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[en] This paper presents a comprehensive exergy analysis of a 10 kW PEM fuel cell at variable operating temperatures, pressures, cell voltages and air stoichiometrics. The calculations of the physical and chemical exergies, mass flow rates and exergetic efficiency are performed at temperature ratios (T/T0) and pressure ratios (P/P0) ranging from 1 to 1.25 and 1 to 3, respectively. In addition, the analysis is conducted on fuel cell operating voltages of 0.5 and 0.6 V and at air stoichiometrics of 2, 3 and 4 in order to determine their effects on the efficiency of the fuel cell. The calculated results illustrate the significance of the operating temperature, pressure, cell voltage and air stoichiometry on the exergetic efficiency of the fuel cell. However, it is recommended that the fuel cell should operate at stoichiometric ratios less than 4 in order to maintain the relative humidity level in the product air and to avoid the membrane drying out at high operating temperatures
[en] Predictions of solar radiation for 17 stations in south-east and north-east Nigeria have been made using only the maximum air temperature and relative humidity. Both yearly and seasonal estimates have been considered. For yearly estimates, the maximum percentage error MPE is about 2% for a majority of the 17 stations and does not exceed 5.4% for any station. The average absolute percentage error AAPE and the sum of percentage errors SPE are less than 1.8% and 0.7%, respectively, while the coefficient of determination R2 > 0.967 for all stations. For seasonal estimates, MPE < 0.3%, SPE < 0.006% while R2 ⩾ 0.9997 for all seasons and all stations. (author)
[en] New empirical models have been developed in this study to facilitate the estimation of global solar radiation for Bahrain from commonly measured meteorological parameters. The objective is to minimize the errors and the number of parameters incorporated in the models over the existing ones. The results obtained are very encouraging. When the estimated values were compared with the measured ones, the models provided very high correlation factors of over 0.99 and absolute maximum errors of only 1.9-7.9%. The best models have been established when the monthly mean daily data were treated at intervals, namely January-June and July-December. These models are dependent on one to three measured inputs, such as temperature, bright sunshine duration, relative humidity and extraterrestrial solar radiation. The present models can, thus, be recommended for the State of Bahrain. (author)
[en] Highlights: ► Demonstrate solar energy misting system for the first time to our knowledge. ► Return on investment for such a system is recovered within two and half years. ► Solar panel tilt position is 25° due south in Medina Munawarah. ► The misting system is capable of lowering ambient temperature over 10 °C. - Abstract: We demonstrated for the first time to our knowledge a misting system that is powered by solar energy. The system was used to cool down an open area in Medina, Saudi Arabia. The ambient and surrounding temperatures were measured and compared for different timing signals that were applied to the misting system. The used solar panel performance is evaluated for different loads, and tilting settings. The return on investment for the misting system is found to be about two years and half.
[en] This paper is concerned with the energy and exergy analyses of the single layer drying process of potato slices via a cyclone type dryer. Using the first law of thermodynamics, an energy analysis was performed to estimate the ratios of energy utilization. An exergy analysis was accomplished to determine the location, type and magnitude of the exergy losses during the drying process by applying the second law of thermodynamics. It was concluded that the exergy losses took place mostly in the 1st tray where the available energy was less utilized during the single layer drying process of potato slices. It is emphasized that the potato slices are sufficiently dried in the ranges between 60 and 80 deg. C and 20-10% relative humidity at 1 and 1.5 ms-1 of drying air velocity during 10-12 h despite the exergy losses of 0-1.796 kJ s-1
[en] In order to obtain data on the behaviour of green facades in buildings as a passive system for energy savings in dry Mediterranean Continental climate a long-term work has been performed. This paper presents the first results of two actions developed during 2009. First, the growth of four different climbing plants as well as their ability to provide shadow was studied. Second, monitoring for a year of a real green facade was carried out. The results confirmed the great capacity of green facades to produce shade, reducing the heat on the facade wall of the building. It was also verified that a microclimate between the wall of the building and the green curtain are created, characterized by slightly lower temperatures and higher relative humidity. This means that the green screen acts as a wind barrier and confirms the evapotranspiration effect of the plants. On the other hand, these results did not allow withdrawing conclusions about the insulation effect of green facades.
[en] The generation of a typical meteorological year is of great importance for calculations concerning many applications in the field of thermal engineering. In this context, method that has been proposed by Hall et al. is selected for generating typical data, and an improved criterion for final selection of typical meteorological month (TMM) was demonstrated. The final selection of the most representative year was done by examining a composite score S. The composite score was calculated as the weighed sum of the scores of the four meteorological parameters used. These parameters are air dry bulb temperature, relative humidity, wind velocity and global solar radiation intensity. Moreover, a new modern software tool using Delphi 6.0 has been developed, utilizing the Filkenstein-Schafer statistical method for the creation of a typical meteorological year for any site of concern. Whereas, an improved criterion for final selection of typical meteorological month was employed. Such tool allows the user to perform this task without an intimate knowledge of all of the computational details. The final alphanumerical and graphical results are presented on screen, and can be saved to a file or printed as a hard copy. Using this software tool, a typical meteorological year was generated for Damascus, capital of Syria, as a test run example. The data processed used were obtained from the Department of Meteorology and cover a period of 10 years (1991-2000)
[en] In this work we evaluated the kinetics of drying of Opuntia's cladodes observing two conditions: complete cladode with the protective cuticle of the intact product and with reduced cuticle (partially removed), using a drying tunnel with forced flow. The temperature of the air was set at 35, 45, and 60 deg. C with velocities of 1.5 and 3.0 m/s. The conditions of the environment were controlled and maintained at 22 deg. C and 30% of relative humidity. The results show that the drying time was considerably reduced when approximately 30% of the cuticle that protects the product was removed. Additionally, the temperature had greater influence than the velocity of the air. The numerical model that best describes the behavior of the drying process is the double logarithmic one, with the imposed restrictions of r close to the unit, the lowest possible χ2 and the RSEM tending to zero. The characteristic drying function of the product resulted in a third-grade exponential curve, where r and SD were the corresponding selection criteria.
[en] Highlights: • Two-step solar radiation zoning method was proposed to identify five solar radiation zones. • Four sunshine-based estimation models using data from 98 stations in China were established. • General models of regional daily estimation for each zone were developed based on cubic models. - Abstract: Classification of solar radiation zones constitutes the prerequisite for the establishment of regional daily global solar radiation () estimation general model. Current zone establishment methods are ordinary based on solar radiation observation stations (SROS) which present a sparse and non-uniform distribution. As a result, the possibility of misclassifications of the stations occurs in cases there are no records of radiation and far away from SROS. Therefore, by using k-means cluster and Support Vector Machine-Genetic Algorithm, a two-step radiation zoning method was proposed in this paper according to: (a) , sunshine duration, temperature and relative humidity from 98 SROS and (b) sunshine duration, temperature and relative humidity from 562 stations without radiation. The method is capable to combine the SROS and the stations without radiation in the process of classification. Thus, these misclassifications have been effectively reduced and the accuracy of each classification has been significantly improved. Based on the method, five radiation zones have been identified. Concurrently, four sunshine-based models were obtained for each SROS and the analysis of statistical indexes indicated that the cubic models presented the best performances in each zone. According to the best site-specific models and radiation zones, the general models of regional estimation were developed by introducing the geographical parameters, including latitude and altitude. The comparative results demonstrated that the general models proposed in this paper had better accuracies and can represent the general models for the estimation of stations without radiation records in China.
[en] The equilibrium moisture contents were determined for prickly pear fruit using the gravimetric static method at t=30, 40 and 50 deg. C over a range of relative humidities from 0.05 to 0.9. The sorption curves of prickly pear fruit decreased with increase in temperature at constant relative humidity. The hysteresis effect was observed. The GAB, modified Halsey, modified Chung-Pfost, modified Oswin and modified Henderson models were tested to fit the experimental data. The GAB model was found to be the most suitable for describing the sorption curves. The monolayer moisture content values for the sorption at different temperatures are calculated using a modified BET equation. The isosteric heats of desorption and adsorption of water were determined from the equilibrium data at different temperatures