Solar Thermochemical Water Splitting Cycles Research Articles

Potential of solar thermochemical water-splitting cycles: A review

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

The role of entropy in the success of nonstoichiometric oxides for two-step thermochemical water and CO2 splitting

Owing to their high theoretical efficiency, two-step solar thermochemical water and CO2 splitting cycles, using a metal oxide intermediate, provide a promising pathway to store solar energy in a stable chemical bond. To date, the most successful demonstrations have utilized nonstoichiometric oxides, a class of materials capable of continuously transitioning from an oxidized to reduced state via formation of oxygen vacancies. The success of nonstoichiometric oxides is typically attributed to their ability to maintain their crystallographic phase, allowing them to be cycled many times without destabilizing–a tremendous practical advantage. In this work, we utilize a combined empirical and statistical thermodynamics approach to present an alternative explanation of the success of nonstoichiometric oxides from an entropy perspective. We first illuminate the importance of the entropy change of the reduction reaction as a key material parameter. We then develop a methodology for plotting nonstoichiometric oxides on the Ellingham diagram, enabling a direct comparison to stoichiometric materials. This analysis reveals the unique ability of nonstoichiometric oxides to achieve a significant solid-state entropy contribution, which results from the disorder generated via oxygen vacancy formation. To quantify the solid-state entropy, we develop a simple configurational entropy model applicable to any oxygen-deficient nonstoichiometric oxide. We compare model predictions to existing data for two important nonstoichiometric oxides, CeO2−δ and La0.6Ca0.4Mn0.6Al0.4O3−δ, to reveal the main trends in entropy vs δ and material composition and discuss the causes and implications of deviations from the theory.

Hydrogen Production by Solar Thermochemical Water-Splitting Cycle via a Beam Down Concentrator

Hydrogen Production by Solar Thermochemical Water-Splitting Cycle via a Beam Down Concentrator

White is the color of hydrogen from concentrated solar energy and thermochemical water splitting cycles

White is the color of hydrogen from concentrated solar energy and thermochemical water splitting cycles

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).

Water-splitting mechanism analysis of Sr/Ca doped LaFeO3 towards commercial efficiency of solar thermochemical H2 production

Solar thermochemical (STC) technology utilizes the entire spectrum of solar energy to decompose water to produce hydrogen. This technology reduces carbonic fuels, nearly only producing hydrogen rather than hydrogen-oxygen mixture. However, low water-splitting activity of redox materials restricts improvement of water-hydrogen conversion ratio and fuel production efficiency. Recently, a kind of perovskite LaFeO3 attracts attention, because of the good performance in photocatalysis hydrogen production. Nevertheless, how LaFeO3 system works in STC water-splitting cycle is rarely studied. In this paper, the first principle method at density functional theory level is adopted to reveal the hydrogen production mechanism of perovskite LaFeO3 doped with 25% Sr/Ca at A site. Hydrogen migration on material surface determines hydrogen generation rate. The activation energy of 25%-Ca-doped LaFeO3 is relatively lower 150.09 kJ/mol. In addition, fuel production efficiency has been calculated. When water to hydrogen conversion ratio is 100%, solar-to-fuel efficiency can reach maximum 0.472. The effect of water-splitting kinetics on hydrogen production is also discussed. The results indicate that when Tred = Toxi = T = 1200K and water to hydrogen conversion ratio is 10%, the dynamic efficiency of La0.75Ca0.25FeO3 can reach 20%. This research can provide index for improving the hydrogen production performance of STC technology.

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.

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%.

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.

Stability of Molten-Phase Cs–V–O Catalysts for SO3 Decomposition in Solar Thermochemical Water Splitting

Supported molten cesium vanadate catalysts (Cs–V–O/SiO2) showed activities comparable to that of a reference Pt catalyst (1 wt % Pt/TiO2) for SO3 decomposition at moderate temperatures (∼600 °C), which is essential as an O2 evolution reaction in solar thermochemical water splitting cycles. Stability testing of the catalyst over a 1000 h continuous reaction at 600 °C resulted in deactivation by ∼20% of the initial activity. Kinetic analysis of the activity versus time-on-stream indicated that the observed deactivation behavior can be divided into an induction period (≤100 h) and an acceleration period (>100 h). The deactivation is mainly caused by the vaporization loss of active components (Cs and V) from the molten phase. At the earliest stage, most vapor is generated in the upstream section of the catalyst bed and then redeposits therebelow. Upon repeating these vaporization and deposition cycles, Cs and V move gradually downstream. During this induction period, the deactivation is not obvious because th...

Catalytic SO3 Decomposition Activity and Stability of Pt Supported on Anatase TiO2 for Solar Thermochemical Water-Splitting Cycles.

Pt-loaded anatase TiO2 (Pt/TiO2-A) was found to be a highly active and stable catalyst for SO3 decomposition at moderate temperatures (∼600 °C), which will prove to be the key for solar thermochemical water-splitting processes used to produce H2. The catalytic activity of Pt/TiO2-A was found to be markedly superior to that of a Pt catalyst supported on rutile TiO2 (Pt/TiO2-R), which has been extensively studied at a higher reaction temperature range (≥800 °C); this superior activity was found despite the two being tested with similar surface areas and metal dispersions after the catalytic reactions. The higher activity of Pt on anatase is in accordance with the abundance of metallic Pt (Pt0) found for this catalyst, which favors the dissociative adsorption of SO3 and the fast removal of the products (SO2 and O2) from the surface. Conversely, Pt was easily oxidized to the much less active PtO2 (Pt4+), with the strong interactions between the oxide and rutile TiO2 forming a fully coherent interface that limited the active sites. A long-term stability test of Pt/TiO2-A conducted for 1000 h at 600 °C demonstrated that there was no indication of noticeable deactivation (activity loss ≤ 4%) over the time period; this was because the phase transformation from anatase to rutile was completely prevented. The small amount of deactivation that occurred was due to the sintering of Pt and TiO2 and the loss of Pt under the harsh reaction atmosphere.

Solar hybrid photo-thermochemical sulfur-ammonia water-splitting cycle: Photocatalytic hydrogen production stage

One of the main limitations of existing solar thermochemical water-splitting cycles (WSC) are that they utilize only thermal component of the solar irradiation neglecting its photonic component. A new hybrid photo-thermochemical sulfur–ammonia (HySA) WSC developed at the Florida Solar Energy Center allows circumventing this shortcoming. In the HySA cycle, water splitting occurs by means of solar beam splitting which enables utilization of the quantum (UV–Vis) portion of the solar spectrum in the hydrogen production stage and the thermal (IR) portion in the oxygen production stage. Present work investigates the photocatalytic hydrogen production step using narrow band gap CdS and CdSZnS composite photocatalysts, and ammonium sulfite as an electron donor. The choice of the electron donor was determined by the considerations of its regenerability in the thermal stages of the HySA cycle. This article examines the impact of photocatalyst and cocatalyst loading, temperature, and light intensity on hydrogen production rates. Photocatalysts, cocatalysts and photoreaction products were analyzed by a number of materials characterization (XRD, SEM, TEM, EDS) and analytical (GC and IC) methods. The experimental data obtained provide guidance for the improved solar photoreactor design.

A comparative thermodynamic analysis of samarium and erbium oxide based solar thermochemical water splitting cycles

This paper reports a thermodynamic comparison between the samarium and erbium oxide based solar thermochemical water splitting cycles. These cycles are a two-step process in which the metal oxide is first thermally reduced into the pure metal, and the produced metal can be used to split water to produce H2. The metal oxides can be reused for multiple cycles without consumption. The effect of water splitting temperature on various thermodynamic parameters which are essential to design the solar reactor system for the production of H2 via water splitting reaction using the samarium and erbium oxides is studied in detail. The total amount of solar energy needed for the thermal reduction of samarium and erbium oxides is estimated. The amount of heat energy released by the water splitting reactor is calculated. Also, the cycle and solar-to-fuel energy conversion efficiency for both cycles are determined by employing heat recuperation. Obtained results indicate that the efficiencies associated with these cycles are comparable to the previously studies thermochemical cycles. It is observed that higher water splitting temperature favors towards higher efficiencies. At constant thermal reduction temperature = 2280 K, by employing 50% heat recuperation, the solar-to-fuel energy conversion efficiency for the samarium cycle (30.98%) is observed to be higher than erbium cycle (28.19%).

Thermodynamic analysis of solar driven SnO2/SnO based thermochemical water splitting cycle

There are many studies related on the SnO2/SnO based solar thermochemical water splitting cycle, however there are still no studies addressing on the detailed thermodynamic analysis of this process using HSC Chemistry software and its thermodynamic database. In this cycle, the first step belongs to the endothermic solar thermal reduction of SnO2 producing gaseous SnO and O2. The second step corresponds to the exothermic production of H2 via water splitting reaction using SnO produced in the first cycle, thereby regenerating SnO2 which can be recycled back to step 1. Thermodynamic equilibrium compositions associated with step 1 and 2 are identified as a function of reaction temperatures and partial pressures of O2 in the inert carrier gas. Furthermore, the thermodynamic efficiency analysis is performed by following the second law of thermodynamics to determine the cycle and solar-to-fuel energy conversion efficiencies associated with the SnO2/SnO based thermochemical water splitting cycle. Effects of thermal reduction and water splitting temperatures on various thermodynamic parameters are also investigated in detail. Obtained results indicate that the higher values of cycle efficiency (41.17%) and solar-to-fuel energy conversion efficiency (49.61%) are achievable by operating this cycle at a thermal reduction temperature of 1780K and water splitting temperature equal to 800K with 50% heat recuperation. This work gives a detailed thermodynamic and efficiency analysis of SnO2/SnO based two-step solar thermochemical water splitting cycle for hydrogen production.

Thermodynamic exergy analysis of dysprosium oxide-based solar thermochemical water-splitting cycle

Thermodynamic exergy analysis of dysprosium oxide-based solar thermochemical water-splitting cycle

Thermodynamic exergy analysis of dysprosium oxide-based solar thermochemical water-splitting cycle

The effect of water splitting temperature on the parameters required for the design of solar thermal reactor to conduct the dysprosium oxide based water splitting (Dy-WS) cycle was investigated by using the HSC Chemistry software. The effect of water splitting temperature on the absorption efficiency of the solar reactor, net solar energy required to run the Dy-WS cycle, re-radiation heat losses from the solar reactor, heat rejected by different coolers and water splitting reactor involved in this cycle, and solar-to-fuel energy conversion efficiency with and without heat recuperation was explored. Obtained results indicate that the Dy-WS cycle carried out by using the thermal reduction temperature = 2280 K and water splitting temperature = 1000 K yields into solar-to-fuel energy conversion efficiency = 10.3%. This efficiency can be increased up to 14.62% by employing heat recuperation (50%).

Catalytic SO3 Decomposition Activity and Stability of A–V–O/SiO2 (A = Na, K, Rb, and Cs) for Solar Thermochemical Water-Splitting Cycles

SiO2-supported molten alkaline metal oxides (A–V–O/SiO2) were studied as SO3 decomposition catalysts for solar thermochemical water splitting. Their catalytic activities at moderate temperatures (≤600 °C), which were superior to those of Cu–V–O/SiO2 catalysts, were dependent on A, exhibiting the following sequence: Cs > Rb > K > Na. These activities increased with the A/V ratio. This result is in accordance with the basicity, which favors the adsorption of SO3 to form sulfate. Another important effect of A is to form molten liquid phases, which dissolve the sulfate and facilitate its decomposition to SO2/O2. However, the molten phase with high A/V ratios led to the collapse of the porous SiO2 structure by a corrosion effect. Consequently, the highest catalytic activity was achieved at a composition of A/V ≈ 1.0 for A = K and Cs. The long-term stability test of K–V–O/SiO2 at 550 °C demonstrated no indication of noticeable deactivation during the first 100 h, whereas 20% deactivation occurred during the fol...

High temperature hydrogen production: Design of a 750 KW demonstration plant for a two step thermochemical cycle

The present work describes the study of a solar reactor for a two-step solar thermo-chemical water splitting cycle concerning the EU-project Hydrosol Plant, which aims to build a plant at the end of 2016 on a solar tower at the Plataforma Solar de Almería with a thermal input power of 750kW to produce 3kg/week of hydrogen. The process applies nickel-ferrite as reactive species, which works optimally at 1100°C for the water splitting step and at 1400°C for the regeneration step. This material is provided in form of monoliths which are used in cars as catalytic converter. On the platform three reactors are placed to reach a volume of about 0.3m3 of active material inside the reactor chambers. During the operations two of these will be regenerated while one will work on water splitting, to reach a quasi-continuous hydrogen production. The design concept of the reactor is taken from the SOLREF reactor, which was originally developed by DLR for methane reforming at 900°C and 10bar. The scheme and the layout of the plant to feed the reactors have been studied, too. A thermodynamic model for the regeneration step has been also developed to check if the thermal power demand of the three reactors can be supplied by the defined thermal input power. The differences to the other HYDROSOL projects are: The Upscaling from 100kW to 750kW, the usage of monoliths completely made of nickel-ferrite and the control strategy with three reactors instead of two.

Solar Hydrogen Production via a Samarium Oxide-Based Thermochemical Water Splitting Cycle

The computational thermodynamic analysis of a samarium oxide-based two-step solar thermochemical water splitting cycle is reported. The analysis is performed using HSC chemistry software and databases. The first (solar-based) step drives the thermal reduction of Sm2O3 into Sm and O2. The second (non-solar) step corresponds to the production of H2 via a water splitting reaction and the oxidation of Sm to Sm2O3. The equilibrium thermodynamic compositions related to the thermal reduction and water splitting steps are determined. The effect of oxygen partial pressure in the inert flushing gas on the thermal reduction temperature (TH) is examined. An analysis based on the second law of thermodynamics is performed to determine the cycle efficiency (ηcycle) and solar-to-fuel energy conversion efficiency (ηsolar−to−fuel) attainable with and without heat recuperation. The results indicate that ηcycle and ηsolar−to−fuel both increase with decreasing TH, due to the reduction in oxygen partial pressure in the inert flushing gas. Furthermore, the recuperation of heat for the operation of the cycle significantly improves the solar reactor efficiency. For instance, in the case where TH = 2280 K, ηcycle = 24.4% and ηsolar−to−fuel = 29.5% (without heat recuperation), while ηcycle = 31.3% and ηsolar−to−fuel = 37.8% (with 40% heat recuperation).

Solar Thermochemical Hydrogen Production via Terbium Oxide Based Redox Reactions

The computational thermodynamic modeling of the terbium oxide based two-step solar thermochemical water splitting (Tb-WS) cycle is reported. The 1st step of the Tb-WS cycle involves thermal reduction of TbO2into Tb and O2, whereas the 2nd step corresponds to the production of H2through Tb oxidation by water splitting reaction. Equilibrium compositions associated with the thermal reduction and water splitting steps were determined via HSC simulations. Influence of oxygen partial pressure in the inert gas on thermal reduction of TbO2and effect of water splitting temperature (TL) on Gibbs free energy related to the H2production step were examined in detail. The cycle (ηcycle) and solar-to-fuel energy conversion (ηsolar-to-fuel) efficiency of the Tb-WS cycle were determined by performing the second-law thermodynamic analysis. Results obtained indicate thatηcycleandηsolar-to-fuelincrease with the decrease in oxygen partial pressure in the inert flushing gas and thermal reduction temperature (TH). It was also realized that the recuperation of the heat released by the water splitting reactor and quench unit further enhances the solar reactor efficiency. AtTH=2280 K, by applying 60% heat recuperation, maximumηcycleof 39.0% andηsolar-to-fuelof 47.1% for the Tb-WS cycle can be attained.