Test operation of a 100 kW pilot plant for solar hydrogen production from water on a solar tower

The present work describes the realisation and successful test operation of a 100 kW pilot plant for two-step solar thermo-chemical water splitting on a solar tower at the Plataforma Solar de Almería, which aims at the demonstration of the feasibility of the process on a solar tower platform under real conditions. The process applies multi-valent iron based mixed metal oxides as reactive species which are coated on honeycomb absorbers inside a receiver-reactor. By the introduction of a two-chamber reactor it is possible to run both process concepts in parallel and thus, the hydrogen production process in a quasi-continuous mode. In summer 2008 an exhaustive thermal qualification of the pilot plant took place, using uncoated ceramic honeycombs as absorbers. Some main aspects of these tests were the development and validation of operational and measurement strategy, the gaining of knowledge on the dynamics of the system, in particular during thermal cycling, the determination of the controllability of the whole system, and the validation of practicability of the control concept. The thermal tests enabled to improve, to refine and finally to prove the process strategy and showed the feasibility of the control concept implemented. It could be shown that rapid changeover between the modules is a central benefit for the performance of the process.In November of 2008 the absorber was replaced and honeycombs coated with redox material were inserted. This allowed carrying out tests of hydrogen production by water splitting. Several hydrogen production cycles and metal oxide reduction cycles could be run without problems. Significant concentrations of hydrogen were produced with a conversion of steam of up to 30%.     

Solar water splitting for hydrogen production with monolithic reactors

The present work proposes the exploitation of solar energy for the dissociation of water and production of hydrogen via an integrated thermo-chemical reactor/receiver system. The basic idea is the use of multi-channelled honeycomb ceramic supports coated with active redox reagent powders, in a configuration similar to that encountered in automobile exhaust catalytic aftertreatment.Iron-oxide-based redox materials were synthesized, capable to operate under a complete redox cycle: they could take oxygen from water producing pure hydrogen at reasonably low temperatures (800 °C) and could be regenerated at temperatures below 1300 °C. Ceramic honeycombs capable of achieving temperatures in that range when heated by concentrated solar radiation were manufactured and incorporated in a dedicated solar receiver/reactor. The operating conditions of the solar reactor were optimised to achieve adjustable, uniform temperatures up to 1300 °C throughout the honeycomb, making thus feasible the operation of the complete cycle by a single solar energy converter.     

Development of a system model for a hydrogen production process on a solar tower

An attractive path to the production of hydrogen from water is a two-step thermo chemical cycle powered by concentrated sunlight from a solar tower system. In the first process step the redox system, a ferrite coated on a monolithic honeycomb absorber, is present in its reduced form while the concentrated solar energy hits the ceramic absorber. When water vapour is fed to the honeycomb at 800 °C, oxygen is abstracted from the water molecules, bond in the redox system and hydrogen is produced. When the metal oxide system is completely oxidised it is heated up for regeneration at 1100–1200 °C in an oxygen-lean atmosphere. Under those conditions and in the second process step, oxygen is set free from the redox system, so the metal oxide is being reduced and after completion of the reaction again capable for water splitting.Since the overall process consists of two core reaction steps, which need to be carried out sequentially in a reactor unit at two different temperature steps, a special process and plant concept had to be developed enabling the continuous supply of product regardless of the alternating nature of the solar reactor operation. The challenge of the process control is to keep the two core reaction temperatures constant and to ensure regular temperature switches after completion of the individual process steps, independent of the weather conditions, like DNI fluctuation, clouds and wind speed. Also start-up, the fast switching after completion of half-cycles and the shutdown must be controlled. State of the art is the manual switching of heliostats to fulfil those control tasks.This paper describes the development and use of a system model of this process. The model consists of three main parts: the simulation of the solar flux distribution at the receiver aperture, the simulation of the temperatures in the reactor modules and the simulation of the hydrogen generation. It can be used for the analysis of the operational behaviour. The model is intended to be used in the future for the control of the whole process.     

Development and test of a solar reactor for decomposition of sulphuric acid in thermochemical hydrogen production

Decomposition of sulphuric acid is a key step of sulphur based thermochemical cycles for hydrogen production by thermal splitting of water. The Hybrid Sulphur Cycle (HyS) consisting of two reaction steps is considered as one of the most promising cycles: firstly, sulphuric acid is decomposed by high temperature heat of 800–1200 °C forming sulphur dioxide, which in a second step is used to electrochemically split water. Compared to conventional water electrolysis only about a tenth of the theoretical voltage is required making the HyS one of the most efficient processes to produce hydrogen by concentrated solar radiation. As a result, this thermochemical cycle has the potential to significantly reduce the amount of energy required for water splitting and to efficiently generate hydrogen free of carbon dioxide emissions. The European research project HycycleS aims at a technical realisation of the HyS. One objective of the project is to develop and qualify a solar interface, meaning a device to couple concentrated solar radiation into the endothermal steps of the chemical process. Therefore, a test reactor for decomposition of sulphuric acid by concentrated solar radiation was developed and tested in the solar furnace of DLR in Cologne. Tests in concentrated solar radiation were carried out for temperatures of the honeycomb up to 950 °C decomposing sulphuric acid of 50 and 96 weight-percent. Mass and energy flow of the process were calculated in order to determine energy efficiency and chemical conversion. The influence of process parameters like temperature, flow rates and space velocity on chemical conversion and reactor efficiency was analysed in detail. If catalysts like iron oxide (Fe₂O₃) and mixed oxides (i.e. CuFe₂O₄) were used a conversion of SO₃ to SO₂ of more than 80% at a thermal efficiency of over 25% could be reached.     

Experimental study on sulfur trioxide decomposition in a volumetric solar receiver–reactor

Process conditions for the direct solar decomposition of sulfur trioxide have been investigated and optimized by using a receiver–reactor in a solar furnace. This decomposition reaction is a key step to couple concentrated solar radiation or solar high‐temperature heat into promising sulfur‐based thermochemical cycles for solar production of hydrogen from water. After proof‐of‐principle a modified design of the reactor was applied. A separated chamber for the evaporation of the sulfuric acid, which is the precursor of sulfur trioxide in the mentioned thermochemical cycles, a higher mass flow of reactants, an independent control and optimization of the decomposition reactor were possible. Higher mass flows of the reactants improve the reactor efficiency because energy losses are almost independent of the mass flow due to the predominant contribution of re‐radiation losses. The influence of absorber temperature, mass flow, reactant initial concentration, acid concentration, and residence time on sulfur trioxide conversion and reactor efficiency has been investigated systematically. The experimental investigation was accompanied by energy balancing of the reactor for typical operational points. The absorber temperature turned out to be the most important parameter with respect to both conversion and efficiency. When the reactor was applied for solar sulfur trioxide decomposition only, reactor efficiencies of up to 40% were achieved at average absorber temperature well below 1000°C. High conversions almost up to the maximum achievable conversion determined by thermodynamic equilibrium were achieved. As the re‐radiation of the absorber is the main contribution to energy losses of the reactor, a cavity design is predicted to be the preferable way to further raise the efficiency. Copyright © 2009 John Wiley & Sons, Ltd.     

Mechanistic and kinetic study of thermolysis reaction with hydrolysis step products in Cu–Cl thermochemical cycle

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–CuCl_2. 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.     

Investigation of hydrogen production performance of a reactor assisted by a solar pond via photoelectrochemical process

In this study, the hydrogen production performance of a reactor assisted by a solar pond by photoelectrochemical method is examined conceptually. The main components of the new integrated system are a solar pond, a photovoltaic panel (PV) and a hybrid chlor-alkali reactor which consists of a semiconductor anot, photocathode and cation exchange membrane. The proposed system produces hydrogen via water splitting reaction and also yields the by products namely chlorine and sodium hydroxide while consumes saturated NaCl solution and pure water. In order to increase the efficiency of the reactor, the saturated hot NaCl solution at the heat storage zone (HSZ) of the solar pond is transferred to the anot section and the heated pure water by heat exchanger in the HSZ is transferred to cathode section. The photoelectrode releases electrons for hydrogen production with diminishing the power requirement from the PV panel that is used as a source of electrical energy for the electrolysis. The results confirm that the thermal performance of the solar pond plays a key role on the hydrogen production efficiency of the reactor.     

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

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

Oxidation reaction kinetics for the steam-iron process in support of hydrogen production

A hydrogen generation research program is focused on solar-driven hydrogen production by means of reactive metal water splitting. In order to dissociate water molecules at significantly reduced thermal energies as well as providing a practical means for efficient hydrogen and oxygen separation, an intermediary reactive material is introduced to realize water splitting in the form of an oxidation reaction. Elemental iron is used as the reactive material in the process commonly referred to as the steam-iron process. In order to exploit the unique characteristics of highly reactive materials and ultimately achieve the potential efficiency gains at the solar reactor scale, a monolithic laboratory-scale reactor has been designed to explore the fundamental kinetic rates during the iron oxidation reaction at temperatures ranging from about 650 to 900 K. Results show hydrogen production rates on the order of 1E-8 g/cm² s. Micro-Raman spectroscopy is used to access information on the exact iron oxide phase produced, and high resolution SEM and electron dispersion spectroscopy (EDS) are used to assess the oxide morphology and further quantify the oxide state, including spatial distributions.     

Catalytic performance of a high-cell-density Ni honeycomb catalyst for methane steam reforming

The catalysis of methane steam reforming (MSR) by pure Ni honeycombs with high cell density of 2300 cells per square inch (cpsi) was investigated to develop efficient and inexpensive catalysts for hydrogen production. The Ni honeycomb catalyst was assembled using 30-μm-thick Ni foils, and showed much higher activity than that of a Ni honeycomb catalyst with cell density of 700 cpsi at a steam-to-carbon ratio of 1.36 and a gas hourly space velocity of 6400 h^?1 in a temperature range of 873–1173 K. Notably, the activity increased approximately proportional to the increasing geometric specific surface area of the honeycombs. The turnover rate of the Ni honeycomb catalyst was higher than that of supported Ni catalysts. The changes in chemical state of the Ni catalyst during hydrogen reduction and MSR reaction were analyzed by in situ X-ray absorption fine structure spectroscopy, which revealed that deactivation was mainly due to oxidation of the surface Ni atoms. These results demonstrated that the high-cell-density Ni honeycomb catalyst exhibits good performance for MSR reaction, and easy regeneration of the deactivated Ni honeycomb catalyst is possible only via hydrogen reduction.     

Hydrogen production via sulfur-based thermochemical cycles: Part 2: Performance evaluation of Fe2O3-based catalysts for the sulfuric acid decomposition step

The sulfuric acid dissociation reaction, via which the production of SO₂ and O₂ is achieved, is the most energy intensive step of the so-called sulfur-based thermochemical cycles for the production of hydrogen. Efforts are focused on the feasibility and effectiveness of performing this reaction with the aid of a high-temperature energy/heat source like the sun. Such coupling can be achieved either directly in a solar reactor by concentrated solar radiation, or indirectly by means of a heat-exchanger/decomposer reactor using a suitable heat transfer fluid. Since a very limited amount of work regarding the potential formulations and sizing of such suitable reactors has been performed so far, the present work addresses further steps necessary for the efficient design, manufacture and operation of such reactors for sulfuric acid decomposition. In this respect, parametric studies on the SO₃ decomposition with iron(III) oxide-based catalysts were performed investigating the effect of temperature, pressure and space velocity on SO₃ conversion. Based on these results, an empirical kinetic law suitable for the reactor design was developed. In parallel, siliconised silicon carbide honeycombs coated with iron(III) oxide were prepared and tested in structured laboratory-scale reactors to evaluate their durability (i.e. activity vs. time) during SO₃ decomposition, with the result of satisfactory and stable performance for up to 100 h of operation. The results in combination with characterization results of “aged” materials can provide valuable input for the design of prototype reactors for sulfuric acid decomposition.     

Thermodynamic analysis of two‐step solar water splitting with mixed iron oxides

A two‐step thermochemical cycle for solar production of hydrogen from water has been developed and investigated. It is based on metal oxide redox pair systems, which can split water molecules by abstracting oxygen atoms and reversibly incorporating them into their lattice. After successful experimental demonstration of several cycles of alternating hydrogen and oxygen production, the present work describes a thermodynamic study aiming at the improvement of process conditions and at the evaluation of the theoretical potential of the process. In order to evaluate the maximum hydrogen production potential of a coating material, theoretical considerations based on thermodynamic laws and properties are useful and faster than actual tests. Through thermodynamic calculations it is possible to predict the theoretical maximum output of H₂ from a specific redox‐material under certain conditions. Calculations were focussed on the two mixed iron oxides nickel–iron‐oxide and zinc–iron‐oxide. In the simulation the amount of oxygen in the redox‐material is calculated before and after the water‐splitting step on the basis of laws of thermodynamics and available material properties for the chosen mixed iron oxides. For the simulation the commercial Software FactSage and available databases for the required material properties were used. The analysis showed that a maximum hydrogen yield is achieved if the reduction temperature is raised to the limits of the operation range, if the temperature for the water splitting is lowered below 800°C and if the partial pressure of oxygen during reduction is decreased to the lower limits of the operational range. The predicted effects of reduction temperature and partial pressure of oxygen could be confirmed in experimental studies. The increased hydrogen yield at lower splitting temperatures of about 800°C could not be confirmed in experimental results, where a higher splitting temperature led to a higher hydrogen yield. As a consequence it can be stated that kinetics must play an important role especially in the splitting step. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical and experimental study on hydrogen production via dimethyl ether steam reforming

This work presents the characteristics of catalytic dimethyl ether (DME)/steam reforming based on a Cu–Zn/γ-Al₂O₃ catalyst for hydrogen production. A kinetic model for a reformer that operates at low temperature (200 °C–500 °C) is simulated using COMSOL 5.2 software. Experimental verification is performed to examine the critical parameters for the reforming process. During the experiment, superior Cu–Zn/γ-Al₂O₃catalysts are manufactured using the sol-gel method, and ceramic honeycombs coated with this catalyst (1.77 g on each honeycomb, five honeycombs in the reactor) are utilized as catalyst bed in the reformer to enhance performance. The steam, DME mass ratio is stabilized at 3:1 using a mass flow controller (MFC) and a generator. The hydrogen production rate can be significantly affected depending on the reactant's mass flow rate and temperature. And the maximum hydrogen yield can reach 90% at 400 °C. Maximum 8% error for the hydrogen yield is achieved between modeling and experimental results. These experiments can be further explored for directly feeding hydrogen to proton exchange membrane fuel cell (PEMFC) under the load variations.     

Performance assessment study of photo-electro-chemical water-splitting reactor designs for hydrogen production

Solar water splitting is considered a greatly promising technique for producing clean hydrogen fuel. However, limited studies have paid attention to the designs of photo-electrochemical (PEC) reactors. In this regard, two different designs of PEC reactor are proposed and studied numerically in the present paper. The effects of important design parameters on the system performance are also investigated. The PEC governing equations of transport phenomena related to water splitting reactor are developed and numerically solved. According to the current results, the rate of the hydrogen volume production and the solar - to - hydrogen conversion efficiency increase as an applied solar incident flux increases for both proposed designs. The solar - to - hydrogen conversion efficiencies are calculated to be 12.65% for design 1 and 12.48% for design 2. The hydrogen volume production rate is performed to achieve 78.3 L/m² h by design 1, and 74.8 L/m² h by design 2.     

Use of solar energy for direct and two-step water decomposition cycles

In this study, the feasibility of using concentrated solar energy at high temperatures to decompose water is experimentally demonstrated. The preliminary studies show that direct decomposition of water at 2000–2500°C is possible and the main development should be directed to reactor design and the separation of product gases. On the other hand, it is shown that two step thermochemical cycles for hydrogen production are feasible when the reactions are carried out at appropriate high temperatures in a solar furnace. The thermal decomposition of zinc oxide, suitable for such a two step cycle, is studied in detail.     

Dissociation of magnetite in a solar furnace for hydrogen production. Tentative production evaluation of a 1000 kW concentrator from small scale (2 kW) experimental results

For experiment results obtained in a 2 kW solar concentrator, the FeO production by thermal dissociation of magnetite (Fe₃O₄) was extrapolated to the 1000 kW solar furnace of Odeillo, France. If this reaction is used in a two step thermochemical water splitting cycle, one can expect an extrapolated value of 137 m³ day^?1 hydrogen production when Fe₃O₄ is dissocated at 2090 K, under an inert atmosphere, during 0.5 min and cooled down by a splat cooling quench technique.     

Syngas and hydrogen production via stepwise methane reforming over Cu‐ferrite/YSZ

This report investigates the effect of an yttria‐stabilized zirconia (YSZ) supported Cu‐ferrite for the production of syngas and hydrogen via stepwise methane reforming and water splitting reactions. The Cu‐ferrite/YSZ samples were prepared by co‐precipitation and impregnation methods. The samples were characterized by X‐ray diffraction spectroscopy and non‐isothermal hydrogen reduction. To investigate syngas and hydrogen production reactivities, isothermal methane reforming and water splitting reactions were performed at 900 °C and 700 °C, respectively. For Cu‐ferrite/YSZ prepared by impregnation, methane conversion was maintained at high levels of ca. 85% and an H₂/CO ratio close to 2 was observed. A lower methane conversion (>30%) was observed for Cu‐ferrite/YSZ prepared by co‐precipitation. No significant deposited carbon and aggregation of Cu‐ferrite/YSZ (prepared by impregnation) were observed over 10 repeated methane reforming and water splitting reactions. Copyright © 2014 John Wiley & Sons, Ltd.     

Direct solar thermal splitting of water and on-site separation of the products—II. Experimental feasibility study

The development of a process of hydrogen production by solar thermal water splitting (HSTWS) presents a formidable technological task. The process has, however, great potential from the thermodynamic point of view and, when combined with fuel cell technology, it can lead to efficient conversion of solar energy to power. In the process under development at the Weizmann Institute of Science, water vapor is partially dissociated in a solar reactor at temperatures approaching 2500 K. Hydrogen is separated from the hot mixture of water splitting products by gas diffusion through a porous ceramic membrane.The paper describes the problems encountered during the development of the HSTWS process. The following topics are discussed in some detail: (a) achievement of very high solar hydrogen reactor temperatures by secondary concentration of solar energy; (b) materials problems encountered in the manufacture of the solar reactor; (c) development of special porous ceramic membranes that resist clogging by sintering at very high temperatures.     

Solar photocatalytic reactor performance for hydrogen production from incident ultraviolet radiation

Solar based hydrogen production is a promising alternative to methods based on fossil fuels, such as steam methane reforming (SMR) and coal gasification. A more economically viable way of producing hydrogen from water is under active investigation by many researchers, to convert solar energy to chemical energy with higher efficiency. In this paper, supramolecular complexes developed by Brewer (2006) for photocatalytic hydrogen production are examined, particularly for larger scale engineering reactors that can use visible light to dissolve the photocatalysts in water, causing the splitting of water molecules into hydrogen and hydroxyl ions. This paper analyzes and optimizes the system parameters associated with this system. A predictive model for the reactor is developed for a batch type photocatalytic reactor. Results are presented and discussed to evaluate how the system parameters affect the hydrogen production rate, and solar to hydrogen efficiency, using a monochromatic LED array and Rhodium based photocatalysts.     

SOLAR HYDROGEN PRODUCTION BY USING FERRITES

Hydrogen production by water splitting with Mn(II) ferrite and CaO (or Na₂CO₃) at 1273 K (or 873 K) was studied. The mixed powder of MnFe₂O₄ and CaO (CaO/MnFe₂O₄>3) (or Na₂CO₃) generates H₂ by reaction with H₂O at 1273 K (or 873 K). This H₂ evolution is caused by the oxidation of Mn(II) ion in the ferrite to the Mn(III) ion. The temperature of 873 K is considerably lower for the solar furnace reaction (O₂ releasing step) in the two-step water splitting (1500–2300 K) process. This lower temperature and economical availability of required elements would permit further progress in the direct solar energy absorption/conversion into H₂.