Browsing by Issue Date, starting with "2008-02-15"
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- Effect of anode flow field design in direct methanol fuel cells: preliminary studiesPublication . Schock, L.; Silva, R. A.; Malaquias, J.; Pinto, A. M. F. R.; Rangel, C. M.The direct methanol fuel cells are promising candidates for portable power sources due to their high energy density, however studies continue in order to give solutions for a number of drawbacks that affect cell performance and efficiency. Achieving good fuel cell performance requires that the flowing streams of fuel and oxidizer are evenly distributed over the entire surface of the catalyst layer and also an efficient removal of reaction products. This is achieved through the optimal design of the flow field, which primarily depend upon channel pattern as well as channel (and rib) shape and size. In this work the effect of anode flow field design on the performance of an own built DMFC is studied. Preliminary results are herein presented.
- Pilhas de combustível e hidrogénio: Kit para fins didácticosPublication . Rangel, C. M.; Silva, R. A.; Paiva Luís, TeresaO cenário energético actual apresenta múltiplas problemáticas que condicionam o nosso futuro próximo. O sistema actual, baseado em combustíveis fósseis, é confrontado com grandes desafios que incluem a identificação de soluções adequadas, em tempo real, à crescente procura de energia e a necessidade de implementação de mais drásticas medidas para a mitigação dos efeitos das alterações climáticas, num quadro em que a segurança no abastecimento associada a instabilidade geopolítica das regiões produtoras impõe uma séria reflexão sobre a dependência e eficiência energéticas a vários níveis. Os investimentos necessários a um novo paradigma passam pela Investigação e Desenvolvimento e a Educação e aprendizagem orientadas, instrumentais na remoção de barreiras técnicas e de aceitação. Neste trabalho, apresenta-se um kit portátil para a divulgação do hidrogénio renovável como combustível em associação com pilhas de combustível, dirigida à população Escolar e Universitária, num pacote que inclui Aula introdutória aos sistemas de energias alternativos com documentação de suporte, demonstração hands-on, que será acompanhado por um Manual de Instruções e problemas a resolver pelos alunos.
- Synthesis of FeTi hydrogen storage material via ball milling: effect of milling energy and atmosphere.Publication . Livramento, Vanessa; Rangel, C. M.; Correia, J.B.; Shohoji, Nobumitsu; Silva, R. A.Attempts were made earlier to synthesize and activate the FeTi intermetallic during ball milling (BM), for H2 storage using sodium boron tetra-hydride (NaBH4) additive as a process controlling agent. Simple reactive milling starting from Fe and Ti powders resulted in heavy agglomeration of powders, due to the self sustaining nature of the reaction following an incubation period. When NaBH4 was used as the process control agent to avoid agglomeration, this resulted in the production of titanium hydride besides FeTi, and as a consequence unfavorable irreversibility in the subsequent hydrogen charging/discharging cycles [1,2,12]. The present work reports on modifications introduced in the synthesis process by changing two processing parameters, namely the milling energy and atmosphere composition
- Materials ageing mechanisms in PEM fuel cellsPublication . Rangel, C. M.; Silva, R. A.; Paiva Luís, TeresaAs fuel cell technology matures and time scale to commercialization decreases, the need for a more comprehensive knowledge of materials ageing mechanisms is essential to attain specified lifetime requirements for applications. In this work, the membrane-electrode assembly (MEA) degradation of a 5 cell PEM stack was evaluated, during and after fuel cell ageing in extreme testing conditions. Cross sections of the membrane catalyst layers and gas diffusion layers were examined indicating that cathode thickness is considerably reduced as a result of ageing. Catalyst particles were found to migrate outwards and located on carbon backings. Fluoride release was considered an indicator of membrane degradation, quantified using an ion selective electrode at gases outlet. MEA degradation mechanisms are discussed.
- Metal hydrides for hydrogen storage at low charging pressuresPublication . Fernades, Vitor R.; Peñalosa, J.; Rangel, C. M.LaNi5 alloys are been extensively study as hydrogen storage material due their easy activation and good kinetics. LaNi5 alloys, containing substitutional elements such as Al or Ce, allow adjustments of the equilibrium pressure and absorption kinetics of the metallic hydrides making them storage candidate materials susceptible to be used in fuel cells systems. In this work, the thermodynamic properties of LaNi5-yAly and LaNi5, La1-xCexNi5 alloys were evaluated using a purpose built Sievert-type apparatus.
- Hydrogen production by hydrolysis of sodium borohydride for PEM fuel cells feedingPublication . Silva, R. A.; Paiva Luís, Teresa; Branquinho, M.; Carvalho, S.; Pinto, A. M. F. R.; Rangel, C. M.In this work, hydrogen is produced by a hydrolysis process that uses sodium borohydride as a hydrogen carrier and storage media. High purity hydrogen is obtained at low temperatures with high volumetric and gravimetric storage efficiencies; reaction products are non-toxic. The produced hydrogen can be supply on-demand at specified flow by tailor made developed catalyst. Hydrogen feeding to a low power fuel cell was accomplished. According to experimental conditions conversion rates of 100% are possible. Catalyst is demonstrated to be reusable.
- Water management in direct methanol fuel cellsPublication . Oliveira, V. B.; Rangel, C. M.; Pinto, A. M. F. R.Direct methanol fuel cell (DMFC) are a promising power source for micro and portable applications due to their high energy density and inherent simplicity of operation with methanol as the liquid fuel. Present state-of-the-art optimised operating conditions are elevated cell temperatures to improve the anode reaction, high air stoichiometries to prevent cathode flooding and dilute methanol solutions to mitigate methanol crossover. These very dilute fuel solutions require the presence of a high amount of water leading to a reduction of the energy density of the system. More concentrated methanol solutions would be preferable in order to achieve energy densities needed for portable power applications. However, the possibility of using highly concentrated methanol solutions at the anode is limited by the significant water loss from the anode to cathode occurring in the DMFC due to electro-osmotic drag and molecular diffusion through the membrane. So, low crossover of both methanol and water through a polymer membrane in a DMFC is essential for using high concentration methanol in portable power applications. In this work, the results of a simulation study using a previous developed model for DMFCś are presented. Particular attention is paid to the water distribution across the cell. The influence of different parameters (such as methanol concentration, membrane thickness and gas diffusion media) over the water transport and on the cell performance is studied. The model used to predict the water transport was validated with recent published data.
- Solar hydrogen production from aqueous solutions of ethanolPublication . Rangel, C. M.; Silva, R. A.; Fernandes, Vitor; Paiva Luís, Teresa; Chesnau, J.In this work, photo-catalytic hydrogen production from water is studied, using ethanol as sacrificial agent. Titania was used as a base catalyst due to its stability and corrosion resistance as well as its environmental friendliness and low cost. The need to decrease the electron-hole recombination rate was accounted for by metal doping and also by using the ethanol molecule as a hole trap. Hydrogen production was achieved using own synthesised sol-gel metal doped titania, with production rate values being larger than published literature data.
- Fuel cell and hydrogen storage development for a wheel chairPublication . Rangel, C. M.; Silva, V.; Luz, Paulo P. da; Franco, Alexandra; Azevedo, F.; Bosukov, L.In this paper are presented, the recent advances of a demonstration project that focuses on the implementation of a hybrid system for a wheel chair incorporating a fuel cell and a metal hydride hydrogen storage system. Advantages regarding weight reduction and greater autonomy are emphasized, apart from the use of clean energy and the drastic reduction in charging time, when compared with the system before conversion.
- Autonomous hydrogen systems: control strategyPublication . Martins, João F.; Viana, Susana; Joyce, A; Pires, Armando; Rangel, C. M.; Sotomayor, João; Castro, RuiThis paper describes the control strategy for a Stand-Alone Energy System Supported by Totally Renewable Hydrogen Production. The basic control strategy considers the pressurized hydrogen gas storage as the energy buffer. The basic logic is that the exceeding renewable energy (solar and wind) is used to accumulate hydrogen, while the fuel cell uses this hydrogen to produce electrical energy within insufficient solar/wind energy. The system is completely autonomous, conceived for off-grid operation. It is composed by solar panels and a wind turbine as primary energy converters, hydrogen tanks as energy buffer and a fuel-cell and an electrolyzer as secondary energy converters.