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.     

Moisture transport in silica gel packed beds—II. Experimental study

Experiments have been performed to obtain the transient response of a thin adiabatic packed bed of silica gel after a step change in inlet air conditions. Comparisons are made with predictions using a solid-side resistance model and a pseudo-gas-side controlled model and better agreement obtained with the former model. An apparent dynamic hysteresis for adsorption/desorption with microporous silica gel is clearly in evidence, which could be due to a solid-side effective diffusion coefficient which decreases with increasing moisture content, or to a lesser extent to a hysteresis in the adsorption isotherm itself.     

Moisture transport in silica gel packed beds—I.Theoretical study

Diffusion mechanisms of moisture within silica gel particles are investigated. It is found that for microporous silica gel surface diffusion is the dominant mechanism of moisture transport, while for macroporous silica gel both Knudsen and surface diffusions are important. A model is proposed for simultaneous heat and mass transfer in a thin packed bed of desiccant particles, which accounts for diffusion of moisture into the particles by both Knudsen and surface diffusions. Using finite difference methods to solve the resulting partial differential equations, predictions are made for the response of thin beds of silica gel particles to a step change in air inlet conditions, and compared to a pseudo-gas-side controlled model commonly used for the design of desiccant dehumidifiers for solar desiccant cooling applications.     

Multi-objective dynamic optimization model for China’s road transport energy technology switching

Deducting the future switching of the road transport energy technology is one of the key preconditions for relative technology development planning. However, one of the difficulties is to address the issue of multi-objective and conflicting constrains, e.g., minimizing the climate mitigation or minimizing economic cost. In this paper, a dynamic optimization model was established, which can be used to analyze the road transport energy technology switching under multi-objective constrains. Through one case study, a series of solutions could be derived to provide decision-makers with the flexibility to choose the appropriate solution with respect to the given situation.     

Multiphysics modeling of water transport in high-intensity lignite drying process on pore scale

The understanding of the moisture transfer process in the pore network is quite important to improve the lignite drying efficiency. Scanning electronic microscopy image was used for construction of pore topology closely approximating the true topology of the real lignite for the heat and mass transfer processes on pore scale by COMSOL simulation. Considering the gas?liquid phase coexistence of water, “Laminar Two-Phase Flow, Phase Field” module and “Liquid Heat Transfer” module were used. The pore size had significant effects on the flow velocity and the larger pores acted as the main pathway for the moisture transport, therefore affected the maximum drying rate. On the other hand, the connection of pores and the throats distribution in the pathway also had a significant effect on the flow velocity, and the moisture between the throats was hard to transfer as a flow, maybe by vapor diffusion. In high-intensity lignite drying process, the moisture vaporization quickly when heated up and vapor pressure was beneficial to keep the pore size and ensure the smooth of moisture flow pathway, thus improving the efficiency of the drying process.     

Extension of the a-Si:H electronic transport model to μc-Si:H: use of the μ0τ0 product to correlate electronic transport properties and solar cell performances

The aim of this communication is to show that it is possible to extend the model of the electronic transport developed for amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H). By describing the electronic transport with the μ⁰τ^R products (mobility×recombination time) as a function of the Fermi level, we observed the same behaviour for both materials, indicating a similar type of recombination. Moreover, applying the normalised μ⁰τ⁰ product (mobility×life-time) obtained by combining the photoconductivity (σ_photo) and the ambipolar diffusion length (L_amb) measured in individual layers, we are able, as in the case of a-Si:H, to predict the quality of the solar cells incorporating these layers as the active 〈i〉 layer.     

Non-equilibrium molecular dynamics study of thermal energy transport in Au–SAM–Au junctions

Non-equilibrium molecular dynamics (NEMD) simulations were performed on Au–SAM (self-assembly monolayer)–Au junctions to study the thermal energy transport across the junctions. Thermal conductance of the Au–SAM interfaces was calculated. Temperature effects, simulated external pressure effects, SAM molecule coverage effects and Au–SAM bond strength effects on the interfacial thermal conductance were studied. It was found that the interfacial thermal conductance increased with temperature increase at temperatures lower than 250 K, but it did not have large changes at temperatures from 250 to 400 K. Such a trend was found to be similar to experimental observations on similar junctions. The simulated external pressure did not affect the interfacial thermal conductance. SAM molecule coverage and Au–SAM bond strength were found to significantly affect on the thermal conductance. The vibration densities of state (VDOS) were calculated to explore the mechanism of thermal energy transport. Interfacial thermal resistance was found mainly due to the limited population of low-frequency vibration modes of the SAM molecule. Ballistic energy transport inside the SAM molecules was confirmed, and the anharmonicity played an important role in energy transport across the junctions. A heat pulse was imposed on the junction substrate, and heat dissipation inside the junction was studied. Analysis of the junction response to the heat pulse showed that the Au–SAM interfacial thermal resistance was much larger than the Au substrate and SAM resistances separately. This work showed that both the Au substrate and SAM molecules transported thermal energy efficiently, and it was the Au–SAM interfaces that dominated the thermal energy transport across the Au–SAM–Au junctions.     

Application of an entry–exit tariff model to the gas transport system in Spain

Under an entry–exit gas tariff system, reservation of capacity is split into entry capacity, to transport gas from the injection points to a virtual balancing point, and exit capacity, to transport gas from the balancing point to the exit points in the system.     

Experimentally measured thermal transport properties of aluminum–polytetrafluoroethylene nanocomposites with graphene and carbon nanotube additives

Reactive materials such as aluminum (Al) and polytetrafluoroethylene (Teflon) are used for energy generation applications and specifically in ordnance technologies. With the advent of nanotechnology various nano-scale additives have become incorporated into reactive material formulations with the hope of enhanced performance. An important component to the study of energy generation is an examination of energy transport through a reactant matrix. This study examines an experimental approach to quantifying thermal properties of an Al/Teflon nanocomposite reactant matrix that has been impregnated with carbon additives. Various structures of carbon are investigated and include amorphous nanoscale carbon spheres (nano C), graphene flakes and unaligned multiwalled carbon nanotubes (CNTs). The additives were selected based on their completely different structures with the hypothesis that the structure of the additive will influence the thermal transport properties of the matrix. Results show graphene has the greatest influence on the thermophysical properties. For example, thermal conductivity of the composites containing graphene increased by 98%. Graphene similarly enhanced the thermal diffusivity and specific heat of the Al/Teflon matrix. Conversely, nano C and CNTs decreased the thermal conductivity and thermal diffusivity of the samples significantly.     

Trends of energy efficiency in Finnish road freight transport 1995–2009 and forecast to 2016

A framework for modeling and analyzing the energy efficiency of road freight transport is presented in this paper. This framework is tested by using the data from the Finnish Goods Transport by Road statistics. The data was enhanced by calculating the fuel consumption for each trip in the data. To calculate this, weight-fuel consumption functions were estimated for each Euro-class vehicles and road type. This is a new method for analyzing the energy efficiency of road freight transport and it could be applied also in other countries gathering freight transport data with continuous company surveys. The analysis show that the energy efficiency of road freight transport in Finland improved during 1995–2002, but has declined since. The major drivers in the development have been the changes in the level of empty running and vehicle fuel efficiency. Extrapolating current statistical trends of factors that influence the energy efficiency show that the target set by the Finnish government for improving energy efficiency by 9% until 2016 will not be achieved. However, the target is possible to be achieved by a combination of small changes to some determinants.     

Which is the preferable transport fuel on a greenhouse gas basis; biomethane or ethanol?

Biomethane and ethanol are both biofuels which are generated from agricultural crops that can be utilised to meet the Biofuels Directive. In Ireland with the demise of the sugar industry 48,000 Ha of land is readily available for biofuel production, without unduly effecting food production. Which biofuel should dominate? This paper investigates biofuel production for three different crop rotations: wheat, barley and sugar beet; wheat, wheat and sugar beet; wheat only. A greenhouse gas balance is performed to determine under what conditions each biofuel is preferable. For both biofuels, the preferred crop on a weight basis is wheat, while on an area basis the preferred crop is sugar beet. Biomethane scenarios produce more gross energy than ethanol scenarios. Under the base assumption (7.41% biogas losses, and biomethane utilised in a converted petrol engine, such as a bi-fuel car, and thus underperforming on a km/MJ basis) ethanol generated more net greenhouse gas savings than biomethane. This was unexpected as biomethane produces twice the net energy per hectare as ethanol. If either biogas losses were reduced or biomethane was utilised in a vehicular engine optimised for biomethane (such as a bus powered solely on gaseous biofuel) then biomethane would generate significantly more net greenhouse gas savings than ethanol. It was found that if biogas losses were eliminated and the biomethane was used in a vehicle optimised for biomethane, then the net greenhouse gas savings are 2.4 times greater than those from ethanol generated from the same feedstock.     

Effective transport properties for polymer electrolyte membrane fuel cells – With a focus on the gas diffusion layer

Multi-phase transport of reactant and product species, momentum, heat (energy), electron and proton in the components of polymer electrolyte membrane (PEM) fuel cells forms the three inter-related circuits for mass, heat (energy) and electricity. These intertwined transport phenomena govern the operation and design, hence the performance, of such cells. The transport processes in the cell are usually determined with their respective effective transport properties due to the porous nature of PEM fuel cell components. These properties include the effective diffusion coefficient for the mass transfer, effective thermal conductivity for heat transfer, effective electronic conductivity for electron transfer, effective protonic conductivity for proton transfer, intrinsic and relative permeability for fluid flow, capillary pressure for liquid water transfer, etc. Accurate determination of these effective transport properties is essential for the operation and design of PEM fuel cells, especially at high current density operation. Thus, it is the focus of intensive research in the recent years. In this article, a review is provided for the determination of these effective transport properties through the various PEM fuel cell components, including the gas diffusion layer, microporous layer, catalyst layer and the electrolyte membrane layer. Given the simplicity of the GDL in structure compared to the other components of the cell, much more work in literature is focused on its transport properties. Hence, its review in this paper is more extensive. Various methods used for the determination of the effective transport properties with and without the presence of liquid water are reviewed, including experimental measurements, numerical modeling and theoretical analyses. Correlations are summarized for these transport properties, where available and further work in this area is provided as a direction for future work.     

Food or fuel? What European farmers can contribute to Europe's transport energy requirements and the Doha Round

Farm support in higher income countries is a testament to the fundamental social and economic importance of agriculture, yet domestic efforts to support this sector can arouse multilateral discord in a world of global food markets. In this paper, we argue that the advent of biofuels offers a new opportunity for agriculture to contribute to society, and to do so in a way that reduces trade rivalry and improves energy security. Holding current agricultural production constant, we find that the EU has the potential to reduce oil imports between 6% and 28% by converting eligible agricultural crops into biofuels under two differing conversion scenarios. Further, 33% of food support could be removed with no net farm revenue loss, using the biofuel premia (compared with food value) of corn and rapeseed to compensate for subsidy reductions. These results can help overcome the current impasse in global trade negotiations by reconciling the needs of EU farmers with those who would gain from more liberal international trade.     

Modeling of the transport phenomena in GMAW using argon–helium mixtures. Part I – The arc

This article presents a numerical investigation on the transient transport phenomena in the arc which include the arc plasma generation and interactions with moving droplets and workpiece for pure argon and three argon–helium mixtures (75% Ar + 25% He, 50% Ar + 50% He, and 25% Ar + 75% He) during the gas metal arc welding (GMAW) process. The results indicate that the arcs in various shielding gases behave very differently due to the significant differences in thermophysical properties, including the ionization potential and the electrical conductivity, thermal conductivity, specific heat, and viscosity at high temperatures. For the same welding power input, it was found the increase of helium content in the mixtures results in (1) the change of plasma arc shape from bell-like to cone-like and (2) the change of arc pressure distribution along the workpiece surface from Gaussian-like to flat-top with decreasing peak value. Detailed explanations to the physics of the very complex but interesting transport phenomena are given.     

The direct collocation meshless method based on a second-order phonon Boltzmann equation for ballistic–diffusive phonon heat transport

A second-order phonon Boltzmann equation (SOPBE) is proposed based on the phonon Boltzmann equation (PBE) under gray relaxation-time approximation, and the direct collocation meshless (DCM) method is employed to solve the SOPBE. Several numerical tests for phonon transport over a broader range of Knudsen numbers under different boundary conditions are carried out. The results show that the SOPBE solved by the DCM method is applicable for different transport regimes. Moreover, it overcomes the numerical error “ray effects” of other numerical methods under the ballistic limit to a certain extent. When modeling phonon transport in materials with inhomogeneous acoustic property, the superiority of SOPBE will be more obvious compared with the PBE. The results demonstrate the capability of our methodology for ballistic–diffusive phonon heat transport.     

Ionic transport and battery characterization studies on NaINa2OB2O3 ternary glassy system

Glasses in the ternary system xNaI-yNa₂O-[100−(x+y)]B₂O₃ prepared by a melt quenching method are characterized by using different experimental techniques such as X-ray diffraction, ionic transference number and conductivity. The conductivity is found to vary in a non-linear manner with change in the NaI/Na₂O ratio. The highest conductivity glass composition is used as an electrolyte in the fabrication of a solid-state electrochemical cell. A decomposition potential of 2.5 V is determined for the electrolyte. The discharge chracteristics of the cell are investigated at ambient temperature and various cell parameters are determined. The open-circuit voltage and short-circuit current of the cell are 2.7 V and 1600 μA, respectively.     

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.