Electrical double-layer capacitors: evaluation of ageing phenomena during cycle life testing

This paper represents an assessment of the main ageing phenomena in electrical double-layer capacitors (EDLCs). In this study the cycle life of the EDLC cells with a rated capacitance of 1,600 F has been investigated at different ambient temperatures and current rates. From the experimental results we can observe that the impact of the high ambient temperature is significant on the cycle life of the cells. Moreover, the results also show the negative impact of the current rate. The internal resistance tests showed that the increase of the resistance is much higher than the decrease of the capacitance. Thus, the ageing of the EDLC during cycling was clearly non-linear. Further the EIS measurements indicated the higher increase of the imaginary part of the impedance at low frequencies during cycling, which indicates the capacitance fade.     

Analogy between adsorption and sorption:An elementary mechanistic approach.I.Monolayer adsorption and sorption without solvent cluster formation

The elementary mechanistic model of adsorption and sorption is based on a simple hypothesis:the adsorption sites are uniformly distributed on the surface of the pore walls in the adsorbent,the sorption sites are uniformly distributed in the volume of the polymer.In this first paper we will analyze the simple case where one solute mol-ecule is only allowed to occupy a single adsorption or sorption site.A common elementary occupation law of the free sites is assumed:the differential increase of the number of the adsorbed/sorbed molecules is proportional to the differential increase of the activity of the solute and the concentration of the free(non-occupied)sites in the solid.The proportionality coefficient is called affinity coefficient depending on the solid/solute couple and on the temperature and independent of the concentration of the solute.In adsorption the concentration of the free sites is a surface concentration on the pore walls and in sorption it is expressed by the molarity.The simple mono-layer adsorption law of Jovanovi?is obtained:n=n0(1?e?KP)where n is the number of moles adsorbed when the pressure is P. n0is the total number of adsorption sites and K the affinity coefficient for adsorption.The sorption law writes:a= 1k? ?1??]+1?rk ln?1+1r?1??]where?,r and k hold respectively for the volume fraction of the solvent in the polymer,for the ratio of the molar volumes of the solvent to the elementary polymer chain containing one single adsorption site and for the sorption affinity coefficient.The confrontation of these equations to experimental isotherms is satisfactory in comparison with the classical Langmuir and Flory-Huggins equations:the best results are obtained for adsorption of vapors on a 5A zeolite and for all analyzed sorption results.     

High-Efficiency Light Coupling in a Submicrometric Silicon-on-Insulator Waveguide

Light coupling into a sub-micrometer-thick waveguide is usually done through a grating coupler. Coupling efficiency is strongly enhanced by addition of a mirror above the grating. This new kind of coupler can be designed to achieve efficiencies as great as 80%. Numerical calculations for a high-angular-spread Gaussian incident beam are compared with experimental results obtained for a standard silicon-on-insulator waveguide.     

Characterization of supercapacitors matrix

This paper treats supercapacitors matrix characterization. In order to cut off transient power peaks and to compensate for the intrinsic limitations in embedded sources, the use of supercapacitors as a storage system is quite suitable, because of their appropriate electrical characteristics (huge capacitance, small series resistance, high specific energy, high specific power), direct storage (energy ready for use), and easy control by power electronic conversion. This use requires supercapacitors modules where several cells connected in serial and/or in parallel, thus a bypass system to balance the charging or the discharging of supercapacitors is required. In the matrix of supercapacitors, six elements of three parallel BCAP0350 supercapacitors in serial connections have been considered. This topology permits to reduce the number of the bypass circuits and it can work in degraded mode. Actually, it allows the system to have more reliability by providing power continually to the load even when there are one or more cells failed. Simulation and experimental results are presented and discussed.     

Régimes d'écoulement et mesure de l'épaisseur d'un film liquide ruisselant autour d'un tube horizontal

The flow characteristics and film thickness have been studied experimentally for the case of a falling liquid film flows on a horizontal cylinder. The optical method based on laser‐induced fluorescence is presented in this paper. All tests have been conducted with the adiabatic water/fluorescence mixture at atmospheric pressure. Five falling‐film modes are observed for different liquid mass flow. The wavelengths have been measured for each film Reynolds number value for droplet and column modes. The transitions between different falling‐film modes are studied and the hysteresis phenomenon is observed. The measured dimensionless wavelengths are compared to the results of Hu and Jacobi (1998). The experimental results show the local evolution of film thickness around the cylinder. They are compared to the results of Rogers and Goindi (1989), Rogers (1986) and Chyu and Bergles (1987). The local measurements of film thickness are correlated by including the effects of film Reynolds number.     

Local convective boiling heat transfer and pressure drop of nanofluid in narrow rectangular channels

This paper reports an experimental study on convective boiling heat transfer of nanofluids and de-ionized water flowing in a multichannel. The test copper plate contains 50 parallel rectangular minichannels of hydraulic diameter 800 μm. Experiments were performed to characterize the local heat transfer coefficients and surface temperature using copper–water nanofluids with very small nanoparticles concentration. Axial distribution of local heat transfer is estimated using a non-intrusive method. Only responses of thermocouples located inside the wall are used to solve inverse heat conduction problem. It is shown that the distribution of the local heat flux, surface temperature, and local heat transfer coefficient is dependent on the axial location and nanoparticles concentration. The local heat transfer coefficients estimated inversely are close to those determined from the correlation of Kandlikar and Balasubramanian [An extension of the flow boiling correlation to transition, laminar and deep laminar flows in minichannels and microchannels, Heat Transfer Eng. 25 (3) (2004) 86–93.] for boiling water. It is shown that the local heat flux, local vapor quality, and local heat transfer coefficient increase with copper nanoparticles concentration. The surface temperature is high for de-ionized water and it decreases with copper nanoparticles concentration.     

Frequency,thermal and voltage supercapacitor characterization and modeling

A simple electrical model has been established to describe supercapacitor behaviour as a function of frequency, voltage and temperature for hybrid vehicle applications. The electrical model consists of 14 RLC elements, which have been determined from experimental data using electrochemical impedance spectroscopy (EIS) applied on a commercial supercapacitor.     

Characterization of the self-heating of AlGaN/GaN HEMTs during an electrical stress by using Raman spectroscopy

We report on the non-invasive measurements of the temperature in active AlGaN/GaN HEMTs grown on sapphire substrate during an electrical stress. The original study permits to highlight the drop of the self-heating in operando during the electrical stress by using Raman spectroscopy. Moreover, a correlation between the decrease of the self-heating and the fall of the drain current during the stress has been demonstrated. This study also highlights that the self-heating of the components and the influence of the ageing test on the self-heating are clearly linked to the position where temperature measurements are carried out.     

Comparative study between exact and metaheuristic approaches for virtual machine placement process as knapsack problem

In cloud computing, the virtual machine placement is a critical process which aims to identify the most appropriate physical machine to host the virtual machine. It has a significant impact on the performance, resource usage and energy consumption of the datacenters. In order to reduce the number of active physical machines in a datacenter, several virtual machine placement schemes have already been designed and proposed. This study investigates how do four different methods compare to each other in terms of accuracy and efficiency for solving the virtual machine placement as a knapsack problem. A new approach has been adopted which focuses on maximizing the use of a server’s central processing unit resource considering a certain capacity threshold. The compared methods are classified; two belong to the category of the exact methods, i.e., branch and bound and dynamic programming, while the other two represent the approximate approach, i.e., genetic algorithm and ant colony optimization algorithm. Experimental results show that the metaheuristic ant colony optimization algorithm outperforms the other three algorithms in terms of efficiency.

     

Boiling local heat transfer enhancement in minichannels using nanofluids

This paper reports an experimental study on nanofluid convective boiling heat transfer in parallel rectangular minichannels of 800 μm hydraulic diameter. Experiments are conducted with pure water and silver nanoparticles suspended in water base fluid. Two small volume fractions of silver nanoparticles suspended in water are tested: 0.000237% and 0.000475%. The experimental results show that the local heat transfer coefficient, local heat flux, and local wall temperature are affected by silver nanoparticle concentration in water base fluid. In addition, different correlations established for boiling flow heat transfer in minichannels or macrochannels are evaluated. It is found that the correlation of Kandlikar and Balasubramanian is the closest to the water boiling heat transfer results. The boiling local heat transfer enhancement by adding silver nanoparticles in base fluid is not uniform along the channel flow. Better performances and highest effect of nanoparticle concentration on the heat transfer are obtained at the minichannels entrance.