Schematic of an iron reduction-oxidation cycle for energy storage, transport and use. Iron and iron oxides are used in a reduction-oxidation cycle as carbon-free carrier of renewable energy. Left: renewable energy is used to reduce iron oxides via electrochemical (i.e., electrolysis) or thermochemical processes (i.e., reacting oxides with green hydrogen), storing the energy. Right: iron is used as a fuel in a combustor to release heat, e.g. in a power plant, to generate electricity, similarly to the traditional combustion of solid fuels. Solid iron oxides (FexOy) can be collected and transported to the reduction facilities, closing the cycle.