Iron as an energy carrier for a climate-neutral circular economy

Clean Circles

Foundation for a metal-energy ecosystem

The vision: iron as a CO₂-free energy carrier

Clean Circles was a research project (2021-2025) that laid the foundation for iron-based energy storage as a key technology for the energy transition. The cluster project established TU Darmstadt, the Karlsruhe Institute of Technology, and other partners as pioneering locations for metallic energy carriers and created the scientific foundation for today’s Metal Energy Hub.

Iron has unique properties as an energy storage medium: it can store large amounts of energy, be transported safely, and produce electricity, heat, or, if required, hydrogen during “combustion” (oxidation) – without CO₂ emissions. The resulting “rust” (iron oxide) can be reduced back to iron using renewable energies – an endlessly repeatable cycle.

Four main research areas

Clean Circles was characterized by the close coupling of engineering, natural science, and social science disciplines. The scientists worked in four closely linked research fields:

Reduction processes
Storage of renewable energy through reduction of iron oxides: Development of innovative processes based on electrolysis (electrochemical) and green hydrogen (thermochemical).
Oxidation processes
Research into controlled and efficient energy release through oxidation under a wide range of process conditions.
Thermodynamic-mathematical analysis
Mathematical modeling of the entire iron cycle for energy optimization and techno-economic evaluation.
Socioeconomic considerations
Analysis of political framework conditions, governance structures, economic potential, and international partners for promotion and market launch.

Milestones and achievements

Clean Circles demonstrated the fundamental feasibility of the concept, which closes an important gap in the energy transition: the long-term storage of large amounts of energy, enabling seasonal balancing and the international transport of renewable energy. Studies have highlighted the retrofit potential of coal-fired power plants, allowing existing infrastructure to continue to be used. Some of the successes achieved in the project are:

Scientific and technological breakthroughs

Over 70 publications in leading journals examine metal-energy cycles from the atomic to the global scale
Experimental verification of the iron-energy cycle on a laboratory scale
Development of the pilot plant for industrial scaling
Estimated electricity-to-electricity efficiency of ~30% – comparable to green hydrogen
Energy system studies rate its economic efficiency positively in comparison with other energy carriers.

Public impact and transfer

Media coverage in newspapers, on television, and on YouTube with on-site visits from hr-science editors and science YouTubers
International conference Organization of the 2nd International Conference on the Metal-enabled Cycle of Renewable Energy in cooperation with TU Eindhoven and strong european and international networking
Serious game “Save Europe’s Energy”: Playful knowledge transfer about energy solutions of the future, not only for the engineers of tomorrow

The project was supported by the unique collaboration of various research groups. In addition to the coordinating Technical University of Darmstadt, the Karlsruhe Institute of Technology, Darmstadt University of Applied Sciences, Johannes Gutenberg-University Mainz, Heidelberg University, the DLR-Institute for Low-Carbon Industrial Processes, and the Max-Planck-Institute for Sustainable Materials were also involved.

Legacy and outlook

Funding from the Hessian Ministry of Science and Art and financial contributions from the project partners laid the foundation for the further development of metals as energy carriers. The scientific principles established are being directly incorporated into new projects and are being pooled at the Metal Energy Hub technology and transfer center. In this way, the interdisciplinary findings are being further developed in demonstration projects and semi-industrial pilot plants – all the way to commercial application.

Clean Circles was more than a research project; it was a strategic cornerstone. And it remains the passion of many project members and researchers: Today, we are working on tomorrow’s solutions for the energy transition.