ETH researchers led by Wendelin Stark have now developed a new storage technology for storing hydrogen seasonally. This type of storage is much safer and cheaper than existing solutions. To do this, the researchers use a well-known technology and the fourth most common element on earth: iron. To store hydrogen better, Stark and his team rely on the iron-steam process, which has been known since the 19th century. If there is too much solar power in the summer months, it can be used to split water to produce hydrogen. This hydrogen is then fed into a 400-degree Celsius stainless steel boiler filled with natural iron ore. There, the hydrogen removes oxygen from the iron ore – which is chemically nothing other than iron oxide – creating elemental iron and water.
“This chemical process is similar to charging a battery. This means that the energy from the hydrogen can be stored as iron and water for a long time with almost no loss,” explains Stark. If the energy is needed again in winter, the researchers reverse the process: They feed hot steam into the boiler, which turns 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. In order to use as little energy as possible for the discharge process, the waste heat from the discharge reaction is used to generate the steam. “The big advantage of the technology is that the starting material, iron ore, is easy to obtain and in large quantities. What’s more, we don’t even have to process it before we put it in the boiler,” says Stark. The researchers also assume that large iron ore storage facilities could be built worldwide without substantially affecting the world market price of iron.
The boiler 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 six millimeters thick. The reaction takes place under normal pressure and the storage capacity increases with each cycle. The boiler with iron oxide can be reused for as many storage cycles as required without having to replace the iron oxide. Another advantage of the technology is that researchers can easily increase the storage capacity. All they have to do is build larger boilers and fill them with more iron ore. All of these advantages make the storage technology estimated to be around ten times cheaper than existing processes.
However, the use of hydrogen also has a disadvantage: its production and conversion are inefficient compared to other energy sources, as up to sixty percent of the energy is lost in the process. Hydrogen is therefore particularly interesting as a storage medium when there is sufficient wind or solar power and other options are not an option. This is especially the case with industrial processes that cannot be electrified. The researchers demonstrated the technical feasibility of the storage technology using a pilot plant on the Hönggerberg campus. This consists of three 1.4 cubic meter stainless steel boilers, which the researchers filled with two to three tons of untreated iron ore available on the market.
“The pilot plant can store around ten megawatt hours of hydrogen in the long term. Depending on how the hydrogen is converted into electricity, this can produce four to six megawatt hours of electricity,” explains Samuel Heiniger, a doctoral student in Wendelin Stark’s research group. This corresponds to the electricity needs of three to five Swiss single-family homes in the winter months. The plant currently runs on electricity from the grid and not on the solar power generated on the Hönggerberg campus.
That is about to change: by 2026, the researchers want to expand the plant and cover a fifth of the electricity needs of the ETH Hönggerberg campus in winter with their own solar power from the summer. This would require boilers with a volume of 2,000 cubic meters that can store around four gigawatt hours of green hydrogen. After being converted into electricity, the stored hydrogen would supply around two gigawatt hours of electricity. “As a seasonal energy storage facility, this plant could replace a small alpine reservoir. For comparison: This would be about a tenth of the capacity of the Nate de Drance pumped storage power plant,” says Stark. In addition, two gigawatt hours of heat would be generated during discharge, which the researchers want to integrate into the campus’s heating system.
But would the technology also work for seasonal energy storage throughout Switzerland? The researchers have made initial calculations: If Switzerland were to obtain around ten terawatt hours of electricity from seasonal hydrogen storage facilities each year in the future, this would require around 15 to 20 terawatt hours of green hydrogen and around ten million cubic meters of iron ore. “This amount of iron corresponds to around two percent of what Australia, the largest producer of iron ore, mines each year,” says Stark.
If tanks were built that could each store around one gigawatt hour of electricity, they would have a volume of around 1,000 cubic meters. This would require building land of around 100 square meters. Switzerland would have to build around 10,000 of these storage tanks in order to obtain ten terawatt hours of electricity in winter, which corresponds to an area of around one square meter per inhabitant.
ETHZ / JOL
