Water Splitting Cycle For Hydrogen Production Research Articles

Process modeling of a hybrid-sulfur thermochemical cycle combined with solid oxide fuel cell/gas turbine system

This work proposes a novel design of the hybrid sulfur (HyS) thermochemical water-splitting cycle for hydrogen production. The hydrogen production plant is combined with a solid oxide fuel cell/gas turbine (SOFC/GT) system to satisfy the demand for the high-temperature heat from the HyS cycle by exploiting the heat loss of the SOFC stack and the demand for electric power for SO2-depolarized electrolysis. A detailed thermodynamic analysis of the HyS cycle and the SOFC/GT system was conducted on a flowsheet basis to determine the energy requirement and to estimate the effects of the operating conditions on the overall efficiency. As a result, it was found that the LHV efficiency of the combined HyS-SOFC/GT system can reach > 29% (34.32% HHV efficiency), with a net electric efficiency of the SOFC/GT system of 38.83% and consumption of 340.3 kJ molH2-1 high-temperature heat, 148.77 kJ molH2-1 low-temperature heat, and 134.35 kJ molH2-1 electric power.

A low-temperature electro-thermochemical water-splitting cycle for hydrogen production based on LiFeO2/Fe redox pair

The thermochemical water-splitting cycles have been paid more attention in recent years because they directly convert thermal energy into stored chemical energy as H2. However, most thermochemical cycles require extremely high temperatures as well as a temperature switch between reduction and oxidation steps, which are the main obstacles for their development. Herein, we introduced an electrochemical reaction into the thermochemical cycle and established a novel two-step water-splitting cycle based on LiFeO2/Fe redox pair. The two-step water-splitting process involves a cyclic operation of electrochemical reduction and water-splitting steps. The feasibility of the water-splitting cycle for the hydrogen production was thermodynamically and experimentally investigated. A mechanism of hydrogen production based on LiFeO2/Fe redox pair was developed. Compared with the traditional high-temperature thermochemical cycles, the electrochemical reduction and water-splitting steps of the process can be isothermally operated in the same cell at a relatively low temperature of 500 °C. The main advantages of the cycle are not only easily available heat sources without involvement of the associated engineering and materials issues, but also without any temperature swings. This is a promising method to achieve water splitting for hydrogen production in the future.

Heliostat field layout optimization for high-temperature solar thermochemical processing

The layout of the heliostat field of solar tower systems is optimized for maximum annual solar-to-chemical energy conversion efficiency in high-temperature thermochemical processes for solar fuels production. The optimization algorithm is based on the performance function that includes heliostat characteristics, secondary optics, and chemical receiver–reactor characteristics at representative time steps for evaluating the annual fuel production rates. Two exemplary applications for solar fuels production are selected: the thermal reduction of zinc oxide as part of a two-step water-splitting cycle for hydrogen production, and the coal gasification for syngas production.

Performance assessment of a combined system to link a supercritical water-cooled nuclear reactor and a thermochemical water splitting cycle for hydrogen production

In this paper, a high-temperature chemical heat pump, which employs the reversible catalytic methane conversion reaction, is proposed. The reaction shift from exothermic to endothermic and back is achieved by changing the steam concentration in the reaction mixture. This heat pump, coupled with the second steam cycle of a supercritical water (SCW) nuclear power plant on one side and a thermochemical water-splitting cycle on the other, permits the transmission of some part of the heat generated in the first steam cycle to a higher temperature level in order to transfer it into the water-splitting cycle. A comparative performance assessment through thermal and hydrogen production efficiencies are conducted of the combined system comprising a SCW nuclear power plant and a chemical heat pump, which provides high-temperature heat to a thermochemical water-splitting cycle for hydrogen production. It is concluded that the proposed chemical heat pump permits the utilization efficiency of nuclear energy to be improved by at least by 2% without jeopardizing nuclear reactor safety. The influence of the steam/methane ratio in the heat pump working medium on the efficiency of the pump is investigated. Based on this analysis, further research appears to be merited on the proposed design of a nuclear power generation plant combined with a chemical heat pump, and implementation in appropriate applications seems worthwhile.

Thermodynamic Analysis of the Use a Chemical Heat Pump to Link a Supercritical Water-Cooled Nuclear Reactor and a Thermochemical Water-Splitting Cycle for Hydrogen Production

Increases in the power generation efficiency of nuclear power plants (NPPs) are mainly limited by the permissible temperatures in nuclear reactors and the corresponding temperatures and pressures of the coolants in reactors. Coolant parameters are limited by the corrosion rates of materials and nuclear-reactor safety constraints. The advanced construction materials for the next generation of CANDU reactors, which employ supercritical water (SCW) as a coolant and heat carrier, permit improved “steam” parameters (outlet temperatures up to 625°C and pressures of about 25 MPa). An increase in the temperature of steam allows it to be utilized in thermochemical water splitting cycles to produce hydrogen. These methods are considered by many to be among the most efficient ways to produce hydrogen from water and to have advantages over traditional low-temperature water electrolysis. However, even lower temperature water splitting cycles (Cu-Cl, UT-3, etc.) require an intensive heat supply at temperatures higher than 550-600°C. A sufficient increase in the heat transfer from the nuclear reactor to a thermochemical water splitting cycle, without jeopardizing nuclear reactor safety, might be effectively achieved by application of a heat pump, which increases the temperature of the heat supplied by virtue of a cyclic process driven by mechanical or electrical work. Here, a high-temperature chemical heat pump, which employs the reversible catalytic methane conversion reaction, is proposed. The reaction shift from exothermic to endothermic and back is achieved by a change of the steam concentration in the reaction mixture. This heat pump, coupled with the second steam cycle of a SCW nuclear power generation plant on one side and a thermochemical water splitting cycle on the other, increases the temperature of the “nuclear” heat and, consequently, the intensity of heat transfer into the water splitting cycle. A comparative preliminary thermodynamic analysis is conducted of the combined system comprising a SCW nuclear power generation plant and a chemical heat pump, which provides high-temperature heat to a thermochemical water splitting cycle for hydrogen production. It is concluded that the proposed chemical heat pump permits the utilization efficiency of nuclear energy to be improved by at least 2% without jeopardizing nuclear reactor safety. Based on this analysis, further research appears to be merited on the proposed advanced design of a nuclear power generation plant combined with a chemical heat pump, and implementation in appropriate applications seems worthwhile.

Modeling of a novel high-temperature solar chemical reactor

Computational fluid dynamics (CFD) is employed in the design and optimization of a novel high-temperature solar chemical reactor. The reaction considered is the thermal reduction of metal oxides, as part of a two-step water-splitting cycle for hydrogen production. The solar reactor uses a flow of metal oxides particles under concentrated solar radiation, that serve simultaneously as energy absorbers and chemical reactants. CFD simulation offers the possibility to calculate velocity, temperature and pressure fields, and particle trajectories, which cannot be measured under the severe flux radiation (above 3000 kW/m 2) and high-temperature (above 1500 K) environment of solar furnace experiments. CFD validation is accomplished by comparison with experimental results in cold operating conditions.