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[en] Probability density functions, such as the Weibull and Rayleigh, are often used in wind speed and wind energy analyses. It is important to establish the goodness of their suitability in modelling a particular measured distribution. The parameters obtained from the distributions (distributional parameters) are required to assess the suitability of the functions. Such distributional parameters that are commonly used are the following: the parameters of the function itself (such as the parameters of the Weibull function), the wind energy output and the correlation coefficient of the probability density distributions. It is shown in the present article that these parameters alone are not sufficient to determine the suitability of a given function. Additional parameters are required to have a complete statistical assessment of the function, such as the standard deviation or, alternatively, the correlation coefficient of the power density distributions. In the present study, the suitability of the Weibull and Rayleigh functions is assessed based on a total of six parameters (of the probability density and power density distributions), calculated from 12 months of hourly time series wind speed data. It is shown that the degree of suitability of a given function changes to a great extent with the parameter on which the judgement is based
[en] The aim of this paper is to highlight various processes for the conversion of biomass into hydrogen gas. Biomass energy has the potential to be 'modernized' worldwide, i.e., produced and used efficiently and cost competitively, generally in the more convenient forms of gases, liquids, or electricity. Biomass will play an important role in the future global energy infrastructure for the generation of power and heat, but also for the production of chemicals and fuels. Biomass and biomass-derived fuels are can be used to produce hydrogen sustainably. The methods available for the hydrogen production from biomass can be divided into two main categories: thermochemical and biological routes.
[en] As a first step of an exhaustive assessment of wind energy potential over Europe, here, we provide a unified description of the wind speed probability distribution both over sea and land. We evaluated surface wind velocity records of the ERA-40 data base covering 44 years with a temporal resolution of 6 hours. We tested the well known distribution functions (Rayleigh, binormal, Weibull, lognormal etc.) and observed that the popular Weibull function performs supremely, however, it fails at many locations over land. We found that the generalized gamma distribution, which has independent shape parameters for both tails, provides an adequate and unified description almost everywhere. The geographical distribution of the fitted parameters reveals the possible climatological origin of different wind speed distributions
[en] Highlights: • Geothermal energy assisted milk powder production line was studied thermodynamically. • The first study on exergy analysis of a milk powder production line. • The overall energy and exergy efficiencies were calculated as 85.4 and 57.45%, respectively. • The evaporator, has the highest exergy destruction rate with 333.6 kW, needs a detailed assessment. - Abstract: Milk has been consumed since time immemorial because of its unique nutritional properties and produced almost 816 million tonnes in the year of 2016. Due to its highly perishable characteristic, milk is processed into more stable milk products such as cheese, yoghurt, and butter and milk powder. Among them, milk powder is distinctive for its longer shelf life and can be stored at ambient temperature. The other advantages of milk powder are less volume requirement during its transportation and higher selling price. Therefore, it is widely used in many food products such as ice cream, bakery products, and sausages. According to a recent study on the statistics from Food and Agriculture Organization, world production of whole dried milk was 3,597,015 tonnes in 2014: Oceania 36.5%, Americas 36.1% and Europe 24.1% of the World production. Milk powder production is a process that requires high energy, especially for evaporation. Recently, reducing energy use has been gaining importance by increasing energy and exergy efficiency. Conventional energy analysis is performed based on the First Law of Thermodynamics. Unlike from the First Law, the Second Law or exergy analysis (defined as useful work) has appeared in the literature, while this analysis not only assesses quantity but also quality of energy. In this study, exergy analysis of a milk powder production system, mainly includes 3 processes (pasteurization, evaporation and spray drying) which will be presented. The aim of the study is to apply a thermodynamic analysis including comprehensive exergy analysis by using different performance parameters such as exergy efficiency, improvement potential rate, sustainability index, relative irreversibility and exergetic factor for the milk powder production system. As a result, exergetic efficiencies of the system components were found in the range of 9–83%. The overall energy and exergy efficiencies of the whole milk powder production system were calculated as 85.4 and 57.45%, respectively. Additionally, it was found that the evaporator and the heater have a higher impact in improvement actions.
[en] Highlight: ► We model wind power density for four locations in Palestine. Second order polynomial better fits the relation between wind power and wind speed. The adjusted R2 of the polynomial fit is 99.8% for all stations except Hebron 70%. The highest mean power value is 37.85 W m−2 for Nablus. In general wind power density is higher during summer and lowers during winter. - Abstract: The daily mean wind speed data for 4 locations in Palestine over a period of 5 years are collected, analyzed, and fitted to the Weibull distribution function. Weibull parameters are derived from the cumulative function of the observed data records (1997–2001), and used to calculate the mean wind speed and variance of the theoretical distribution. A quadratic polynomial is employed to fit the relationship between the wind power and the mean wind speed. The monthly mean wind power density found to be higher during summer and lowers during winter in most locations. However, in Hebron district the winter months have higher values than those for summer. The highest mean power values were 32.98 W m−2 in January and 37.85 W m−2 in July for Hebron and Nablus, respectively. The lowest mean power was 1.66 W m−2 in January for Jericho. The adjusted R2 of the polynomial fit was about 99% for all stations except for Hebron was about 70%.
[en] In a lot of sites, at best, only global irradiations on horizontal planes are available. As most systems using solar energy are tilted, it is obvious that these data are not sufficient. The purpose of our work consists in predicting the solar irradiation on inclined surfaces based on horizontal measurements. To reach this goal, two previous studies were performed. They consisted in quantifying the accuracy of various well-known models of the literature: the first type of models (seven correlations) calculated diffuse horizontal irradiations from global ones and the second type (fifteen models) computed global irradiations on inclined planes from diffuse and global components on a horizontal surface. This study combines the two classes of models and calculates the adequacy of each association for experimental hourly data collected on the French Mediterranean site of Ajaccio. We compared these combinations with the Olmo model, transforming global irradiations directly onto tilted planes from horizontal ones
[en] Highlights: • An open-source optimization tool for solar home systems (SHSs) design is developed. • The optimization tool is written in MS Excel-VBA. • The optimization tool is validated with a commercial and open-source software. • The optimization tool has the potential of improving future SHS installations. - Abstract: Solar home systems (SHSs) represent a viable technical solution for providing electricity to households and improving standard of living conditions in areas not reached by the national grid or local grids. For this reason, several rural electrification programmes in developing countries, including Namibia, have been relying on SHSs to electrify rural off-grid communities. However, the limited technical know-how of service providers, often resulting in over- or under-sized SHSs, is an issue that has to be solved to avoid dissatisfaction of SHSs’ users. The solution presented here is to develop an open-source software that service providers can use to optimally design SHSs components based on the specific electricity requirements of the end-user. The aim of this study is to develop and validate an optimization model written in MS Excel-VBA which calculates the optimal SHSs components capacities guaranteeing the minimum costs and the maximum system reliability. The results obtained with the developed tool showed good agreement with a commercial software and a computational code used in research activities. When applying the developed optimization tool to existing systems, the results identified that several components were incorrectly sized. The tool has thus the potentials of improving future SHSs installations, contributing to increasing satisfaction of end-users.
[en] Sustainable energy generation is becoming increasingly important due to the expected limitations in current energy resources and to reduce pollution. Wave energy generation has seen significant development in recent years. This paper describes an innovative system for generating energy from wave power. A complete description of the system is presented including the general concept, configurations, mechanical design, electrical system, simulation techniques and expected power output of the system. The results from the hydraulic linear wave simulator, using a real wave profiles captured at a location in the UK using an ultrasound system, it was seen that a ±0.8 m wave at 10 s time period, produced a conditioned power output of approximately 22 kW at optimum load conditions for the tested 3-phase 44 kW permanent magnet generator type STK500. The results indicate that this new technology could provide an efficient and low cost method of generating electricity from waves.
[en] As the objective of this study, artificial neural network (ANN) and Markov chain (MC) are used to develop a new ANN-MC model for forecasting wind speed in very short-term time scale. For prediction of very short-term wind speed in a few seconds in the future, data patterns for short-term (about an hour) and very short-term (about minutes or seconds) recorded prior to current time are considered. In this study, the short-term patterns in wind speed data are captured by ANN and the long-term patterns are considered utilizing MC approach and four neighborhood indices. The results are validated and the effectiveness of the new ANN-MC model is demonstrated. It is found that the prediction errors can be decreased, while the uncertainty of the predictions and calculation time are reduced.
[en] In this paper, a direct torque control (DTC) strategy for the brushless doubly fed machine (BDFM) is presented. After analyzing the mathematical model of this machine, the voltage vectors look-up table of classical DTC techniques is derived. Then, the behavior of the machine is studied when it is controlled by the developed DTC technique, concluding that under some specific operation conditions, a BDFM could present a time interval where the torque and the flux can not be controlled simultaneously. In these cases, two different control solutions have been defined: 'flux priority' and 'torque priority'. Finally, simulation and experimental results validate the effectiveness of the proposed control algorithms.