  Effective usage of renewable energy requires ways of storage and delivery to
balance energy demand and availability divergences. Carbon-free chemical energy
carriers are proposed solutions, converting clean electricity into stable media
for storage and long-distance energy trade. Hydrogen (H$_2$) is the subject of
significant investment and research. Metal fuels, such as iron (Fe), are
promising solutions for a clean energy supply, but establishing an
interconnected ecosystem still requires considerable research and development.
A model is proposed to assess the supply chain of hydrogen and iron as clean,
carbon-free energy carriers and then examines case studies of possible trade
routes between the potential energy exporters Morocco, Saudi Arabia, and
Australia and importers Germany and Japan. The work comprehends the assessment
of economic (levelized cost of electricity - LCOE), energetic (thermodynamic
efficiency) and environmental (CO$_2$ emissions) aspects, quantified by the
comprehensive model accounting for the most critical processes in the supply
chain. Sensitivity and uncertainty analyses identify the main drivers for
energy costs. Iron is shown to be lower-cost and more efficient to transport in
longer routes and for long-term storage, but potentially more expensive and
less efficient than H$_2$ to produce and convert. Uncertainties related to the
supply chain specifications and the sensitivity to the used variables indicate
that the path to viable energy carriers fundamentally depends on efficient
synthesis, conversion, storage, and transport. A break-even analysis
demonstrated that clean energy carriers could be competitive with conventional
energy carriers at low renewable energy prices, while carbon taxes might be
needed to level the playing field. Thereby, green iron is an important
potential energy carrier for long-distance trade in a globalized clean energy
market.
