Modeling of a solar receiver–reactor for sulfur‐based thermochemical cycles for hydrogen generation

Sulfur‐based thermochemical cycles for hydrogen generation from water have one reaction step in common, which is the decomposition of sulfuric acid, which is one of the most energy‐consuming steps. The present work deals with the development of a dynamic mathematical model of a solar reactor for this key step. One of the core parts of the model is a partial model of the reaction kinetics of the decomposition of sulfur trioxide, which is based on experiments investigating the kinetics of the used catalyst platinum coated on a ceramic solar absorber. Other partial models describe, e.g. the absorption of solar radiation, heat conduction in the absorber, convection between gas and the absorber walls and energy losses due to heat radiation. A comprehensive validation of the reactor model is performed using measured data, which is gained in experiments with a prototype reactor. The operating behavior of the real reactor is compared with the results of the numerical simulation with the model. The validation is, in particular, performed by reproducing the influences of individual parameters on the chemical conversion and the reactor efficiency. The relative deviations between the experimental data and the simulation results are mostly within the range of measurement accuracy. In particular, the good agreement of calculated values of the derived parameters, SO₃ conversion and reactor efficiency with those determined from the experiments qualifies the model for optimization purposes. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical analysis of operation conditions and design aspects of a sulfur trioxide decomposer for solar energy conversion

A basic concept for a receiver–reactor for solar sulfuric acid decomposition as the key step of the Hybrid Sulfur Cycle for hydrogen production has been developed and realized. A prototype reactor has been built and is specialized for the second part of the reaction, the decomposition of sulfur trioxide. For a detailed understanding of the operational behavior of the developed reactor type a mathematical model was developed. The reactor model was validated using experimental data from the test operation with a prototype reactor. The present work deals with the optimization of process and design parameters and the evaluation of the achievable performance of the reactor type. Furthermore the reactor model is used for numerical simulations to predict specific operational points of the prototype reactor and the performance of a large‐scale reactor on a solar tower. Influences of operational parameters like absorber temperature, feed mass flow, residence time and initial concentration of the acid are analyzed. In many cases those analyses reveal the existence of an optimum of reactor efficiency. When varying the absorber temperature an optimum of reactor efficiency emerges due to two compensating effects: chemical conversion increases with temperature, whereas re‐radiation losses increase disproportionately at the same time. This matches the experimental findings very well. A large‐scale tower receiver–reactor consisting of several individual modules is modeled and simulated. The main differences to the prototype system are the reduced gradients of solar flux distribution on the receiver front face and the reduced thermal conduction losses due to the presence of several neighbor modules at a comparable temperature level. This leads to higher chemical conversions and better efficiencies. Reactor efficiencies up to 75% are predicted. Copyright © 2011 John Wiley & Sons, Ltd.     

Heat transfer model and scale-up of an entrained-flow solar reactor for the thermal decomposition of methane

The solar thermochemical decomposition of CH₄ is carried out in a solar reactor consisting of a cavity-receiver containing an array of tubular absorbers, through which CH₄ flows and thermally decomposes to H₂ and carbon particles. A reactor model is formulated by coupling radiation/convection/conduction heat transfer and chemical kinetics for a two-phase solid-gas reacting flow. Experimental validation is accomplished by comparing measured and simulated absorber temperatures and H₂ concentrations for a 10 kW prototype reactor tested in a solar furnace. The model is applied to optimize the design and simulate the performance of a 10 MW commercial-scale reactor mounted on a solar tower system configuration. Complete conversion is predicted for a maximum CH₄ mass flow rate of 0.70 kg s⁻¹ and a desired outlet temperature of 1870 K, yielding a solar-to-chemical energy conversion efficiency of 42% and a solar-to-thermal energy conversion efficiency of 75%.     

Numerical analysis of the biomimetic leaf-type hierarchical porous structure to improve the energy storage efficiency of solar driven steam methane reforming

Due the energy resource comes from solar energy, resulting in a high working temperature, radiation field has a significant influence on the energy storage efficiency of the high temperature solar thermochemistry. In order to promote the solar energy conversion efficiency of solar driven steam methane reforming (SMR), the idea of regulate the radiation field to be in accordance with the energy conversion on-demand is proposed and the biomimetic leaf-type hierarchical porous structure solar thermochemical reactor is introduced, which can regulate the spatial distribution of solar radiation intensity and optimize the temperature field. Combined with thermochemical kinetics and Finite Volume Method (FVM), the numerical calculation model of the SMR reaction in a biomimetic solar thermochemical reactor is established to optimize the temperature field. The effects of different reaction conditions and reactor parameters on steam methane reforming hydrogen production are analyzed. The results show that methane conversion in the biomimetic leaf-type solar thermochemical reactor is increased by 4.5%.     

Numerical modeling of a bayonet heat exchanger-based reactor for sulfuric acid decomposition in thermochemical hydrogen production processes

Sulfur-based thermochemical hydrogen production cycles represent one of the most appealing options to produce hydrogen from water on a large scale. The Hybrid Sulfur is one of the most advanced thermochemical cycles. The high temperature section of the process, common to all sulfur-based cycles, operates the sulfuric acid thermal decomposition reaction at temperatures on the order of 800 °C. The paper shows and discusses the modeling results obtained for a bayonet heat exchanger based high temperature reactor that decomposes the sulfur compounds into sulfur dioxide and oxygen. A detailed transport phenomena model, including suitable decomposition kinetics, has been set up using a finite volume numerical approach. A preliminary configuration of the reactor, established based on process simulation results and on the initial reactor prototype developed at Sandia National Laboratory, has been examined and simulated. Results, obtained for a reactor driven by thermal power provided by helium flow, demonstrate the effective decomposition performance at maximum temperatures on the order of 800 °C and pressures of 14 bar. For a laminar flow configuration a sulfur dioxide production yield of about 28 wt% (with sulfur trioxide reduction from 69 wt% to approximately 33 wt%) has been achieved, representing decomposition rates practically equal to the corresponding equilibrium values. Limited pressure drops of approximately 2500 Pa have also been achieved in the sulfur mixture region.     

Heat transfer analysis of thermal disassociation of ZnO for solar hydrogen production

Hydrogen production by the two-step solar thermochemical cycle has high cycle efficiency, low cost, and a great development space. Of special interest is the solar thermochemical cycle based on ZnO/Zn redox reactions since its high theoretical hydrogen yield and relatively low endothermic reaction temperature. In this paper, a steady heat transfer model for thermal ZnO dissociation in a solar thermochemical reactor is developed, coupling conduction, convection and radiation with chemical reaction. Accuracy was evaluated by comparison of results obtained from other references. Based on the new proposed reactor, the model is adopted to analyze the operating parameter effect on the conversion rate and fluid feature inside the solar reactor. The results show that the mass flow rate of ZnO and aperture gas temperature have a positive relation with ZnO conversion rate, however, the diameter of particles and aperture gas velocity has an inverse relation with ZnO conversion rate under specific condition. The results will provide useful foundation for improving the solar-to-fuel conversion rate in the near future.     

Performance improvement of a solar volumetric reactor with passive thermal management under different solar radiation conditions

To alleviate the effect of solar radiation fluctuation on the solar volumetric reactor, phase change material (PCM) is applied to buffer the temperature vibration and improve the stability of thermochemical reactions. In this work, we analyzed the heat flow and distribution characteristics of the conventional double-walled volumetric reactor filled with PCMs (SVR1). We then proposed a novel solar volumetric reactor design (SVR2) to solve the problems of local high temperature, slow charging-discharging rate, and fluctuating methane conversion in various radiation conditions. The heat and mass transfer model coupled with thermochemical reaction kinetics was established to compare the performance of SVR1 and SVR2 under steady state, heat charging-discharging mode, and actual solar radiation fluctuation, respectively. The results show that compared to SVR1, the maximum temperature of SVR2 decreases by 106.3 K, and the minimum methane conversion rate increases from 77.4% to 93.6% under natural solar radiation fluctuation.     

A thermochemical reactor design with better thermal management and improved performance for methane/carbon dioxide dry reforming

A solar thermochemical reactor with better thermal management is proposed to improve the performance for dry reforming of methane. Conical cavity is introduced in the thermochemical reactor to adjust incident solar radiation distribution. Preheating area is adopted to recover sensible heat from gas outlet. Multiphysical model is presented for analyzing the overall performance of the reactor under different inlet flow rates. Also, local ideal reaction temperature required for maximizing local hydrogen production is analyzed according to the reaction kinetics. It is shown that better synergy between real temperature distribution and ideal temperature requirement can be achieved in this new reactor. Compared with conventional reactor, the present reactor exhibits the better performance in terms of reactant conversion, energy storage efficiency and hydrogen yield. Particularly, hydrogen yield is increased by 4.31%–17.12% at inlet flow rates between 6 and 12 L min^?1.     

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.     

Kinetic and thermodynamic analyses of mid/low-temperature ammonia decomposition in solar-driven hydrogen permeation membrane reactor

It is a promising method for hydrogen generation without carbon emitting by ammonia decomposition in a catalytic palladium membrane reactor driven by solar energy, which could also store and convert solar energy into chemical energy. In this study, kinetic and thermodynamic analyses of mid/low-temperature solar thermochemical ammonia decomposition for hydrogen generation in membrane reactor are conducted. Hydrogen permeation membrane reactor can separate the product and shift the reaction equilibrium forward for high conversion rate in a single step. The variation of conversion rate and thermodynamic efficiency with different characteristic parameters, such as reaction temperature (100–300 °C), tube length, and separation pressure (0.01–0.25 bar), are studied and analyzed. A near-complete conversion of ammonia decomposition is theoretically researched. The first-law thermodynamic efficiency, net solar-to-fuel efficiency, and exergy efficiency can reach as high as 86.86%, 40.08%, and 72.07%, respectively. The results of this study show the feasibility of integrating ammonia decomposition for hydrogen generation with mid/low-temperature solar thermal technologies.     

Analysis of biomimetic hierarchical porous structure regulating radiation field to improve solar thermochemical performance based on minimum Gibbs free energy

Solar energy is the source of energy required for the dry methane reforming (DMR). In the high temperature field induced by concentrated solar energy, the spatial distribution of radiation intensity has a significant impact on the solar thermochemical performance. Based on principle of minimum Gibbs free energy (G_min), the idea of regulating radiation field to match solar thermochemical energy conversion on-demand is proposed. To improve solar thermochemical conversion efficiency, biomimetic hierarchical porous structure is introduced as solar thermochemical reactor, which can optimize both the radiation field and temperature field. The analysis model of solar driven DRM is established, combined with user-defined functions (UDFs). The effects of pore diameter combinations along the direction of L and R on the reforming properties are studied. The results show that by designing biomimetic hierarchical porous structure, the temperature of solar thermochemical reactor can approach the thermodynamic temperature (1050 K), and finally the methane conversion is improved by up to 7%.     

Catalytic thermochemical reactor/receiver for solar reforming of natural gas: Design and performance

The advantages of thermochemical conversion of concentrated solar energy using catalytic processes are discussed. The design of a solar volumetric thermochemical reactor/receiver (TCRR) with catalytic absorber, method for synthesis of catalytically activated ceramics, and preparation of catalytic absorber have been described. The prototype TCRR was tested in the high flux solar furnace at the DAC, Cologne by using the dioxide reforming of methane. The tests were performed to check the main concept of the TCRR design and catalytic absorber, to study the influence of solar flux distribution, the reagent flows and their ratio on the productivity or conversion, determine the reagent's conversion depending on the focal point disposition with respect to the absorber, and to study the efficiency of the thermochemical conversion. The chemical and total efficiencies of the CO₂–methane conversion were calculated using the experimentally measured concentrations of the reaction products. The highest overall efficiency achieved in these experiments was 30% with the Ni–Cr catalytic absorber.     

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.     

Heat transfer analysis of solar-driven high-temperature thermochemical reactor using NiFe-Aluminate RPCs

Converting solar energy efficiently into hydrogen is a promising way for renewable fuels technology. However, high-temperature heat transfer enhancement of solar thermochemical process is still a pertinent challenge for solar energy conversion into fuels. In this paper, high-temperature heat transfer enhancement accounting for radiation, conduction, and convection heat transfer in porous-medium reactor filled with application in hydrogen generation has been investigated. NiFe-Aluminate porous media is synthesized and used as solar radiant absorber and redox material. Experiments combined with numerical models are performed for analyzing thermal characteristics and chemical changes in solar receiver. The reacting medium is most heated by radiation heat transfer and higher temperature distribution is observed in the region exposed to high radiation heat flux. Heat distribution, O₂ and H₂ yield in the reacting medium are facilitated by convective reactive gas moving through the medium's pores. The temperature gradient caused by thermal transition at fluid-solid interface could be more decreased as much as the reaction chamber can store the transferred high-temperature heat flux. However, thermal losses due to radiation flux lost at the quartz glass are obviously inevitable.     

Thermodynamic analysis of solar thermochemical energy absorption

Thermochemical energy conversion is analysed on a thermodynamic basis with particular interest in obtaining guidance as to solar thermochemical absorber design at an early stage of development when technical and cost information is often unreliable. An earlier-used thermodynamic equivalence technique which is equally applicable to both the endothermic and exothermic reactions has been developed to the point where it gives a clear insight into all sources of heat and work. The method is applied in particular to separating endothermic reactions of which ammonia dissociation is a prime example. A reversibility ratio is defined as the ratio of irreversible to reversible work and it is shown that in a practical solar thermochemical absorber design the reversibility ratio should be minimized, corresponding to reaction temperature minimization and therefore to tower energy losses and to potential for use of lower cost reactor materials and more active catalysts. Values of reversibility ratio are calculated for the ammonia system and are discussed in relation to solar thermochemical absorber design. In a final analysis employing the thermodynamic equivalence technique, it is shown that the apparent paradox between liquid and alternative gas pump work requirements in a liquid/gas thermochemical system is thermodynamically consistent with the internal generation of effective work from the heat source used to drive the endothermic reaction system.     

Dynamic model of a solar thermochemical water-splitting reactor with integrated energy collection and storage

Water-splitting solar thermochemical cycles are important in meeting the challenges of global climate change and limited fossil fuels. However, solar radiation varies in availability, leading to unsteady state operation. We propose a solar receiver-reactor with integrated energy collection and storage. The reactor consists of a double-pipe heat exchanger placed at the focal line of a parabolic trough solar concentrator. Molten salt passes through the jacket, absorbing energy from the irradiated outer surface while driving the endothermic oxygen production step of the copper-chlorine water-splitting cycle in the reactor bore. Excess energy is stored in a thermal storage tank to buffer the reactor from changes in insolation. Dynamic simulation indicates that the reactor can sustain steady 100% conversion during 24/7 operation with a reasonable plant layout. The technology employed is extant and mature. This is important in view of the urgency to reduce dependency upon fossil fuels as primary energy sources.     

Numerical modeling of high-temperature shell-and-tube heat exchanger and chemical decomposer for hydrogen production

Numerical simulations of shell-and-tube heat exchanger and chemical decomposer with straight tube configuration and porous media were performed using FLUENT6.2.16 to examine the percentage decomposition of sulfur trioxide. The decomposition process can be a part of sulfur–iodine (S–I) thermochemical water splitting cycle, which is one of the most studied cycles for hydrogen production. A steady-state, laminar, two-dimensional axisymmetric shell-and-tube model with counter flow and parallel flow arrangements and simple uniform cubical packing was developed using porous medium approach to investigate the fluid flow, heat transfer and chemical reactions in the decomposer. As per the investigation, the decomposition percentage of sulfur trioxide for counter flow arrangement was found to be 93% and that of parallel flow was 92%. Also, a high pressure drop was observed in counter flow arrangement compared to parallel flow. The effects of inlet velocity, temperature and the porous medium properties on the pressure drop across the porous medium were studied. The influence of geometric parameters mainly the diameter of the tube, diameter of the shell and the length of the porous zone on the percentage decomposition of sulfur trioxide in the tube was investigated as well. A preliminary parametric study of the mentioned configuration is conducted to explore effects of varying parameters on the decomposition of sulfur trioxide. From the performed calculations, it was found that the Reynolds number played a significant role in affecting the sulfur trioxide decomposition. The percentage decomposition decreases with an increase in Reynolds number. Surface-to-volume area ratio and activation energy were also the important parameters that influenced the decomposition percentage.     

Performance investigation of magnesium–chloride hybrid thermochemical cycle for hydrogen production

In this paper, we study the yields of reactants in hydrolysis and chlorination chemical processes of the low temperature Mg–Cl hybrid thermochemical cycle to investigate the requirements of temperature, pressure and product ratios for individual reactors of the cycle. A simulation of both hydrolysis and chlorination processes is conducted using the Aspen Plus software. A Mg–Cl cycle is developed by considering the results obtained from the present simulations. Both energy and exergy efficiencies of Mg–Cl cycle are comparatively evaluated under varying system and environmental parameters, and an efficiency comparison of the cycle with other promising thermochemical water splitting cycles is conducted. The results show that, compared to other cycles, lower pressure, higher temperature and higher steam to magnesium–chloride ratio are required for full conversion of reactants in the hydrolysis step; and hence, lower temperature, higher pressure and higher chlorine to magnesium oxide ratio is required for full conversion in chlorination reactor. The efficiency results show that Mg–Cl cycle can compete with other low temperature thermochemical water splitting cycles and under influence of various internal and external parameters.     

Flux and temperature distribution in the receiver of parabolic solar furnaces

In this study, a mathematical analysis is presented on the complete interface problem between solar concentration systems and high temperature thermochemical processes. This includes the thermal process starting from the incoming solar radiation up to the heat transfer to a heat carrier fluid or reactants in a given reactor. The system considered comprises a heliostat, a parabolic concentrator and a receiver. The hourly incoming radiation, the hourly reflection and absorption losses on the heliostat and concentrator systems, the radiation flux density distribution in the receiver space, the solar and IR bands radiation exchange and the useful heat transfer are all considered in the analysis. The parameters such as temperature distribution in the receiver as well as thermal efficiency can be calculated for a given case. The model has been verified using the experimental results obtained in two different systems. In addition, a parametric study has been carried out on the global receiver efficiency with respect to temperature.