The potential is enormous and could become hugely significant worldwide: iron as a storage medium for electrical energy from renewable sources. Surplus solar and wind energy can convert rust – or, to be chemically correct, iron oxide – into iron. The energy stored in it is easy and clean to transport and can be released again through combustion. The key advantage is that this combustion can be carried out without any problems in decommissioned coal-fired power stations. ‘Burning coal in the traditional way or burning iron dust as an alternative is comparable,’ says Hasse.

This year, the team wants to prove in the demonstration power plant on the grounds of the Technical University of Darmstadt what already works in the small laboratory reactor. The three-storey power plant has been used for experiments with waste materials and biomass and is now expected to supply one megawatt of “iron energy”. The next step is already in preparation: Together with an energy supplier, Clean Circles wants to finalise plans to convert a central thermal power plant in Berlin, which has previously been powered by coal, to iron combustion. It supplies an entire district with local heating. ‘We want to show that iron is an option for the energy supply of the future,’ says Dreizler.

A major advantage is the ease with which iron and rust can be transported: 27 million tonnes of iron and 1.6 billion tonnes of rust or iron ore are already shipped by rail and ship worldwide every year. ‘This would bring us much closer to the goal of energy security,’ emphasises Hasse. After all, energy storage – whether overnight or throughout the winter – is the central issue of the energy transition. Iron is an ideal solution for this, as the metal is not consumed and can be recharged like a battery. In addition, no carbon is involved, so no CO2 is released.

With an additional application idea, the team recently won two prizes at the TU Ideas Competition 2024: Iron energy storage can not only provide a lot of heat, but also the coveted hydrogen. When iron is oxidised with water vapour – i.e. H2O – hydrogen (H2) is produced in addition to iron oxide. The team named this research idea ‘MetalH2eat’ for metal to H2 and heat. The team, led by Marius Schmidt, who holds a doctorate in engineering from TU Darmstadt and is managing director of Clean Circles, impressed the jury with its sustainable idea. According to Schmidt, this could be another building block for the energy transition and hydrogen strategy – ‘for example, for a glassworks in the deep Black Forest that can use energy in the form of hydrogen and heat but is not connected to the relevant networks’. Such businesses could use decentralised energy from iron.

Schmidt hopes that the ideas will lead to a spin-off within the next three years. At the moment, TU Darmstadt is ideal for combining basic and applied research on a small to large scale. In addition to the large-scale facilities available at TU Darmstadt, Schmidt praises the successful simulations thanks to efficient high-performance computers and the technical know-how in areas such as burners for the best possible energy conversion. The team has also begun to secure patents for some of its ideas.

Hasse sees many questions that can be clarified in a targeted manner through basic research. For example, they are experimenting with the size of iron dust particles and with the recovery of iron from iron oxide. This is because these ‘rusty crumbs’ are not only about 40 percent larger than pure iron, but also porous and, in some cases, not ideally reusable. ‘Precise process control will be crucial here,’ says Hasse. There are also ideas to link recovery with green steel production processes.

The scientists at Clean Circles are certain that the future of energy supply lies in a mix of technologies. Long-term storage is an important aspect of this, which iron can enable, according to Schmidt. ‘It can be used to store extremely large amounts of energy over a long period of time,’ emphasises Hasse – for example, for the famous dark doldrums in the European winter. Large-scale iron storage is ideal for these purposes. ‘The first conversions of coal-fired power plants will take place from 2030 onwards,’ Hasse is certain.

Unlike hydrogen, the everyday problems – especially transport – have already been solved. This is because iron is an everyday material with a functioning infrastructure – from raw ore from Australia and Brazil to recycling as scrap metal. Rainer Hofmann, the new team member responsible for transfer management, contributes his experience in the field of hydrogen. The political scientist has ‘converted’ from hydrogen to iron: ‘There is so much potential in iron storage! Many questions that are still unanswered in the field of hydrogen have already been solved here.‘ An important goal is to address and convince decision-makers and target groups from business and politics. “We want to become more visible,” emphasises Hofmann.

’We plan successfully using case studies and incorporate political and social issues,’ says Dreizler. Clean Circles also takes a socio-economic perspective. The scientists hope that their idea will find its way into economic policy concepts: ‘Iron should also be included in technology openness.’ In any case, the next generation is enthusiastic about the topic: ‘They like to come to us and are eager to work on this future-oriented topic,’ says Schmidt happily.