Search

Microbial electrolysis cell (MEC) is a promising reactor. However, currently, the reactor cannot be adapted for industrial-scale biohydrogen production. Nevertheless, this drawback can be overcome by modeling studies based on mathematical equations. The limitation of analytical instrumentation to record the non-linearity of the dynamic behavior for biohydrogen processes in an MEC has led to the introduction of computational approach that has the potential to reduce time constraints and optimize experimental costs. Reviews of comparative studies on bioelectrochemical models are widely reported, but there is less emphasis on the MEC model. Therefore, in this paper, a comprehensive review of the MEC mathematical model will be further discussed. The classification of the model with respect to the assumptions, model improvement, and extensive studies based on the model application will be critically analyzed to establish a methodology algorithm flow chart as a guideline for future implementation. [Display omitted] • Advancement of MEC mathematical models. • Classification of MEC mathematical models. • Proposed guideline for the MEC mathematical model implementation. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a novel prediction model of the effective hydrogen diffusivity for the gas diffusion layer (GDL) in proton exchange membrane fuel cell (PEMFC) by using fractal theory to characterize microstructure. With the consideration of pore-size distribution and Knudsen diffusion effect, a relationship between micro-structural parameters and effective hydrogen diffusivity of GDL is deduced. The prediction of effective hydrogen diffusivities of two samples shows that Knudsen diffusion effect makes the effective diffusivity value decrease, and after being treated with polytetrafluoroethylene (PTFE), carbon paper, a basal material of the GDL, exhibits a lower effective diffusivity value due to the decrease in the pore space and porosity. From the parametric effect study, it can be concluded that effective diffusivity has a positive correlation with pore area fractal dimension D p or porosity ɛ, whereas it has a negative correlation with tortuosity fractal dimension D t. [Copyright &y& Elsevier]

An experimental study on small-scale for solar hydrogen production system via a Proton Exchange Membrane electrolysis under a desert climatic condition in Ouargla region (South-East of Algeria) has been carried out, the target of this study has been first to evaluate hydrogen production by water analysis and to store the solar energy which has had the form of a hydride-metal hydrogen, after that, to investigate the performance of sophisticated commercial electrolyser (HG-60)powered by photovoltaic panels via the Power Management Unit (PMU) as a power conditioner, this paper has also a mathematical models based on real-time experiments were used to simulate both the photovoltaic system and PEM electrolyser work, along with attempting to direct linking strategy with the same experimental components of photovoltaic panels and commercial electrolyser, it was found through this study, the addition of the number of commercial electrolyser with the bank of four HG-60 stacks in series. More effective considering the improving voltage matching, with power transfer efficiency reach to 99%, also another factor is the photovoltaic panels slope on panel output power and hydrogen productivity are theoretically examined, where the proper selection of optimal tilt angle has an importance for collecting the maximum hydrogen amount, eventually, over the experiment span, the real-amount of hydrogen vented over experiment course is around 92.54l. • Experiments were used to modelling the solar hydrogen production system components. • The performance of PEM electrolyser powered by PV system was investigated. • A direct and indirect coupling PV/HG-60 water electrolyser system is investigated. • Hourly hydrogen production simulations for various panels slope levels are presented. • Panels slope angles effect on the PV/HG-60 performance is examined. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a semi-empirical mathematical model for predicting the electrochemical behavior of an alkaline water electrolysis system, based on the polarization curve and Faraday efficiency as a function of the current density under different operating conditions, such as, temperature and pressure. Also, the gas impurities of hydrogen in oxygen have been modeled for safety reasons due to its importance when the electrolyzer is dynamically operated using renewable energy sources. The different parameters defined in the model have been calculated by MATLAB, using a non-linear regression, on the basis of experimental data obtained in a 15 kW alkaline test bench. The simulated and measured values have been compared to ensure the accuracy and validity of the proposed model. In this sense, the error has been evaluated for the voltage with an average result of 5.67 mV per cell and for the Faraday efficiency and the gas impurities of hydrogen in oxygen with a value lower than 1%. These results show an excellent correlation between experimental and modeled data, so the model is a useful design and optimization tool for alkaline electrolyzers. Also, a sensitivity analysis has been used to determine the most influential operating variables in the performance of the electrolyzer. Highlights • A model for predicting the behavior of an alkaline electrolyzer has been developed. • The model is based on the polarization curve, Faraday efficiency and gas impurities. • Results of the model were validated using an alkaline water electrolysis test bench. • The model describes accurately a 15 kW alkaline water electrolyzer. • Sensitivity analysis was used to quantify the influence of the variables considered. [ABSTRACT FROM AUTHOR]

Hydrogen as an energy currency, carrier and storage medium can contribute to solve the problem of intermittent availability of renewable energies. In this paper, we study the production of hydrogen by a proton exchange membrane (PEM) electrolyzer (WE) using photovoltaic energy (PV) as source of electricity. Experiments were performed to model and optimize the direct-coupling system. Mathematical and empirical models based on experiments were used to simulate the system. RMSE was about 2% for the PV and WE. Optimization of direct connection was carried out to improve the system efficiency. However, it was noticed that the simulation of the system does not fit well the experimental results. Nevertheless, the RMSE is about 7%. In this study, we emphasize the need to develop appropriate models for hydrogen production system that operates in direct connection mode. [ABSTRACT FROM AUTHOR]

This paper presents a novel quasi -autothermal hydrogen production process. The proposed layout couples a Chemical Looping Combustion (CLC) section and a Steam Methane Reforming (SMR) one. In CLC section, four packed-beds are operated using Ni as oxygen carrier and CH 4 as fuel to continuously produce a hot gaseous mixture of H 2 O and CO 2 . In SMR section, two fixed-beds filled with Ni-based catalyst convert CH 4 and H 2 O into a H 2 -rich syngas. Four heat exchangers were employed to recover residual heat content of all the exhaust gas currents. By means of a previously developed 1D numerical model, a dynamic simulation study was carried out to evaluate feasibility of the proposed system in terms of methane conversion (100% circa), hydrogen yield (about 0.65 mol H2 /mol CH4 ) and selectivity (about 70%), and syngas ratio (about 2.3 mol H2 /mol CO ). Energetic and environmental analyses of the system performed with respect to conventional steam methane reforming, highlights an energy saving of about 98% and avoided CO 2 emission of about 99%. [ABSTRACT FROM AUTHOR]

This paper covers the research activities performed at the CNR Institute for Advanced Energy Technologies “Nicola Giordano”, aimed at developing and testing a biogas steam reforming reactor. A mathematical model has been developed in order to describe, the performance of the above-cited steam reforming reactor (packed bed). To study the effects on reaction performance, a parametric analysis was performed varying operating conditions such as inlet temperature and reagent molar ratio. The model was validated by comparing the calculated data with the experimental data obtained with a proprietary Ni/CeO 2 based catalyst in packet bed micro-scale reactor at different T = 700–900 °C, S/C = 1–5 and GHSV = 30,000 h −1 , showing a good agreement between the experimental and theoretical results. [ABSTRACT FROM AUTHOR]

This paper presents a dynamics mathematical model to simulate the steam reforming of methane that take place in conventional fixed bed reactor (FBR) as well in fixed bed membrane reactor (FBMR) with steam added both with co-current mode. The model covers all aspects of main chemical kinetics, heat and mass phenomena in the membrane reactor with hydrogen permeation in radial direction across a Pd-based membrane. Firstly, a dynamics study was made for describing that temperatures of gaseous and solid phases reach to steady-state as well as molar flow rates. The effect several parameters including the axial position (z) divided by the reactor length L z , reaction temperature and hydrogen partial pressure ( P H 2 = P pz ) in permeation side were investigated. The conversion of methane is significantly enhanced by the partial removal of hydrogen from the reaction zone as a result of diffusion through the Pd-based membrane. Simulation results showed that a conversion from 99.85% could be achieved in a FBMR at reaction temperature of 600 °C relative to a conversion from 88.87% to 950 °C in a FBR. Besides, results showed that the yield of H 2 reached to level from 1.548 (dynamics-state) and 1.626 (steady-state) in a FBMR at reaction temperature of 550 °C while the yield of H 2 achieved to level from 1.261 (dynamics-state) and 1.445 (steady-state) in a FBR at reaction temperature of 725 °C. [ABSTRACT FROM AUTHOR]

This paper investigates the feasibility of a microalgae derived hydrogen process at a pilot scale. For that, a general transient mathematical model for managing microalgae derived hydrogen production, with temperature dependence of the cultivation medium is developed. The tool allows for the determination of the resulting whole system temperature, and mass fractions distribution. The simplified physical model combines principles of classical thermodynamics, mass, species and heat transfer, resulting in a system of differential equations which are discretized in space using a three-dimensional cell-centered finite volume scheme, namely a volume element model (VEM). A Michaelis–Menten type expression is proposed for modeling the rate of H2 production with dependence on O2 inhibition. Tridimensional simulations are performed in order to determine the mass fractions distributions inside a compact photobioreactor (PBR), under different operating conditions. A relatively coarse mesh was used (6048 volume elements) to obtain converged results for a large compact PBR computational domain (2 m × 5 m × 8 m). The largest computational time required for obtaining results was 560 s, i.e., less than 10 min. The numerical results for microalgal growth are validated by direct comparison to experimental measurements. Hydrogen production simulations are conducted to demonstrate PBR intermittent operation (aerobic and anaerobic stages) feasibility and adequate species evolution trends in an indirect biophotolysis approach. Therefore, after experimental validation for a particular H2 production system, it is reasonable to state that the model could be used as an efficient tool for PBR systems thermal design, control and optimization for maximum H2 production. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this paper is to conduct a parametric study to show the best steam to carbon ratio that produces the maximum system performance of an integrated gasifier for hydrogen production. The study focuses on the energy and exergetic efficiency of the system and hydrogen production. The work is completed using computer simulation models in Engineering Equation Solver software package. This software is used for its extensive thermodynamic properties library. An equilibrium based model is used to determine the performance of the system. The data is presented in graphs which show the chemical composition in molar fractions of the syngas, the overall energy and exergy efficiency of the system, and the hydrogen production rates. A study of these parameters is conducted by varying the steam to carbon ratio entering the gasifier and the ambient temperature. It is observed that the higher the steam to carbon ratio that is achieved the more hydrogen and more power the plant is able to produce. Because of this, the exergy and energy efficiency of the system increases as the steam to carbon ratio increases as well. It is also observed that the system favors a lower ambient temperature for maximum exergy efficiency and hydrogen production. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to present and evaluate a proposal for designing an off-grid offshore electrolysis plant powered by wave energy. This plant includes PEM electrolyzers, a Reverse Osmosis system to produce water with adequate conductivity, a compression unit to store the hydrogen for transport, and batteries for temporary storage of electricity for short-time balances. First, the systems that compose the proposed plant are justified and described. Then a proposal for sizing these subsystems is given, based on using buoy-measured data at the expected location and simple mathematical models of the different sections of the plant. Finally the performance of the plant in a specific location is tested in detailed by using measured data, studying the influence of sizing on the expected performance. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental and modelling study of the kinetics of hydrogen production through the methanol steam reforming (MSR) process using a membrane reactor with a self-made, porous PTFE-nafion membrane. The operating conditions were optimised through an analysis of the hydrogen recovery and the CO selectivity. The membrane reactor was modelled based on the hydrogen production rate, which was linked (chain) to the changes in the reactant concentrations. The analysis of the reaction kinetics provided an overview of the reaction process and the factors that affect the process. Moreover, the size of the membrane was estimated using the distribution of hydrogen concentrations along the reactor. The separation factors, particularly the separation of hydrogen from CO and CO2, are some of the factors that can influence the performance of a membrane reactor. [Copyright &y& Elsevier]

Abstract: The kinetic modeling of homogeneous decomposition of hydrogen iodide (HI) and HI/H2O vapors with the addition of diatomic iodine (I2) using the mechanism proposed in the companion work (part I) in the sulfur–iodine cycle was investigated in this paper. Thermodynamic results calculated by FactSage and the kinetic experiment verified the applicability of the mechanism. The effect of temperature, residence time, pressure, HI/H2O/I2 molar ratio, HI/I2 molar ratio, and sensitivity analysis on the HI conversion was observed in the modeling process. The addition of small amount of diatomic iodine greatly decreases the HI conversion, and the overall pressure could promote the HI decomposition rate in the kinetic process. Sensitivity analysis shows that hydrogen yield was most sensitive to reactions (4) HI + H = H2 + I, (1) HI + HI = H2 + I2, (5) HI + I = I2 + H, and (8) HI + OH = H2O + I. The existence of diatomic iodine increases the reverse reaction of (1) and (5). [Copyright &y& Elsevier]

Abstract: In the last decade, production of biogas from biomass degradation has attracted the attention of several research groups. The interest on this hydrogen source is focused on the potential use of this gas as raw material to supply high temperature fuel cells (HTFC). This paper reports a wide research investigation carried out at CNR-ITAE on biogas reforming processes (steam reforming, autothermal reforming and partial oxidation). A mathematical model was developed, in Aspen Plus, and an experimental validation was made in order to confirm model results. Simulations were performed to determine the reformed gas composition and the system energy balance as a function of process temperature and pressure. The value of Gas Hour Space Velocity (GHSV) was selected for calculating compositions at full equilibrium, as it is expected in operative large scale plant. To obtain a realistic evaluation of the reforming processes efficiency, the energy balance for each examined process was developed as available energy of outlet syngas on inlet required energy ratio. The comparison between values of efficiency process gives useful indication about their reliability to be integrate with fuel cell systems. [Copyright &y& Elsevier]

Abstract: Due to the advantages of high hydrogen storage capacity, low cost of production, and long life cycle, activated carbon used for adsorption hydrogen storage has become a research focus in recent years. Cryo-adsorption of hydrogen on activated carbon is presently promising because it reaches a reasonable gravimetric density of more than 10 wt% and a volumetric density of 41 kg/m3. This paper builds a mathematical model based on a set of partial differential equations (PDEs) controlling the balances or conservations of mass, momentum and energy in the tank and simulates the charge–discharge cycle of cryo-adsorptive hydrogen storage on activated carbon, which is implemented in the finite element analysis software COMSOL Multiphysics™. A modified Dubinin-Astakhov (D-A) model is used to express the relationship between hydrogen adsorption density and the pressure and the temperature in the tank. The constant isosteric heat of adsorption is modified by variational isosteric heat of adsorption based on the Dubinin-Astakhov isotherm of adsorption. The stainless steel container packed with activated carbon is cooled by liquid nitrogen at 80 K. As the liquid nitrogen is evaporated slowly in the whole experiment process, in order to simulate the realistic environment we use a dynamic boundary condition in the model. To monitor the variations of pressure, temperature and adsorption density in the tank, we choose eight monitoring points along axial and radial directions of the hydrogen storage tank. The simulated results are compared with experimental data obtained at the Hydrogen Research Institute of the University of Quebec at Trois-Rivieres. A good agreement between the experiments and simulations has been achieved. [Copyright &y& Elsevier]

Abstract: The metal hydride heat transformer shows great potential for the low-grade heat recovery. However, the heat output is variable, and these dynamics have to be taken into account in realistic applications. In this paper, a new index called true temperature boost is introduced together with conventional indices to assess the performance of the heat transformer system. Detailed analyses are carried out using a mathematical model that authors developed, investigating effects of key design and operating factors on the system’s performance. The amount of hydrogen transferred between coupled reactors is found to be crucial to obtain a high coefficient of performance, while specific heating power is very sensitive to the cycle time. On the true temperature boost, the driving force of reaction and the heat transfer conditions exert the most significant effects. [Copyright &y& Elsevier]

Abstract: This paper describes the design, fabrication and performance evaluation of a high efficiency, compact heater that uses the catalytic oxidation of hydrogen to provide heat to a hydrogen storage system. The heater was designed to transfer up to 30kW of heat from the catalytic reaction to the hydrogen storage system via a recirculating heat transfer fluid. The catalytic heater consists of three main parts: 1) the reactor, 2) the gas heat recuperator, and 3) oil and gas flow distribution manifolds. The reactor and recuperator are integrated, compact, finned-plate heat exchangers to maximize heat transfer efficiency and minimize mass and volume. Detailed, three-dimensional, multi-physics computational models were used to design and optimize the system. At full power the heater was able to catalytically combust a 10% hydrogen/air mixture flowing at over 80 cubic feet per minute and transfer 30kW of heat to a 30 gallon per minute flow of oil over a temperature range from 100°C to 220°C. The total efficiency of the catalytic heater, defined as the heat transferred to the oil divided by the inlet hydrogen chemical energy, was determined to exceed the design goal of 80% for oil temperatures from 60°C to 165°C. [Copyright &y& Elsevier]

Abstract: Thermally coupling of endothermic and exothermic reactions in a membrane reactor improves thermal efficiency and production rate in the processes, reduces the size of reactors and decreases purification cost. This paper focuses on modeling and optimization of a thermally coupled dual-membrane reactor for simultaneous production of hydrogen, dimethyl ether (DME) and benzene. A steady state heterogeneous mathematical model is developed to predict the performance of this novel configuration. The catalytic methanol dehydration reaction takes place in the exothermic side that supplies the necessary heat for the catalytic dehydrogenation of cyclohexane to benzene in the endothermic side. Selective permeation of hydrogen and water vapor through the Pd/Ag and composite membranes are achieved by co-current flow of sweep gas through the membrane wall. The differential evolution method is applied to optimize the thermally coupled dual-membrane reactor considering the summation of DME and benzene mole fractions from reaction sides and hydrogen mole fraction in the permeation side as the main objectives. The optimization results are compared with corresponding predictions for an industrial methanol dehydration adiabatic reactor operated at the same feed conditions. Methanol conversion enhances about 5.5% in the optimized thermally coupled dual-membrane reactor relative to the conventional DME reactor. The results suggest that coupling of these reactions in the proposed configuration could be feasible and beneficial. [Copyright &y& Elsevier]

Abstract: In many visions and roadmaps, there is a broad agreement that fuel cells – both for stationary and mobile applications – are the key technology to allow the development of a hydrogen infrastructure. Furthermore, this development is generally thought to be based on a gradual, decentralised evolution. Nevertheless, in this paper it is argued that, taking into account the entire hydrogen chain (production, transport, storage, distribution and end-use), this decentralised fuel-cell based philosophy shows some serious flaws. Therefore, a new hydrogen-transition approach was pushed forward: mixing in of hydrogen into the natural-gas bulk. Using Flanders – the Northern part of Belgium – as a case study, the development of a transitory hydrogen infrastructure has been studied, taking into account the entire hydrogen chain and its dynamics, from production to end use. In a next step, this transition is being quantified. An optimisation model has been developed using Matlab and the commercial solvers GAMS and CPLEX. Following a mixed-integer linear-programming approach, this model is able to determine the economically optimal hydrogen-production mix and operational behaviour of each hydrogen-production plant separately. The model then allows gaining valuable insights in the importance of storage and the influence of fuel prices and carbon taxes with regard to the development of an early hydrogen economy. [Copyright &y& Elsevier]

Abstract: Development of the hydrogen economy will require a better understanding of the potential for fires and explosions associated with the unintended release of hydrogen within a structure. The ability to predict the mixing and dispersion behavior of hydrogen, when accidentally released in a partially confined space (e.g. hydrogen leak from automobiles parked in a residential garage) is critical to the safe use of hydrogen products. Hydrogen release and dispersion in a garage can be simulated using computational fluid dynamic (CFD) tools. However, CFD software needs to be validated with experimental data before it can be used reliably for development of codes and standards appropriate for hydrogen fire safety. This paper assesses the capability of a CFD software package to simulate a set of experiments on the mixing and dispersion behavior of highly buoyant gases in a partially confined geometry. Simulation results accurately captured the overall trend measured in experiments conducted in a reduced scale enclosure with idealized leaks. The difference between experimentally measured peak concentrations and numerical simulation results, averaged over various heights was 2.3%. Sensitivity of the computed results on various model parameters was determined. Results indicate that the size of the leak has a small effect on the predicted concentrations, but the location of the leaks in the garage has a very significant effect on the computed results. This result has important implications on future modeling efforts as well as codes and standards related to hydrogen fire safety. [Copyright &y& Elsevier]

Abstract: The paper presents key results of the studies on energy-technology installations for combined coal-based production of hydrogen and electricity. Special attention is paid to the development of systems, as part of ETI, for cryogenic removal of CO2 from combustion products. Consideration is given to the issues of mathematical modeling of the installations and nonlinear technical and economic optimization of their parameters with regard to investments in the CO2 removal systems. Comparison of efficiency of ETI for hydrogen production and ETI for production of alternative energy carriers (methanol and dimethyl ether) with CO2 removal is presented. [Copyright &y& Elsevier]

Abstract: This paper employs societal lifetime cost for evaluating hydrogen fuel cell vehicles (FCVs) from a societal welfare perspective as compared to conventional gasoline vehicles. We employ a learning-curve model for fuel-cell system cost estimates over time. The delivered hydrogen fuel cost is estimated using the UC Davis SSCHISM hydrogen supply pathway model, and most vehicle costs are estimated using the Advanced Vehicle Cost and Energy-Use Model (AVCEM). To estimate external costs, we use AVCEM and the Lifecycle Emissions Model (LEM). We examine hydrogen transition costs for a range of market penetration rates, externality evaluations, technology assumptions, and oil prices. Our results show that although the cost difference between FCVs and gasoline vehicles is initially very large, FCVs eventually become lifetime cost competitive with gasoline vehicles as their production volume increases, even without accounting for externalities. High valuation of externalities and high oil price could reduce the buy-down cost (the cumulative investment needed to bring hydrogen FCVs to lifetime cost parity with gasoline vehicles) by $10 billion relative to our reference case. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the results of a theoretical investigation whose aim was the development of a simulation tool for performance prediction of a steam reforming hydrogen production plant, and particularly of its overall energetic efficiency. A 1500 Nm3/h hydrogen production plant was simulated. Field data coming from an industrial plant were used for model validation in both design and off design operating conditions. To evaluate the plant performances in terms of energetic efficiency, a particular attention was paid to the simulation of all plant auxiliaries consumptions. Nevertheless the large uncertainty in most of the field data values, the model was able to capture all the relevant phenomena taking place in all the plant components, from reformer reactor up to CO2 sequestration unit, in the investigated plant capacity range (40–100%). [Copyright &y& Elsevier]

Abstract: In this paper hydrogen production via reforming of ethanol has been studied in a novel hybrid plant consisting in a ethanol reformer and a concentrating solar power (CSP) plant using molten salt as heat carrier fluid. The heat needed for the reforming of ethanol has been supplied to the system by molten salts heated up by solar energy. The molten salt stream temperature drop for supplying hydrogen production process heat duty is less than 20K, making the molten salt stream still suitable for steam and electricity production in a co-generative plant (clean hydrogen and electricity). A 2D mathematical model has been formulated, validated and used in order to simulate the performance of the reformer coupled with heat integration through molten salt. Firstly, the effect of operating conditions on the ethanol conversion and on hydrogen production have been evaluated in order to find the optimal conditions for the solar reforming reaction. Then, a reactor design is proposed in order to achieve almost complete ethanol conversion. The results show that the hybrid plant proposed here is a good solution for “green” hydrogen production. Almost complete conversion (99%) can be achieved with an optimized set of operating conditions (GHSV=1115h−1, Pressure=10 bar, steam-to-carbon ratio=5, inlet reactants temperature=823K) and with a 4.5m long reactor. [Copyright &y& Elsevier]

Abstract: A well-designed control system with a high utilization ratio of hydrogen and a fast filling speed are two critical objectives to ensure the reduction of cost and time required in the refueling process. In this paper, the popular three-stage refueling process is modeled with the aim to address both objectives. Using the real gas law of hydrogen, the utilization ratio of hydrogen filling is analyzed and the filling flowrate and time of each stage are evaluated. A multi-objective iterative optimization model is established and an optimization algorithm for the filling process is proposed to achieve both fast refueling and high utilization. Numerical results can be applied to the optimization of an actual hydrogen filling process. Besides, the tests show that an optimized control method can significantly improve the utilization ratio and allow refueling in a widely acceptable time. [Copyright &y& Elsevier]

Abstract: Partial oxidative gasification in supercritical water is a new technology for hydrogen production from biomass. Firstly in this paper, supercritical water partial oxidative gasification process was analyzed from the perspective of theory and chemical equilibrium gaseous product was calculated using the thermodynamic model. Secondly, the influence of oxidant equivalent ratio on partial oxidative gasification of model compounds (glucose, lignin) and real biomass (corn cob) in supercritical water was investigated in a fluidized bed system. Experimental results show that oxidant can improve the gasification efficiency, and an appropriate addition of oxidant can improve the yield of hydrogen in certain reaction condition. When ER equaled 0.4, the gasification efficiency of lignin was 3.1 times of that without oxidant. When ER equaled 0.1, the yield of hydrogen from lignin increased by 25.8% compared with that without oxidant. Thirdly, the effects of operation parameters including temperature, pressure, concentration, and flow rate of feedstock on the gasification were investigated. The optimal operation parameters for supercritical water partial oxidative gasification were obtained. [Copyright &y& Elsevier]

Abstract: A simple and fast empirical design model for a 5kW proton exchange membrane (PEM) stack is presented in this paper. The performance analysis of the PEM stack operating on a membrane humidifying method is made through a series of experiments, including current–voltage–power characteristics, uniformity of cell unit voltages, gas pressure impact and air flux impact. Based on the above analysis, an empirical predicted model for the PEM stack has been developed by the combination of mechanistic and empirical modeling approaches to characterize and predict the voltage–current characteristics without examining in depth all physical/chemical phenomena. The good agreement between the predicted and experimental results covering a range of optimal operating conditions shows that the proposed model provides an accurate representation of the behavior for the PEM stack. [Copyright &y& Elsevier]

Abstract: In this paper, specific cases for the interaction between the future electricity-generation mix and a newly-developing hydrogen-production infrastructure is modelled with the model E-simulate. Namely, flexible integrated-gasification combined-cycle units (IGCC) are capable of producing both electricity and hydrogen in different ratios. When these units are part of the electricity-generation mix and when they are not operating at full load, they could be used to produce a certain amount of hydrogen, avoiding the costly installation of new IGCC units for hydrogen production. The same goes for the massive introduction of renewable energies (especially wind), possibly generating excess electricity from time to time, which could then perhaps be used to produce hydrogen electrolytically. However, although contra-intuitive, the interaction between both ‘systems’ turns out to be almost negligible. Firstly, it is shown that it is more beneficial to use IGCC facilities to produce hydrogen with, rather than (excess) wind-generated electricity due to the necessary electrolyser investment costs. But even flexible IGCC facilities do not seem to contribute substantially to the early development of a hydrogen economy. Namely, in most scenarios – which are combinations of a wide range of fuel prices and carbon taxes – one primary-energy carrier (natural gas or coal) seems to be dominant, pushing the other, and the corresponding technologies such as reformers or IGCCs, out of the market. [Copyright &y& Elsevier]

Abstract: Understanding how a bioreactor functions is a necessary precursor for successful reactor design and operation. This paper describes a two-dimensional computational fluid dynamics simulation of three-phase gas–liquid–solid flow in an expanded granular sludge bed (EGSB) reactor used for biohydrogen production. An Eulerian–Eulerian model was formulated to simulate reaction zone hydrodynamics in an EGSB reactor with various hydraulic retention times (HRT). The three-phase system displayed a very heterogeneous flow pattern especially at long HRTs. The core-annulus structure developed may lead to back-mixing and internal circulation behavior, which in turn gives poor velocity distribution. The force balance between the solid and gas phases is a particular illustration of the importance of the interphase rules in determining the efficiency of biohydrogen production. The nature of gas bubble formation influences velocity distribution and hence sludge particle movement. The model demonstrates a qualitative relationship between hydrodynamics and biohydrogen production, implying that controlling hydraulic retention time is a critical factor in biohydrogen-production. [Copyright &y& Elsevier]

Abstract: The objective of this paper is to mathematically model a stand-alone renewable power system, referred to as “Photovoltaic–Fuel Cell (PVFC) hybrid system”, which maximizes the use of a renewable energy source. It comprises a photovoltaic generator (PV), a water electrolyzer, a hydrogen tank, and a proton exchange membrane (PEM) fuel cell generator. A multi-domain simulation platform Simplorer is employed to model the PVFC hybrid systems. Electrical power from the PV generator meets the user loads when there is sufficient solar radiation. The excess power from the PV generator is then used for water electrolysis to produce hydrogen. The fuel cell generator works as a backup generator to supplement the load demands when the PV energy is deficient during a period of low solar radiation, which keeps the system''s reliability at the same level as for the conventional system. Case studies using the present model have shown that the present hybrid system has successfully tracked the daily power consumption in a typical family. It also verifies the effectiveness of the proposed management approach for operation of a stand-alone hybrid system, which is essential for determining a control strategy to ensure efficient and reliable operation of each part of the hybrid system. The present model scheme can be helpful in the design and performance analysis of a complex hybrid-power system prior to practical realization. [Copyright &y& Elsevier]

Abstract: This paper presents a two-dimensional mathematical model to evaluate transient heat and mass transfer in a metal hydride tank (hereinafter MHT) with metal foam heat exchanger. The model is validated by comparison with experimental data. A good agreement is obtained. A study of the geometric and the operating parameters of the metal foam (such as base material, pore size and relative density) has been carried out to identify their influence on the performance of the charging process of the MHT. After that, the model was used as a tool to predict the performance of the hydrogen-charging process of two configurations of MHTs with metal foam heat exchangers. The obtained results showed that the reduction of the storage time due to the metal foam heat exchanger can be more important when the MHT is equipped with a concentric heat exchanger tube. [Copyright &y& Elsevier]

Abstract: In this paper, the similarity form is developed of a hydrogen–air buoyant jet ‘forced plume’ resulting from an unignited small-scale hydrogen leakage in the air. As the domain temperature is assumed to be constant and therefore the density of the mixture is a function of the concentration only, the hydrogen–air mixture is assumed to be of a linear mixing type. The rate of entrainment is assumed to be a function of the plume centerline velocity and the ratio of the mean plume and ambient densities. The local rate of entrainment may be considered to be consisted of two components; one is the component of entrainment due to jet momentum while the other is the component of entrainment due to buoyancy. Consequently, the entrainment coefficient is taken as a variable not constant. The density will be considered as a variable in all terms of the equations of continuity, momentum and concentration. The integral models of the mass, momentum and concentration fluxes are obtained. Non-dimensional transformations known as similarity transformations and used to transform the integral model to a set of ordinary differential equations called similarity equations which are solved numerically. Under various values of Froude number, the hydrogen centerline mass fraction, jet width and centerline velocity are introduced and compared with published experimental results for a slow leak vertical hydrogen jet. [Copyright &y& Elsevier]

Abstract: The main focus of this paper is the modeling, simulation, and analysis of the calcium bromide hydrolysis reactor stage in the calcium–bromine thermochemical water-splitting cycle for nuclear hydrogen production. One reactor concept is to use a spray of calcium bromide into steam, in which the heat of fusion supplies the heat of reaction. Droplet models were built up in a series of steps incorporating various physical phenomena, including droplet flow, heat transfer, phase change, and reaction, separately. Given the large heat reservoir contained in a pool of molten calcium bromide that allows bubbles to rise easily, using a bubble column reactor for the hydrolysis appears to be a feasible and promising alternative to the spray reactor concept. The two limiting cases of bubble geometry, spherical and spherical-cap, are considered in the modeling. Results for both droplet and bubble modeling with COMSOL MULTIPHYSICS™ are presented, with recommendations for the path forward. [Copyright &y& Elsevier]

Abstract: In this paper, typical and Pd–Ag membrane methanol loop reactors have been analyzed. In the proposed models all basic equipments in the methanol loop were included. Detailed dynamic models described by set of ordinary differential and algebraic equations were developed to predict the behavior of the overall processes. The conventional model was validated against plant data, and then the results of the hydrogen-permselective membrane loop are compared with the conventional model. Using this novel model, diffusion by membrane tubes compensates reduction of production rate due to catalyst deactivation. By use of the membrane model, dynamic optimization of temperatures was performed for improving overall methanol production. Here, differential evolution (DE) method was applied as powerful method for optimization of procedure. Optimal inlet temperatures of membrane tube, steam drum and both of them were determined. The optimization approaches enhanced additional yield throughout 4 years of operation as catalyst lifetime. Therefore, the methanol synthesis loop can be deduced to redesign based on this study. [Copyright &y& Elsevier]

Abstract: In this paper, the homogeneous decomposition of methane and ethane is modeled in a well stirred flow reactor. The kinetics of this process is represented by a reaction mechanism of 242 reactions and 75 species, based on a mechanism developed for hydrocarbon combustion and soot formation. It is shown that this model correctly predicts the hydrogen yield from pyrolysis in a temperature range of 600–1600°C, and pressure range of 0.1–10atm. Furthermore, the effect of temperature, pressure and residence time on the amount of hydrogen produced from the decomposition of methane, ethane, natural gas, and a mixture of methane and argon is studied. The model predicts that the use of ethane or its addition to methane increases the speed of hydrogen production at low temperatures and pressures. The addition of a noble gas like argon also increases the yield of hydrogen at high pressures. [Copyright &y& Elsevier]

Abstract: In this paper, energy and exergy analyses for a 1.2kWp Nexa PEM fuel cell unit in a solar-based hydrogen production system is undertaken to investigate the performance of the system for different operating conditions using experimental setup and thermodynamic model. From the model results, it is found that there are reductions in energy and exergy efficiencies (about 14%) with increase in current density. These are consistent with the experimental data for the same operating conditions. A parametric study on the system and its parameters is undertaken to investigate the changes in the efficiencies for variations in temperature, pressure and anode stoichiometry. The energy and exergy efficiencies increase with pressure by 23% and 15%, respectively. No noticeable changes are observed in energy and exergy efficiencies with increase in temperature. The energy and exergy efficiencies decrease with increase in anode stoichiometry by 17% and 14%, respectively. These observations are reported for the given range of current density as 0.047–0.4A/cm2. The results and analyses show that the PEM fuel-cell system has lower exergy efficiencies than the corresponding energy efficiencies due to the irreversibilities that are not considered by energy analysis. In comparison with experimental data, the model is accurate in predicting the performance of the proposed fuel-cell system. The parametric and multivariable analyses show that the option of selecting appropriate set of conditions plays a significant role in improving performance of existing fuel-cell systems. [Copyright &y& Elsevier]

Abstract: In this paper a study of the thermal performance of a commercial alkaline water electrolyzer (HySTAT from Hydrogenics) designed for a rated hydrogen production of 1Nm3 H2/h at an overall power consumption of 4.90kWh/Nm3 H2 is presented. The thermal behaviour of the electrolyzer has been analyzed under different operating conditions with an IR camera and several thermocouples placed on the external surface of the main electrolyzer components. It has been found that the power dissipated as heat can be reduced by 50–67% replacing the commercial electric power supply unit provided together with the electrolyzer by an electronic converter capable of supplying the electrolyzer with a truly constant DC current. A lumped capacitance method has been adopted to mathematically describe the thermal performance of the electrolyzer, resulting in a thermal capacitance of 174kJ°C−1. The effect of the AC/DC converter characteristics on the power dissipated as heat has been considered. Heat losses to the ambient were governed by natural convection and have been modeled through an overall heat transfer coefficient that has been found to be 4.3Wm−2 °C−1. The model has been implemented using ANSYS® V10.0 software code, reasonably describing the performance of the electrolyzer. A significant portion of the energy dissipated as heat allows the electrolyzer operating at temperatures suitable to reduce the cell overvoltages. [Copyright &y& Elsevier]