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

This investigation reports the thermodynamic exploration of a novel three-step GeO₂/GeO water splitting (WS) cycle. The thermodynamic computations were performed by using the data obtained from HSC Chemistry thermodynamic software. Numerous process parameters allied with the GeO₂/GeO WS cycle were estimated by drifting the thermal reduction ($T_H$) and water splitting temperature ($T_L$). The entire analysis was divided into two section: a) equilibrium analysis and b) efficiency analysis. The equilibrium analysis was useful to determine the $T_H$ and $T_L$ required for the initiation of the thermal reduction (TR) of GeO₂ and re-oxidation of GeO via WS reaction. Furthermore, the influence of $P_{O_2}$ on the $T_H$ required for the comprehensive dissociation of GeO₂ into GeO and O₂ was also studied. The efficiency analysis was conducted by drifting the $T_H$ and $T_L$ in the range of 2080 to 1280 K and 500–1000 K, respectively. Obtained results indicate that the minimum ${\dot{Q}}_{solar-cycle}=624.3\text{kW}$ and maximum ${\eta }_{solar-to-fuel}=45.7\text{\%}$ in case of the GeO₂/GeO WS cycle can be attained when the TR of GeO₂ was carried out at 1280 K and the WS reaction was performed at 1000 K. This ${\eta }_{solar-to-fuel}=45.7\text{\%}$ was observed to be higher than the SnO₂/SnO WS cycle (39.3%) and lower than the ZnO/Zn WS cycle (49.3%). The ${\dot{Q}}_{solar-cycle}$ can be further decreased to 463.9 kW and the ${\eta }_{solar-to-fuel}$ can be upsurged up to 61.5% by applying 50% heat recuperation.     

Solar thermochemical ZnO/ZnSO4 water splitting cycle for hydrogen production

In this paper, solar reactor efficiency analysis of the solar thermochemical two-step zinc oxide–zinc sulfate (ZnO–ZnSO₄) water splitting cycle. In step-1, the ZnSO₄ is thermally decomposed into ZnO, SO₂, and O₂ using solar energy input. In step-2, the ZnO is re-oxidized into ZnSO₄ via water splitting reaction producing H₂. The ZnSO₄ is recycled back to the solar reactor and hence can be re-used in multiple cycles. The equilibrium compositions associated with the thermal reduction and water-splitting steps are identified by performing HSC simulations. The effect of Ar towards decreasing the required thermal reduction temperature is also explored. The total solar energy input and the re-radiation losses from the ZnO–ZnSO₄ water splitting cycle are estimated. Likewise, the amount of heat energy released by different coolers and water splitting reactor is also determined. Thermodynamic calculations indicate that the cycle (η_cycle) and solar-to-fuel energy conversion efficiency (η_{solar-to-fuel}) of the ZnO–ZnSO₄ water splitting cycle are equal to 40.6% and 48.9% (without heat recuperation). These efficiency values are higher than previously investigated thermochemical water splitting cycles and can be increased further by employing heat recuperation.     

Analysis of sulfur–iodine thermochemical cycle for solar hydrogen production. Part I: decomposition of sulfuric acid

The sulfur–iodine (S–I) thermochemical water splitting cycle is one of the most studied cycles for hydrogen (H₂) production. S–I cycle consists of four sections: (I) acid production and separation and oxygen purification, (II) sulfuric acid concentration and decomposition, (III) hydroiodic acid (HI) concentration, and (IV) HI decomposition and H₂ purification. Section II of the cycle is an endothermic reaction driven by the heat input from a high temperature source. Analysis of the S–I cycle in the past thirty years have been focused mostly on the utilization of nuclear power as the high temperature heat source for the sulfuric acid decomposition step. Thermodynamic as well as kinetic considerations indicate that both the extent and rate of sulfuric acid decomposition can be improved at very high temperatures (in excess of 1000 °C) available only from solar concentrators. The beneficial effect of high temperature solar heat for decomposition of sulfuric acid in the S–I cycle is described in this paper. We used Aspen Technologies' HYSYS chemical process simulator (CPS) to develop flowsheets for sulfuric acid (H₂SO₄) decomposition that include all mass and heat balances. Based on the HYSYS analyses, two new process flowsheets were developed. These new sulfuric acid decomposition processes are simpler and more stable than previous processes and yield higher conversion efficiencies for the sulfuric acid decomposition and sulfur dioxide and oxygen formation.     

Operational strategy of a two-step thermochemical process for solar hydrogen production

A two-step thermochemical cycle process for solar hydrogen production from water has been developed using ferrite-based redox systems at moderate temperatures. The cycle offers promising properties concerning thermodynamics and efficiency and produces pure hydrogen without need for product separation.     

Two-step water splitting thermochemical cycle based on iron oxide redox pair for solar hydrogen production

This study deals with solar hydrogen production from the two-step iron oxide thermochemical cycle (Fe₃O₄/FeO). This cycle involves the endothermic solar-driven reduction of the metal oxide (magnetite) at high temperature followed by the exothermic steam hydrolysis of the reduced metal oxide (wustite) for hydrogen generation. Thermodynamic and experimental investigations have been performed to quantify the performances of this cycle for hydrogen production. High-temperature decomposition reaction (metal oxide reduction) was performed in a solar reactor set at the focus of a laboratory-scale solar furnace. The operating conditions for obtaining the complete reduction of magnetite into wustite were defined. An inert atmosphere is required to prevent re-oxidation of Fe(II) oxide during quenching. The water-splitting reaction with iron(II) oxide producing hydrogen was studied to determine the chemical kinetics, and the influence of temperature and particles size on the chemical conversion. A conversion of 83% was obtained for the hydrolysis reaction of non-stoichiometric solar wustite Fe_(1−y)O at 575 °C.     

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.     

Two-step thermochemical electrolysis: An approach for green hydrogen production

Electrolysis and thermochemical water splitting are approaches to produce green hydrogen that use either an electrical potential (electrolysis) or a chemical potential (thermochemical water splitting) to split water. Electrolysis is technologically mature when applied at low temperatures, but it requires large quantities of electrical energy. In contrast to electrolysis, thermochemical water splitting uses thermal energy, as thermal energy can typically be supplied at a lower unit cost than electrical energy using concentrating solar power. Thermochemical water splitting, however, typically suffers from high thermal losses at the extremely high process temperatures required, substantially increasing the total energy required. We show how, by combining electrical and chemical potentials, a novel and cost-efficient water splitting process can be envisioned that overcomes some of the challenges faced by conventional electrolysis and thermochemical water splitting. It uses a mixed ionic and electronic conducting perovskite with temperature-dependent oxygen non-stoichiometry as an anode in an electrolyzer. If solar energy is used as the primary source of all energy required in the process, the cost of the energy required to produce hydrogen could be lower than in high-temperature electrolysis by up to 7%.     

Novel two-step thermochemical cycle for hydrogen production from water using germanium oxide: KIER 4 thermochemical cycle

This paper proposes a novel two-step thermochemical cycle for hydrogen production from water using germanium oxide. The thermochemical cycle is herein referred to as KIER 4. KIER 4 consists of two reaction steps: the first is the decomposition of GeO₂ to GeO at approximately 1400–1800 °C, the second is hydrogen production by hydrolysis of GeO below 700 °C. A 2nd-law analysis was performed on the KIER 4 cycle and a maximum exergy conversion efficiency was estimated at 34.6%. Thermodynamic analysis of GeO₂ decomposition and hydrolysis of GeO confirmed the possibility of this cycle. To demonstrate the cycle, the thermal reduction of GeO₂ was performed in a TGA with mass-spectroscopy. Results suggest GeO₂ decomposition and oxygen gas evolution. To confirm the thermal decomposition of GeO₂, the effluent from GeO₂ decomposition was quenched, filtered and analyzed. SEM analysis revealed the formation of nano-sized particles. XRD analysis for the condensed-filtered particles showed the presence of Ge and GeO₂ phases. The result can be explained by thermodynamic instability of GeO. It is believed that GeO gas disproportionates to ½Ge and ½GeO₂ during quenching. 224 ml hydrogen gas per gram of reduced GeO₂ was produced from the hydrolysis reaction.     

Optimizing the operational strategy of a solar-driven reactor for thermochemical hydrogen production

In this paper the operational strategy of a pilot plant for regenerative hydrogen production based on two-step thermochemical redox cycles is investigated with focus on optimal operational parameters for highest solar-to-fuel efficiency. The current plant consists of a solar driven large-scale 250 kW thermochemical inert gas reactor using ceria as reactive material for water splitting and an efficient fluid heat recovery system.Here we analyse the most important process conditions, which are operating temperatures, mass flow rates and duration times for both steps in the cycle. A highly accurate and detailed simulation model combined with well-suited optimization routines reveals new insights in most efficient operational strategies. Within the optimization material and technical limits of the used components are considered, thereby yielding reliable practical results.Optimal operational parameters are found by using a temperature swing strategy with corresponding solar-to-fuel plant efficiency determined by up to 1.1%.     

Comparing the solar-to-fuel energy conversion efficiency of ceria and perovskite based thermochemical redox cycles for splitting H2O and CO2

A thermodynamic analysis was conducted on a solar thermochemical plant for syngas generation via H₂O/CO₂-splitting redox cycles in order to determine the performance of six candidate redox materials under an array of operation conditions. The values obtained for the solar-to-fuel energy conversion efficiency are higher in relative order Zr-doped CeO₂ > undoped CeO₂ > La_0.6Ca_0.4MnO₃ > La_0.6Ca_0.4Mn_0.6Al_0.4O₃ > La_0.6Sr_0.4MnO₃ > La_0.6Sr_0.4Mn_0.6Al_0.4O₃. This ordering is attributed to their relative reducibility and re-oxidizability, where the doped and undoped ceria, that favor oxidation, outperform perovskites, that favor reduction and therefore require high flowrates of excess H₂O and CO₂ during re-oxidation. Solids-solid heat recuperation during the temperature swing between the redox steps is crucial, particularly for ceria because of its low specific oxygen exchange capacity per mole and cycle. Conversely, the efficiencies of the perovskites are more dependent on gas-gas heat recuperation due to the massive excess of H₂O/CO₂. Redox material thermodynamics and plant/reactor performance are closely coupled, and must be considered together to maximize efficiency. Overall, we find that Zr-CeO₂ is the most promising redox material, while perovskites which seem promising due to high H₂/CO production capacities under large H₂O/CO₂ flow rates, perform poorly from an efficiency perspective due to the high heating duties, especially for steam.     

Thermochemical H2 production via solar driven hybrid SrO/SrSO4 water splitting cycle

This article reports the thermodynamic efficiency analysis of the strontium oxide – strontium sulfate (SrO-SrS) water splitting cycle by applying the principles of the second law of thermodynamics and by utilizing the commercially available HSC Chemistry software. Initially, the thermodynamic equilibrium compositions allied with a) the thermal reduction of SrSO₄, b) H₂ production via water splitting reaction (through SrO re-oxidation) are recognized. Moreover, the temperatures desirable for performing the thermal reduction and the water splitting steps are determined. The consequence of the molar flow rate of Ar on the thermal reduction of SrSO₄ is also examined in detail. The effect of the thermal reduction and water splitting temperatures on the total solar energy input mandatory to run the cycle, re-radiation shortfalls from the cycle, heat energy emitted by the coolers and the water splitting reactor, and the cycle and the solar-to-fuel energy conversion efficiency (with heat recuperation) is scrutinized in detail. The attained outcomes specify that the cycle and the solar-to-fuel energy conversion efficiency up to 18.9 and 22.8% can be accomplished if the thermal reduction and the water splitting steps are conducted at 2380 and 1400 K (with 30% heat recuperation).     

Efficient solar hydrogen production by photocatalytic water splitting: From fundamental study to pilot demonstration

Photocatalytic water splitting with solar light is one of the most promising technologies for solar hydrogen production. From a systematic point of view, whether it is photocatalyst and reaction system development or the reactor-related design, the essentials could be summarized as: photon transfer limitations and mass transfer limitations (in the case of liquid phase reactions). Optimization of these two issues are therefore given special attention throughout our study. In this review, the state of the art for the research of photocatalytic hydrogen production, both outcomes and challenges in this field, were briefly reviewed. Research progress of our lab, from fundamental study of photocatalyst preparation to reactor configuration and pilot level demonstration, were introduced, showing the complete process of our effort for this technology to be economic viable in the near future. Our systematic and continuous study in this field lead to the development of a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar rector for the first time. We have demonstrated the feasibility for efficient photocatalytic hydrogen production under direct solar light. The exiting challenges and difficulties for this technology to proceed from successful laboratory photocatalysis set-up up to an industrially relevant scale are also proposed. These issues have been the object of our research and would also be the direction of our study in future.     

Thermodynamic assessment of hydrogen production via solar thermochemical cycle based on MoO2/Mo by methane reduction

Inspired by the promising hydrogen production in the solar thermochemical (STC) cycle based on non-stoichiometric oxides and the operation temperature decreasing effect of methane reduction, a high-fuel-selectivity and CH₄-introduced solar thermochemical cycle based on MoO₂/Mo is studied. By performing HSC simulations, the energy upgradation and energy conversion potential under isothermal and non-isothermal operating conditions are compared. In the reduction step, MoO₂: CH₄ = 2 and 1020 K<T_red<1600 K are found to be most favorable for syngas selectivity and methane conversion. Compared to the STC cycle without CH₄, the introduction of methane yields a much higher hydrogen production, especially at the lower temperature range and atmospheric pressure. In the oxidation step, a moderately excessive water is beneficial for energy conversion whether in isothermal or non-isothermal operations, especially at H₂O: Mo= 4. In the whole STC cycle, the maximum non-isothermal and isothermal efficiency can reach 0.417 and 0.391 respectively. In addition, the predicted efficiency of the second cycle is also as high as 0.454 at T_red = 1200 K and T_oxi = 400 K, indicating that MoO₂ could be a new and potential candidate for obtaining solar fuel by methane reduction.     

Kinetic investigations of the hydrogen production step of a thermochemical cycle using mixed iron oxides coated on ceramic substrates

A two‐step thermochemical cycle for solar hydrogen production using mixed iron oxides as the metal oxide redox system has been investigated. The ferrite is coated on a honeycomb structure, which serves as the absorber for solar irradiation and provides the surface for the chemical reaction. Coated honeycomb structures have already been tested in a solar receiver reactor in the solar furnace of DLR in Cologne with respect to their water splitting capability and their long‐term stability. The concept of this new reactor design has proven feasible and constant hydrogen production during repeated cycles has been shown. For a further optimization of the process and in order to gain reliable performance predictions more information about the process especially concerning the kinetics of the oxidation and the reduction step are essential. To examine the hydrogen production during the water splitting step a test rig has been built up on a laboratory scale. In this test rig small coated honeycombs are heated by an electric furnace. The honeycomb is placed inside a tube reactor and can be flushed with water vapour or with an inert gas. A homogeneous temperature within the sample is reached and testing conditions are reproducible. Through analysis of the product gas the hydrogen production is monitored and a reaction rate describing the hydrogen production rate per gram ferrite can be formulated. Using this test set‐up, SiC honeycombs coated with zinc ferrite have been tested. The influences of the temperature and the water concentration on the hydrogen production during the water splitting step have been investigated. An analysis of the ferrite conversion was performed using the Shrinking Core Model. A mathematical approach for the peak reaction rate at the beginning of the water splitting step was formulated and the activation energy was calculated from the experimental data. An activation energy of 110 kJ mol⁻¹ was found. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

Experimental assessment of the cyclability of the Mn2O3/MnO thermochemical cycle for solar hydrogen production

The cyclability of Mn₂O₃/MnO thermochemical cycle for solar hydrogen production has been experimentally evaluated. The results of three consecutive cycles show a stable hydrogen production per mass of initial solid that is in agreement with the maximum expected amount according to the stoichiometry of the process. The characterization of the material recovered after each cycle shows a mixture of different manganese oxide phases that are completely converted into MnO in the subsequent thermal reduction, maintaining the productivity of the cycles. Based on that, a modification of the thermochemical cycle scheme is proposed taking into account the differences observed between the first cycle and the following ones. MnO₂/MnO thermochemical cycle appears as a promising alternative, working in the same temperature range but with a theoretical hydrogen production per unit mass of solid manganese oxide almost twice than that obtained with the conventional Mn₂O₃/MnO cycle. Finally, the results of exergy efficiency of the complete cycle give new insights into the commercial possibilities of the cycle for hydrogen production, demonstrating the sustainable cyclability of the process regarding the manganese containing materials at lower temperatures than those theoretically reported in literature and consequently with higher exergy efficiencies that the common values associated to this cycle.     

Study of the first step of the Mn2O3/MnO thermochemical cycle for solar hydrogen production

In this work, a complete thermodynamic study of the first step of the Mn₂O₃/MnO thermochemical cycle for solar hydrogen production has been performed. The thermal reduction of Mn₂O₃ takes place through a sequential mechanism of two reaction steps. The first step (reduction of Mn₂O₃ to Mn₃O₄) takes place at teomperatures above 700 °C, whereas the second reaction step (reduction of Mn₃O₄ to MnO) requires temperatures above 1350 °C to achieve satisfactory reaction rates and conversions. Equilibrium can be displaced to lower temperatures by increasing the inert gas/Mn₂O₃ ratio or decreasing the pressure. The thermodynamic calculations have been validated by thermogravimetric experiments carried out in a high temperature tubular furnace. Experimental results corroborate the theoretical predictions although a dramatically influence of chemical kinetics and diffusion process has been also demonstrated, displacing the reactions to higher temperatures than those predicted by thermodynamics. Finally, this work demonstrates that the first step of the manganese oxide thermochemical cycle for hydrogen production can be carried out with total conversion at temperatures compatible with solar energy concentration devices. The range of required temperatures is lower than those commonly reported in literature for the manganese oxide cycle obtained from theoretical and thermodynamic studies.