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High Temperature Thermochemical Cycles (HYTHEC)

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Massive hydrogen production from sulphuric acid

The EU is placing ever greater emphasis on development of hydrogen-based energy for large-scale power production. Water is one of the best long-term candidates for producing hydrogen via electrolysis in so-called thermochemical cycles.


The ‘High temperature thermochemical cycles’ (Hythec) project was developed to evaluate the potential of two specific thermochemical cycles, both based on high temperature sulphuric acid (H2SO4) decomposition that was carried out via a solar primary energy source, to produce hydrogen on a very large scale for short-, medium- and long-term energy needs. The researchers chose the sulphur-iodine (S-I) cycle and the hybrid-sulphur (HyS) cycle, also known as the Westinghouse cycle, given their widespread use and thus detailed description in the literature. They gave particular emphasis to the hydrogen production step in the S-I cycle as well as to analysis of various membranes as alternative, low-energy separation methods to enhance the efficiency of pure hydrogen production. In addition to technical characterisations of the S-I cycle with particular emphasis on efficiency, the researchers carried out detailed analyses of cost and safety factors. They also constructed a membrane database and carried out flow-sheet modelling to determine optimal process parameters. Furthermore, the investigators successfully tested and demonstrated the feasibility of solar splitting of H2SO4 with reactor efficiency up to 40 %. Building on their work on the S-I cycle, the project team carried out similar analyses on the Westinghouse cycle based on coupling to a solar and/or nuclear (VHTR, very high temperature reactor) heat source. A careful cost analysis demonstrated that solar-only plants are best for small power requirements, nuclear only is most economical for large-scale production and hybrid plants using both solar and nuclear thermal energy are most advantageous for the range in between. In summary, the Hythec project proved the feasibility of splitting sulphuric acid, H2SO4, with solar energy for large-scale hydrogen production. They found that two important thermochemical cycles based on decomposition of H2SO4 and requiring very high temperatures could thus be implemented with solar heat, coupling to a VHTR or a combination of the two to efficiently and massively produce hydrogen. Commercialisation of the project outcomes should significantly decrease our dependence on fossil fuels for electricity and power, with important economic and environmental benefits.

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