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Open Access Publications from the University of California

A Continuous Solar Thermochemical Hydrogen Production Plant Design

  • Author(s): Luc, Wesley Wai
  • et al.

The sulfur-ammonia thermochemical water-splitting cycle for hydrogen production driven by solar thermal energy is a promising technology for large-scale commercial production of hydrogen. Hydrogen is an attractive alternative to fossil fuels because it is environmentally friendly, transportable, and can be manufactured. The process utilizes the electrolytic oxidation of aqueous ammonium sulfite in the hydrogen producing half-cycle and the thermal decomposition of molten potassium pyrosulfate and gaseous sulfur trioxide in the oxygen producing half- cycle. The thermochemical cycle is an all-fluid cycle driven by solar thermal energy captured from a heliostat array focused on a receiver and required electricity is generated internally from waste heat. The only input into the process is water, and the only products are oxygen and hydrogen gas. A sulfur-ammonia thermochemical plant was designed and modeled with a chemical process simulator, Aspen Plus. The plant was designed to operate continuously by using a phase-change thermal-storage system with NaCl which provides large thermal capacity at 800°C. The plant model generates ̃1.7 X 10⁵ kg of hydrogen per day, which is equivalent to ̃268 MW thermal equivalent on a lower heating value basis, with a US Department of Energy efficiency of 13%. Various parameters, such as reactor operating temperature, plant pressure, and salt concentration, were varied to study to their effects on plant efficiency and performance. Plant cost estimation was also performed to estimate the projected costs of hydrogen to determine the viability of the sulfur-ammonia thermochemical plant

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