Valter Silva

Optimizing the DMFC operating conditions using a response surface method

Abstract The power density of a Direct Methanol Fuel Cell (DMFC) as a function of temperature, methanol concentration, air flow rate, methanol flow rate and air relative humidity was studied using a Response Surface Methodology (RSM). For a DMFC equipped with a membrane of Nafion 112, it was observed that only the temperature, methanol concentration and air flow rate were relevant factors or operating variables. A new design of experiments was done for a narrower range of these variables and the operating values that optimise the power density were obtained using the software JMP 7.0 (SAS). The predicted power density values were in agreement with the experimental results obtained for the optimized operating conditions. Then, the RSM was applied to membranes with different thicknesses, Nafion 112, Nafion 1135 and Nafion 117, and as a function of the temperature and methanol concentration. The DMFC was characterized for the open circuit voltage (OCV), methanol crossover at the OC, power density and global efficiency. The membrane showing the best compromise between power density and efficiency was Nafion 117.

Pre‐treatment Effect on the Sulfonated Poly(ether ether ketone) Membrane Transport Properties and Direct Methanol Fuel Cell Performance

Abstract In this paper a critical study is presented on how the direct methanol fuel cell (DMFC) membrane properties are affected by a pre‐treatment. Membranes based on sulfonated poly(ether ether ketone) (sPEEK) with sulfonation degree (SD) of 42% were used, plain and modified inorganically with 1 wt.% of zirconium oxide. Nafion® 1135 was used as reference. The results obtained show that the sPEEK polymer (SD=42%) enables the preparation of proton exchange membranes with improved properties compared to Nafion® and to the ZrO2‐modified sPEEK membrane, mainly due to its improved relation between proton conductivity and methanol permeation. After the pre‐treatment, the plain sPEEK membrane had a performance 6.4 times higher in terms of maximum power output when used in the DMFC.

Exergy Analysis in Hydrogen-Air Detonation

The main goal of this paper is to analyze the exergy losses during the shock and rarefaction wave of hydrogen-air mixture. First, detonation parameters (pressure, temperature, density, and species mass fraction) are calculated for three cases where the hydrogen mass fraction in air is 1.5%, 2.5%, and 5%. Then, exergy efficiency is used as objective criteria of performance evaluation. A two-dimensional computational fluid dynamic code is developed using Finite volume discretization method coupled with implicit scheme for the time discretization (Euler system equations). A seven-species and five-step global reactions mechanism is used. Implicit total variation diminishing (TVD) algorithm, based on Riemann solver, is solved. The typical diagrams of exergy balances of hydrogen detonation in air are calculated for each case. The energy balance shows a successive conversion of kinetic energy, and total enthalpy, however, does not indicate consequent losses. On the other hand, exergy losses increase with the augment of hydrogen concentration in air. It obtained an exergetic efficiency of 77.2%, 73.4% and 69.7% for the hydrogen concentrations of 1.5%, 2.5%, and 5%, respectively.

Non-Fluorinated Membranes Thickness Effect on the DMFC Performance

Abstract In order to study the influence of the proton exchange membrane thickness on the direct methanol fuel cell (DMFC) performance, sulfonated poly (ether ether ketone) (sPEEK) membranes with a sulfonation degree (SD) of 42% and thicknesses of 25, 40, and 55 µm were prepared, characterized, and tested in a DMFC. These polymeric membranes were tested in a DMFC at several temperatures by evaluating the current-voltage polarization curve, the open circuit voltage (OCV) and the constant voltage current (CV, 35 mV). The CO2 concentration at the cathode outlet was also measured. The thinnest sPEEK membrane proved to have the best DMFC performance, although having lower Faraday efficiency (lower ohmic losses but higher methanol permeation). In contrast, the thickest membrane presented improved properties in terms of methanol permeation (lower methanol crossover). DMFC tests results for this membrane showed 30% global efficiency, obtained with pure oxygen at the cathode feed.

An Impedance Study on the sPEEK/ZrO2 Membranes for Direct Methanol Fuel Cell Applications

Electrochemical impedance experiments were carried out in order to study the influence of the ZrO2 inorganic incorporation on the proton conductivity of sulfonated poly(ether ether ketone) (sPEEK) membranes. The impedance data was fitted to an extension of Randles’ circuit, within the inorganic content and temperature ranges considered. The model fits quite well for ZrO2 loads up to 10 wt.%. Such a model allows for characterizing the diffusion phenomena (Warburg) of the membrane electrode assembly (MEA), membrane and electrodes resistances, capacitive and inductive behavior. Proton conductivity was obtained from the impedance spectra and it was observed that it increases with temperature and decreases with the inorganic content. As a general trend, the Warburg parameter decreases slightly with the temperature, except for the 5 wt. % ZrO2 membrane that suffers a more pronounced influence. The Warburg parameter also decreases with the ZrO2 content.

Assessment of Municipal Solid Wastes Gasification Through CFD Simulation

A two dimensional CFD model for MSW gasification has been used to predict and analyze the viability of the hydrogen generation from MSW gasification. The model is based in an Eulerian-Eulerian approach to describe the transport of mass, momentum and energy for the solid and gas phases. The model is applied to a fluidized bed gasifier to full predict and analyze the viability of the hydrogen generation from MSW gasification taking into account the equivalence ratio and steam-to-waste ratio. Conclusion could be drawn that the increase of equivalence ratio has a negative effect on hydrogen production because the oxidation reactions are favored. The introduction of steam to MSW gasification is favorable for improving hydrogen yield, because it increases the partial pressure of steam inside the reactor which favors the gas-phase reactions.

Modeling Biomass Substrates for Syngas Generation by Using CFD Approaches

Recent reports from top universities state that in spite of having great national importance, there are dozens of fields of study that are suffering due to a lack of funding. Perhaps the greatest tool available to assist researchers with this regard is numerical simulation. This tool allows cutting costs, decreasing the necessary design cycle and allows an enormous amount of physical insight on the process itself). Numerical models ability to correctly predict a complex system was tested in this chapter by drawing from a previously developed computational fluid dynamics model for biomass gasification. Numerical results were compared with both experimental results (pilot scale plant) and available literature. Results from common Portuguese biomass substrates were found to be within a satisfactory margin of error of 20%. Influence of all major operational conditions was then investigated and the model was once again able to predict all the expected trends. All the relevant process products were also analyzed. Finally, the numerical model was coupled with an optimization model. Maximum efficiency value was found at 900 C with a SBR of 1.5 for MSWand 1 for forest residues. Results showed that numerical models could have a preponderant impact on biomass gasification field.

Methane Combustion: An Exergy Analysis

A VBA (Visual Basic for Applications) code was developed to determine the exergy associated to the methane combustion. It was considered as the main sub‐processes for each stage of reaction: the combined reactant mixing, the fuel oxidation, the internal thermal energy exchange (heat transfer), and the product mixing process. The exergetic efficiency and the temperature of the products were computed as a function of the percentage of the excess air. It was verified that the internal thermal energy exchange is the sub‐process where the larger exergy destruction occurs.