So far, there has been a lack of technology to efficiently store electricity from renewable energies and hydrogen for longer periods of time. Researchers are now proposing a method that is more than 120 years old: the iron-steam process. In this method, hydrogen reduces iron ore pellets to metallic iron. If the hydrogen is needed again later, hot steam is passed over it, which oxidizes the iron again and releases the hydrogen. Tests in a pilot plant showed that this method is more space-saving and cheaper than most other currently tested storage technologies for hydrogen gas. However, after deducting the necessary energy supply, the efficiency was still rather low at just over eleven percent. However, the researchers assume that an efficiency of up to 79 percent could be achieved in larger plants. Such systems could therefore contribute to the seasonal storage of hydrogen and thus to the energy transition.
Electricity from renewable energies such as sun and wind is considered an important pillar of the energy transition. However, both energy sources are not available at the same time. In particular, the solar energy that can be used via photovoltaics fluctuates greatly with the seasons in our latitudes. While solar systems often generate more electricity in summer than the grid can absorb, there is a shortage in winter. For some time now, people have been looking for the most efficient methods possible to temporarily store the excess electricity in seasonal storage facilities. In addition to large battery storage systems, one option is to use the excess electrical energy for reversible chemical conversion processes, for example by using the electricity to generate hydrogen using electrolysis. This can then be used again in fuel cells or via combustion to generate electricity and heat in winter or at other times when there is a shortage of electricity. But for this to happen, the hydrogen must also be stored safely and efficiently for months.
Ore and hydrogen become iron and steam
This is where the pilot test by Samuel Heiniger and his colleagues from ETH Zurich comes in. For their storage method, they used a process that was already in commercial use in 1900: the iron-steam process. “This process was used to produce high-purity hydrogen from iron and water vapor,” explain the researchers. In their pilot plant, they used the reverse process to store hydrogen for longer periods and then recover it. “This chemical process is similar to charging a battery. In this way, the energy of the hydrogen can be stored as iron and water for a long time with almost no loss,” explains senior author Wendelin Stark from ETH. Specifically, the plant consists of several stainless steel tanks, each of which was initially filled with 250 kilograms of iron oxide powder – ground iron ore that had not been pretreated in any other way.
“The big advantage of the technology is that the starting material iron ore is easy to obtain and in large quantities. In addition, we don’t even have to process it before we put it in the boiler,” says Stark. The iron ore powder is then heated to around 400 degrees and the hydrogen previously produced by electrolysis using electricity is fed into it. In practice, this would happen in the summer when there is a surplus of solar power, as the team explains. A chemical reaction then takes place in the reactor in which the iron oxide is reduced to metallic iron and water vapor. “To promote the reaction, the water is drained off until the majority of the iron oxide has been converted to iron,” explain Heiniger and his colleagues. “This process represents the loading of the reactor.” In the tests, around 88 percent of the iron oxide was reduced to iron. “This corresponds to an effective volumetric storage density of 30.1 kilograms of H2 per cubic meter,” reports the team.
To discharge the hydrogen storage tank, the process is reversed: hot steam is introduced into the tank, which oxidizes the iron to iron oxide, and the remaining hydrogen is released again. In order to use as little energy as possible, the researchers used the waste heat from the discharge reaction, which also takes place at around 400 degrees, to generate the steam required for this. However, this discharge is not suitable for rapid hydrogen demand: in the test reactor, it took around a month for around half of the steam introduced to be converted into hydrogen. However, the tests showed that the iron ore can withstand repeated charging and discharging without too much degradation. The system can therefore be used for numerous storage cycles without having to be replaced, as the team reports. In addition, the tank used – unlike hydrogen tanks – does not have to meet any special safety requirements because the reaction takes place under normal pressure and there is hardly any free hydrogen left in the fully charged state.
How competitive is the process?
The crucial question, however, is how cost-effective and efficient hydrogen storage using the iron-steam process is. To determine this, Heiniger and his colleagues compared it with other common hydrogen storage methods, including storage in a liquefied state and conversion into ammonia, methane or liquid hydrocarbons. A major advantage of the iron-steam method over other hydrogen storage methods is its low cost: “The simplicity of the demonstrated process, the mild process conditions and the low price of iron ore make it a financially attractive option,” the researchers write. According to their calculations, the cost of a 400 gigawatt-hour plant is US$0.57 per kilowatt-hour of hydrogen, and a smaller 100 megawatt-hour plant is US$1.95 per kilowatt-hour.
However, the technology still has one catch: “Admittedly, the efficiency of the current, non-optimized and still experimental system is very low,” the researchers admit. If the total energy requirement is included, it is only around 11.4 percent. However, most of this energy was consumed by heat loss through the (still) uninsulated outer wall of the stainless steel tanks. “Since heat loss also decreases with the size of the tanks, efficiency can reach up to 79 percent if the system is scaled up and properly insulated,” explain Heiniger and his team. They have already built the first large pilot plant consisting of three tanks, each measuring 1.4 cubic meters. “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 will produce four to six megawatt hours of electricity,” explains Heiniger. This corresponds to the electricity requirements of three to five single-family homes in the winter months. The researchers want to expand the plant further by 2026 and use it to cover a fifth of the electricity needs of the ETH Hönggerberg campus in winter. This would require storing around four gigawatt hours of green hydrogen in summer. In winter, around two gigawatt hours of electricity could then be generated from it. In addition, two gigawatt hours of heat would be generated during the discharge, which the researchers want to integrate into the campus’s heating system.
Source: Samuel Heiniger et al. (Swiss Federal Institute of Technology in Zurich (ETH Zurich), Sustainable Energy & Fuels, doi: 10.1039/D3SE01228J