Thermochemical Water Splitting Cycle Research Articles

A 100 kW cavity-receiver reactor with an integrated two-step thermochemical cycle: Thermal performance under solar transients

A future sustainable economy based on hydrogen will require large-scale hydrogen production processes in a CO2 emission-free way. Within this context, solar-driven thermochemical water splitting cycles have been proposed as a promising alternative for hydrogen mass production with the use of concentrated solar energy as the principal source to provide the process heat.Numerous solar thermochemical water-splitting cycles have been investigated for hydrogen production, each with different sets of operating conditions, engineering challenges, etc. Challenges remain in the demonstration of commercially viable thermochemical cycles and reactors, in particular, efficient and robust reactor designs compatible with solar concentrating systems. A 100 kWth multi-tubular cavity reactor for hydrogen production integrated in a solar tower has been used as test bed to demonstrate the technological feasibility of a thermochemical process with incident solar transients.The work presented on this paper dealt with integration of a cavity reactor into a solar concentrating system running a ferrite based solar thermochemical water splitting cycle. This work also explores the thermal performance of a cavity reactor beyond optimal operational conditions governed by transient cloud events. These conditions emerge as a relevant matter to be considered because they have not been studied so far.

Effect of an Alumina Supported Palladium Catalyst on the Magnesium Sulfate Decomposition Kinetics

The sulfur related thermochemical water-splitting cycles are an important class of chemical processes considered for hydrogen production. Recently, the magnesium sulfate thermal decomposition has been reported as a potential unit operation in one of these cycles. Therefore, some interest has been observed in the use of catalysts to lower the activation energy of such reactions. In this context, the present manuscript reports the thermodynamics and kinetics modeling results associated with this reaction system in the presence of a Pd catalyst supported over γ-Al2O3. The presence of such species is responsible for shifting the decomposition temperature to lower values in at least 100 °C. It was observed that the magnesium content is still oriented towards MgO formation. The obtained results indicate that the Pd/Al2O3 catalyst could be a good alternative in reducing the thermal decomposition temperature as its presence was responsible for diminishing the process activation energy from 368.2 to 258.8 kJ.mol−1.

Investigation of Zr, Gd/Zr, and Pr/Zr – doped ceria for the redox splitting of water

There is a renewed interest in CeO2 for use in solar-driven, two-step thermochemical cycles for water splitting. However, despite fast reduction/oxidation kinetics and high thermal stability of ceria, the cycle capacity of CeO2 is low due to thermodynamic limitations. In an effort to increase cycle capacity and reduce thermal reduction temperature, we have studied binary zirconium-substituted ceria (ZrxCe1-xO2, x = 0.1, 0.15, 0.25) and ternary praseodymium/gadolinium-doped Zr-ceria (M0.1Zr0.25Ce0.65O2, M = Pr, Gd). We evaluate the oxygen cycle capacity and water splitting performance of crystallographically and morphologically stable powders that are thermally reduced by laser irradiation in a stagnation flow reactor. The addition of zirconium dopant into the ceria lattice improves O2 cycle capacity and H2 production by approximately 30% and 11%, respectively. This improvement is independent of the Zr dopant level, up to 25%, suggesting that above 10% Zr dopant level, Zr might be displaced during the high temperature annealing process. The addition of Pr and Gd to the binary Zr-ceria mixed oxide, on the other hand, is detrimental to H2 production. A kinetic analysis is performed using a model-based analytical approach to account for effects of mixing and dispersion, and to identify the rate controlling mechanism of the water splitting process. We find that the water splitting reaction at 1000 °C and with 30 vol% H2O, for all doped ceria samples, is surface limited and best described by a deceleratory power law model (F-model), similar to undoped CeO2. Additionally, we used density functional theory (DFT) calculations to examine the role of Zr, Pr, and Gd. We find that the addition of Pr and Gd induce non-redox active sites and, therefore, are detrimental to H2 production, in agreement with experimental work. The calculated surface H2 formation step was found to be rate limiting, having activation barriers greater than bulk O diffusion, for all materials. This agrees with and further explains experimental findings.

Integration of thermochemical water splitting with CO2 direct air capture

Renewable production of fuels and chemicals from direct air capture (DAC) of CO2 is a highly desired goal. Here, we report the integration of the DAC of CO2 with the thermochemical splitting of water to produce CO2, H2, O2, and electricity. The produced CO2 and H2 can be converted to value-added chemicals via existing technologies. The integrated process uses thermal solar energy as the only energy input and has the potential to provide the dual benefits of combating anthropogenic climate change while creating renewable chemicals. A sodium-manganese-carbonate (Mn-Na-CO2) thermochemical water-splitting cycle that simultaneously drives renewable H2 production and DAC of CO2 is demonstrated. An integrated reactor is designed and fabricated to conduct all steps of the thermochemical water-splitting cycle that produces close to stoichiometric amounts (∼90%) of H2 and O2 (illustrated with 6 consecutive cycles). The ability of the cycle to capture 75% of the ∼400 ppm CO2 from air is demonstrated also. A technoeconomic analysis of the integrated process for the renewable production of H2, O2, and electricity, as well as DAC of CO2 shows that the proposed scheme of solar-driven H2 production from thermochemical water splitting coupled with CO2 DAC may be economically viable under certain circumstances.

Kinetic and hydrodynamic analyses of chemically reacting gas-particle flow in cupric chloride hydrolysis for the Cu-Cl cycle

The non-catalytic reaction of cupric chloride with steam to produce copper-oxy-chloride solid and hydrogen chloride gas is one of the most challenging steps in the copper-chlorine (Cu–Cl) thermochemical water splitting cycle of hydrogen production. Researchers at the University of Ontario Institute of Technology have designed a reactor in which a CuCl2 (aq) solution is atomized and reacted in a furnace where superheated steam is drawn in a counter-current stream. This study develops a new predictive model of the transport mechanisms of the hydrolysis reaction to predict the conversion of CuCl2(s) into the products. The hydrodynamic model estimates the residency time of the reactants in the reaction, and the kinetics predicts the gas-solid reaction through the shrinking core diffusion model (SCM). It is shown that the maximum conversion of CuCl2 is limited by the reactant velocity, reaction temperature, steam partial pressure and particle size. Results of the lab-scale experimental hydrolysis unit are presented and discussed.

Thermochemical two-step water splitting cycle using perovskite oxides based on LaSrMnO3 redox system for solar H2 production

A thermochemical two-step water-splitting cycle using perovskite oxides based on LaSrMnAlO3 was examined for hydrogen production from water using concentrated solar radiation. Concurrent Sr and Mn substitutions in La1-xSrxMnxAl1-xO3, Sr and Al substitutions in La1-ySryMn1-yAlyO3, and substitution of Al by Cr in La0.7Sr0.3Mn1-zCrzO3 were examined, and their kinetics, oxygen/hydrogen productivity, and repeatability were compared against LaSrMnAlO3. The impact of the chemical compositions of the perovskite oxides was systematically examined at a thermal reduction temperature of 1350 °C and water decomposition temperatures of 1000–1200 °C. From the viewpoints of average amounts of oxygen and hydrogen produced and the H2/O2 ratios, La0.7Sr0.3Mn0.9Cr0.1O3 and La0.7Sr0.3Mn0.8Cr0.2O3 provided the most reproducible productions of oxygen and hydrogen in the thermochemical two-step water-splitting cycle.

Enhanced CANDU reactor with heat upgrade for combined power and hydrogen production

Nuclear energy is considered a key alternative to overcome the environmental issues caused by fossil fuels. It offers opportunities with an improved operating efficiency and safety for producing power, synthetic fuels, delivering process heat and for multigeneration applications. The high-temperature nuclear reactors, although possess great potential for integration with thermochemical water-splitting cycles for hydrogen production, are not yet commercially established. Current nuclear reactor designs providing heat at relatively low temperature can be utilized to produce hydrogen using thermochemical cycles if the temperature of their thermal heat is increased. In this paper, a hybrid chemical-mechanical heat pump system is proposed for upgrading the heat of the Enhanced CANDU (EC6) reactor design to the quality required for the copper-chlorine (Cu–Cl) hybrid thermochemical water splitting cycle operating at 550–600 °C. A modification to the heat pump is proposed to bring the heat to temperature higher than 650 °C with operating coefficient of performance estimated as 0.65.

Mn-ferrite based solar thermochemical water splitting cycle: A thermodynamic evaluation

By utilizing the data obtained from the HSC Chemistry software, the thermodynamic equilibrium and efficiency analysis of the Mn-ferrite based water splitting (MFWS) cycle was conducted. All the thermodynamic calculations were performed by varying the partial pressure of O2 (PO2), thermal reduction (TH), and water splitting temperature (TL). The degree of non-stoichiometry (δ) allied with the Mn-ferrite was observed to be increased as the PO2 was decreased from 10−1 to 10−5 atm and the TH was upsurged from 1500 to 2100 K. This rise in the δ resulted into a higher amount of H2 production via water splitting (WS) reaction. To achieve upper δ, elevated TH was required and hence the solar energy required to run the MFWS cycle (Q̇solar-cycle) was observed to be higher. In contrast, as the TL was increased, due to the drop in the temperature gap between the TH and TL, the Q̇solar-cycle was reduced. The solar-to-fuel energy conversion efficiency (ηsolar-to-fuel) of the MFWS cycle was observed to be maximum when the PO2 and TH were lowest and the TL was the highest. For instance, at PO2 = 10−5 atm, TH = 1500 K, and TL = 1400 K, the uppermost ηsolar-to-fuel = 39.8% was attained in case of MFWS cycle which was higher than Ni-ferrite based water splitting (NFWS) cycle by 8.5%.

A novel three-step GeO2/GeO thermochemical water splitting cycle for solar hydrogen production

This investigation reports the thermodynamic exploration of a novel three-step GeO2/GeO water splitting (WS) cycle. The thermodynamic computations were performed by using the data obtained from HSC Chemistry thermodynamic software. Numerous process parameters allied with the GeO2/GeO WS cycle were estimated by drifting the thermal reduction (TH) and water splitting temperature (TL). The entire analysis was divided into two section: a) equilibrium analysis and b) efficiency analysis. The equilibrium analysis was useful to determine the TH and TL required for the initiation of the thermal reduction (TR) of GeO2 and re-oxidation of GeO via WS reaction. Furthermore, the influence of PO2 on the TH required for the comprehensive dissociation of GeO2 into GeO and O2 was also studied. The efficiency analysis was conducted by drifting the TH and TL in the range of 2080 to 1280 K and 500–1000 K, respectively. Obtained results indicate that the minimum Q˙solar−cycle=624.3kW and maximum ηsolar−to−fuel=45.7% in case of the GeO2/GeO WS cycle can be attained when the TR of GeO2 was carried out at 1280 K and the WS reaction was performed at 1000 K. This ηsolar−to−fuel=45.7% was observed to be higher than the SnO2/SnO WS cycle (39.3%) and lower than the ZnO/Zn WS cycle (49.3%). The Q˙solar−cycle can be further decreased to 463.9 kW and the ηsolar−to−fuel can be upsurged up to 61.5% by applying 50% heat recuperation.

Understanding water-splitting thermochemical cycles based on nickel and cobalt ferrites for hydrogen production

Two step water-splitting cycles by using metal ferrites are considered as a clean and sustainable hydrogen production method, when concentrated solar energy is used to drive the thermochemical reactions. This process involves the reduction at very high temperature of the ferrite, followed by the water reoxidation to the original phase at moderate temperature, with the release of hydrogen. In order to decrease the temperature required to decompose the oxide, mixed ferrites of the type MFe2O4 with spinel crystal structure have been examined. In this sense, ferrites with the partial substitution of Co and Ni for Fe appear as successful materials in terms of hydrogen production and cyclability. In this work, commercial Ni and synthetic Co ferrites have been subjected to two water splitting cycles. The solid products obtained after thermal reduction and water decomposition reactions have been chemically and structurally characterized by WDXRF, XRD, XPS and SEM techniques, in order to get a deeper understanding of the mechanisms controlling the water splitting process. This knowledge contributes to improve the process involved in thermochemical cycles and to understand the lower efficiencies (H2/O2) for Co ferrite thermochemical cycles in comparison with those corresponding to Ni ferrite.

Gibbs free energy [formula omitted] analysis for the Na[sbnd]O[sbnd]H (sodium-oxygen-hydrogen) thermochemical water splitting cycle

The main aim of the paper is to determine a theoretical range of operational conditions for the three chemical reactions of the NaOH (sodium-oxygen-hydrogen) thermochemical water splitting cycle by performing a Gibbs free energy (ΔG) analysis. ΔG is a thermodynamic parameter that allows determining if a chemical reaction is able to occur due to its enthalpy (ΔH) and entropy (ΔS) changes under specific conditions of pressure (p) and temperature (T). The methodology consists of implementing in Engineering Equation Solver (EES) software equations related to ΔH and ΔS for the three chemical reactions of the NaOH process, considering specific conditions of p and T with proper thermodynamic data to calculate ΔG. The main obtained results are: reaction 1, hydrogen production step, has ΔG < 0 for any values of T and p considered in the study; it can be performed at 100 °C under 1 bar; reaction 2, metal separation step, only can proceed under vacuum atmosphere at around 500 °C; reaction 3, hydrolysis step, always has negative Gibbs free energy despite pressure and temperature values and it can be carried out under 1 bar at 25 °C. Moreover, the methodology described in this paper could be used to investigate operational conditions for thermochemical processes.

Solar driven CdSO4/CdO thermochemical water splitting cycle for hydrogen generation

The CdSO4/CdO thermochemical water splitting (WS) cycle was thermodynamically scrutinized by utilizing the HSC Chemistry software and the obtained results are reported in this paper. Thermodynamic equilibrium analysis was performed to identify a) the temperatures needed to initiate the thermal reduction (TR) of CdSO4 and WS reaction and b) the equilibrium molar compositions associated with both steps. The obtained results shows that an increase in the molar flowrate of Ar (n˙Ar) from 1 to 50 mol/s significantly reduced the temperature (TH) required for the complete TR of CdSO4 from 1489 to 1286 K. The efficiency analysis was carried out to estimate the various process parameters as a function of variation in the TH, n˙Ar, and WS temperature (TL). The solar-to-fuel energy conversion efficiency (ηsolar−to−fuel) of this cycle was decreased with the drop in the TH and increased with the upsurge in the TL. The maximum ηsolar−to−fuel = 46.5% was achieved when the TR and WS reactions were conducted at 1489 K and 550 K, respectively. The application of heat recuperation up to 10%, 30%, and 50% was responsible for further upturn in theηsolar−to−fuel up to 48.8%, 54.4%, and 61.3%, correspondingly.

Simulation study on the microscopic characteristics of electrochemical Bunsen reaction in the sulfur–iodine cycle for renewable hydrogen production

The sulfur–iodine (SI) thermochemical water-splitting cycle is a promising hydrogen production process with high thermal efficiency and low pollution. The Bunsen reaction in the cycle using an electrochemical cell (EC) has been proposed and R&D on the cell has been carried out. To fill the knowledge gap in the microscopic characteristics of the electrochemical Bunsen reaction, a numerical study was conducted in this work. A two-dimensional, steady state, laminar and isothermal model of EC with detailed processes of flow, species transfer and electrode reactions was developed, and the simulated results showed good agreement with experimental data. The reaction rates, indicated in the form of species molar flux variation, increase with rising current density due to more electron transfer at a certain time, along with the increase of electric energy consumption from 9.1 W/m2 to 227.9 W/m2. Increasing the temperature inhibits the conversion and generation of species. Although higher flow rate reduces the flow residence time as well as the concentration variation of species, faster flow and higher bulk reactants concentration around the electrode surface lead to the increase of overall reaction rates. The consumption of pumping power also increases from 0.264 W/m2 to 6.661 W/m2 with rising flow rate. The optimal operating conditions of 4–5 A/dm2, 303–313 K and 0.01–0.02 m/s are obtained. The simulation favors a better understanding of the phenomena occurring in the EC and its further optimization.

Thermodynamic feasibility analysis of a water-splitting thermochemical cycle based on sodium carbonate decomposition

A novel water-splitting thermochemical cycle for the production of green hydrogen is introduced. This three-step cycle is based on the thermal decomposition of sodium carbonate. In this work, thermodynamic equilibrium analysis of the cycle's constituent reactions is performed, for a variety of temperatures, pressures, and dilution ratios. Feasible temperature/pressure/dilution operating windows are identified for the cycle's reactions such that the maximum temperature of the cycle can be generated using current concentrated solar power (CSP) technologies, thus enabling the replacement of steam methane reforming (SMR) as the major industrial production method of hydrogen.

First-principles investigation of BiVO3 for thermochemical water splitting

Thermochemical water splitting is a promising clean method of hydrogen production of high relevance in a society heavily reliant on fossil fuels. Using evolutionary methods and density functional theory, we predict the structure and electronic properties of BiVO3. We build on previous literature to develop a framework to evaluate the thermodynamics of thermochemical water splitting cycles for hydrogen production. We use these results to consider the feasibility of BiVO3 as a catalyst for thermochemical water splitting. We show that for BiVO3, both the thermal reduction and gas splitting reactions are thermodynamically favorable under typical temperature conditions. We predict that thermochemical water splitting cycles employing BiVO3 as a catalyst produce hydrogen yields comparable to those of commonly used catalysts.

Hydrogen production methods efficiency coupled to an advanced high-temperature accelerator driven system

Hydrogen, rather than oil, must be produced in volumes not provided by the currently employed methods. In this work, two high-temperature hydrogen production methods coupled with an advanced nuclear system are presented. A new design of a pebble-bed accelerator nuclear-driven system called TADSEA (Transmutation Advanced Device for Sustainable Energy Applications) was chosen because of the advantages in transmutation and safety. A detailed flowsheet of the high-temperature electrolysis process coupled to TADSEA through a Brayton gas cycle was developed using chemical process simulation software: Aspen HYSYS®. It is obtained 0.1627 kg/s of hydrogen with the model with optimized operating conditions, resulting in an overall process efficiency of 34.51%, a value in the range of results reported by other authors. A conceptual design of a plant using the iodine-sulfur thermochemical water splitting cycle was carried out producing 5.66e-2 kg/s and electric energy in cogeneration. The overall efficiency was calculated performing an energy balance resulting in 22.56%. A brief hydrogen production cost estimation was performed for both methods obtaining 5.96$/kg for the sulfur-iodine (SI) and 4.8 $/kg for the high-temperature electrolysis (HTE) process.

Metal sulfate decomposition using green Pd-based catalysts supported on γAl2O3 and SiC: A common step in sulfur-family thermochemical cycles

Thermochemical water splitting cycles (TWSCs) are processes with the potential for large-scale production of carbon-free hydrogen. Among these, the sulfur-family thermochemical cycles are considered the most promising due to both, the use of readily affordable chemical reagents and the temperature required to thermally decompose oxygenated sulfur compounds, which is achievable by solar means. Indeed, solar heat assisted metal sulfate decomposition is a key step, where catalysis can be employed to reduce decomposition temperature. Here we present a green route to synthesize Ag-Pd and Fe-Pd intermetallic alloy catalysts supported over γ-Al2O3 and Si-C by a microwave-assisted method using glycerol both as a solvent and as a reducing and stabilizing agent. The obtained supported catalysts were physicochemically characterized. Fe-Pd/Al2O3 catalyst exhibited the best performance, abating the zinc sulfate decomposition temperature by ca. 85 °C in comparison with other reported catalysts.

Sustainability analysis of different hydrogen production options using hesitant fuzzy AHP

Hydrogen is seen as the key component of energy systems for a sustainable future. In the literature, there has been extensive efforts on making hydrogen energy systems more sustainable. True sustainability of such systems requires hydrogen to be produced in clean, reliable, affordable, and safe manners without harming neither the environment nor the societies. In the literature, there is a lack of studies focusing on a complete technical, environmental, social, and economic evaluation of hydrogen production systems by taking availability and reliability into account. Therefore, the primary aim of this study is to provide a comprehensive review and investigation on sustainability of hydrogen production systems, which could potentially guide researchers, policy makers, different industries, and energy market customers. The selected hydrogen production methods are grid electrolysis (electricity from fossil fuels), wind electrolysis, PV electrolysis, nuclear thermochemical water splitting cycles, solar thermochemical water splitting cycles and photoelectrochemical cells. To deal with the ambiguity and vagueness in the evaluation process, and to overcome the observed hesitancy in decision makers’ preferences, a novel approach, hesitant fuzzy AHP, is used to evaluate sustainability of the selected hydrogen production methods. In the proposed model, five criteria; economic performance (initial cost and running cost), environmental performance (GHG emissions, land use, water discharge quality, and solid waste), social performance (impact on public health, employment and training opportunities, and public acceptance), technical performance (energy and exergy efficiencies, process control, and raw material input), and availability/reliability (dependence on imported resources, predictability, and scalability) are taken into account. The results show that grid electrolysis is expected to hold the key of sustainable hydrogen production in the near future while the technologies of other methods advance and their associated costs decrease.

Thermodynamic development and design of a concentrating solar thermochemical water-splitting process for co-production of hydrogen and electricity

A concentrating solar plant is proposed for a thermochemical water-splitting process with excess heat used for electricity generation in an organic Rankine cycle. The quasi-steady state thermodynamic model consisting of 23 components and 45 states uses adjustable design parameters to optimize hydrogen production and system efficiency. The plant design and associated thermodynamic model demonstrate that cerium oxide is suitable for thermochemical water-splitting cycles involving the co-production of hydrogen and electricity. Design point analyses at 900 W/m2 DNI indicate that a single tower with solar radiation input of 27.74 MW and an aperture area of 9.424 m2 yields 10.96 MW total output comprised of 5.55 MW hydrogen (Gibbs free energy) and 5.41 MW net electricity after subtracting off 22.0% of total power generation for auxiliary loads. Pure hydrogen output amounts to 522 tonne/year at 20.73 GWh/year (HHV) or 17.20 GWh/year (Gibbs free energy) with net electricity generation at 14.52 GWh/year using TMY3 data from Daggett, California, USA. Annual average system efficiency is 38.2% with the constituent hydrogen fraction and electrical fraction being 54.2% and 45.8%, respectively. Sensitivity analyses illustrate that increases in particle loop recuperator effectiveness create an increase in hydrogen production and a decrease in electricity generation. Further, recuperator effectiveness has a measurable effect on hydrogen production, but has limited impact on total system efficiency given that 81.1% of excess heat is recuperated within the system for electricity generation.

Catalytic performance of semi-coke on hydrogen iodide decomposition in sulfur-iodine thermochemical cycle for carbon dioxide-free hydrogen production

Sulfur-iodine thermochemical water splitting cycle is a promising and carbon dioxide-free method for hydrogen production. Among the reactions in this cycle, hydrogen iodide catalytic decomposition is the rate-determining step. In this study, catalytic reactivity test combined with characterizations of nitrogen physisorption, X-Ray diffraction, and Raman spectroscopy provide evidences that hydrofluoric acid modified semi-coke is a promising catalyst candidate for hydrogen iodide decomposition because of its high active and low cost. The raw semi-coke enhanced the hydrogen iodide conversion rate. After modification of hydrofluoric acid, the catalytic activity of semi-coke increased largely. Semi-coke modified by 40 wt% hydrofluoric acid showed better performance than commercial activated carbon catalyst. Combine the characterization results and catalytic reactivity, the graphitic edge carbon atoms in semi-coke are the active sites for hydrogen iodide decomposition. This finding pointed out the direction of carbon material catalyst design for hydrogen iodide decomposition.