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- Estratégias fotónicas para fotovoltaico : novos avanços para além da óticaPublication . Mendes, Manuel Joao; Sanchez-Sobrado, O.; Haque, S.; Centeno, Pedro; Alexandre, Miguel; Ribeiro, Guilherme; Boane, J.; Mateus, T.; Mouquinho, Ana; Menda, U.D.; Águas, H.; Fortunato, Elvira; Martins, RodrigoRESUMO: Estruturas fotónicas com tamanhos comparáveis aos comprimentos de onda da luz solar são as soluções preferenciais para melhorar a eficiência de dispositivos fotovoltaicos através de aprisionamento de luz. As micro-estruturas fotónicas aqui desenvolvidas operam no regime de ótica de ondas, pelo que foram construídos modelos eletromagnéticos que permitiram encontrar os parâmetros ótimos para aplicação no contacto frontal de diferentes tipos de tecnologias, nomeadamente em células de filme fino baseadas em silício ou perovskite. Desta forma, foram obtidas diferentes arquiteturas fotónicas de células, demonstrando melhoras de até 50% na eficiência relativamente a células de referência planas. Os resultados mostram que as vantagens da aplicação de estruturas fotónicas não estão só limitadas a ganhos óticos de melhora da absorção, mas também possibilitam outros benefícios importantes tais como: ganhos elétricos devido à melhora dos contactos transparentes, e melhor desempenho em condições ambientais devido a um encapsulamento avançado dos dispositivos que confere até propriedades de auto-limpeza dos mesmos.
- Slow pyrolysis of cork granules under nitrogen atmosphere: by-products characterization and their potential valorizationPublication . Costa, Paula; Barreiros, M. Alexandra; Mouquinho, Ana; Silva, P. Oliveira e; Paradela, Filipe; Oliveira, Fernando Almeida CostaABSTRACT: Cork granules (Quercus suber L.) were slowly pyrolyzed at temperatures between 400-700 degrees C and under N-2 flow. While preserving its structure, some cells of the cork biochar became interconnected, allowing such carbon residue to be used as templates for manufacturing ceria redox materials. The pyrolytic char morphology was similar to that of the natural precursor. The produced cork biochar belonged to Class 1 (C > 60%) and possessed a high heating value of 32 MJ kg(-1). Other pyrolysis-derived compounds were identified and quantified through GC-FID and GC-MS analyses. The yield of gases released during cork pyrolysis was strongly dependent on the temperature used due to the thermal decomposition reactions involved in the degradation of cork. In particular, rising pyrolysis temperature from 500 to 700 T resulted in reducing the total hydrocarbon gases from 74 to 24 vol%. On the other hand, the yield of H-2 increased from 0 to 58% by increasing the pyrolysis temperature from 400 to 700 T. Due to the presence of suberin in cork, the composition and yield of bio-oil could be regulated by the pyrolysis temperature. Cork bio-oil was found to consist of long-chain hydrocarbons (from C11 to C24). The bio-oil resulting from the slow pyrolysis of cork residues is suitable as an appropriate feedstock for producing aliphatic-rich pyrolytic biofuels or as a source of olefms. Overall, the findings of this study suggest that Quercus suber L. could be a promising feedstock for biochar and biofuel production through the pyrolytic route and could contribute to the environmental and economic sustainability of the cork production industry.
- High performance cork-templated ceria for solar thermochemical hydrogen production via two-step water-splitting cyclesPublication . Oliveira, Fernando Almeida Costa; Barreiros, M. Alexandra; Haeussler, Anita; Caetano, Ana P. F.; Mouquinho, Ana; Silva, P. Oliveira e; Novais, Rui M.; Pullar, Robert C.; Abanades, StéphaneABSTRACT: Water splitting by solar energy-driven two-step thermochemical cycles is a promising approach for large-scale production of renewable fuels (e.g. hydrogen). The key challenge is developing materials capable of withstanding the harsh environmental conditions and to ensure high reliability in use, particularly in terms of redox kinetics and better activity at low operation temperatures. In this work, we demonstrate that cork-templated ceria can significantly enhance the hydrogen production performance under solar irradiation heating. Three types of ceria morphologies were synthesised and investigated in two-step thermochemical redox cycles, namely ceria granules (ecoceramics) prepared from cork templates based on either a green water-based or an acetone solvent-based approach, as well as ceria foams replicated from polyurethane templates. These materials were cycled in a high-temperature indirectly-irradiated solar tubular reactor, heated via concentrated solar light, using a temperature-swing process. Samples were typically thermally reduced at 1400-1450 degrees C and subsequently re-oxidised with H2O between 950-1150 degrees C. The green synthesis ceria granules had up to 25% and 32% higher average H-2 production yields than the acetone-based ecoceramics and replicated ceria foams, respectively. On average, H-2 production rates for cork-templated ceria granules (1.3 +/- 0.2 mL min(-1) g(-1)) were up to similar to 60% higher than for ceria foams (0.8 +/- 0.3 mL min(-1) g(-1)), indicating that the morphology of this three-dimensionally ordered macroporous (3-DOM) CeO2 improves the reaction kinetics. This is attributed to the smaller mean cell size of the cork-derived ecoceramic (25 mu m) compared to that of the replicated ceria foam (575 mu m), suggesting that their semi-closed wall cells enhanced reaction rates. The increase in reduction temperature from 1400 to 1450 degrees C resulted in the highest H-2 production rate (1.6 mL min(-1) g(-1)) reported so far for 3-DOM ceria. Neither loss in redox performance nor change in grain morphology was observed from the first to the last cycle. These findings show that cork-like structural features are key to engineering efficient materials for enhanced solar thermochemical fuel production.
- Pressureless sintering of Ceria [Resumo]Publication . Silva, P. Oliveira e; Mouquinho, Ana; Barreiros, M. Alexandra; Oliveira, Fernando Almeida CostaABSTRACT: Cerium oxide (CeO2), known as ceria, is widely used in several applications, such as automobile exhaust catalysts, electrolytes in solid oxide fuel cells, electrodes in gas sensors, and ultraviolet absorbents, owing to superior thermal and chemical stability, high ionic conductivity, good oxygen storage or release capacity and strong UV absorption.
- Novos avanços no processo de fabrico de espumas de céria para produção de hidrogénio pela via termoquímica solarPublication . Oliveira, Fernando Almeida Costa; Mouquinho, Ana; Barreiros, M. AlexandraRESUMO: Este trabalho reporta um processo alternativo ao utilizado anteriormente para a produção de espumas de céria, pelo método de replicação de esponjas polimérica patenteado, em 1963, por Schwartzwalder e Somers. As referidas espumas destinam-se a servir de catalisadores à reação de separação da água pela via termoquímica solar. Esta via envolve um ciclo redox, em que a redução endotérmica da céria ocorre a T~1400ºC, em atmosfera de árgon, e a oxidação tem lugar a T~1000ºC, em presença de vapor de água. O aquecimento foi efetuado por radiação solar concentrada em fornos solares de 1,5 kW, concebidos para o efeito, disponíveis no Laboratório PROMES-CNRS (Odeillo, França). A adição de 5% PVA à suspensão permitiu evitar o colapso da estrutura celular durante a etapa de queima do substrato de PU, permitindo obter uma microestrutura mais porosa, que promoveu a formação de H2 pela via termoquímica solar. Ficou assim demonstrado que a microestrutura deste novo material teve um efeito benéfico na produção de H2, ou seja, estima-se que o H2 produzido por este material (CF5) seja cerca do dobro do anterior (CF), com base nos teores de CO e O2 registados.
- Pyrolysis of cork granules suitable for production of ceria ecoceramics used in water splitting under concentrated solar energy [Resumo]Publication . Mouquinho, Ana; Silva, P. Oliveira e; Costa, Paula; Barreiros, M. Alexandra; Oliveira, Fernando Almeida CostaABSTRACT: Developing solar technologies for converting H2O into H2 is a great challenge, as it leads to a sustainable alternative to the production of transportation fuels. The solar thermochemical approach seems particulary interesting since it uses high temperature process heat to drive a two-step cycle base on metal oxide redox reactions.