Chemistry and light - part 2: light and energy

The conversion of solar energy into more useful forms of energy, such as chemical fuels or electricity, is one of the central problems facing modern science. Progress in photochemistry and chemical synthesis has led to a point where light energy conversion by means of artificial molecular devices can be rationally attempted. In this article, a general approach towards this challenging goal is presented.

Eletrocatálise e poluição ambiental

This work considers mainly the problem of environmental pollution due to the production of energy by burning fossil fuels, particularly in urban vehicles. Electrochemical energy conversion is proposed as a partial solution to this problem, through the use of hydrogen in fuel cells. In both the production of hydrogen by electrolysis and in the electrochemical processes in fuel cells, the key factor is electrocatalysis. The concept of electrocatalysis, the production of catalysts and supported catalysts and their use in practical systems for the conversion of energy is discussed.

Métodos de preparação de nanopartículas metálicas suportadas em carbono de alta área superficial, como eletrocatalisadores em células a combustível com membrana trocadora de prótons

Fuel cells are attracting much interest as efficient and clean energy conversion devices. The main components of low temperature fuel cells are the electrocatalysts used to promote the anodic and cathodic reactions, which are based on platinum and platinum alloys. These electrocatalysts are normally prepared in the form of metal nanoparticles supported on a conductive material, usually high surface area carbon, to improve catalyst utilization and reduce cost. This work presents and comments some methods used presently to produce these electrocatalysts. The performances of the produced electrocatalysts are compared to that of state-of-the-art commercial E-TEK electrocatalysts.

Avaliação da eficiência de uma célula a combustível estacionária de ácido fosfórico

Operation and performance of a commercial PAFC power plant were analyzed. Processes influencing energy conversion efficiency were studied in each module of the fuel cell power plant. The main processes were simulated using mass and energy balance equations, and the results were validated by means of experimental data. It was concluded that the electrical efficiency is higher in comparison with microturbines. The main result achieved is a better understanding of balance of plant processes, knowledge necessary for fuel cell power plant development.

Celdas solares fotoelectroquímicas basadas en Bi2WO6

In this study, photoelectrochemical solar cells based on bismuth tungstate electrodes were evaluated. Bi2WO6 was synthesized by a hydrothermal method and characterized by scanning electron microscopy, UV-Vis reflectance spectroscopy, and X-ray powder diffraction. For comparison, solar cells based on TiO2 semiconductor electrodes were evaluated. Photoelectrochemical response of Grätzel-type solar cells based on these semiconductors and their corresponding sensitization with two inexpensive phthalocyanines dyes were determined. Bi2WO6-based solar cells presented higher values of photocurrent and efficiency than those obtained with TiO2 electrodes, even without sensitization. These results portray solar cells based on Bi2WO6 as promising devices for solar energy conversion owing to lower cost of production and ease of acquisition.

PYROLIGNEOUS LIQUOR PRODUCED FROM ACACIA MEARNSII DE WILD WOOD UNDER CONTROLLED CONDITIONS AS A RENEWABLE SOURCE OF CHEMICALS

Acacia mearnsii de Wild (black wattle) is one of the most important trees planted in Southern Brazil for tannin extraction and charcoal production. The pyrolysis of the black wattle wood used for obtaining charcoal is performed in brick ovens, with the gas fraction being sent directly into the environment. The present study examines the condensable compounds present in the liquor produced from black wattle wood at different thermal degradation conditions, using gas chromatography coupled with mass spectrometry (GC/MS). Branches of black wattle were thermally degraded at controlled ambient and temperature conditions. Overall, a higher variety of compounds were obtained under atmospheric air pressure than under synthetic air pressure. Most of the tentatively identified compounds, such as carboxylic acids, phenols, aldehydes, and low molecular mass lignin fragments, such as guayacol, syringol, and eugenol, were products of lignin thermoconversion. Substituted aromatic compounds, such as vanillin, ethyl vanillin, and 2-methoxy-4-propeny-phenol, were also identified. At temperatures above 200 ºC, furan, 2-acetylfuran, methyl-2-furoate, and furfural, amongst others, were identified as polysaccharide derivatives from cellulose and hemicellulose depolymerization. This study evidences the need for adequate management of the condensable by-products of charcoal production, both for economic reasons and for controlling their potential environmental impact.

CÉLULAS SOLARES SENSIBILIZADAS POR CORANTES NATURAIS: UM EXPERIMENTO INTRODUTÓRIO SOBRE ENERGIA RENOVÁVEL PARA ALUNOS DE GRADUAÇÃO

An interesting practical experiment about the preparation of dye–sensitized solar cells (DSSC) using natural dyes were carried out by the undergraduate students in the chemistry course at UNICAMP . Natural dyes were extracted from blueberries (Vaccinium myrtillus L.), jabuticabas (Myrciaria cauliflora), raw and cooked beets (Beta vulgaris L.), and annattos (Bixa orellana L.), which were used to sensitize TiO2 films that composed the photoanode in the DSSC. A polymer electrolyte containing an iodide/triiodide redox couple was used in lieu of the use of liquid solutions to prevent any leakage in the devices. A maximum solar-to-electric energy conversion of 0.26 ± 0.02% was obtained for the solar cell prepared with annatto extracts. This experiment was an effective way to illustrate to the undergraduate students how to apply some of the chemical concepts that they learned during their chemistry course to produce electric energy from a clean and renewable energy source. Teachers could also exploit the basics of the electronic transitions in inorganic and organic compounds (e.g., metal-to-ligand charge transfer and ϖ-ϖ* transitions), thermodynamics (e.g., Gibbs free energy), acid–base reactions in the oxide solid surface and electrolyte, and band theory (i.e., the importance of the Fermi level energy).

Activity and functional interaction of alternative oxidase and uncoupling protein in mitochondria from tomato fruit

Cyanide-resistant alternative oxidase (AOX) is not limited to plant mitochondria and is widespread among several types of protists. The uncoupling protein (UCP) is much more widespread than previously believed, not only in tissues of higher animals but also in plants and in an amoeboid protozoan. The redox energy-dissipating pathway (AOX) and the proton electrochemical gradient energy-dissipating pathway (UCP) lead to the same final effect, i.e., a decrease in ATP synthesis and an increase in heat production. Studies with green tomato fruit mitochondria show that both proteins are present simultaneously in the membrane. This raises the question of a specific physiological role for each energy-dissipating system and of a possible functional connection between them (shared regulation). Linoleic acid, an abundant free fatty acid in plants which activates UCP, strongly inhibits cyanide-resistant respiration mediated by AOX. Moreover, studies of the evolution of AOX and UCP protein expression and of their activities during post-harvest ripening of tomato fruit show that AOX and plant UCP work sequentially: AOX activity decreases in early post-growing stages and UCP activity is decreased in late ripening stages. Electron partitioning between the alternative oxidase and the cytochrome pathway as well as H+ gradient partitioning between ATP synthase and UCP can be evaluated by the ADP/O method. This method facilitates description of the kinetics of energy-dissipating pathways and of ATP synthase when state 3 respiration is decreased by limitation of oxidizable substrate.

Energy balance in the poultry-shed system and its influence on broiler performance

ABSTRACTThe objective of this study was to determine the energy balance of the poultry-shed system and its effect on broiler performance during the production cycle. The experimental design was completely random with sub-divided blocks. The blocks were composed of five different types of sheds and the sub-blocks of the evaluation times (00:00 h to 23:00 h), allowing an analysis of variance and a comparison between means with the Tukey test. There were no significant differences between the mean values of the exchanges of sensible, latent and total heat between the poultry sheds but the differences for the evaluation times were significant (P<0.05). There was no significant difference between sheds 1 and 4 for broiler productive performance regarding weight gain, feed consumption and feed conversion. Bird performance was significant (P<0.05) for the remaining poultry sheds. The productive indexes remained below the ranges considered ideal for broilers and values in the final weeks were characterized by the poor installation efficiency in controlling temperature variations and, consequently, the energy balance in the system, which adversely affected bird productive performance.

Energy balance measurements over a banana orchard in the Semiarid region in the Northeast of Brazil

The objective of this work was to evaluate the reliability of eddy covariance measurements, analyzing the energy balance components, evapotranspiration and energy balance closure in dry and wet growing seasons, in a banana orchard. The experiment was carried out at a farm located within the irrigation district of Quixeré, in the Lower Jaguaribe basin, in Ceará state, Brazil. An eddy covariance system was used to measure the turbulent flux. An automatic weather station was installed in a grass field to obtain the reference evapotranspiration (ET0) from the combined FAO-Penman-Monteith method. Wind speed and vapor pressure deficit are the most important variables on the evaporative process in both growing seasons. In the dry season, the heat fluxes have a similar order of magnitude, and during the wet season the latent heat flux is the largest. The eddy covariance system had acceptable reliability in measuring heat flux, with actual evapotranspiration results comparing well with those obtained by using the water balance method. The energy balance closure had good results for the study area, with mean values of 0.93 and 0.86 for the dry and wet growing seasons respectively.

Performance of small wind turbines: simulation of electricity supply to loads connected to the public or isolated grid

ABSTRACT The successful in the implementation of wind turbines depends on several factors, including: the wind resource at the installation site, the equipment used, project acquisition and operational costs. In this paper, the production of electricity from two small wind turbines was compared through simulation using the computer software HOMER - a national model of 6kW and an imported one of 5kW. The wind resources in three different cities were considered: Campinas (SP/BR), Cubatão (São Paulo/BR) and Roscoe (Texas/ USA). A wind power system connected to the grid and a wind isolated system - batteries were evaluated. The results showed that the energy cost ($/kWh) is strongly dependent on the windmill characteristics and local wind resource. Regarding the isolated wind system – batteries, the full supply guarantee to the simulated electrical load is only achieved with a battery bank with many units and high number of wind turbines, due to the intermittency of wind power.

Energy system contribution in a maximal incremental test: correlations with pacing and overall performance in a 10-km running trial

This study aimed to verify the association between the contribution of energy systems during an incremental exercise test (IET), pacing, and performance during a 10-km running time trial. Thirteen male recreational runners completed an incremental exercise test on a treadmill to determine the respiratory compensation point (RCP), maximal oxygen uptake (V˙O2max), peak treadmill speed (PTS), and energy systems contribution; and a 10-km running time trial (T10-km) to determine endurance performance. The fractions of the aerobic (WAER) and glycolytic (WGLYCOL) contributions were calculated for each stage based on the oxygen uptake and the oxygen energy equivalents derived by blood lactate accumulation, respectively. Total metabolic demand (WTOTAL) was the sum of these two energy systems. Endurance performance during the T10-km was moderately correlated with RCP, V˙O2maxand PTS (P<@0.05), and moderate-to-highly correlated with WAER, WGLYCOL, and WTOTAL (P<0.05). In addition, WAER, WGLYCOL, and WTOTAL were also significantly correlated with running speed in the middle (P<0.01) and final (P<0.01) sections of the T10-km. These findings suggest that the assessment of energy contribution during IET is potentially useful as an alternative variable in the evaluation of endurance runners, especially because of its relationship with specific parts of a long-distance race.