Swiss scientists have developed a process for using iron to store hydrogen safely and over the long term. Energy storage systems using this technology could in future balance out the fluctuation of renewable energies.
One challenge of the energy transition with renewable energies in Switzerland, Germany and elsewhere is that, due to the unpredictable fluctuating output of renewable energies, electricity shortages must be compensated for by gas-fired power plants and imports, for example.
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, extremely volatile and makes many materials brittle. Storing the gas from summer until winter calls for special pressurised containers and cooling technology. These require a lot of energy, while the many safety precautions that must be followed make building such storage facilities very expensive. What’s more, hydrogen tanks are never completely leak-proof, which harms the environment and adds to the costs.
Now researchers at ETH Zurich led by Wendelin Stark, Professor of Functional Materials at the Department of Chemistry and Applied Biosciences, 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.
To store hydrogen better, Stark and his team are relying on the steam-iron process, which has been understood since the 19th century. If there is a surplus of solar power available in the summer months, 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 degrees 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 similar to 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,” Stark says. 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.
“The big advantage of this technology is that the raw material, iron ore, is easy to procure in large quantities. Plus it doesn’t even need processing before we put it in the reactor,” Stark says. 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 in which the reaction takes place doesn’t have to fulfil any special safety requirements either. It consists of stainless steel walls just 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 any number of storage cycles without having to replace its contents. Another advantage of the technology is that the researchers can easily expand the storage capacity. It’s simply 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 sources of energy, as up to 60 percent of its energy is lost in the process. This means that as a storage medium, hydrogen is most attractive 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.
The researchers have demonstrated the technical feasibility of their storage technology using a pilot plant on the Hönggerberg campus. This is soon set to change: the researchers want to expand the system such that by 2026, the ETH Hönggerberg campus can meet one-fifth of its winter electricity requirements using its own solar power from the summer.
Source: ETH Zurich
