without any substantial impact on the global market price of iron. The reactor within which the reaction takes place also does not have any special safety requirements. It is just stainless steel walls and six millimeters thick, with the reaction take place at normal pressure and the storage capacity increasing with each cycle.
Once the reactor has been filled with iron oxide, it can be reused for any number of storage cycles with no need to replace its contents. The capacity can also be expanded with bigger reactors built and filled with more iron ore, with the researchers tipping their technology to be an estimated ten times cheaper than existing methods.
The research team have built a pilot plant on the Hönggerberg campus – one of two at ETH Zürich’s main location – which is made up of three stainless-steel reactors, each with a capacity of 1.4 cubic metres, each filled with between 2-3 tonnes of iron ore. From this, they have been able to store around 10MWh of hydrogen for long periods, corresponding to the electricity demand from three to five Swiss single-family homes in the winter months. The system has been running on electricity from the grid however, rather than solar power being generated on campus.
Looking ahead, the researchers want to expand their system by 2026, paving the way for the Hönggerberg campus to meet a fifth of its winter electricity requirements using its own solar power generated from the summer. This will require reactors that have a volume of 2,000 cubic metres, however, allowing for 4GWh of hydrogen to be stored. Converted back into electricity, this would see 2GWh of power, with the discharging process generating 2GWh of heat – something the researchers want to integrate into the campus’ heating system.
For Switzerland as a whole, the researchers believe seasonal hydrogen storage systems could provide it with 10TWh of electricity each year, meaning 15-20TWh of green hydrogen. This would require 10 million cubic metres of iron ore, however, which is substantial given it equates to 2% of what the largest producer of iron ore on earth – Australia – mines each year.
Building reactors capable of storing around 1GWh of electricity each would see them with a volume of around 1,000 cubic metres. This would require around 100 square metres of building land, meaning 10,000 would be needed for 10GWh of electricity in the winter. This would work out around 1 square metre per inhabitant.
Image credit: ETH Zurich
