Water Splitting Cycle Research Articles

Modeling Development of a Receiver–Reactor of Type R2Mx for Thermochemical Water Splitting

Abstract This work reports on the development of a transient heat transfer model for a prototype reactor of type R2Mx for thermochemical water splitting by temperature and pressure swing of ceria. Key aspects of the R2Mx concept, which are also incorporated in the prototype design, include a movable monolithic redox structure combined with a linear transport system, a reduction reactor, as well as a dedicated oxidation reactor. With the model, the operation of the prototype is simulated for consecutive water splitting cycles, in which ceria is reduced in a continuously heated reactor, oxidized in a separate oxidation reactor, and transported in between the reaction zones. A 2D axisymmetric numerical model of the prototype reactor was developed in ansys mechanical. The model includes heat transfer calculations in combination with an approximated simulation of the transport of the redox material during cyclic operation. It incorporates the chemical reaction by means of a modified heat capacity for ceria and accounts for internal radiation heat transfer inside the porous redox material by applying effective heat transfer properties. A parametric analysis has been undertaken to evaluate different modes of operation of the oxidation reactor. Model results are used to size the power demand of the reduction reactor and vacuum pump, to define durations of the process steps, as well as to assess operational parameters with respect to achieved temperatures. Findings suggest that suitable operation of the prototype reactor involves reduction durations ranging from 8 to 10 min and oxidations of 6 to 10 min.

Insights into the effect of operating parameters and hydrodynamics on hydrolysis reaction in Copper–Chlorine thermochemical water splitting cycle for hydrogen production: Modelling and experimental validation

Copper–Chlorine (Cu–Cl) thermochemical cycle is being explored as one of the promising technologies for large scale production of clean hydrogen from water using nuclear or renewable energy. In this method, water splitting is achieved through a series of reactions in a loop producing only hydrogen and oxygen with complete recycle of all the intermediate copper and chlorine compounds. The first step of the cycle is the hydrolysis reaction wherein steam reacts with solid CuCl2 to form solid Cu2OCl2 (copper oxychloride) and HCl gas. The products enter the subsequent steps of the cycle and are recycled back to complete the loop. Multiple side reactions in the hydrolysis step result in reduced yield and loss of chemicals from the cycle. Efficient reactor design and optimal operating conditions of hydrolysis reactor is imperative towards achieving sustained closed loop operation and reducing the energy demand of Cu–Cl cycle. This paper presents a mathematical model for the simulation of hydrolysis reaction in a bubbling fluidized bed reactor. The one-dimensional model is developed based on modified two-phase theory of fluidization and is coupled with kinetics of multiple side reactions to investigate the effect of operating parameters and hydrodynamics on the conversion and yield of Cu2OCl2. The model results are validated with experimental data in 50 mm and 100 mm semi-batch fluidized bed reactors. The developed model clearly brings out the effect of different process parameters for process optimization and scale-up of the hydrolysis reactor for efficient integration in the cycle.

Towards chemical equilibrium in thermochemical water splitting. Part 2: Re-oxidation

Chemical equilibrium represents the highest efficiency achievable by a thermochemical cycle under specific operational conditions. This study delves into two-step thermochemical water splitting cycles, which dissociate water into hydrogen (H2) and oxygen (O2) via sequential thermal reduction and re-oxidation processes. Building on the thermal reduction analysis presented in Part 1, this paper zeroes in on the re-oxidation reaction within the optimal reactor framework – the counter-current reactor. It elucidates the equilibrium pathway, delineating the progression of chemical equilibrium at each incremental stage of re-oxidation. Using ceria (CeO2) as a model redox-active metal oxide, the investigation analyzes the influence of operational parameters on the re-oxidation reaction. Findings reveal the theoretical feasibility of maintaining near constant temperature (±2.5 °C) during re-oxidation, achieving a total extent of reduction of 0.038 and a hydrogen conversion yield of 32%. While near adiabatic operation is also achievable, practicality is constrained by a maximum total extent of reduction of 6.5·10−3. The study also explores the economic implications of thermochemical water splitting, particularly focusing on the steam-to-hydrogen output ratio. It highlights the severe economic hurdles associated with hydrogen yields below approximately 1%. Furthermore, the investigation reveals that the addition of inert gas to the re-oxidation step offers no significant advantages. Concluding the analysis, a comparative assessment exposes negligible differences in outcomes when substituting carbon dioxide (CO2) for water (H2O) as the oxidant in low-temperature scenarios (800 °C). This comprehensive study not only advances the understanding of re-oxidation dynamics in thermochemical water splitting but also informs practical and economic considerations crucial for the advancement of this technology.

Chemical Potential Analysis as an Alternative to the van't Hoff Method: Hypothetical Limits of Solar Thermochemical Hydrogen.

The van't Hoff method is a standard approach for determining reaction enthalpies and entropies, e.g., in the thermochemical reduction of oxides, which is an important process for solar thermochemical fuels and numerous other applications. However, by analyzing the oxygen partial pressure pO2, e.g., as measured by thermogravimetric analysis (TGA), this method convolutes the properties of the probe gas with the solid-state properties of the examined oxides, which define their suitability for specific applications. The "chemical potential method" is here proposed as an alternative. Using the oxygen chemical potential ΔμO instead of pO2 for the analysis, this method does not only decouple gas-phase and solid-state contributions but also affords a simple and transparent approach to extracting the temperature dependence of the reduction enthalpy and entropy, which carries important information about the defect mechanism. For demonstration of the approach, this work considers three model systems; (1) a generic oxide with noninteracting, charge-neutral oxygen vacancy defects, (2) Sr0.86Ce0.14MnO3(1-δ) alloys with interacting vacancies, and (3) a model for charged vacancy formation in CeO2, which reproduces the extensive experimental TGA data available in the literature. The reduction behavior of these model systems obtained from the chemical potential method is correlated with simulated results for the thermochemical water splitting cycle, highlighting the exceptional behavior of CeO2, which originates from defect ionization. The theoretical performance limits for solar thermochemical hydrogen within the charged defect mechanism are assessed by considering hypothetical materials described by a variation of the CeO2 model parameters within a plausible range.

Thermocatalytic Hydrogen Production from Water at Boiling Condition.

Thermochemical water-splitting cycles are technically feasible for hydrogen production from water. However, the ultrahigh operation temperature and low efficiency seriously restrict their practical application. Herein, one-step and one-pot thermocatalytic water-splitting process is reported at water boiling condition catalyzed by single atomic Pt on defective In2O3. Water splitting into hydrogen is verified by D2O isotopic experiment, with an optimized hydrogen production rate of 36.4 mmol·h-1·g-1 as calculated on Pt active sites. It is revealed that three-centered Pt1In2 surrounding oxygen vacancy as catalytic ensembles promote the dissociation of the adsorbed water into H, which transfers to singlet atomic Pt sites for H2 production. Remaining OH groups on adjacent In sites from Pt1In2 ensembles undergoes O─O bonding, hyperoxide formation and diminishing via triethylamine oxidation, water re-adsorption for completing the catalytic cycle. Current work represents an isothermal and continuous thermocatalytic water splitting under mild condition, which can re-awaken the research interest to produce H2 from water using low-grade heat and competes with photocatalytic, electrolytic, and photoelectric reactions.

An integrated alkali metal thermoelectric converter with sodium hydroxide thermoelectric water splitter and organic Rankine cycle for efficient power and hydrogen production

This study proposes the integration of the NaOH thermochemical water splitting cycle with both an alkali metal thermoelectric converter and an organic Rankine cycle. The objective of the proposed model is to enhance the overall efficiency by utilizing the waste heat generated by the NaOH thermochemical water splitting process and redirecting the heat from the alkali metal thermoelectric converter using bypass when hydrogen demand is low.Performance parameters such as system efficiency, system power, hydrogen production, AMTEC power production, and ORC power production were examined in the study. The results indicated an increase in power and hydrogen production with higher hot-end temperatures, along with an overall reduction in system efficiency for low values of load resistance. The system achieved a maximum efficiency of 53.37%.The system efficiency reached its peak at an AMTEC hot end temperature of 1173 °C, achieving 53.37 % efficiency at an RL value of 1.01 O. When the system operates at its maximum AMTEC power production, the AMTEC modules supply 12.81 kW, while the ORC generates 5.57 kW, and the hydrogen compressor consumes 0.13 kW. The total electrical power generation is 18.38 kW. The findings also indicate that the utilization of the bypass led to a reduction in hydrogen production while increasing the power output of the organic Rankine cycle.

Continuous multiphase Bunsen reactor of iodine-sulfur thermochemical water splitting cycles for hydrogen production: Experimental, Modelling and Design Insights

Thermochemical water splitting cycles (TWSCs) hold promise for sustainable H2 production in future energy systems, with Iodine-Sulfur (IS) and Nickel-Iodine-Sulfur (NIS) processes standing out as efficient options. The core of both these processes lies in the Bunsen reaction, underscoring the need to identify optimal operational conditions and plant solutions for this crucial reaction section. Thus, in this study, a continuous counter-current flow Bunsen reactor fed with 5 NL h−1 of SO2 was constructed and tested. An inventive aspect was the introduction of solid I2 pellets from the top to enhance saturation at the liquid-liquid interface between sulfuric and hydriodic product phases, promoting segregation. This ingenious design achieved nearly complete SO2 conversion and optimal H2SO4 and HI concentrations in the respective product phases, while avoiding undesirable byproduct formation. The study further advanced the modelling of the experimental reactor by employing innovative kinetic and liquid-liquid equilibrium description approaches, and demonstrating exceptional validation accuracy of R2 = 0.9988. Additionally, with the aim of obtaining design charts as a potential tool for Bunsen reactor design, a microscopic model describing the gas phase trend along bubble or packed Bunsen-type reactors was developed. The model’s dimensionless analysis revealed a characteristic times ratio (Bu) comprehensively describing all the phenomena occurring to the gas phase within the Bunsen reaction section, and identified an optimal Bu value of 0.4 for pure SO2 gas inlet. Overall, the study's innovative strategies and models contribute significantly to the advancement of Bunsen reactor engineering.

Thermodynamic analysis on CSP integrated cerium oxide (CeO2 − CeO1.72/1.83) water splitting cycle for hydrogen production

Solar thermochemical CeO2-based H2O splitting cycle was thermodynamically analyzed to ascertain the optimal thermal reduction (TH) and re-oxidation (TL) temperatures and evaluate the solar-to-fuel conversion efficiency (ηsolar−to−fuel) of the cycle. The equilibrium composition of CeO2, CeO1.72,CeO1.83 and O2 exhibited that the thermal reduction initiated at 1400 K thereby attaining 100 % TR at 2734 K. The observed variations in Gibbs free energy (ΔG) correspond to the feasibility of the re-oxidation step of CeO1.83 and CeO1.72 at 1050 K and 1200 K respectively. It was reported that the absorption efficiency of solar reactor (ηabs−solar−reactor−WS) decreases from 95.6 % to 37 %, as the TH increases from 1400 K to 2734 K. The results show that ηsolar−to−fuel reached the maximum value of 7.45 % for CeO1.72 and 7.69 % for CeO1.83 at 61.72 % TR of CeO2 without heat recuperation. Further, the ηsolar−to−fuel attained the maximum value of 14.92 % and 13.54 % for CeO1.83 and CeO1.72, respectively at the %TR of CeO2 of 80 % with 50 % heat recuperation. The solar-to-fuel conversion efficiency (ηsolar−to−fuel) further can be increased with the optimization of oxygen partial pressure (PO2) and molar flow rate of reduction regents (Ar or N2).

H2 producing hybrid solar thermochemical ZnSO4/ZnO water splitting cycle: Thermodynamic efficiency analysis

This investigation reports the thermodynamic efficiency analysis of a hybrid ZnSO4/ZnO water-splitting cycle. The data required for efficiency calculations are gathered from HSC Chemistry Software. The prime goal is to examine the effects of the thermal energy needed for heating inert sweep gas and separating the gaseous components in the cycle on the solar-to-fuel energy conversion efficiency. Complete dissociation of ZnSO4 into ZnO, SO2, and O2 is considered. Besides, a comprehensive re-oxidation of ZnO in the presence of SO2 and H2O is assumed. A thermodynamic process model, which includes reduction and oxidation reactors, two separators, multiple gas-to-gas heat exchangers, two heaters, and one ideal H2/O2 fuel cell, is developed and used to analyze the efficiency. The energy required for the separation of inert/O2/SO2 gas mixture is observed to be one of the significant contributors to the energy needed to drive the ZnSO4/ZnO WS cycle. The solar power required (989.9 kW) is recorded to be the lowest for the inert gas molar flow rate equal to 10 mol/s. Because of the low solar energy requirement, the maximum solar-to-fuel energy conversion efficiency (28.4 %) is attained at the thermal reduction temperature of 1445 K.

Thermal design and performance optimization of the four-step Cu–Cl cycle coupled with clean energy for hydrogen production

The four-step Cu–Cl cycle, one of the most promising thermo-electrochemical water-splitting cycles for producing hydrogen, is improved with its energy utilization intensified based on the integration with the Organic Rankine Cycle. Equations are deduced to analyze the influence of reaction temperatures on the system performance. With the reaction thermodynamic equilibrium and the two phase-transitions of molten CuCl considered, the mathematical model is developed and can be applied to conduct a comprehensive analysis of utilities, reaction temperatures, current density, energy/exergy efficiency, and economic performance and facilitate the evaluation and optimization of the improved system. The improved system's heating utility consumption, cooling utility consumption, and energy efficiency are 43.68 MW, 28.76 MW, and 13.57%, respectively. After the optimization, the heating and cooling utility consumptions are reduced by 12.41% and 18.08%, respectively, and the energy efficiency is increased by 14%. Six different clean-energy-based power generation systems are integrated with the improved process. Their optimal LCOHs (Levelized cost of hydrogen) are reduced by 13.29%, 15.35%, 16.05%, 16.53%, 22.93%, and 20.44%, respectively. The integrated system with power generated based on hybrid bifacial photovoltaic (BiPV) and geothermal has the highest exergy efficiency (63.24%) and minimum LCOH (3.30 $/kg); the corresponding hydrolysis, decomposition, electrolysis, and drying temperatures, and current density are 400 °C, 520 °C, 76 °C, 100 °C and 2,000 A/m2. The results can guide the further improvement of the Cu–Cl cycle.

Energy and exergy analysis of a novel solar-hydrogen production system with S–I thermochemical cycle

A novel hydrogen production system based on a sulfur-iodine (S–I) thermochemical water-splitting cycle driven by 100% solar energy is proposed in this paper. A solar power tower (SPT) with thermal energy storage (TES) is used to supply heat stably. Considering the large heat recovery potential in the S–I cycle, a supercritical carbon dioxide (S–CO2) Brayton cycle is used to improve the system performance. A complete thermodynamic model and detailed description of the energy and exergy flow from the sun to SPT, TES, S–I cycle, S–CO2 Brayton cycle and hydrogen is obtained. The system performance in the base case and actual weather conditions are obtained. The influence of major parameters on the system performance is investigated. The results show that the hydrogen production rate is 3.52 mol s−1 in the base case. Using the S–CO2 Brayton cycle, the overall energy and exergy efficiency can be increased from 10.6%, 11.1% to 30.5%, 32.4%, respectively. The solar part accounts for 48.2% of the total energy loss and 76.1% of the exergy destruction. In addition, increasing DNI, the maximum reaction temperature, the regeneration ratio of the S–CO2 Brayton cycle (RR) can improve the system performance, and the system efficiency is more sensitive to RR.

Investigation of the hydrogen production of the PACER fusion blanket integrated with Fe–Cl thermochemical water splitting cycle

In order to meet the energy demand, energy production must be done continuously. Hydrogen seems to be the best alternative for this energy production, because it is both an environmentally friendly and renewable energy source. In this study, the hydrogen fuel production of the peaceful nuclear explosives (PACER) fusion blanket as the energy source integrated with Fe–Cl thermochemical water splitting cycle have been investigated. Firstly, neutronic analyzes of the PACER fusion blanket were performed. Necessary neutronic studies were performed in the Monte Carlo calculation method. Molten salt fuel has been considered mole-fractions of heavy metal salt (ThF4, UF4 and ThF4+UF4) by 2, 6 and 12 mol. % with Flibe as the main constituent. Secondly, potential of the hydrogen fuel production as a result of the neutronic evaluations of the PACER fusion blanket integrated with Fe–Cl thermochemical cycle have been performed. In these calculations, tritium breeding (TBR), energy multiplication factor (M), thermal power ratio (1−ψ), total thermal power (Phpf) and mass flow rate of hydrogen (m˙H2) have been computed. As a results, the amount of the hydrogen production (m˙H2) have been obtained in the range of 232.24x106 kg/year and 345.79 x106 kg/year for the all mole-fractions of heavy metal salts using in the blanket.

Thermodynamic analysis of a novel two-step high temperature thermo-electrochemical water splitting cycle

Thermodynamic analysis of a novel two-step high temperature thermo-electrochemical water splitting cycle

Advancing production of hydrogen using nuclear cycles - integration of high temperature gas-cooled reactors with thermochemical water splitting cycles

The usage of heat from the nuclear cogeneration plant and hydrogen production technologies based on thermochemical water splitting was analyzed. Two most promising ones – copper-chlorine (Cu–Cl) and sulfur-iodine (S–I) cycles have been studied. The main advantage of the two analyzed thermochemical technologies is that they can be completely or partially independent from the electricity required for the process. The Cu–Cl method has a high ability to reduce the enthalpy of technological heat even to temperatures of up to 120 °C, enabling high output of the produced hydrogen. It is not demonstrated by the S–I technology, which can reduce the temperature of the working medium to approximately 380 °C, although it exhibits the limited demand for electricity. Considering the working conditions of the planned demonstration plant it is recommended to install Cu–Cl cycle due to its high level of utilization of the available heat and no requirement for additional medium heating.

Hydrogen production by isothermal thermochemical cycles using La0.8Ca0.2MeO3±δ (Me = Co, Ni, Fe and Cu) perovskites

Solar-driven thermochemical water splitting has the potential to transform concentrated solar energy into green hydrogen and other solar fuels. In this work, La0.8Ca0.2MeO3±δ (Me = Co, Ni, Fe and Cu) perovskites have been synthesised by a modified Pechini method and evaluated as materials for hydrogen production by two step thermochemical water splitting cycles. Performing the thermal reduction at temperatures of 1200 and 1000 °C, while the oxidation is done at 800 °C, allows a remarkable and stable hydrogen production after 5 consecutive cycles. However, the perovskites suffer changes in the structure after each redox cycle, with potential effects in the long-term cyclic operation. On the contrary, the isothermal thermochemical cycles at 800 °C produce a stable amount of hydrogen with each consecutive cycle maintaining the perovskite structure. This hydrogen production ranges from 3.60 cm3 STP/gmaterial·cycle for the material with the lowest productivity (La0.8Ca0.2FeO3±δ) to 5.02 cm3 STP/gmaterial·cycle for the one with the highest activity (La0.8Ca0.2NiO3±δ). Particularly the Ni-based material shows the highest H2 productivity accompanied by very good material stability after 15 consecutive cycles, being possible to combine with current solar thermal facilities based on concentrated solar power technologies like plants with central receivers.

Heat transfer characteristics and design aspects of thermolysis reactor in copper-chlorine thermochemical water splitting cycle

Thermolysis of copper oxychloride is the highest temperature reaction step of Copper–Chlorine cycle which forms cuprous chloride and oxygen gas as products. In the present study, important design aspects of thermolysis reactor have been investigated experimentally and a mathematical model has been developed to understand the effect of critical parameters on conversion. Experiments were conducted in tubular reactors with three different diameters of 25 mm, 50 mm and 100 mm to study the effect of reactor geometry on heat transfer and conversion. Modelling of batch thermolysis reactor has been carried out using governing heat transport equations coupled with reaction kinetics and validated against experimental data. As per the experimental and modelling results, enhancement in conversion could be achieved with increasing aspect ratio and heat transfer. The studies provide clear insights into the design and scale-up of thermolysis reactor to address the issues of low conversion observed during the multi-step thermal decomposition reaction.

Simulation and optimization of an innovative thermochemical water splitting cycle for the production of greenhydrogen

Highly efficient and cost-effective green Hydrogen production is a crucial challenge in a future sustainable economy. The innovative Nickel–Iodine-Sulphur (NIS) thermochemical cycle is a potential candidate for this purpose. For the first time in literature, this paper provides a complete Aspen Plus V10 simulation of the closed NIS cycle. Whitin the cycle, the Bunsen reactor was simulated as a PFR with a kinetic constant k = 1.38 m3/mol·s, while the other four reactors were considered at the thermodynamic equilibrium. A simulation of the conventional IS cycle was used as a benchmark for the NIS cycle, whose process parameters were optimized through a sensitivity study. Finally, the thermal efficiency without heat recovery of both cycles was calculated through an energy balance. The simulation results shows that the NIS cycle simulation achieves the same stoichiometric hydrogen yield as the IS cycle and a 17% higher thermal efficiency than the latter.

The water splitting cycle for hydrogen production at photo-induced oxygen vacancies using solar energy: experiments and DFT calculation on pure and metal-doped CeO2

Water splitting driven by photo-induced oxygen vacancies for H2 production was proved to be facile on metal doped CeO2.

Potential of solar thermochemical water-splitting cycles: A review

Hydrogen fuel is a valuable tool to achieve the energy transition process, and according to the 2050 net zero emissions scenario its demand is expected to increase by more than 530 Mt H2. This article discusses several routes available to produce hydrogen fuel, with a special focus on solar thermochemical cycles for Water Splitting (WS). Solar thermochemical WS cycles are a potential technology to produce green hydrogen and CO in the future due to their great potential to become a commercial scale process. This technology is still under development and some challenges related to components and sub-processes are being addressed by several research groups in order to make green hydrogen production by this route technically and economically feasible. Specific technological aspects such as particle heating methods, suitability and properties of redox materials for two-step cycles, methods to achieve low oxygen pressure in the reduction chamber, as well as the importance of implementing heat recovery are analysed in detail, as their performance have a significant effect on the overall efficiency and economic of the whole process. Current progress indicates that the realisation of a sufficiently efficient thermochemical cycle is possible within the few years, if the mentioned limitations are overcome.

The waste heat of a biofuel-powered SOFC for green hydrogen production using thermochemical cycle; Economic, environmental analysis, and tri-criteria optimization

Biofuels are becoming more and more attractive recently due to their environmental benefits and various applications. In transport section and automotive industries, the biofuels and hydrogen energy are considered promising alternatives for decarburization. In the present research, two biofuel-driven systems for cogeneration of hydrogen and electricity, are suggested and investigated, in which biomass digestion or biomass gasification are compared to provide the required biofuel. The suggested schemes are comprised of SOFC units and thermochemical water splitting cycles, fueled with either biogas or syngas. To comprehensively compare the two schemes, thermoeconomic investigations are established to assess thermodynamic efficiencies as well as produced electricity and hydrogen costs. To examine adverse impacts on the environment, the greenhouse gas emission index is also considered. A sensitivity appraisal is presented for studying the influence of vital design/operation factors on systems’ responses and performances. Finally, triple-objective optimization is implemented to detect the best practical operation of the systems for achieving maximum efficiency and minimum emission and product costs. The obtained results under the optimum operation of the systems revealed an interesting feature. It is concluded that each of the two examined systems might be preferred to the other one regarding the desired objective. For instance, if higher hydrogen production (with lower prices) is favored over electricity production, then the gasification-derived scheme is chosen; however, the digestion-based scheme must be selected to gain more electricity.