4.23 Solar Thermochemical Energy Conversion

Thermochemical hydrogen cogeneration using heat of molten salt nuclear reactors (MSRs) is discussed in this paper. Sulfur-iodine and copper-chlorine cycles are taken as typical examples for analysis and discussion. It is found that the heat exchanger design is predominately determined by the maximum and range of temperatures of themochemical hydrogen production cycles with MSRs. Copper-chlorine (Cu-Cl) thermochemical cycles can link with most MSRs, but sulfur-iodine (S-I) cycles can only link with very high temperature MSRs. The location of extracted heat from MSRs to S-I and Cu-Cl cycles is investigated, and its influence on the layout of nuclear reactor coolant loop is discussed. Some conceptual designs of heat exchangers are proposed to transfer heat from MSRs to Cu-Cl and S-I cycles. The available heat quantity at different hours of a day and corresponding hydrogen production scales are determined. It is found that the available heat at most hours of power demand in a day is equivalent to the hydrogen cogeneration capacity of an industrial scale steam methane reforming plant, if an MSR power station is operating at an invariable maximum power, independent of an electrical load throughout a day or year.

Energy and exergy analyses of magnesium-chlorine (Mg-Cl) thermochemical cycle

A review of the integration of the copper-chlorine cycle with other systems for hydrogen production

Simulation study about the effect of pressure on purification of H2SO4 and HIx phases in the iodine–sulfur hydrogen production cycle

A sulfur family chemical cycle having ammonia as the working fluid and reagent was developed as a cost-effective and efficient hydrogen production technology based on a solar thermochemical water-splitting cycle. The sulfur ammonia (SA) cycle is a renewable and sustainable process that is unique in that it is an all-fluid cycle (i.e., with no solids handling). It uses a moderate temperature solar plant with the solar receiver operating at 800°C. All electricity needed is generated internally from recovered heat. The plant would operate continuously with low cost storage and it is a good potential solar thermochemical hydrogen production cycle for reaching the DOE cost goals. Two approaches were considered for the hydrogen production step of the SA cycle: (1) photocatalytic, and (2) electrolytic oxidation of ammonium sulfite to ammonium sulfate in aqueous solutions. Also, two sub-cycles were evaluated for the oxygen evolution side of the SA cycle: (1) zinc sulfate/zinc oxide, and (2) potassium sulfate/potassium pyrosulfate. The laboratory testing and optimization of all the process steps for each version of the SA cycle were proven in the laboratory or have been fully demonstrated by others, but further optimization is still possible and needed. The solar configuration evolved to a 50 MW(thermal) central receiver system with a North heliostat field, a cavity receiver, and NaCl molten salt storage to allow continuous operation. The H2A economic model was used to optimize and trade-off SA cycle configurations. Parametric studies of chemical plant performance have indicated process efficiencies of ~20%. Although the current process efficiency is technically acceptable, an increased efficiency is needed if the DOE cost targets are to be reached. There are two interrelated areas in which there is the potential for significant efficiency improvements: electrolysis cell voltage and excessive water vaporization. Methods to significantly reduce water evaporation are proposed for future activities. Electrolysis membranes that permit higher temperatures and lower voltages are attainable. The oxygen half cycle will need further development and improvement.

Economic comparison of solar hydrogen generation by means of thermochemical cycles and electrolysis

A dual-scale transport model of the porous ceria based on solar thermochemical cycle water splitting hydrogen production

Inspired by the promising hydrogen production in the solar thermochemical (STC) cycle based on non-stoichiometric oxides and the operation temperature decreasing effect of methane reduction, a high-fuel-selectivity and CH4-introduced solar thermochemical cycle based on MoO2/Mo is studied. By performing HSC simulations, the energy upgradation and energy conversion potential under isothermal and non-isothermal operating conditions are compared. In the reduction step, MoO2:CH4 = 2 and 1020 K < Tred < 1600 K are found to be most favorable for syngas selectivity and methane conversion. Compared to the STC cycle without CH4, the introduction of methane yields a much higher hydrogen production, especially at the lower temperature range and atmospheric pressure. In the oxidation step, a moderately excessive water is beneficial for energy conversion whether in isothermal or non-isothermal operations, especially at H2O: Mo = 4. In the whole STC cycle, the maximum non-isothermal and isothermal efficiency can reach 0.417 and 0.391 respectively. In addition, the predicted efficiency of the second cycle is also as high as 0.454 at Tred = 1200 K and Toxi = 400 K, indicating that MoO2 could be a new and potential candidate for obtaining solar fuel by methane reduction.

Aluminum-doped strontium ferrites for a two-step solar thermochemical air separation cycle: Thermodynamic characterization and cycle analysis

Process optimization and safety assessment on a pilot-scale Bunsen process in sulfur–iodine cycle

Canadian advances in the copper–chlorine thermochemical cycle for clean hydrogen production: A focus on electrolysis

The DOE Nuclear Hydrogen Initiative has selected two sulphur cycles, the sulphur iodine (SI) cycle and the HyS process, as the first priority thermochemical processes for development and potential demonstration with the next generation nuclear plant. Both cycles share a common high temperature reaction step – the catalytic thermal decomposition of sulphuric acid. However, they are fundamentally different in the methods used for the hydrogen production step.

Structure and SO3 decomposition activity of CeVO4/SiO2 catalysts for solar thermochemical water splitting cycles

Dewatering of HIx solutions by pervaporation through Nafion® membranes

Water splitting by Sulfur–Iodine (S–I) cycle is one of the promising thermochemical processes for hydrogen production due to its high efficiency. The decomposition of H2SO4 to produce SO2 is the reaction with the highest energy demand in the S–I cycle and it shows a large kinetic barrier. Sulfuric acid is highly corrosive and its endothermic decomposition needs elevated temperatures (>800 °C). Henceforth, before the scale-up of the process plant there is a need to explore various materials of construction under very harsh acidic environments and phase changing conditions. Corrosion studies on some of the possible materials of construction (SS-304, SS-310, SS-316, Inconel-800, Alloy-20, Inconel-600, Incoloy-800H, Hastelloy C-276) were performed in detail and the most corrosion resistant material is suggested for the construction of sulfuric acid decomposition unit. The studies were performed at low temperatures (60 °C and 120 °C) as well as at high temperatures (700 °C, 800 °C and 900 °C). The corrosion rates were determined using weight loss method at low as well as high temperature and by using electrochemical method at low temperature (80 °C). The phase changing condition was more severe and resulted in higher corrosion rate. Hastelloy C-276 showed the least corrosion rate. SYNOPSIS In order to identify the suitable materials of construction for sulfuric acid decomposition in the S–I cycle, corrosion rates of different materials were determined. Materials were exposed to the various phases of sulfuric acid. The phase changing conditions had the most adverse effect on the materials.