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Marcos J.,Public University of Navarra | Marroyo L.,Public University of Navarra | Lorenzo E.,Institute Energia Solar | Garcia M.,Public University of Navarra
Progress in Photovoltaics: Research and Applications | Year: 2012

The quality and the reliability of the power generated by large grid-connected photovoltaic (PV) plants are negatively affected by the source characteristic variability. This paper deals with the smoothing of power fluctuations because of geographical dispersion of PV systems. The fluctuation frequency and the maximum fluctuation registered at a PV plant ensemble are analyzed to study these effects. We propose an empirical expression to compare the fluctuation attenuation because of both the size and the number of PV plants grouped. The convolution of single PV plants frequency distribution functions has turned out to be a successful tool to statistically describe the behavior of an ensemble of PV plants and determine their maximum output fluctuation. Our work is based on experimental 1-s data collected throughout 2009 from seven PV plants, 20MWp in total, separated between 6 and 360km. Copyright © 2011 John Wiley & Sons, Ltd. Source


Tablero C.,Institute Energia Solar
Solar Energy Materials and Solar Cells | Year: 2014

The substitution of cation atoms by V, Cr and Ir in the natural and synthetic quaternary Cu2ZnSnS4 semiconductor is analyzed using first-principles methods. In most of the substitutions, the electronic structure of these modified CZTS is characterized for intermediate bands with different occupation and position within of the energy band gap. A study of the symmetry and composition of these intermediate bands is carried out for all substitutions. These bands permit additional photon absorption and emission channels depending on their occupation. The optical properties are obtained and analyzed. The absorption coefficients are split into contributions from the different absorption channels and from the inter- and intra-atomic components. The sub bandgap transitions are significant in many cases because the anion states contribute to the valence, conduction and intermediates bands. These properties could therefore be used for novel optoelectronic devices. © 2014 Elsevier B.V. All rights reserved. Source


Tablero C.,Institute Energia Solar
Journal of Alloys and Compounds | Year: 2014

The substitution of Cu, Sn or Zn in the quaternary Cu2ZnSnS 4 semiconductor by impurities that introduce intermediate states in the energy bandgap could have important implications either for photovoltaic or spintronic applications. This allows more generation-recombination channels than for the host semiconductor. We explore and discuss this possibility by obtaining the ionization energies from total energy first-principles calculations. The three substitutions of Cu, Sn and Zn by impurities are analyzed. From these results we have found that several impurities have an amphoteric behavior with the donor and acceptor energies in the energy bandgap. In order to analyze the role of the ionization energies in both the radiative and non-radiative processes, the host energy bandgap and the acceptor and the donor energies have been obtained as a function of the inward and outward impurity-S displacements. We carried out the analysis for both the natural and synthetic CZTS. The results show that the ionization energies are similar, whereas the energy band gaps are different. © 2013 Elsevier B.V. All rights reserved. Source


Perpinan O.,Institute Energia Solar | Perpinan O.,Technical University of Madrid | Marcos J.,Public University of Navarra | Lorenzo E.,Institute Energia Solar
Solar Energy | Year: 2013

This paper analyzes the correlation between the fluctuations of the electrical power generated by the ensemble of 70. DC/AC inverters from a 45.6. MW PV plant. The use of real electrical power time series from a large collection of photovoltaic inverters of a same plant is an important contribution in the context of models built upon simplified assumptions to overcome the absence of such data. This data set is divided into three different fluctuation categories with a clustering procedure which performs correctly with the clearness index and the wavelet variances. Afterwards, the time dependent correlation between the electrical power time series of the inverters is estimated with the wavelet transform. The wavelet correlation depends on the distance between the inverters, the wavelet time scales and the daily fluctuation level. Correlation values for time scales below one minute are low without dependence on the daily fluctuation level. For time scales above 20. min, positive high correlation values are obtained, and the decay rate with the distance depends on the daily fluctuation level. At intermediate time scales the correlation depends strongly on the daily fluctuation level. The proposed methods have been implemented using free software. Source code is available as supplementary material. © 2012 Elsevier Ltd. Source


Almeida M.P.,University of Sao Paulo | Perpinan O.,Technical University of Madrid | Perpinan O.,Institute Energia Solar | Narvarte L.,Institute Energia Solar
Solar Energy | Year: 2015

Forecasting the AC power output of a PV plant accurately is important both for plant owners and electric system operators. Two main categories of PV modeling are available: the parametric and the nonparametric. In this paper, a methodology using a nonparametric PV model is proposed, using as inputs several forecasts of meteorological variables from a Numerical Weather Forecast model, and actual AC power measurements of PV plants. The methodology was built upon the R environment and uses Quantile Regression Forests as machine learning tool to forecast AC power with a confidence interval. Real data from five PV plants was used to validate the methodology, and results show that daily production is predicted with an absolute cvMBE lower than 1.3%. © 2015 Elsevier Ltd. Source

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