A novel flow field with controllable pressure gradient to enhance mass transport and water removal of PEM fuel cells

An easily machined novel flow field with controllable pressure gradient across adjacent channels was designed and a two dimensional, across-the-channel, two-phase model was developed to study the gas transport and water removal of the novel configuration. The effect of channel-rib width ratio, GDL thickness and pressure gradient on the profiles of oxygen concentration and water saturation within the GDL were investigated. Special attention was paid to the mechanisms of the promoted mass transport and water removal rates under a pressure gradient. The model was validated by experiments with various channel-rib ratios and GDL thicknesses at different operating pressure. The results revealed that, oxygen concentration was increased, and the water saturation was reduced under the rib with a pressure gradient generated across the adjacent channels. The optimal pressure gradient is between 0.1 to 0.2 atm for the studied channel geometry and configuration. The mechanisms of the improved cell performance were elucidated.     

On non-linear dynamics of shells: implementation of energy-momentum conserving algorithm for a finite rotation shell model

Continuum and numerical formulations for non-linear dynamics of thin shells are presented in this work. An elastodynamic shell model is developed from the three-dimensional continuum by employing standard assumptions of the first-order shear-deformation theories. Motion of the shell-director is described by a singularity-free formulation based on the rotation vector. Temporal discretization is performed by an implicit, one-step, second-order accurate, time-integration scheme. In this work, an energy and momentum conserving algorithm, which exactly preserves the fundamental constants of the shell motion and guaranties unconditional algorithmic stability, is used. It may be regarded as a modification of the standard mid-point rule. Spatial discretization is based on the four-noded isoparametric element. Particular attention is devoted to the consistent linearization of the weak form of the initial boundary value problem discretized in time and space, in order to achieve a quadratic rate of asymptotic convergence typical for the Newton–Raphson based solution procedures. An unconditionally stable time finite element formulation suitable for the long-term dynamic computations of flexible shell-like structures, which may be undergoing large displacements, large rotations and large motions is therefore obtained. A set of numerical examples is presented to illustrate the present approach and the performance of the isoparametric four-noded shell finite element in conjunction with the implicit energy and momentum conserving time-integration algorithm. © 1998 John Wiley & Sons, Ltd.     

Conceptual evaluation of fluid–structure interaction effects coupled to a seismic event in an innovative liquid metal nuclear reactor

The seismic response analysis of such liquid storage systems, especially liquid metal reactors, as for example the eXperimental Accelerator Driven System (XADS), was examined taking into account mainly the coupling effects of the fluid–structure interaction and their influence on its relevant internal systems and components.Therefore this paper deals with the structural analyses of the seismically induced hydrodynamic responses, in the event of a safe shutdown earthquake (SSE), and the free oscillation (known as sloshing waves) of a metal liquid coolant as well as the dynamic buckling effects on involved structures.To the mentioned purpose the interaction and coupling effects among the main reactor vessel structures and the primary coolant response were investigated by means of a numerical evaluation (with a qualified finite element code) because of the lack of analytical linear theories that in any case are not adequate to describe all the complex phenomena related to the seismic loading.For the numerical modelling procedure, 3D finite element models were set up to analyse the propagation of seismic waves as well as its derived structural effects, such as the fluid steep waves motion, the local buckling bulges, etc., taking into account the geometrical and material nonlinearities of the RPV and the considered simplified internals.The obtained numerical results in terms of stress intensity and of the capability of the structures to resist relevant seismic loads are, thus, presented and discussed. Moreover the performed analyses allowed to highlight the structures mostly affected by the assumed loading conditions in order to achieve data useful for an upgrading of the design geometry, if any, for the considered reactor.     

Preliminary evaluation of the seismic response of the ELSY LFR

The main goal of the present study is the preliminary evaluation of the seismic demand of a LFR with reference to European Lead System project (ELSY) considered one of the most promising innovative Generation IV reactor. The safety aspects of the ELSY reactor in the event of a Safe Shutdown Earthquake, taking into account also the effects of the possible fluid–structure interaction, have been analyzed.To the purpose to determine the seismic demand, in according with the international rules, a non-linear dynamic analysis method was used with rather refined 3-D model of LFR for the foreseen structural analyses and simulations of the plant and of the reactor internals behaviour. In this report numerical results are presented and discussed highlighting the relevance of the fluid–structure interaction in terms of structural integrity as well as the isolation technique effectiveness, which is expected to increase the safety margin of the reactor structures during a seismic event, if the isolators frequency is far from that of the reactor.The present work has been performed within the 6th European Framework Project.     

Experimental validation of variable temperature flow field concept for proton exchange membrane fuel cells

Variable temperature flow field concept allows maintaining close to 100% relative humidity along the entire flow field of the anode and the cathode side without external humidification using water generated during fuel cell operation for internal reactant humidification. This work deals with the experimental validation of the variable temperature flow field concept on a five-segment single cell. The experimental setup provides insight into the membrane water transport, temperature distribution on the current collectors and inside the channels, and the current density distribution along the cell. Variable temperature flow field operation with dry reactants is compared to isothermal operation with partially and fully humidified reactants. The polarization curve comparison shows that the variable temperature flow field operating efficiency is similar or better than the commonly used isothermal configuration with fully humidified reactants. The main contribution of the variable temperature flow field concept, when compared to isothermal operation, is the reduction of the mass transport losses at higher currents, since the generated water is evaporated in the stream of reactants, thereby minimizing the problem of liquid water removal from the cell.     

A method for optimal sizing of an electrolyzer directly connected to a PV module

A method for optimal sizing of an electrolyzer directly connected to a PV module or array is presented. By combining the electrolyzer cells in series and in parallel it is possible to closely match the electrolyzer polarization curve to the curve connecting PV system’s maximum power points at different irradiation levels. The method presented here is based on linear approximation of both curves. With such a method it is possible to achieve the power transfer efficiency of up to 99%. The effect of PV temperature on optimum electrolyzer sizing is also investigated. The optimum electrolyzer size decreases with the increase of the PV temperature for the same PV system size. It was found that it is better (in terms of the system efficiency and the hydrogen generation rate) to size the EL system for a higher PV operating temperature.     

Transition to renewable energy systems with hydrogen as an energy carrier

Unlike the present energy system based on fossil fuels, an energy system based on renewable energy sources with hydrogen and electricity as energy carriers would be sustainable. However, the renewable energy sources in general have less emergy than the fossil fuels, and their carriers have lower net emergy. Because of that they would not be able to support continuous economic growth, and would eventually result in some kind of a steady-state economy. An early transition to renewable energy sources may prove to be beneficial in the long term, i.e., it may result in a steady state at a higher level than in the case of a transition that starts later. Once the economy starts declining it will not be able to afford transition to a more expensive energy system, and transition would only accelerate the decline. Similarly, if a transition is too fast it may weaken and drain economy too much and may result in a lower steady state. If a transition is too slow, global economy may be weakened by the problems related to utilization of fossil fuels (such as global warming and its consequences) before transition is completed and the result again would be a lower steady state. Therefore, there must be an optimal transition rate; however, its determination would require very complex models and constant monitoring and adjustment of parameters.     

A randomized pilot trial of low-dose combination lipid-lowering therapy following coronary artery bypass grafting

Vein graft atherosclerosis is a common and serious complication of coronary artery bypass grafting (CABG). There is mounting evidence that lipoprotein abnormalities play an equally important role in the development of lesions in saphenous vein grafts after CABG as in native coronary vessel disease. The potential benefit of low-dose lipid lowering combination therapy in these patients has not been investigated. In a randomized, double-blind, placebo-controlled study, we compared the efficacy and safety of a low-dose combination of colestipol 10 g and simvastatin 10 mg/day (CS) to colestipol 10 mg and bezafibrate 400 mg/day (CB) for 2 months in 33 patients with serum total cholesterol > 6.5 mmol/l and triglyceride < 4.5 mmol/l who had undergone CABG for severe coronary artery disease. In the CS group, total cholesterol decreased by 29% and low-density lipoprotein (LDL) cholesterol by 42%; similarly, CB reduced total cholesterol by 17%, LDL cholesterol by 23%, triglyceride by 19%, and increased high-density lipoprotein (HDL) cholesterol by 14%. Lipoprotein (a) and hemostatic factors were unaffected by either therapy in this study. Both combination therapies were well tolerated with no significant clinical or biochemical side effects. We conclude that low-dose combinations of colestipol and simvastatin or colestipol and bezafibrate are effective and well tolerated in the management of moderate hyperlipidemia in patients who had undergone CABG.