| Summary: | Zero emission fuels such as hydrogen and its carriers are essential for decarbonizing hard-to-electrify sectors like industrial process heat, long-distance transportation and seasonal energy storage. Green hydrogen can also decarbonize chemical processes in the fertilizer, petrochemicals and metal refining industries. While electrochemical water splitting has high TRL its cost is higher than future targets, especially when coupled with transient renewables.
In this thesis we consider solar thermochemical hydrogen (STCH) with redox cycles using non-stoichiometric metal oxides. This pathway uses renewable heat, including concentrated solar, coupled with thermal energy storage for continuous, round-the-clock green hydrogen production. High-temperature heat (> 1300℃) is used to reduce metal oxides like doped ferrites or ceria in a low oxygen environment. The reduced metal oxide is then used to split water at ~ 800℃ and atmospheric pressure. The large temperature and pressure swings severely penalize heat-to-fuel conversion efficiency if measures like heat recovery and pressure cascading are not implemented. Demonstrated state-of-the-art STCH systems have ~7% heat-to-fuel efficiency.
In this thesis we present a roadmap for achieving 40% heat-to-fuel STCH efficiency. At the core of our innovation is the novel Reactor Train System (RTS). This system consists of multiple sealed and insulated STCH reactors that move between reduction and oxidation zones. The RTS is modular, scalable and compatible with any high-temperature heat source and thermal energy storage. Our system, designed around the RTS, achieves high efficiency by:
i. Heat recovery from reactors between reduction and oxidation temperatures via a radiative counterflow heat exchanger (> 75% effectiveness).
ii. Radiative waste heat recovery in the oxidation zone for on-site electricity production to drive oxygen removal and hydrogen separation.
iii. Staged oxygen removal whereby the oxygen partial pressure in a reactor is reduced gradually as it traverses the reduction zone. This reduces power consumption and pump capex by 70%.
iv. Replacing mechanical pumps by thermochemical oxygen pumping (TcOP), wherein a second redox cycle is used to absorb oxygen released during the STCH reduction. TcOP increases hydrogen yield per cycle, enabling 40% heat-to-fuel efficiency.
v. Using composites of ceria and metal ferrites as the redox material. The composites combine the large oxygen carrying capacity of ferrites with superior kinetics and stability of ceria.
The work involved the development of comprehensive and computationally efficient models for the design and optimization of the RTS, that couple radiative heat exchange between reactors; heat and transfer mass and defect chemistry within porous redox material, and energy and mass integration among components that minimize entropy generation. New high-fidelity modeling tools were developed for transient radiative heat transfer in reactors having porous redox materials have low optical thickness, highly anisotropic scattering and non-uniform morphology. Our method GREENER (Generalized Radiation ExchangE factors and NEt Radiation method) achieves the same accuracy as Monte Carlo Ray Tracing with several orders of magnitude lower computational cost.
In this thesis we proposed and tested novel composite redox materials containing ceria and magnesium ferrite. Composite pellets were fabricated. A STCH test system was assembled where pellets were cycled in an infrared furnace at STCH-relevant conditions. Our result show that ceria-ferrite composites are a promising class of redox materials that can improve both efficiency and productivity compared to state-of-the-art single-phase materials.
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