Modelagem analítica e numérica semiempírica de células fotovoltaicas

Abstract

The purpose of this dissertation is to present and evaluate an effective system for the characterization and electrical modeling of silicon solar cells, by means of current-voltage characteristic curves (IxV) extracted with a solar simulator and natural sunlight, and further analysis of obtained parameters responsible for qualifying such devices. It is intended to use these parameters to feed numerical (Spice®) and analytical models, capable of faithfully representing a cell, enabling its performance evaluation under different temperatures and irradiances. Accurate models are important in order to predict the behavior of solar cell arrangements, and systems composed not only by panels, but also by inverters, maximum-power-point tracking circuits, charge controllers and battery banks. The characterization demands information regarding the spectrum, light source intensity and homogeneity; cell material; electrodes and cell geometry; cell temperature; equipment employed for the IxV curve measurement; electrical connections and time consumed for data acquisition. In addition, the full characterization should be performed for forward bias higher than the open-circuit voltage, allowing the extraction of a more accurate parasitic series resistance. Conventional IxV curves, for photovoltaic cells, range from the short-circuitcondition, with maximum current, to the open-circuit condition, with maximum voltage; presenting a certain fill factor, and a maximum power point. The curve shape gives information regarding dominance regions of drift and diffusion mechanisms for photogenerated charge carriers; irradiance magnitude; temperature; the finite conduction of the depletion region; quasi-neutral and contact regions parasitic series resistance; and presence of defects. The extraction of parameters is performedthrough different methods. However, the most reliable are the interactive methods, where the Shockley equation, either with one or two exponentials, is used to fit experimental data. Parameters, like the series resistance, can be incorporated to this equation considering the cell internal structure in the surroundings of the PN junction, and the charge-carrier mobilities. Comparisons of measurements under differentsolarimetric and temperature conditions are performed after the conversion to Standard Test Condition (STC). This work presents electrical tests performed with both mono- and polycrystalline silicon cells of different geometries, under distinct measuring conditionsviii and with different equipment. The experimental curves are compared to their simulated counterparts with the extracted parameters. These procedures allowed to identify several hindering factors and necessary caution, in order to obtain a reliable model, that is scalable regarding temperature, irradiance and cell geometry.