The sustainable future energy system will need carbon-free, clean, and flexible energy carriers as links between a range of conventional and renewable sources and clean (low- and zero-emissions) and highly efficient end-use applications. In the future it will be even necessary to replace carbon-containing energy sources with carbon-free energy sources. In the next decades, coal and oil, at the least, should be replaced; later on, natural gas should be too; and, at the same time, biomass and bio-based fuels should be used with caution and only insofar as their sustainability is beyond doubt. When the various options are being considered, it appears that basically only two energy carriers have the desired characteristics: electricity and hydrogen. Together, these two energy carriers will become the backbone for the future highly decarbonized energy system.

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