J.L. Torres

Selecting a suitable model for characterizing photovoltaic devices

The aim of this work is to evaluate a simple analytical method for extracting parameters involved in the photovoltaic module behaviour equation. Based on a series of experimental voltage–intensity curves obtained under various temperature and irradiance conditions, values are obtained to extract the model parameters, giving rise to adjustment errors at data points (short circuit current, open circuit voltage and voltage at maximum power point) and in the entire curve that are less than 1%. It has also been confirmed that assigning suitable values for series and parallel resistance avoids having to know beforehand the slope value of the characteristic curve, which is not normally indicated in the solar module specifications; this gives rise to good adjustment results between the experimental curves and the theoretical model, even when the theoretical parameters are adapted to other temperature and irradiance conditions.

Fitting wind speed distributions : A case study

Abstract The wind speed data represented in the form of frequency curves show the shape of a potential model. The Weibull and Lognormal models are used for this purpose, with hourly mean wind speed data. This study deals with the estimation of the annual Weibull and Lognormal parameters from 20 locations in Navarre. The suitability of both distributions is judged from the R 2 coefficient with a linear regression for the Weibull distribution and a nonlinear regression for the Lognormal distribution. Both approaches give a good fit, giving better results for the Weibull distribution. A comparison between the estimation and the production for a wind farm is offered.

Development and testing of a prototype of low-power water–ammonia absorption equipment for solar energy applications

This article outlines the development and testing of a prototype of a water–ammonia absorption system designed for solar-powered refrigeration in small rural operations. The objective is to design a 2 kW refrigeration equipment for isolated areas with a high level of solar radiation to meet refrigeration requirements. The equipment has been designed to operate with a concentrating solar power system to obtain the required temperatures. The heat exchangers, which act as condensers, as well as the absorber and evaporator, are all made of galvanized steel piping having fins. The design uses natural convection; the generator and energy-saving heat exchanger have a multi-tubular arrangement and a transfer tank is used in place of a pump for displacing the ammonia solution. All of the operations are manually controlled. Overall, the test results showed unsatisfactory operation of the equipment having low efficiency. Nevertheless, the proposed objectives have been met and it is evident that with several important modifications the equipment will operate satisfactorily. In conclusion, based on the development phase, it is apparent that future prototypes must be more compact and more efficient.

Dielectric properties of grape juice at 0.2 and 3 GHz

Abstract The usual characterization of musts and wines is based on chemical composition and sensory considerations. We aim to describe the grape juice using mainly dielectric parameters, as well as examining the data using statistical methods in order to find out a possible connection among these values and the variety or the area where the samples were collected from. Red wine grape juice samples were used for measuring dielectric constant and loss factor at 0.2 and 3 GHz together with the electrical conductivity. The main varieties chosen were merlot, tempranillo and cabernet sauvignon, because these are the most important types of grape used in Navarra for barrel aging wines. We observed the existence of differences among the three varieties in relation to the values of total and dipolar losses at 3 GHz, but these preliminary results are not decisive enough. More measurements are considered as necessary.

Effects of the model selected for the power curve on the site effectiveness and the capacity factor of a pitch regulated wind turbine

The present work is based on the fact that site effectiveness is a relevant value when analysing the suitability of a location for constant-speed wind turbines. An analytical expression is developed to calculate the said effectiveness by means of a potential adjustment model to the ascending segment of the power curve in pitch regulated wind turbines. Upon analysis of the results obtained for maximum effectiveness in a given location, using both the potential model and the quadratic model applied in earlier studies, no significant differences were observed in wind conditions of interest for wind energy applications. Notwithstanding, when the cut-in wind speed was compared, corresponding to the maximum effectiveness values at the same location according to the two models indicated, noticeable differences were observed. It can therefore be deduced that, when selecting this design parameter, a careful study should be made in order to determine which model best fits the power curve of the wind turbine under analysis for a specific location.

Characterization of wind speed data according to wind direction

Knowledge of the wind speed distribution and the most frequent wind directions is important when choosing wind turbines and when locating them. For this reason wind evaluation and characterization are important when forecasting output power. The data used here were collected from eleven meteorological stations distributed in Navarre, Spain. We obtained data for the period extending from 1992 to 1995, with each datum encompassing 10 minutes of time. Wind speed data of each station were gathered in eight directional sectors, each one extended over 45 degrees according to the direction from which the wind blows. The stations were grouped in two blocks: those under the influence of the Ebro valley and those in mountainous areas. For each group the Weibull parameters were estimated, (according to the Weibull probability paper because the Weibull distribution gives the best fit in this region). Kurtosis and skewness coefficients were estimated as well. The Weibull parameters, especially the scale parameter c, depend strongly on the direction considered, and both Weibull parameters show an increasing trend as the direction considered moves to the more dominant direction, while both kurtosis and skewness show a corresponding decreasing trend.