Solar thermochemical hydrogen (STCH) Processes

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There is a significant opportunity to store solar energy using hydrogen if a suitable thermochemical process can be developed. Although there are literally hundreds of cycles to choose from, there are only two real ones. One, the direct thermochemical cycle based on zinc oxide, has garnered significant attention in recent years, and development has proceeded far enough that a pilot-scale reactor has been developed. However, extremely high temperatures required present significant materials challenges that may not be solvable in the near-term, and the rapid quenching step limits process efficiency.The second, the hybrid thermochemical cycle based on sulfur dioxide, combines a low-temperature electrolysis step with a higher temperature decomposition step. Recent techno-economic studies demonstrate solar to hydrogen efficiencies on the order of 15-20% and costs on the order $4.80/kg with potential to reach values of $2.00/kg for large scale solar hydrogen production using the HyS cycle. In the future, additional advances in materials and operating parameters for all feasible thermochemical cycles will need to be demonstrated for commercial adoption of these processes.