Search

Most thermochemical cycles require complex thermal processes at very high temperatures, which restrict the production and the use of hydrogen on a large scale. Recently, thermochemical cycles producing hydrogen at relatively low temperatures have been developed in order to be competitive with other kinds of energies, especially those of fossil origin. The low temperatures required by those cycles allow them to work with heats recovered by thermal, nuclear and solar power plants. In this work, a new thermochemical cycle is proposed. This cycle uses the chemical elements Magnesium-Chlorine (Mg-Cl) to dissociate the water molecule. The configuration consists of three chemical reactions or three physical steps and uses mainly thermal energy to achieve its objectives. The highest temperature of the process is that of the production of hydrochloric acid, HCl, estimated between 350-450℃. A thermodynamic analysis was performed according to the first and second laws by using Engineering Equation Solver (EES) software and the efficiency of the proposed cycle was found to be 12.7%. In order to improve the efficiency of this cycle and make it more competitive, an electro-thermochemical version should be studied.

To achieve carbon-neutral energy vectors, researchers have investigated various sulfur-based thermochemical cycles. The sulfur–iodine cycle has emerged as a cost-effective global process with massi...

Copper–Chlorine cycle has been identified as the most prospective among the low temperature thermochemical cycles for hydrogen production. The cycle consists of two thermal reaction steps, one electrochemical step and a physical separation step. The two thermal reaction steps, hydrolysis and thermolysis are carried out in series for water splitting and oxygen production, respectively. The solid product from hydrolysis step Cu2OCl2 enters the thermolysis step where it undergoes decomposition to CuCl and O2. In the present work, thermolysis experiments were carried out in a laboratory scale horizontal furnace reactor with CuO–CuCl2 equimolar mixture and Cu2OCl2 in the temperature range of 470–575 °C. Experiments in furnace reactor show that, under otherwise same conditions, similar conversions are obtained with Cu2OCl2 as well as with the equimolar mixture of CuO–CuCl2. It was also observed that the conversion increased with an increase in CuCl2 percentage in the reaction mixture. From the experimental data, an attempt has been made to provide insights into the reaction mechanism and kinetics. These results are expected to be useful for the design and scale-up of the thermolysis reactor.

In this work, a novel approach related to the production of hydrogen using a polymer electrolyte membrane electrolysis powered by a renewable hybrid system is proposed. The investigation is carried out by establishing energy balances in the different components constituting the combined renewable system. A mathematical model to predict the production of electricity and hydrogen is proposed. The discrepancies between the numerical results and those from the literature review do not exceed 7%. The results show that the overall efficiency and the capacity factor of the combined renewable system without thermal storage are 20 and 34%, respectively. The levelized cost of hydrogen also is 6.86 US$/kg. The effect of certain physical parameters such as optical efficiency, water electrolysis temperature, unit electrolysis capital cost and solar multiple on the performance of the combined system is investigated. The results show that the performance of hydrogen production is optimal when the solar installation is three times oversized. The results also show that the levelized cost of hydrogen for the optimal sized is 4.07 US$/kg. Finally, the proposed combined system can produce low cost hydrogen and compete with hybrid sulfur thermochemical cycles, conventional photovoltaic installations, concentrated photovoltaic thermal systems and wind farms developed in all regions of the world.

In recent years, oxygen storage materials (OSMs) have been widely used in many fields. It would be particularly important for researchers to design high-oxygen-uptake/release-rate materials. In this study, various synthesis processes were used to successfully synthesize YBaCo4O7+δ and comprehensively investigate their potential applications. Compared with traditional solid-state reaction method and co-precipitation method, the results demonstrated that the utilization of mechanical ball milling treatment on co-precipitated precursors could lead to samples with reversible oxygen uptake/release under an oxidative atmosphere at low temperatures. The resultant materials exhibited fast oxygen absorption/desorption rate that could uptake/release oxygen directly to the equilibrium state within 9 min and 20 min, respectively. The mechanochemically ball-milled sample possessed outstanding oxygen storage performance, which could be attributed to their small particle size, the active outer surface of particles, large specific surface area, and relatively low activation energy. Moreover, the ball-milled sample also exhibited excellent cycling stability during relatively short time spacing. TG results also demonstrated that the ball-milled samples could reversibly uptake/release 2.90 wt.% of excess oxygen (while only 0.70 wt.% for solid-state samples) by adjusting the ambient temperature under pure O2 atmosphere, which would make them promising candidates in various applications. This research demonstrated that mechanical treatment could be an effective strategy to tune the properties and oxygen storage capacity(OSC) performances of YBaCo4O7+δ.

Solar-to-fuel technology promises to play a key role in realizing a carbon-neutral future by enabling renewable fuel processing for capacity-independent storage beyond current battery technologies....

Two-step solar thermochemical cycles based on reversible reactions of SrFeO3−δ and (Ba,La)0.15Sr0.85FeO3−δ perovskites were considered for air separation. The cycle steps encompass (1) the thermal reduction of SrFeO3−δ or (Ba,La)0.15Sr0.85FeO3−δ perovskites driven by concentrated solar irradiation and (2) oxidation in air to remove O2 and produce N2. Rate limiting mechanisms were examined for both reactions using a combination of isothermal and non-isothermal thermogravimetry for temperature-swings between 673 and 1373 K, heating rates of 10, 20, and 50 K min−1, and O2 pressure-swings between 20% O2/Ar and 100% Ar at atmospheric pressure. Evolved O2 and associated lag due to transport behavior were measured with gas chromatography and used with measured sample temperatures to predict equilibrium compositions from a compound energy formalism thermodynamic model. Measured and predicted chemical equilibrium changes in deviation from stoichiometry were compared. Rapid chemical kinetics were observed as the samples equilibrated rapidly for all conditions, indicative that heat and mass transfer were the rate limiting mechanisms. The effects of bulk diffusion (or gas diffusion through the bed or pellet) were examined using pelletized and loose powdered samples and determined to have no discernable impact.

This perspective highlights and evaluates recent key developments in the thermodynamic approach used to analyze trends in acid and base strength variation. According to this approach, acid and base strength ranking can be interpreted by using thermodynamic or thermochemical cycles. Each cycle generally consists of three independent but well-defined steps. The modus operandi described here entails the identification of the dominant step and the rationalization of its free energy/enthalpy/energy change along a selected series in terms of known structural chemical concepts. Developments in this approach are described by focusing on two related series of bases and two series of acids. In the case of the former the protonation of a series of N-heterocyclic amine bases together with their methyl-substituted analogs receives particular attention while in the case of acids, the acidic properties of aqua dications of elements in period 4 and group 2 are probed. It is illustrated how significant progress in computational chemistry and mass spectrometric techniques can be employed to compare ‘inherent’ basicity or acidity in the selected families of compounds by using simple gas-phase energy cycles. Unique, dual functions for both electronegativity (element and orbital) and charge density (for aqua cations) indicators are identified and used to evaluate these cycles. Solvent effects (in aqueous solution) are accommodated by including dehydration and hydration changes in appropriately-extended, three-step free energy cycles. It is further suggested that the dominant step in the extended thermodynamic cycle for monomeric aqua cations is the transfer of M(H2O)n2+ complex hydrates from the gas-phase to bulk water. Charge density of the aqua cations again features prominently in proposed rationalizations. Finally, this article also sheds light on salient relationships that exist between empirically and quantum-chemically estimated enthalpy and entropy changes for the aforementioned transfer process.

Hydrogen is a clean energy carrier that can help mitigate greenhouse gas (GHG) emissions if it is used to replace fossil fuels for power production. One way to produce hydrogen on a large scale is through the use of water splitting thermochemical cycles such as the hybrid copper chlorine (Cu–Cl) cycle. Canadian Nuclear Laboratories Ltd. (CNL) chose to develop the Cu–Cl cycle because the highest temperature required by this cycle is about 530 °C, compatible with the Canadian Super Critical Water Reactor (SCWR) or some small modular reactors (SMR). The on-going effort at CNL is to demonstrate a fully integrated Cu–Cl cycle at laboratory scale with a hydrogen production rate of 50 L/h. Some recent experimental results of the electrolysis step, one of the main steps of the cycle, are discussed in this paper. The anode reaction of CuCl oxidation was investigated using a three-electrode electrochemical cell. Half-cell experiments found that CuCl oxidation did not require noble metals as catalyst. The CuCl oxidation on carbon was found to be a mass-transfer controlled process. Hence the limiting current density increased with increasing turbulence on the electrode surface. Increasing the CuCl concentration and the solution temperature also resulted in higher limiting current densities. A current density of 0.53 A/cm2 was achieved for a 1.0 M CuCl solution at 80 °C. Single cells with electrode areas up to 100 cm2 were used to establish the operating conditions for the electrolysis step. The effects of flow rate, temperature, and current density on the cell voltage were studied. A hydrogen production rate of 50 L/h was successfully achieved at 0.4 A/cm2 in a 2.0 M CuCl solution at 80 °C. The electrolysis step is fully developed for integration in a laboratory-scale demonstration of the Cu–Cl cycle.

Solar thermochemical water splitting was successfully demonstrated with monolithic receiver-reactors in field at 50 kW scale. Since monolithic receiver-reactors are limited in size, several of the reactors will have to be combined in receiver-reactor arrays for large-scale plants. In this study, the yearly performance of solar thermochemical plants for hydrogen production implementing receiver-reactor arrays is investigated. Thereto, a transient receiver-reactor model is used in combination with realistic hourly flux profiles from dedicated MW high temperature solar concentrator systems. The batched operation of receiver-reactors leads to particular requirements of the array. Therefore, an array efficiency is introduced and different control strategies for the solar field are analyzed for performance optimization. Advanced strategies have the potential to substantially (~46%) improve the overall performance compared to the base case. Further design and operational optimization approaches are discussed, which allow approaching the theoretical array performance limit.

Concentrated solar thermal technology is considered a very promising renewable energy technology due to its capability of producing heat and electricity and of its straightforward coupling to thermal storage devices. Conventionally, this approach is mostly used for power generation. When coupled with the right conversion process, it can be also used to produce methanol. Indeed methanol is a good alternative fuel for high compression ratio engines. Its high burning velocity and the large expansion occurring during combustion leads to higher efficiency compared to operation with conventional fuels. This study is focused on the system level modeling of methanol production using hydrogen and carbon monoxide produced with cerium oxide solar thermochemical cycle which is expected to be CO2 free. A techno-economic assessment of the overall process is done for the first time. The thermochemical redox cycle is operated in a solar receiver-reactor with concentrated solar heat to produce hydrogen and carbon monoxide as the main constituents of synthesis gas. Afterwards, the synthesis gas is turned into methanol whereas the methanol production process is CO2 free. The production pathway was modeled and simulations were carried out using process simulation software for MW-scale methanol production plant. The methanol production from synthesis gas utilizes plug-flow reactor. Optimum parameters of reactors are calculated. The solar methanol production plant is designed for the location Almeria, Spain. To assess the plant, economic analysis has been carried out. The results of the simulation show that it is possible to produce 27.81 million liter methanol with a 350 MWth solar tower plant. It is found out that to operate this plant at base case scenario, 880685 m2 of mirror's facets are needed with a solar tower height of 220 m. In this scenario a production cost of 1.14 €/l Methanol is predicted.

Apart from being a major feedstock for chemical production, hydrogen is also a very promising energy carrier for the future energy. Currently hydrogen is predominantly produced via fossil routes, but as green energy sources are gaining a larger role in the energy mix, novel and green production routes are emerging. The most abundant renewable hydrogen sources are water and biomass, which allow several possible processing routes, such as electrolysis, thermochemical cycles and gasification. By introducing heat to the process the required electricity demand can be reduced (high temperature electrolysis) or practically eliminated (thermochemical cycles). Each renewable hydrogen production route has its own strength and weaknesses; the choice of the most suitable method is always dependent on the economical potentials and the location. The aim of this paper is to evaluate the different high temperature, renewable hydrogen production technologies.

In this paper, an atmospheric-pressure distillation system is designed and constructed for partial to separation of hydrochloric acid and water. The system concentrates HCl(aq) between the electrolyzer and hydrolysis processes of the Copper–Chlorine (Cu–Cl) cycle for hydrogen production. The motivation behind this study is to investigate azeotropic separation of HCl(aq), as needed for integration of unit operations in the Cu–Cl cycle. The separation is only partial, as the mixture is unable to cross the azeotrope with only a single pressure. The distillation system consists primarily of one packed distillation column, which employs heating tapes and thermocouples to achieve a desired axial temperature profile. The column can be operated in batch or continuous mode. The distillate is H 2O(l) and the bottoms is HCl(aq) near the azeotropic concentration; feed concentrations are less than azeotrope. Thus, the degree of separation is determined to be independent of the feed concentration. The bottoms concentration varies from experiment to experiment, but does so independently of feed concentration, likely the result of corrosion impurities affecting the calculation of its concentration. It is found that HCl(aq) can be concentrated up to approximately 0.1068 mol/mol from an initial concentration of 0.0191 mol/mol. A simulation of pressure-swing distillation (PSD) is also performed, but due to safety constraints (a column operating at 10 atm must be certified to CSA B51), a single-pressure (single-column) distillation is physically performed. A single-pressure column is beneficial to the Cu–Cl cycle because it partially recycles HCl, which reduces the cost of the cycle, and still provides valuable results for analysis. The maximum HCl concentration achieved experimentally is 0.1068 mol/mol and the maximum HCl concentration determined from simulation is 0.11 mol/mol (the azeotropic concentration). The novelty of this research is that the experimental column built to study HCl partial separation is designed to be simple yet safe to integrate within the Cu–Cl cycle for hydrogen production.

In this paper, a theoretical and experimental study on a novel cobalt-chlorine thermochemical cycle for hydrogen production is presented. The cobalt-chlorine cycle comprises a closed loop of four thermochemical reactions occurring at 700 °C that is a reaction temperature compatible with the present generation of high-temperature gas-cooled reactors. Firstly, a thermodynamic analysis was done for determining whether this cycle is attractive for hydrogen production in terms of both energy and exergy efficiencies. Following, proof-of-principle experiments were carried out at laboratory scale in a batch reactor at temperatures in the range from 550 °C to 950 °C and holding times between 1 h and 72 h. Experimental results complemented by the characterization of condensed compounds deposited on the reactor walls allowed confirm the reaction pathway of thermochemical reactions originally proposed, define the slowest step of the global process, and explain the beneficial effect of increasing the system pressure on the hydrogen yield. Even both performance assessment and proof-of-principle experimental results look like promising more research will be required in the future to confirm these preliminary findings. Finally, a modified version of the cobalt-chlorine cycle is proposed for enhancing the global kinetics, based on the experimental evidence found in the proof-of-principle tests.

HIx processing section of Iodine-Sulphur (IS) thermochemical cycle dictates the overall efficiency of the cycle, which poses extremely corrosive HI–H2O–I2 environment, coupled with a very low equilibrium conversion (~22%) of HI to hydrogen at 450 °C. Here, we report the fabrication, characterization and operation of a 4-tube packed bed catalytic tantalum (Ta) membrane reactor (MR) for enhanced HI decomposition. Gamma coated clay-alumina tubes were used as supports for fabrication of Ta membranes. Clay-alumina base support was fabricated with 92% alumina (~8 μm particle size) and 8% clay (~10 μm particle size), sintered at a temperature of 1400 °C. An intermediate gamma alumina coating was provided with 4% polyvinyl butyral as binder for a 10% solid loading. Composite alumina tubes were coated with thin films of Ta metal of thickness 80% single-pass conversion of HI to hydrogen at 450 °C. The hydrogen throughput of the reactor was ~30 LPH at a 2 bar trans-membrane pressure, with >99.95% purity. This is the first time a muti-tube MR is reported for HIx processing section of IS process.

In this study, an experimental lab-scale copper-chlorine (Cu–Cl) cycle of hydrogen production is examined and optimized in terms of exergy efficiency and operational costs of produced hydrogen. The integrated process is modeled and simulated in Aspen Plus incorporating the reaction kinetic parameters with a sensitivity analysis of a range of operating conditions. An artificial neural network (ANN) method with machine learning is used to generate a mathematical function that is optimized based on a multi-objective genetic algorithm (MOGA) method. A sensitivity analysis of variations of each design parameter for both the objective functions and the effectiveness of exergy performance relative to operational costs of produced hydrogen is demonstrated. The sensitivity analysis and optimization results are presented and discussed.

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.

Thermochemical water splitting cycles are recognized as one of the promising pathways for sustainable hydrogen production. In the present study, Iron-chlorine (Fe–Cl) cycle as one of the chlorine family thermochemical cycles where iron chloride is consumed for hydrogen production from water, is considered for a study. This four-step cycle is modelled by Aspen Plus software package and analyzed for performance investigation of each reaction step and system's components. The parametric studies are also performed to assess the effect of operation conditions such as temperature, pressure and steam to feed ratio on the reaction products and conversion rates. Results indicated that although the effect of pressure is not significant on reaction's production rates, an increase in temperature favors oxygen production in reverse deacon reaction and magnetite production in hydrolysis and lowers hydrogen production in the hydrolysis step. On the other hand, steam to chlorine (Cl2) ratio is directly correlated with hydrochloric acid (HCl) and oxygen production in reverse deacon reaction and hydrogen production in hydrolysis.

Hydrolysis of CuCl₂ is the water splitting step of the Cu–Cl thermochemical cycle, where CuCl₂ reacts with steam to produce Cu₂OCl₂ and HCl. In the present work, this gas–solid reaction was investigated to understand the mechanism and kinetics. Experiments were conducted in a semibatch fluidized bed reactor to study the effect of temperature (275–375 °C), steam mole fraction (0.4–0.9), and reaction time (0–3 h). The challenges due to the hygroscopic nature of the reactant, product agglomeration, and multiple side reactions to achieve smooth and consistent reactor performance were overcome by the addition of inert additives during fluidization. The analysis of the mechanism showed that the desired product Cu₂OCl₂ is formed initially and further undergoes decomposition to CuO and CuCl₂. Also, with increasing temperatures, the yield of Cu₂OCl₂ decreases because of the formation of CuCl from reactant decomposition. The results indicate that a minimum steam mole fraction of 0.5 is required to prevent the formation of side product CuCl in the temperature range of 300–325 °C. The minimum steam requirement for maximum yield to Cu₂OCl₂ was found to increase with increase in temperature.

Solar thermochemical reactors have been considered in recent studies because of converting the solar energy to a fuel, which is called solar fuel. In such reactors, heat transfer is a dominant phenomenon in generating products. Providing the optimum thermal energy for the solar thermochemical cycle can be gained by adjusting the size of the solar concentrator. In this study, the sizing of the solar concentrator is studied and the best size of the cavity is calculated by the Monte Carlo method. In this reactor using solar energy, the intermediate metal is converted to solar fuel. ZnO/Zn is considered to be the intermediate metal for the reaction. Next, the solar reactor is modeled in three dimensions and all types of heat transfer mechanisms, i.e., conduction, convection, and radiation along with chemical reaction conditions, are also considered. Sensitivity analysis is done based on the solar concentrator size and the aperture cavity. The results show that the optimum size of the dish collector is 5.168 m and the aperture cavity diameter was gained 5 cm for 10 kWth solar reactor. Nanofluid is used as cooling fluid, with the best modeled fluid flow rate for this structure, the ratio of annual fluid flow to nanofluid being 1. By examining the hydrogen production reactor, the amount of hydrogen produced in the system is 34 mol m−3. Also, the irradiation distribution of the cavity receiver and the temperature distribution of the solar reactor were modeled and analyzed.

Sulfur-based thermochemical cycles, such as the hybrid sulfur-ammonia (HySA) cycle, offer a valuable approach in which hydrogen is produced by exploiting sulfur dioxide (potentially pollutant emissions) through the electrochemical oxidation of aqueous sulfite. In this study, the effect of pH on electrooxidation rate was assessed by comparing different reaction scenarios. Then, a Central Composite Design (CCD) combined with a Response Surface Methodology (RSM) was used to optimize batch electrooxidation of ammonium sulfite at near-neutral pH. Results show that the use of an anion exchange membrane (AEM) greatly improves sulfite electrooxidation rate while pH is effectively stabilized. Furthermore, a second-order model that relates applied potential and sulfite concentration with the normalized half-life of the reaction was obtained and verified experimentally at long-term batch electrooxidations. A good agreement between the model and experimental tests, adequate hydrogen recoveries and low sulfur crossover through the membrane demonstrate practical robustness of this approach.

The sulfur–iodine (SI or IS) thermochemical cycle assembled with solar or nuclear energy has been proposed as a large-scale, clean and renewable hydrogen production method. In present work, an improved SI cycle integrated with HI electrolysis for hydrogen production was developed according to experiments and simulation. The mathematical models of HI electrolysis using proton exchange membrane (PEM) electrolytic cell was developed, and then the user-defined module of HI electrolysis was set up through Aspen Plus and verified by experimental data. After designing and simulating the new flowsheet of the SI cycle based on HI electrolysis, 10 L/h of H2 and 5 L/h of O2 were obtained. The theoretic thermal efficiency of flowsheet reached 25–42% in terms of the utilization of waste heat. An ideal thermal efficiency of 33.3% through the proper internal heat exchange in the flowsheet was determined. Sensitivity analyses of parameters in the system were conducted. Increasing proton transfer number of PEM electrolytic cell in HI section improved the thermal efficiency of SI cycle. The ratio of distillate to feed rate and the plate number of distillation column in H2SO4 section were the most sensitive factors to the heat duty of overall SI cycle. The proposed new flowsheet for SI cycle is competitive to the flowsheets previously proposed in the field of flowsheet simplification.

Solar thermal-driven thermochemical H2O and CO2 splitting offers a carbon-neutral path to produce feedstocks for synthetic fuel production such as hydrogen or synthesis gas. This paper assesses research outcomes for perovskite materials in two-step thermochemical cycles. Experimental, computational and thermodynamic studies are summarized and critically discussed, identifying key attributes for future research. In addition to the critical review, a fast method for the classification of effective thermochemical properties (oxygen vacancy formation enthalpy and entropy) in a wide range of operational temperatures is provided. These properties together with a high-grade of sintering resistance and fast kinetics are the main characteristics required to maximize the solar-to-fuel efficiency of the process. The discovery of optimum material compositions for this application could be effectively achieved by a combination of machine learning, DFT, experimental testing and system modelling, and will require an extensive international research effort. If successful, this could lead to the ultimate development and practical application of thermochemical cycles for fuel production.

Thermochemical water splitting cycles are considered an attractive option to produce H2, a substance with many important applications for society such as ammonia production because they do not require fossil fuels. In contrast, they obtain H2 decomposing water by means of cyclic chemical reactions. There are a wide variety of thermochemical ones under research, including the relatively new Na–O–H (sodium–oxygen–hydrogen) cycle proposed in 2012 by a research group. Despite introducing a new potential thermochemical cycle, those previous researchers do not investigate the thermal performance of it. Such task can be done through an exergy analysis. So, the aim of the paper is to evaluate the exergy performance of the Na–O–H cycle and its chemical reactions. This goal is accomplished by implementing in EES (Engineering Equation Solver) software exergy efficiency ( e 1 ) and exergy destroyed (ED_1) balances for the cycle and their reactions considering specific conditions of pressure (p) and temperature (T). This approach allows understanding how these two variables influence the system performance, which could compromise its actual implementation and economic feasibility if it has low thermal efficiency. According to the results: reaction 1 (hydrogen production step) has mean e 1 = 96 % and ED_1 = 8.5 kJ under 100–300 °C at 0–1 bar; reaction 2 (metal separation step) has ED_2 = 280 kJ and e 2 = 56 % at 450 °C considering vacuum condition; hydrolysis step (reaction 3) has average e 3 = 87 % and ED_3 = 18 kJ at 25–200 °C under 0–1 bar. Then, the Na–O–H cycle has overall e = 82 % and ED = 306 kJ to produce 1 mol of H2. These values of e and ED are theoretical and maximized ones. In practical situations, such amounts probably will reduce in function of unavoidable irreversibility present in actual systems such as pressure drop and heat loss that were neglected in the work to facilitate its development. Finally, it concludes the Na–O–H cycle has relatively high exergy efficiency, making it a potential hydrogen production method. However, its practical implementation in the future must overcome some of its drawbacks, by means of more research, like the low thermal efficiency of reaction 2 when compared to reactions 1 and 3 beyond the low pressure needed to perform such step.

The two-step cycle for solar-thermochemical fuels production and thermochemical energy storage benefits from low oxygen partial pressure in the high-temperature thermal reduction step. To be practical, low oxygen partial pressures must be reached by energetically efficient and economically affordable methods—a challenge currently not met by either mechanical vacuum pumping or by inert gas sweeping. To address this challenge, we have examined a promising, thermally-driven surface adsorption/desorption approach. Providing that appropriately designed materials can be identified as expected, this approach would substantially advance solar-thermochemical fuel production (water and carbon dioxide splitting), thermochemical energy storage, and related technologies.

Using solid oxide membrane, this paper presents the theoretical modeling of the high temperature electrolysis of hydrogen bromide gas for hydrogen production. The electrolysis of hydrogen halides such as hydrogen bromide is an attractive process, which can be coupled to hybrid thermochemical cycles. The high temperature electrolyzer model developed in the present study includes concentration, ohmic, and activation losses. Exergy efficiency, as well as energy efficiency parameters, are used to express the thermodynamic performance of the electrolyzer. Moreover, a detailed parametric study is performed to observe the effects of various parameters such as current density and operating temperature on the overall system behavior. The results show that in order to produce 1 mol of hydrogen, 1.1 V of the applied potential is required, which is approximately 0.8 V less compared to high temperature steam electrolysis under same conditions (current density of 1000 A/m2 and temperature of 1073 K). Furthermore, it is found that with the use of the presented electrolyzer, one can achieve energy and exergy efficiencies of about 56.7% and 53.8%, respectively. The results presented in this study suggest that, by employing the proposed electrolyzer, two-step thermochemical cycle for hydrogen production may become more attractive especially for nuclear- and concentrated solar-to-hydrogen conversion applications.

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.

Dissociation of CO2 to form CO can play a key role in decarbonizing our energy system. We report here a two-step thermochemical cycle using a variety of iron-poor (Fe-poor) ferrites (FeyM1−yOx where y < 2/3) that produce CO with unusually high yield using Fe as the redox active species. Conventional wisdom suggests that increasing the Fe fraction would increase the capacity for CO2 dissociation. Here, we report the opposite result: at partial pressure ratio CO : CO2 = 1 : 100, we demonstrated CO yields of 8.0 ± 1.0 mL-CO per gram from Fe0.35Ni0.65Ox, and 3.7 ± 1.0 mL-CO per gram from Fe0.45Co0.55Ox, at a thermal reduction temperature of 1300 °C; remarkably, these CO2 dissociation capacities are significantly higher than those of state-of-the-art materials such as spinel ferrites (Fe2MO4), (substituted) ceria, and Mn-based perovskite oxides. Optimization of the kinetics of Fe-poor ferrites with a ZrO2 support resulted in higher CO yields per gram of ferrite. The unexpected CO yield vs. Fe ratio trend is consistent with the prediction of calculated ternary phase diagrams, which suggest a swing between spinel and rocksalt phases. These Fe-poor ferrites open new opportunities for tuning the redox properties of oxygen exchange materials.

© 2021 Hydrogen Energy Publications LLCIn the presented paper, energy and exergy analysis is performed for thermochemical hydrogen (H2) production facility based on solar power. Thermal power used in thermochemical cycles and electricity production is obtained from concentrated solar power systems. In order to investigate the effect of thermochemical cycles on hydrogen production, three different cycles which are low temperature Mg–Cl, H2SO4 and UT-3 cycles are compared. Reheat-regenerative Rankine and recompression S–CO2 Brayton power cycles are considered to supply electricity needed in the Mg–Cl and H2SO4 thermochemical cycles. Furthermore, the effects of instant solar radiation and concentration ratio on the system performance are investigated. The integration of S–CO2 Brayton power cycle instead of reheat-regenerative Rankine enhances the system performance. The maximum exergy efficiency which is obtained in the system with Mg–Cl thermochemical and recompression S–CO2 Brayton power cycles is 27%. Although the energy and exergy efficiencies decrease with the increase of the solar radiation, they increase with the increase of the concentration ratio. The highest exergy destruction occurred in the solar energy unit.

The focus of this chapter is on thermochemical water-splitting cycles, with an emphasis on solar energy as the input. However, other sources of thermal and electrical energy (e.g., nuclear) can drive these cycles. Further emphasis will be placed on comparing those with no electrochemical step to those that take advantage of one. In general, water splitting that employes an electrochemical step is less complex but often less efficient. The simplest example is water electrolysis, where water is split directly into oxygen and hydrogen using electricity. However, since there are thermodynamic inefficiencies in generating electricity from thermal energy, thermal water splitting would be attractive from an efficiency standpoint. Unfortunately, thermal dissociation of water occurs at temperatures in excess of 2500°C. Therefore, hundreds of different thermochemical cycles involving chemical species in addition to water are under investigation. These various thermochemical cycles split water at lower temperatures (∼500–1000°C), with the other chemical species recycled in the system. Thermochemical cycles can be divided into two broad categories: (1) direct processes (i.e., all chemical steps); or (2) hybrid processes (i.e., a combination of chemical and electrochemical steps).

Nowadays, the framework of the hydrogen economy is been considered by many stakeholders as a way to improve the security, cost, and environmental concerns of the fuels and technologies used in vehicles. Therefore, this work aims to provide novel solutions in the refueling of hydrogen gas into fuel cell vehicles, as well as green production of hydrogen from sun power via thermochemical cycles. First, a novel methodology for the fill-up of hydrogen is developed, which yields strategies that significantly reduce the fill-up time and cooling needs while satisfying safety concerns. Then, the methodology is applied to the case of methane and compressed natural gas, leading to similar results as in the case of hydrogen. Finally, a novel water-splitting thermochemical cycle, powered by concentrated solar power, is studied at a fundamental level in order to provide operating conditions for its reactions.