Thermochemical Water Splitting Cycle Research Articles

Exergoenvironmental analysis of the integrated copper-chlorine cycle for hydrogen production

Thermochemical water-splitting cycles have the potential of producing large-scale and cost-effective sustainable hydrogen in an environmentally benign manner. Limited exergoenvironmental assessments have been reported in the literature which generally considers thermochemical hydrogen production and specifically the copper-chlorine (Cu–Cl) cycle in thid regard. Thus, the present investigation aims at assessing the four-step integrated Cu–Cl thermochemical cycle set-up located at the Ontario Tech University thereby contributing to the environmental impact assessment of thermochemical hydrogen production. In this study, the exergy analysis of the cycle is first performed by thermodynamically modeling and simulating the cycle in Aspen-plus. The fundamental principles of the exergoenvironmental analysis, analogous to the specific exergy costing (SPECO) methodology of the exergoeconomic evaluation, are applied to the cycle and the balanced environmental impact equations for all cycle components are developed. Based on the balanced environmental impact equations, the environmental impact rate, and the specific environmental impact at each state point is evaluated. The environmental impact levels for all the major cycle components are also obtained. The exergoenvironmental factor, the relative difference of the specific environmental impacts, and the environmental impact rate of exergy destruction for various cycle components are in this regard evaluated. Furthermore, the influence of various parameters on the cumulative and component-related rate of environmental impact as well as the exergoenvironmental factor is analyzed by conducting a detailed sensitivity analysis. According to the obtained results, the rate of environmental impact corresponding to exergy destruction dominates the component-related rate of environmental impact and thus the potential reduction in the cumulative environmental impact for various components could be achieved by improving their exergy efficiencies. The hydrolysis step accounts for 60% of the component-related environmental impact while the thermolysis step accounts for the highest environmental impact of exergy destruction ranging between 41 and 42%. The percentage difference between fossil fuel-based and renewable electricity in terms of the global warming potential ranges between 80% and 98% with the highest difference for wind and nuclear-based electricity.

Thermo-chemical water splitting: Selection of priority reversible redox reactions by multi-attribute decision making

Hydrogen is a top chemical and potential fuel. Its sustainable production from biomass or water splitting gains importance. Whereas sole thermal water splitting requires too high temperatures, thermo-chemical water splitting cycles offer a solution at moderate temperatures. Such cycles were investigated over the past decades, mostly by small-scale experiments and mainly to prove their concept without judgment of their practical, economic, environmental and cyclic performance. To facilitate the decision making and to guide future priority research, these multiple aspects can be combined in a global screening system that applies the improved Analytic Hierarchy Process (AHP) and grey relational TOPSIS, together with the use of linear and non-linear combination weighing. The assessment is quantitative and comprehensive, emphasizing the complex relationship between energy efficiency, conversion, recyclability, economy and environmental quality. The total index combines systematics and flexibility through its multi-objective and multi-level nature. The index helps users, system manufacturers, researchers and governments to select the most appropriate future schemes.Very high temperature reactions of e.g. metal-metal oxides, metal-metal hydroxides, perovskites or doped ceria were not included. At the required temperatures, concentrated solar energy is the evident heat source, although applicable temperatures should meet the mechanical and thermal constraints of the solar receiver-reactor construction materials. The experimental set-up that will be used for subsequent pilot-scale solar testing is briefly described. As a result of the multi-attribute assessment, 4 out of 24 oxidation/reduction reactions are selected for further laboratory or preferably pilot-scale application, including the MnFe2O4, MnO/NaMnO2 and ZnO/Fe3O4/ZnFe2O4 redox reactions.

Catalytic SO3 Decomposition Activity of SiO2-Supported Alkaline Earth Vanadates for Solar Thermochemical Water Splitting Cycles

Alkaline earth vanadates (Ae–V: Ae = Ca, Sr, and Ba) were supported on mesoporous SiO2 by a wet impregnation method. The catalytic activity of the prepared materials for the decomposition of SO3 into SO2 and O2, which is a key step in solar thermochemical water splitting cycles, was investigated. In the temperature range 700–800 °C, the Ae–V/SiO2 catalysts exhibited remarkably high activities, which were superior to those of supported Pt catalysts in a wide range of weight hourly space velocities (55–220 g-H2SO4 g–1 h–1). Despite the melting points of the materials exceeding 1000 °C, the high activity was determined to be closely related to the unusual melting behavior of Ae–V. Under the reaction atmosphere, the Ae–V phase was converted to AeSO4 and molten V2O5 (melting point = 690 °C) via facile solid–gas reactions between SO3 and alkaline earth elements displaying high basicity. Notably, upon contact with the molten V2O5 phase, the as-deposited AeSO4 was immediately decomposed into SO2 and O2 to regenerate the Ae–V phase. The catalyst, which solidified at lower temperatures (<690 °C), could not decompose the sulfate and was therefore unable to drive the catalytic cycles. Consequently, the SO3 decomposition rate at <690 °C was lower than that of an alkaline vanadate (Cs–V) with a melting point as low as 500 °C but higher than that of a rare earth vanadate (La–V) with the highest melting point (>1800 °C).

Energy optimization of a Sulfur–Iodine thermochemical nuclear hydrogen production cycle

The use of nuclear reactors is a large studied possible solution for thermochemical water splitting cycles. Nevertheless, there are several problems that have to be solved. One of them is to increase the efficiency of the cycles. Hence, in this paper, a thermal energy optimization of a Sulfur–Iodine nuclear hydrogen production cycle was performed by means a heuristic method with the aim of minimizing the energy targets of the heat exchanger network at different minimum temperature differences. With this method, four different heat exchanger networks are proposed. A reduction of the energy requirements for cooling ranges between 58.9-59.8% and 52.6-53.3% heating, compared to the reference design with no heat exchanger network. With this reduction, the thermal efficiency of the cycle increased in about 10% in average compared to the reference efficiency. This improves the use of thermal energy of the cycle.

Investigations on a platinum catalyzed membrane for electrolysis step of copper‐chlorine thermochemical cycle for hydrogen production

SummaryConsidering the low temperature demand (~550°C maximum) and high efficiency (~43%), Cu–Cl thermochemical water splitting cycle has immense potential as a futuristic scalable hydrogen generation process. CuCl/HCl electrolysis is the crucial hydrogen generation step in the Cu–Cl thermochemical cycle. The efficiency of this electrolysis process is highly dependent on the performance of the membrane electrode assembly (MEA) employed. To explore the suitability of the platinized membrane for this step, in this study, we have investigated a Pt/Nafion‐117 MEA prepared by in‐situ impregnation‐reduction method for CuCl/HCl electrolysis. A single‐cell PEM‐type electrolyser (4 cm2 active area) consisting of serpentine channels (2 cm × 2 cm) grooved in graphite flow field plates was designed and fabricated to carry out the CuCl/HCl electrolysis. Platinum electrocatalyst was deposited on Nafion‐117 membrane on the cathodic side by reduction of hexachloroplatinic acid (H2PtCl6) with sodium borohydride (NaBH4) by the impregnation‐reduction method which served as the MEA. Monodispersed nanocrystalline Pt particles formed a ~30 μm thick electrocatalyst layer on the membrane. Pt/Nafion‐117 MEA was found to be effective for CuCl /HCl electrolysis. In a PEM‐type electrolyser with Ar‐purged 0.2 M CuCl in ~2 M HCl solution as anolyte flowing at ~ 8.8 mLmin−1, a current density of ~100 mA cm−2 was achieved at a cell voltage of 1 V. However, undesired Cu‐crossover could be noticed from anode to cathode side during operation.

Techno-economic analysis of thermochemical water-splitting system for Co-production of hydrogen and electricity

The two-step thermochemical metal oxide water-splitting cycle with the state-of-the-art material ceria inevitably produces unutilized high-quality heat, in addition to hydrogen (H2). This study explores whether the ceria cycle can be of greater value by using the excess heat for co-production of electricity. Specially, this technoeconomic study estimates the H2 production cost in a hybrid ceria cycle, in which excess heat produces electricity in an organic Rankine cycle, to increase revenue and decrease H2 cost. The estimated H2 cost from such a co-generation multi-tower plant is still relatively high at $4.55/kg, with an average H2 production of 1431 kg/day per 27.74 MWth tower. Sensitivity analyses show opportunities and challenges to achieving $2/kg H2 through improvements such as increased solar field efficiency, increased revenue from electricity sales, and a decreased capital recovery factor from baseline assumptions. While co-production improves overall system efficiency and economics, achieving $2/kg H2 remains challenging with ceria as the active material and likely will require a new material.

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

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.

Efficient generation of hydrogen by two-step thermochemical cycles: Successive thermal reduction and water splitting reactions using equal-power microwave irradiation and a high entropy material

Hydrogen production through two-step thermochemical water splitting cycle based on metal oxide has emerged as a promising strategy to store dilute and intermittent solar energy. However, a typical reaction time of the two-step thermochemical water splitting cycle is lengthy, with at least 0.5 h for thermal reduction step and 1 h for water splitting step, and the energy required in the thermal reduction process for hydrogen regeneration is higher than the generated hydrogen energy. In this work, to overcome the problem of an energy efficiency imbalance, we investigated the possibility of the rapid and successive reactions of thermal reduction and water splitting, using short-term irradiation from a low-energy microwave. To this end, a high entropy material, as a poly-metal oxide used to generate hydrogen, was produced by simultaneously introducing four cations onto a SiC foam – (FeMgCoNi)O1.2@SiC. It was found that the oxygen vacancy of the (FeMgCoNi)O1.2@SiC could be significantly increased by short-term microwave irradiation, and hence the thermal reduction process took only 4 min, which is much less than normal. High H2 generation rates were achieved by re-oxidation of the Fe (II) to Fe (III) of the (FeMgCoNi)O1.2@SiC. In addition, the microwave plasma generated by microwave irradiation induced (FeMgCoNi)O1.2@SiC discharge could enhance the water splitting process. The maximum hydrogen yield was 122 mL/g at 700 W, due to the coupling effect of the thermochemical cycle and microwave plasma. In this way, the power consumption of microwave process is only 3% of that of conventional high-temperature heat treatment during thermal reduction process.

Thermodynamic analysis and optimization of an integrated solar thermochemical hydrogen production system

In this study, thermodynamic analysis of solar-based hydrogen production via copper-chlorine (Cu–Cl) thermochemical water splitting cycle is presented. The integrated system utilizes air as the heat transfer fluid of a cavity-pressurized solar power tower to supply heat to the Cu–Cl cycle reactors and heat exchangers. To achieve continuous operation of the system, phase change material based on eutectic fluoride salt is used as the thermal energy storage medium. A heat recovery system is also proposed to use the potential waste heat of the Cu–Cl cycle to produce electricity and steam. The system components are investigated thoroughly and system hotspots, exergy destructions and overall system performance are evaluated. The effects of varying major input parameters on the overall system performance are also investigated. For the baseline, the integrated system produces 343.01 kg/h of hydrogen, 41.68 MW of electricity and 11.39 kg/s of steam. Overall system energy and exergy efficiencies are 45.07% and 49.04%, respectively. Using Genetic Algorithm (GA), an optimization is performed to evaluate the maximum amount of produced hydrogen. The optimization results show that by selecting appropriate input parameters, hydrogen production rate of 491.26 kg/h is achieved.

Regeneration of a metal oxide catalyst with polyvinylpyrrolidone under xenon flash irradiation for repetitive hydrogen generation

In thermochemical water splitting cycles using an oxide catalyst, the temperature required for the catalyst reduction process (1000 °C–2500 °C) is much higher than that required for the hydrogen generation process (800 °C–1500 °C). Accordingly, there is an issue of inefficiency because the energy required to generate hydrogen is higher than the generated hydrogen energy. In this study, we investigated a method to decrease the energy required for the reduction of an oxide catalyst through the combination of polyvinylpyrrolidone as a reducing agent and Xe flash irradiation. A three-dimensional (3D) microporous nickel oxide (NiOx) was used as the oxide catalyst to maximize the surface area of the reaction and, hence, the amount of hydrogen generated from water. During the repeated hydrogen generation cycles, the maximum rate of hydrogen generation (10.6 µmol min−1 g−1) and the total amount of hydrogen generated (642 µmol g−1) were stably maintained without the degradation of the 3D microporous NiOx catalyst structure. The reduction method proposed here is expected to provide insights to resolve the issue of the inefficiency of energy in the catalyst reduction process during the hydrogen generation and regeneration processes using an oxide catalyst.

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

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

A study on the Fe–Cl thermochemical water splitting cycle for hydrogen production

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.

Utilization of Niobium Pentoxide based redox reactions for solar hydrogen generation via thermochemical water splitting cycle

A solar-driven Nb2O5-based water splitting (Nb-WS) cycle thermodynamically scrutinized and the obtained results reported in this paper. The thermodynamic analysis conducted in two phases: phase-1) equilibrium analysis and phase-2) efficiency analysis. According to the equilibrium analysis, partial thermal reduction (TR) of Nb2O5 from 0.7% up to 100% was feasible in the temperature range of 1700 K–2083 K. The obtained results further indicate that the WS reaction was favorable at any temperature below 1500 K. As per the outcomes associated to the efficiency analysis, the Q˙solar-reactor-Nb-WS and Q˙solar-steam generator-Nb-WS increased by 2321.7 kW and 87.2 k due to the rise in the TH from 1700 K to 2083 K. This upsurge in the Q˙solar-reactor-Nb-WS and Q˙solar-steam generator-Nb-WS resulted in an upturn in the Q˙solar-cycle-Nb-WS by 2408.9 kW. The ηsolar-to-fuel-Nb-WS also amplified from 1.9% up to 11.4% as the TH surged from 1700 K up to 2040 K. A further rise in the TH from 2040 K up to 2083 K does not yield any improvement in the ηsolar-to-fuel-Nb-WS. A 50% heat recuperation resulted in the attainment of the maximumηsolar-to-fuel-HR-Nb-WS equal to 15.8% at TH=2020 K.

Hydrogen production by water splitting with Mn3-xCoxO4 mixed oxides thermochemical cycles: A thermodynamic analysis

The high temperature required for hydrogen production by solar driven thermochemical cycles is a critical factor hindering full-scale applications. The thermochemical cycle based on Mn3O4/MnO redox pair is one of the most studied despite the high operating temperature required for complete de reduction step (1623–1723 K). The combination of Mn3O4 with Co3O4, a metal oxide with lower reduction temperature than Mn3O4 that cannot be used for hydrogen production due to thermodynamic limitations, is presented as a way to decrease significantly the energy demand of the cycle. In this work, a complete thermodynamic study of thermochemical cycles with different Mn/Co mixed oxides (Mn3-xCoxO4, 0.9 < x < 2.7) for hydrogen production has been performed. The study of the variation of Gibbs energy with temperature allowed to determine that the thermal reduction of the metal oxide (Mn3-xCoxO4) takes place at temperatures between 1048 and 1173 K, which can be achieved by conventional solar concentration technologies. Unfortunately, the oxidation of the reduced metal oxide (Mn3-xCoxO3) with water to produce H2 is not feasible from a thermodynamically point of view, so a stronger oxidizing agent, as sodium hydroxide, is required. The optimum temperature for the oxidation with NaOH was found to be 1373 K, meaning that this reaction takes place at higher temperatures than those actually required for the reduction, something uncommon in thermochemical cycles for water splitting. On the other hand, after the study of the variables affecting the equilibrium like the inert gas/solid ratio, it can be concluded that in both reactions, thermal requirements can be reduced by operating at lower temperatures by means of a higher inert gas/Mn3-xCoxO4 ratio. Finally, energy and exergy analysis of the system based on the solar absorption efficiency and the energy requirements predicts a solar-to-fuel efficiency and exergy efficiency of 40% and 23%, respectively. These values are comparable or even higher than those found in literature for other metal oxides thermochemical cycles for water splitting, thus with the advantage of working at a considerable lower temperature (1373 K).

An ageing protocol for testing high temperature solar materials for thermochemical applications

Future's 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.Thermochemical processes require thermal and chemically stable reactor wall materials, which can withstand severe operating conditions, suitable for specific solar fuels production. Several unresolved issues are involved in predicting the effect of environmental degradation on the long-term performance of innovative materials for next generation solar chemical reactors to guarantee their reliability and durability under demanding working conditions.The research project ARROPAR-CEX involves a multidisciplinary team for studying novel receiver concepts and new materials, working under extreme conditions (in excess of 1000 kW/m2, and very high temperatures, typically above 1000 °C).The main objective of this work has been to develop and propose as candidate an accelerated ageing method for solar receiver’ materials, in order to estimate their degradation during lifetime under extreme solar operating conditions. A set of four selected ceramic materials have been submitted to these accelerated ageing test procedures and some preliminary conclusions on their suitability have been extracted.

Thermodynamic performance assessment of boron based thermochemical water splitting cycle for renewable hydrogen production

The present study is related with the thermodynamic performance assessment of renewable hydrogen production through Boron thermochemical water splitting cycle. Therefore, all step efficiencies and overall cycle efficiency are calculated based on complete reaction. Additionally, a parametric study is conducted to determine the effect of the reference environment temperature on the overall cycle efficiency. In this regard, exergy efficiencies, exergy destruction rates and also inlet and outlet exergy rates of the cycle are calculated and presented for various reference temperatures. The exergy efficiency of the cycle is calculated as 0.4393 based on complete reaction and occurs at 298 K. This study has shown that Boron thermochemical water splitting cycle has a great potential due to cycle performance. As a result, Boron based thermochemical water splitting cycle can help achieve better environment and sustainability due to high exergetic efficiency. By the way, economic and technical issues of the storage and transportation of the hydrogen can find a proper solution if the hydrogen production reaction of the Boron thermochemical water splitting cycle takes place on-board of a vehicle.

CeTi2O6-A Promising Oxide for Solar Thermochemical Hydrogen Production.

A large entropy of reduction is crucial in achieving materials capable of high-efficiency solar thermochemical hydrogen (STCH) production through two-step thermochemical water splitting cycles. We have recently demonstrated that the onsite electronic entropy of reduction attains an extreme value of 4.26 kB at 1500 K in Ce4+ → Ce3+ redox reactions, which explains the high performance and uniqueness of CeO2 as an archetypal STCH material. However, ceria requires high temperatures (T > 1500 °C) to achieve a reasonable reduction extent because of its large reduction enthalpy, which is a major obstacle in practical applications. Therefore, new materials with a large entropy of reduction and lower reduction enthalpy are required. Here, we perform a systematic screening to search for Ce4+-based oxides which possess thermodynamics superior to CeO2 for STCH production. We first search the Inorganic Crystal Structure Database (ICSD) and literature for Ce4+-based oxides and subsequently use density functional theory to compute their reduction enthalpies (i.e., oxygen vacancy formation energies). We find that CeTi2O6 with the brannerite structure is the most promising candidate for STCH because it possesses three essential characteristics of an STCH material: (i) a smaller reduction enthalpy compared to ceria yet large enough to split water, (ii) a high thermal stability, as reported experimentally, and (iii) a large entropy of reduction associated with Ce4+ → Ce3+ redox. Our proposed design strategy suggests that further exploration of Ce4+ oxides for STCH production is warranted.

Exergy analysis for the Na-O-H (sodium-oxygen-hydrogen) thermochemical water splitting cycle

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 (ε1) 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 ε1=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 ε2=56% at 450 °C considering vacuum condition; hydrolysis step (reaction 3) has average ε3=87% and ED_3 = 18 kJ at 25–200 °C under 0–1 bar. Then, the Na–O–H cycle has overall ε=82% and ED = 306 kJ to produce 1 mol of H2. These values of ε 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.

Solar hydrogen production via ZnO/Zn based thermochemical water splitting cycle: Effect of partial reduction of ZnO

This investigation reports the examination of the influence of partial reduction of ZnO on the thermodynamic parameters allied with the ZnO/Zn WS cycle. The efficiency analysis was conducted by developing and following the ZnO/Zn process flow configuration. By estimating the equilibrium compositions, the percentage of ZnO reduction (ZnO-R) as a function of the thermal reduction (TR) temperature (TH) was determined. After identifying the ZnO-R percentages, the influence of TH and ZnO-R on the thermodynamic process parameters was explored. Overall results of the efficiency analysis indicate that the operation of the partially reduced ZnO/Zn WS cycle, in the temperature range of 1040 K–1240 K, was more useful rather than operating at 1340 K. ZnO-R equal to 57.2% (TH = 1240 K) was favorable to operate the ZnO/Zn WS cycle as a further rise in the ZnO-R did not yield any significant improvement in the solar-to-fuel energy conversion efficiency. At ZnO-R equal to 57.2% (TH = 1240 K), by applying 50% HR, a value of 62.6% as the efficiency of solar to fuel can be achieved.

Thermodynamic study of a novel trigeneration process of hydrogen, electricity and desalinated water: The case of Na-O-H thermochemical cycle, SCWR nuclear power plant and MED desalination installation

Hydrogen (H2) is a substance with a wide range of applications, especially ammonia (NH3) production. It is the key chemical used to make fertilizers, a pillar of agriculture industry, an important activity for society. H2 can be obtained according to different processes, including the recent method Na-O-H (sodium-oxygen-hydrogen) thermochemical water splitting cycle that produce it by cracking H2O (water) molecules into H2 and O2 (oxygen) through cyclic chemical reactions based on the elements Na, O and H. Such reactions are sustained by a heat source at specified temperature level. In this context, nuclear reactors, such as Supercritical Water Reactor (SCWR), are suitable energy options for this kind of process because they are designed to support electricity together with high temperature applications like H2 production. At the same time, water could be get by means of MED (Multi-Effect Distillation), a desalination method able to harvest waste heat from thermal systems. This approach allows saving the available potable water to be used in essential human needs such as drinking and agriculture while the desalinated one would serve as hydrogen source in the sodium-oxygen-hydrogen technique. Then, H2 production based on the technologies Na-O-H cycle through the heat supplied by SCWR coupled to a MED unit enables the trigeneration of electricity-hydrogen-water, three important goods for current society, what justifies the development of a paper covering this multigeneration approach. So, the main aim of this work is to evaluate the thermodynamic performance of this trigeneration case, a task never done before because those three systems were not coupled together aiming this multigeneration case as it was checked in the literature. It was considered a SCWR with output thermal power equal to 1000 MWth as the heat source to both Na-O-H hydrogen production facility plus MED installation. The work is developed by implementing mass and energy balances in Engineering Equation Solver (EES) software to determine the amount of H2, electricity and desalinated H2O acquired. According to the main results, this trigeneration process has potential to produce a maximum of 5 kg/s of hydrogen, 400 MW of electricity and 960 kg/s of desalinated H2O.