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Lucero M.J.,Rice University | Aguilera I.,Institute Energia Solar | Diaconu C.V.,Rice University | Palacios P.,Institute Energia Solar | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The cadmium and magnesium indium sulfides are medium-gap semiconductors demonstrating a propensity to form intermediate band materials when doped with transition metals. The inherent structural diversity exhibited by M +2In2S4 thiospinels and related AB 2X4 compounds often precludes definitive experimental determination of the band-gap width and type of transition. Employing a series of traditional semilocal functionals (e.g., the local spin density approximation; the Perdew, Burke, and Enzerhof functional; and the Tao, Perdew, Staroverov, and Scuseria functional) the screened hybrid of Heyd, Scuseria, and Ernzerhof (HSE), band gaps, projected densities of states, and band structures are calculated for the normal, full inverse, and intermediate configurations of [Cd/Mg]8In16S32. Band structures and band gaps are also obtained via self-consistent many-body methods, using the static Coulomb-hole and screened exchange approximation to GW as a starting point for perturbative G0W0 calculations. Comparison to experiment indicates that HSE provides an accurate, computationally efficient, and relatively rapid means for predicting band-gap properties in spinel-type photovoltaic materials. © 2011 American Physical Society.

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.

Antonanzas-Torres F.,University of La Rioja | Sanz-Garcia A.,University of La Rioja | Martinez-de-Pison F.J.,University of La Rioja | Perpinan-Lamigueiro O.,Technical University of Madrid | Perpinan-Lamigueiro O.,Institute Energia Solar
Renewable Energy | Year: 2013

This paper presents a new methodology to build parametric models to estimate global solar irradiation adjusted to specific on-site characteristics based on the evaluation of variable importance. Thus, those variables highly correlated to solar irradiation on a site are implemented in the model and therefore, different models might be proposed under different climates. This methodology is applied in a study case in La Rioja region (northern Spain). A new model is proposed and evaluated on stability and accuracy against a review of twenty-two already existing parametric models based on temperatures and rainfall in seventeen meteorological stations in La Rioja. The methodology of model evaluation is based on bootstrapping, which leads to achieve a high level of confidence in model calibration and validation from short time series (in this case five years, from 2007 to 2011).The model proposed improves the estimates of the other twenty-two models with average mean absolute error (MAE) of 2.195MJ/m2day and average confidence interval width (95% C.I., n=100) of 0.261MJ/m2day. 41.65% of the daily residuals in the case of SIAR and 20.12% in that of SOS Rioja fall within the uncertainty tolerance of the pyranometers of the two networks (10% and 5%, respectively). Relative differences between measured and estimated irradiation on an annual cumulative basis are below 4.82%. Thus, the proposed model might be useful to estimate annual sums of global solar irradiation, reaching insignificant differences between measurements from pyranometers. © 2013 Elsevier Ltd.

Antonanzas-Torres F.,University of La Rioja | Canizares F.,SOLUTE Ingenieros | Perpinan O.,Technical University of Madrid | Perpinan O.,Institute Energia Solar
Renewable and Sustainable Energy Reviews | Year: 2013

An analysis and comparison of daily and yearly solar irradiation from the satellite CM SAF database and a set of 301 stations from the Spanish SIAR network is performed using data of 2010 and 2011. This analysis is completed with the comparison of the estimations of effective irradiation incident on three different tilted planes (fixed, two axis tracking, and north-south horizontal axis) using irradiation from these two data sources. Finally, a new map of yearly values of irradiation both on the horizontal plane and on inclined planes is produced mixing both sources with geostatistical techniques (kriging with external drift, KED). The Mean Absolute Difference (MAD) between CM SAF and SIAR is approximately 4% for the irradiation on the horizontal plane and is comprised between 5% and 6% for the irradiation incident on the inclined planes. The MAD between KED and SIAR, and KED and CM SAF is approximately 3% for the irradiation on the horizontal plane and is comprised between 3% and 4% for the irradiation incident on the inclined planes. The methods have been implemented using free software, available as supplementary material, and the data sources are freely available without restrictions. © 2013 Elsevier Ltd.

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.

Palacios P.,Institute Energia Solar | Aguilera I.,Institute Energia Solar | Wahnon P.,Institute Energia Solar
Thin Solid Films | Year: 2010

ZnO doped with some transitions metals (ZnO:M) has several significant potential application. ZnO:Co is proposed to be used in advanced spintronic devices due to its high Curie temperature and large magnetic moments per transition metal. ZnO:Cd has potential applications in short-wavelength optoelectronic devices. This work focuses on an ab-initio study of the electronic and optical properties of ZnO:M doped with Co, and Cd. Theoretical calculations have been done with different computational codes, using the density functional theory (DFT) at the GGA and GGA+U level. The latter introduces a Hubbard term correction in the "d" levels of the Zn and Co. We used different supercells in order to have different realistic dilution levels which can be achieved in experiments. Doping effects on the features of the optical absorption are also studied and analysed in this work. © 2009 Elsevier B.V. All rights reserved.

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.

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.

Aguilera I.,Institute Energia Solar | Palacios P.,Institute Energia Solar | Sanchez K.,Institute Energia Solar | Wahnon P.,Institute Energia Solar
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We analyzed and compared two families of intermediate-band materials derived from indium thiospinels: MgIn2 S4, having an inverse spinel structure, and CdIn2 S4, a direct spinel. First-principles studies of the electronic structures of these two parent semiconductors were carried out to understand the nature of their band gaps. Optical properties were also analyzed and we found good agreement with experiments. As derivatives of these semiconductors, alloys where transition metals (M=Ti and V) substitute for In atoms at octahedral sites are presented as a class of intermediate-band materials. First, the effect of the substitution on structural parameters is assessed. Then, electronic structures are determined for Mg2 M In3 S8 and Cd2 M In3 S8 to show that the t2g d states of the transition metal form a partially filled localized band within the band gap of the host semiconductor. The suitability of these compounds as photovoltaic high-efficiency absorbers is discussed. An increase in absorption is assessed by studying the contribution of the transition-metal band toward their optical properties, in the range of higher solar emission, and comparing them with those of the host semiconductors. An analysis of transmittance spectra is carried out to predict the range of optimum thicknesses for samples of this type of thin film absorber. We compare, by means of structural, electronic, and optical behavior, Ti and V as substituents, to evaluate the resulting alloys for potential photovoltaic applications. © 2010 The American Physical Society.

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.

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