Abstracr-As fuel cells are likely to he used in many future applications, dedicated power converters must he developed and optimized. A thorough knowledge nl the fuel cell operation is thus required for power-electronics engineers. This paper proposes a theoretical and experimental study of the behaviour of a fuel cell stack subject to current harmonics. The fundamental role of the internal double layer capacitor is demonstrated.

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The presence of non-erodible roughness elements on erodible surfaces has the effect of absorbing part of the wind shear stress and thus protecting the erodible surface from wind erosion. This paper examines the shear stress distribution over roughness arrays of varying density, representing the progress of erosion on a bed of erodible and non-erodible particles. Threedimensional numerical simulations, simulating wind fl ow over a bed of particles covered by roughness elements, were conducted in order to investigate the effect of roughness elements on the shear stress near the surface. The results of these simulations confi rm that the erosion of soil by wind is strongly attenuated by the presence of roughness elements on the surface and depends on the geometric properties of the roughness elements. Based on the new numerical results obtained, a refi nement of existing theoretical approaches is developed to describe the dependence of the friction velocity upon roughness frontal area and real exposed cover rate. The new formulation proposed will allow a more accurate evaluation of shear stress partitioning as a function of topographic changes.

The amount of hydrogen present in porous silicon (PS) nanostructures is analyzed in detail. Concentration of atomic hydrogen chemically bound to the specific surface of PS is quantitatively evaluated by means of attenuated total reflection infrared (ATR-IR) spectroscopy and temperature-programmed desorption (TPD) spectroscopy. The concentration values are correlated to the PS nanoscale morphology. In particular, the influence of porosity, silicon nanocrystallite dimension, and shape on hydrogen concentration values is described. Hydrogen concentrations in fresh, aged, as well as in chemically and thermally treated PS layers are measured. Maximal hydrogen concentration of 66 mmol/g is detected in nanoporous layers with high (>95%) porosity consisting of nanocrystallites with dimensions of about 2 nm. Mass energy density that can be potentially obtained from this amount of hydrogen through a low-temperature fuel cell is estimated to be about 2176 W-h/kg and is found to be comparable with other substances containing hydrogen, such as hydride materials and methanol, which are usually used as hydrogen reservoirs.

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