Effects of water induced pore blockage and mitigation strategies in low temperature PEM fuel cells – A simulation study

Ineffective water management in proton exchange membrane fuel cells (PEMFCs) can cause performance degradation. A simple mathematical model capturing the effect of water on the overall performance of the fuel cell system is of immense use in developing tools for water management. In this work, a computationally efficient first principles dynamic model for PEMFC system simulations and concomitant water management studies are developed. The steady-state version of this model is validated with experimental data. The effect of various operating conditions and design parameters on the performance of the fuel cell is studied using this model. Various control strategies for improving fuel cell performance in the presence of flooding are evaluated using the model. The simplicity and adequate predictive capability of the model make it amenable to be used in a model-based feedback control framework for online water management.     

Modeling transport of membrane water in PEMFC and energy loss analysis at −20 °C based on mathematical boundedness

This work analyzes why the classic cold start-up model cannot match experimental data well in high initial membrane water content due to the unboundedness of membrane water content conservation equation, and a mathematical framework to ensure the boundedness of membrane water content is developed to solve these issues. Based on vehicle mechanism and Grotthuss mechanism, a “traffic jam” model is applied to describe the transport of membrane water reaching its maximum content. To keep the membrane water content within the upper bound, a new governing equation is proposed and the corresponding boundary conditions and discretization schemes are set. This model has been proved in eleven cases by comparing with experimental data including different initial membrane water content and different current density. Some of these cases are considered impossible to exist in classical cold start-up theory. The results illustrate that the boundedness model can show a clearer physical process and obtain more accurate and reasonable calculation results.     

Coupled continuum and condensation–evaporation pore network model of the cathode in polymer-electrolyte fuel cell

A model of the cathode side of a Proton Exchange Membrane Fuel Cell coupling the transfers in the GDL with the phenomena taking place in the cathode catalyst layer and the protonic transport in the membrane is presented. This model combines the efficiency of pore network models to simulate the liquid water formation in the fibrous substrate of the gas diffusion layer (GDL) and the simplicity of a continuum approach in the micro-porous layer (MPL). The model allows simulating the liquid pattern inside the cathode GDL taking into account condensation and evaporation phenomena under the assumption that the water produced by the electro-chemical reactions enters the MPL in vapor form from the catalyst layer. Results show the importance of the coupling between the transfers within the various layers, especially when liquid water forms as the result of condensation in the region of the GDL fibrous substrate located below the rib.     

Magnetic resonance imaging investigation of water accumulation and transport in graphite flow fields in a polymer electrolyte membrane fuel cell: Do defects control transport?

Water management remains a leading challenge in the implementation of polymer electrolyte membrane (PEM) fuel cells. At present there are many excellent models for the distribution of water within PEM fuel cells, but little quantitative data on the water distribution that can be compared to models. In this paper magnetic resonance imaging (MRI) is used to examine and quantify the flow of water in graphite coated flow fields in a miniature PEM (hydrogen) fuel cell. It was found as with Teflon^® flow fields, that the water accumulated in waves along the bottom of the flow field. The water waves moved very slowly through the flow channels and seem to get stuck on tiny defects in the flow field. The water accumulated at the defect, until the wave nearly bridged the gap between the cathode and the bottom of the flow field. Then the water wave was pushed along to the next defect. Surprisingly, the current out of the cell was nearly constant as waves accumulated and were swept away, even though the flow was clearly not at steady state. These results show that small defects in the wall of the flow field play a critical role in water transport in the flow fields.     

Influence of pyrolysis conditions on nitrogen speciation in a biochar ‘preparation-application’ process

To investigate biochar nitrogen conversion in a ‘preparation-application’ system and the response of its transportation in plants, biochar samples were produced from rice straw at different pyrolysis temperatures (400 °C and 800 °C) and atmospheres (N₂ and CO₂). Subsequently, biochar was synthesized under CO₂ atmosphere to explore its nitrogen nutrient characteristics and further improve the chemical and physical properties of soil. Nitrogen speciation of the biochar and plant root samples were evaluated by X-ray photoelectron spectroscopy. Research has shown that organic nitrogen such as protein-N, free amino acid-N, and alkaloid-N in rice straw is converted into organic (nitrile-N, pyridine-N, amino-N, and pyrrole-N) and inorganic (NH₄⁺-N, NO₂^?-N, and NO₃^?-N) species in biochar during the biomass pyrolysis process. In turn, biochar nitrogen is transported to plants as protein-N, free amino acid-N, alkaloid-N, NH₄⁺-N, NO₂^?-N, and NO₃^?-N. Comprehensive consideration of the biochar quality and preparation cost indicated the lower pyrolysis temperature (400 °C) under CO₂ atmosphere as the best conditions for biochar preparation.     

Effects of economies of scale and experience on the costs of energy-efficient technologies – case study of electric motors in Germany

Increasing energy efficiency is discussed as an effective way to protect the climate, even though this is frequently associated with additional (investment) costs when compared to standard technologies. However, the investment costs of emerging energy-efficient technologies can be reduced by economies of scale and experience curve effects. This also brings about higher market penetration by lowering market barriers. Experience curves have already been analyzed in detail for renewable energy technologies, but are not as well documented for energy-efficient technologies despite their significance for energy and climate policy decisions. This work provides empirical evidence for effects of economies of scale and experience on the costs of energy-efficient electric motors. We apply a new methodology to the estimation of learning effects that is particularly promising for energy-efficient technologies where the very low data availability did not allow calculations of learning rates so far. Energy-efficient electric motors are a highly relevant energy technology that is responsible for about 55% of German electricity consumption. The analysis consists of three main steps. First, the calculation of composite price indices based on gross value added statistics for Germany which show the changes in cost components of electric motors over the period 1995 to 2006; second, an estimation of the corresponding learning rate which is, in a third step, compared with learning rates observed for other energy-efficient technologies in a literature review. Due to restrictions of data availability, it was not possible to calculate a learning rate for the differential costs of energy-efficient motors compared to standard motors. Still, we estimated a learning rate of 9% for “Eff2” motors in a period when they penetrated the market and replaced the less efficient “Eff3” motors. Furthermore, we showed the contribution of different effects to these cost reductions, like a reduction of material use per motor produced by 15% and an improvement of labor productivity of 43%.     

Thermal conductivity variation on natural convection flow of water–alumina nanofluid in an annulus

This work is focused on the numerical modeling of steady laminar natural convection flow in an annulus filled with water–alumina nanofluid. The inner surface of the annulus is heated uniformly by a uniform heat flux q and the outer boundary is kept at a constant temperature T_c. Two thermal conductivity models namely, the Chon et al. model and the Maxwell Garnett model, are used to evaluate the heat transfer enhancement in the annulus. The governing equations are solved numerically subject to appropriate boundary conditions by a penalty finite-element method. A parametric study is conducted and a selective set of graphical results is presented and discussed to illustrate the effects of the presence of nanoparticles, the Prandtl number and the Grashof number on the flow and heat transfer characteristics for both nanofluid models. It is found that significant heat transfer enhancement can be obtained due to the presence of nanoparticles and that this is accentuated by increasing the nanoparticles volume fraction and Prandtl number at moderate and large Grashof number using both models. However, for the Chon et al. model the greatest heat transfer rate is obtained.     

Stochastic modeling of polymer electrolyte membrane fuel cell gas diffusion layers – Part 2: A comprehensive substrate model with pore size distribution and heterogeneity effects

A stochastic modeling algorithm was developed that accounts for porosity distribution, fiber diameter, fiber co-alignment, fiber pitch, and binder and/or polytetrafluorethylene fractions. Materials representative of a commercially available GDL were digitally generated based on empirical measurements of these various properties. Materials made with varying fiber diameters and binder/fiber volume ratios were compared with a generated reference material through porosity heterogeneity calculations and mercury intrusion porosimetry simulations. Fiber diameters and binder/fiber ratios were found to be key modeling parameters that exhibited non-negligible impacts on the pore space. These key parameters were found to positively correlate with heterogeneity and mean pore diameter and exhibit a complementary relationship in their impact on the pore space. Because both parameters directly impacted the number of fibers added to the domain, modeling techniques and parameters pertaining to fiber count must be considered carefully.     

Investigations on the influence of ethanol and water injection techniques on engine's behavior of a hydrogen - biofuel based dual fuel engine

This work explores the influence of hydrogen and ethanol on improving engine's behavior of Maduca longifolia oil (MO) based dual fuel diesel engine. A mono cylinder diesel engine was tested in dual fuel mode of operation at the rated power output of 3.7 kW under variable hydrogen energy shares from 0 to the maximum allowable limit (until severe knocking i.e. upto 20%). The knock limit was further extended by injecting water and ethanol at the intake manifold and the engine's performance, emission and combustion characteristics were analyzed. In addition ethanol was also injected and introduced along with the intake air for comparison with hydrogen dual fuel mode. Dual fuel operation increased the BTE from 25.2% with neat MO to a maximum of 28.5% and 30% respectively with hydrogen and ethanol for the energy share of 15% and 38% where as the BTE was 30.8% with ND. The smoke opacity was reduced from 78% with neat MO to 58% for the hydrogen energy share of 15% which is the MEP (maximum efficiency point) whereas the smoke emission was noted as 51% with ND operation. However, hydrogen induction increased the NO (nitric oxide) emission. Injection of water and ethanol at the inlet was observed to extend the knocking limit with improved BTE. The BTE reached a maximum of 30.1% with 5% water and 30.8% with 10% ethanol injection. The MEPs were arrived as 31% and 30% hydrogen energy shares respectively with 5% water and 10% ethanol injection. It was concluded that hydrogen induction can be very effective in improving the diesel engine's performance when using MO as base fuel when operating on dual fuel mode. The performance could be improved by extending the knock limit by injecting ethanol and water along with hydrogen.     

Enhancement of heat transport in thermosyphon air preheater at high temperature with binary working fluid: A case study of TEG–water

In this research, the critical heat flux (CHF) due to flooding limit of thermosyphon heat pipe using triethylene glycol (TEG)–water mixture has been investigated. From the experiment it is found that, use of TEG–water mixture can extend the heat transport limitation compared with pure water and higher heat transfer is obtained compared with pure TEG at high temperature applications. Moreover it is found that ESDU equation is appropriate to predict the CHF of the thermosyphon in case of TEG–water mixture.For thermosyphon air preheater at high temperature applications, it is found that with selected mixture content of TEG–water in each row of the thermosyphon the performance of the system could be increased approximately 30–80% compared with pure TEG for parallel flow and 60–115% for counter flow configurations. The performances also increase approximately 80–160% for parallel flow and 140–220% for counter flow compared with those of pure dowtherm A which is the common working fluid at high temperature applications.     

Effect of structural evolution of aluminum powder during ball milling on hydrogen generation in aluminum–water reaction

Effect of milling time on structure of pure Al powder and consequently on efficiency of Al-water reaction and hydrogen production is studied. Progress in milling process is studied considering changes in morphology, size, lattice imperfections and grains orientation of Al particles. Morphology of particles before and after the reaction is studied to establish a role of size and shape of particles on kinetics of the reaction. It is demonstrated that the formation of interlayer spaces within the investigated Al particles effectively increases hydrogen yield. Moreover, prolonged ball milling eliminates the interlayer spaces and decreases the rate of reaction of water with the powders.     

Hydrogen quality for fuel cell vehicles – A modeling study of the sensitivity of impurity content in hydrogen to the process variables in the SMR–PSA pathway

As fuel cell vehicles approach wide-scale deployment, the issue of the quality of hydrogen dispensed to the vehicles has become increasingly important. The various factors that must be considered include the effects of different contaminants on fuel cell performance and durability, the production and purification of hydrogen to meet fuel quality guidelines, and the associated costs of providing hydrogen of that quality to the fuel cell vehicles. In this paper, we describe the development of a model to track the formation and removal of several contaminants over the various steps of hydrogen production by steam-methane reforming (SMR) of natural gas, followed by purification by pressure-swing adsorption (PSA). We have used the model to evaluate the effects of setting varying levels of these contaminants in the product hydrogen on the production/purification efficiency, hydrogen recovery, and the cost of the hydrogen. The model can be used to track contaminants such as CO₂, CO, N₂, CH₄, and H₂S in the process. The results indicate that a suggested specification of 0.2 ppm CO would limit the maximum hydrogen recovery from the PSA under typical design and operating conditions. The steam-to-carbon ratio and the process pressure are found to have a significant impact on the process efficiency. Varying the CO specification from 0.1 to 1 ppm is not expected to affect the cost of hydrogen significantly, although the cost of gas analysis to comply with such stringent requirements may add 2–10 cents/kg to the cost of hydrogen.     

Disclosure of the internal transport phenomena in an air-cooled proton exchange membrane fuel cell—part III: Performance research based on qualitative comparisons between modeling and experimental results

The validation of a computational fluid dynamics model for a proton exchange membrane fuel cell (PEMFC) is normally conducted by the experimental I–V performance curve. However, it seems this method is not solid enough. In the meantime, it's difficult to conduct the item-to-item quantitative comparisons between the internal distributions acquired from numerical and testing results. Therefore, in this paper, as the first attempt, qualitative comparisons between the modeling and experimental results are conducted based on the three important parameters of an air-cooled PEMFC (air stoichiometric ratio, air relative humidity, cathode flow field design) to explore the trends of the related fuel cell I–V performance curves and internal resistances. The internal resistances are tested using the EIS technique and differentiated by an equivalent circuit model. Conclusions show that the qualitative comparisons between the numerical and testing results support each other well and new results are found based on the comparisons. Finally, discussions on the sensitivity based on the experimental EIS results are conducted to explore the response degree of the total resistance to the air stoichiometric ratio, the air relative humidity and the cathode flow field design.     

Molecular dynamics simulation on the effect of water uptake on hydrogen bond network for OH− conduction in imidazolium-g-PPO membrane

OH⁻ conduction involved in the hydrophilic channel of anion exchange membrane strongly depends on the water uptake. To investigate the effect of water uptake on the hydrogen bond network for OH⁻ conduction, a series of molecular dynamics simulations based on all-atom force field were performed on the hydrated imidazolium-g-PPO membranes with different water uptakes. The systems were well verified by comparing the membrane density and OH⁻ conductivity with previous experiments. By means of local structural properties and pair-potential energy, reasonable hydrogen bond criteria were determined to describe the hydrogen bond network confined in the membrane. Increasing water uptake enhances the hydration structures of water and OH⁻, and facilitates the reorganization of the hydrogen bond network. Water and OH⁻ are nearly saturated with water when the water uptake reaches λ = 10, where well-connected hydrogen bond network is produced. Further increasing water uptake has much less contribution to improving the hydrogen bond network, but inevitably swells the membrane channel. This work provides a molecular-level insight into the effect of water uptake on the hydrogen bonding structures and dynamics of OH⁻ and water confined in the imidazolium-g-PPO membrane.     

Experimental study on the purification of HIx phase in the iodine–sulfur thermochemical hydrogen production process

Purification of hydriodic phase (HIx) plays an important role in avoiding the undesirable side reactions between HI and H₂SO₄ in the sulfur–iodine thermochemical cycle. In this paper, a series of experiments on HIx phase purification were conducted by means of a stirred reactor using N₂ as stripping gas. The effects of the iodine concentration, reaction temperature, and the striping gas flow rate on HIx phase purification were investigated systematically in terms of the conversion of H₂SO₄ and the reaction types during purification. It was observed that the iodine concentration played a significant role in dictating the reactions during purification. The quantitative analysis of the compositions of the initial and purified HIx phases showed that not only the conversion of H₂SO₄ was enhanced but also the side reactions were effectively impeded by increasing the iodine concentration, temperature and the stripping gas flow rate. Based on the experimental data, the suitable operating conditions for HIx phase purification were proposed.     

Research into the formation process of hydrogen–air mixture in hydrogen fueled engines based on CFD

The density of hydrogen is much smaller than that of air, so it is hard for hydrogen and air to form high grade mixture. Furthermore, the diffusing speed of hydrogen is so high that the formation state of mixture changes rapidly. Therefore it will become more difficult to carry through the further research of mixture space–time distributing rule. In order to investigate the formation rule of hydrogen–air mixture and improve the mixture quality, in this paper, computation fluid dynamics (CFD) mode is adopted to carry through three-dimensional numerical simulation research of flow field in hydrogen fueled engine cylinder. The numerical simulation is done in a two-stroke hydrogen fueled engine, and the mixture forming state at different hydrogen-injecting time is contrasted. The evolvement rule of flow field in cylinder and mixture forming state is shown in the result. The simulation results show that, when hydrogen-injecting begins at 260 °CA, the forming quality of the mixture is better than other two states, this is the same as the experimental results. It indicates that CFD is one of the effective methods to analyze the formation of mixture in hydrogen fueled engine.     

Effect of sludge recirculation on characteristics of hydrogen production in a two-stage hydrogen–methane fermentation process treating food wastes

The two-stage hydrogen–methane fermentation process with different patterns of recirculation was investigated. Operations with the circulation of heat-treated sludge performed considerably better than those with the recirculation of raw sludge with respect to both the hydrogen production rate and yield. In addition, the results of the batch tests demonstrated that circulated sludge was capable of consuming hydrogen under acidogenic pH while the heat-treated sludge was not. These results suggest that the recirculation of active methanogenic sludge had an inhibitive effect on the hydrogen production, which can likely be attributed to the high hydrogen-consuming activity of microorganisms present in the circulated sludge. On the other hand, operations without any sludge recirculation did not perform well in terms of hydrogen production or carbohydrates degradation compared to those with recirculation, perhaps due to a shortage of available nitrogen. This suggests that sludge recirculation in effect supplemented the NH₄⁺ in the hydrogen reactor.     

Pressure loss and compensation in the combustion process of Al–CuO nanoenergetics on a microheater chip

This study investigates pressure loss and compensation in the combustion process of Al–CuO metastable intermolecular composite (MIC) on a microheater chip. A ball cell model of pressure change in the combustion process is proposed to show the effects of pressure loss on the reaction rate and efficiency of energy output at microscale. An effective compensation method for pressure loss is then developed by integrating Al–CuO MIC with CL-20 (2,4,6,8,10,12-hexanitrohexaazaisowurtzitane) onto a SiO₂/Cr/Pt/Au microheater chip. The combustion processes of Al–CuO MIC with different weight percentages of fine CL-20 particles on the microheater chips are observed by high-speed video recording. Results indicate that the reaction of Al–CuO MIC is a slow combustion process that turns into intense deflagration after adding fine CL-20 particles to Al–CuO MIC. The pressure–time characteristics indicate higher maximum pressure and pressurization rate for Al–CuO/CL-20 because the pressure loss at microscale is well compensated by the addition of fine CL-20. This study proves the importance of pressure loss in the combustion process of MIC at microscale and provides an efficient compensation strategy for pressure loss to improve the reaction rate and efficiency of energy output at microscale environment.     

Development of process for aluminising nickel alloy 718 strips for tube bundle support structures in liquid sodium–water steam generators

Abstract

The tubes in liquid sodium–water steam generators of the Indian prototype fast breeder reactor (PFBR) will be supported by corrugated nickel alloy 718 strips. Aluminisation of nickel alloy 718 strips has been chosen for this application because of the excellent performance of aluminide coatings in reducing impact fretting wear of the tubes due to flow induced vibrations and compatibility of the coating with liquid sodium at the operating temperature of the steam generators. Aluminisation of nickel alloy 718 strips for steam generator tube bundle support structures has been developed using a procedure involving thermal spraying of aluminium followed by diffusion heat treatment in vacuum atmosphere. One of the advantages of the technique is that it will coat only the desired surfaces of the strips, whereas in conventional pack cementation process, significant precautions have to be ensured. Furthermore, this process has enabled aluminisation to be carried out at a much lower cost than the conventional process of pack aluminising. The problems encountered during the initial trials and technology development, such as coating thickness and distortion, are discussed. A process flow chart for this procedure to take the job on an industrial scale is also reported. This process (under patenting) has been adopted for the aluminisation of corrugated strips for the support structures of one steam generator module and the steam generator for a test facility during the technology development phase, as also of all the steam generators being fabricated for the PFBR.     

Production of ultrapure hydrogen in a Pd–Ag membrane reactor using noble metals supported on La–Si oxides. Heterogeneous modeling for the water gas shift reaction

Two different catalysts, Rh(0.6% wt/wt)/La₂O₃(27% wt/wt)·SiO₂ and Pt(0.6% wt/wt)/La₂O₃(27%)·SiO₂, were tested in the WGS reaction. Their performances were first studied in a conventional fixed-bed reactor. Their activities were similar and they were both very stable. However, as Pt(0.6)/La₂O₃(27)·SiO₂ showed a much higher selectivity to the desired reaction, the performance of a membrane reactor employing this catalyst was studied. The effects of the H₂O/CO ratio, space velocity, sweep gas flow rate and size of the catalyst particle on CO conversion and H₂ recovery were studied at laboratory scale under isothermal conditions. A 1-D heterogeneous model was developed in order to properly reproduce the experimental results obtaining good agreement between the simulation results and laboratory data. The experimental and theoretical results confirm the existence of significant external mass-transfer limitations in the fluid-particle interface for these very active formulations.