Van der Waals contribution to the inelastic atom-surface scattering

A calculation of the inelastic scattering rate of Xe atoms on Cu(111) is presented. We focus in the regimes of low and intermediate velocities, where the energy loss is mainly associated to the excitation electron-hole pairs in the substrate. We consider trajectories parallel to the surface and restrict ourselves to the Van der Waals contribution. The decay rate is calculated within a self-energy formulation. The effect of the response function of the substrate is studied by comparing the results obtained with two different approaches: the Specular Reflection Model and the Random Phase Approximation. In the latter, the surface is described by a finite slab and the wave functions are obtained from a one-dimensional model potential that describes the main features of the surface electronic structure while correctly retains the image-like asymptotic behaviour. We have also studied the influence of the surface state on the calculation, finding that it represents around 50% of the total probability of electron-hole pairs excitation.Comment: 7 pages, 4 figure

Modelo óptico para acristalamientos multicapa

En el caso del vidrio laminado, dadas las múltiples configuraciones que pueden concebirse con propósitos arquitectónicos varios en la fase de diseño (estéticos, de eficiencia energética, seguridad o aislamiento acústico, principalmente), resulta de gran utilidad poder disponer de simulaciones computacionales de las propiedades ópticas en función de las propias de sus constituyentes (sustratos vítreos, láminas poliméricas y capas selectivas a la radiación solar). Los procedimientos para la determinación por cálculo de las magnitudes de caracterización óptica de este tipo de vidrios se encuentran actualmente en proceso de normalización por el CEN (Comité Europeo de Normalización). El estudio de vidrios laminados con capa selectiva interna, en contacto con la lámina polimérica, tiene un valor teórico especialmente significativo, al introducir reflectividad en el interior del sistema óptico multicapa. En este trabajo se presenta un modelo teórico basado en el método de matrices de transferencia particularizado para el cálculo de las propiedades ópticas del vidrio laminado con capas externas e internas. Los resultados de la simulación se comparan con datos experimentales obtenidos mediante espectrofotometría UV-VIS-NIR

Antireflective mesoporous silica coatings by optimization of water content in acid-catalyzed sol-gel method for application in glass covers of concentrated photovoltaic modules

Porous silica layers with outstanding antireflective properties have been prepared by acid-catalyzed sol-gel process in presence of organic phases as structure directing agents (SDA) and excess water, with the aim of offering a cost-competitive, easy up-scaling and high efficiency process that contributes to reduce current levelized cost of energy (LCOE) of concentrating photovoltaics (CPV). The process has been optimized by controlling the water/alkoxide ratio, which is an important structure-regulating tool, having a strong influence in the structural properties of sol-gel synthesized materials. Hydrolysis of the inorganic precursor has been accomplished in high water/alkoxide conditions and in the presence of SDAs. Evaporation induced self-assembly (EISA) during coating deposition and the scanning of four types of SDAs have permitted to select the coating that fulfilled specific thickness and refractive index values with, in parallel, excellent results on sol stability. The final optimization has produced mesoporous coatings with ∼9 nm mean pore size, leading to an enhancement in transmittance up to 7.4% over bare glass in the 300–1500 nm wavelength range. The transmittance spectra have been used as inputs for the theoretical calculation of the short-circuit current density of a commercially available multijunction solar cell for CPV applications.This work was supported by the Basque Government for EMAI-TEK 2017 program as well as theELKARTEK projects FRONTIERS-2(contract number KK2016-00093) and FRONTIERS-3 (contractnumber KK2017-00096). The authors thank ICV-CSIC, Yolanda Cas-tro and Alicia Durán for ellipsometry and EEP measurements. Theauthors thank Miguel Pérez-Aradros for the help with graphicalabstract illustration

Polyfluoroalkyl-silica porous coatings with high antireflection properties and low surface free energy for glass in solar energy application

Polyfluoroalkyl-silica porous coating stacks with durable antireflection (AR) properties have been obtained for photovoltaic (PV) application. An acid-catalyzed sol-gel process combined with evaporation induced self-assembly and the presence of a non-hydrolyzable polyfluoroalkyl group linked to the central atom of the silicon alkoxide was conducted. The aim was to obtain a low surface energy coating, devised to mitigate soiling adherence, without losing the AR properties of a baseline coating. In particular, the influence of polyfluoroalkyl chain length on the thickness, the water contact angle and optical transmission properties was first analyzed. The optimized polyfluoroalkyl-silica porous coating presented low surface energy < 20 mJ/m2, even with the desired low roughness values required for obtaining a negligible scattering of the incoming solar radiation. This coating was studied as an AR mono-layer and as an external coating in an AR bi-layer stack, with the presence of an inner dense-structured silica layer, that contributed to both the optical performance and durability, acting as an alkali diffusion preventing layer. The AR bi-layer stack deposited on two sides of glass provided a transmittance gain of 7.1%. Those optical properties were inalterable after accelerated aging tests, which sustains the reliability of the materials for solar energy applications.This work was supported by the Basque Government through EMAITEK 2017 program as well as the ELKARTEK projects FRONTIERS-2 (contract number KK2016-00093 ) and FRONTIERS-3 (contract number KK2017-00096 ). The authors thank ICV-CSIC, Yolanda Castro and Alicia Durán for their support with ellipsometry and EEP measurements. The authors thank Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) for their collaboration with the aging tests. The authors thank Miguel Pérez-Aradros for the help with graphical abstract illustration

PVSITES: Building-Integrated Photovoltaic Technologies and Systems for Large-Scale Market Deployment

The large potential for energy savings in buildings led the EU Commission to adopt the 2010/31/EU Directive on the energy performance of buildings with the objective that all new buildings are Nearly Zero Energy Buildings (NZEB) by 2020. Renewable energy technologies, and in particular the integration of photovoltaic systems in the building environment offer many possibilities to play a key role within the NZEB scenario. The objective of PVSITES project is to drive BIPV technology to a large market deployment by demonstrating an ambitious portfolio of building-integrated solar technologies and systems, giving a forceful, reliable answer to the market requirements identified by the industrial members of the consortium in their day-to-day activity.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 691768

Electrons in Dry DNA from Density Functional Calculations

The electronic structure of an infinite poly-guanine - poly-cytosine DNA molecule in its dry A-helix structure is studied by means of density-functional calculations. An extensive study of 30 nucleic base pairs is performed to validate the method. The electronic energy bands of DNA close to the Fermi level are then analyzed in order to clarify the electron transport properties in this particularly simple DNA realization, probably the best suited candidate for conduction. The energy scale found for the relevant band widths, as compared with the energy fluctuations of vibrational or genetic-sequence origin, makes highly implausible the coherent transport of electrons in this system. The possibility of diffusive transport with sub-nanometer mean free paths is, however, still open. Information for model Hamiltonians for conduction is provided.Comment: 8 pages, 4 figure

Thermal analysis of a BIPV system by various modelling approaches

This work presents various models developed and implemented within the SOPHIA European project in order to thermally characterize PV modules in a rooftop BIPV configuration. Different approaches have been considered, including a linear model, lumped elements models and models that make use of commercial software solvers. The validation of the models performed by comparing the results of simulations with experimental data recorded on a test bench over an entire year is presented and discussed on a seasonal basis. The results have shown that all the models implemented allow achieving a good prediction of the PV modules back surface temperature, with the minimum value of the coefficient of determination R2 around 95% on a yearly basis. Moreover, the influence of season weather conditions and of the incident solar irradiance magnitude on the accuracy of the considered thermal models is highlighted. The major result of the present study is represented by the fact that it has been possible to perform a better thermal characterization of the BIPV module by tuning some of the heat transfer coefficients, such as those relative to the effects of the wind velocity, and to the evaluation of sky temperature.The experimental data used for the thermal simulation of BIPV system behavior were obtained in the framework of the project Performance BIPV supported by the French research agency (ANR), within the research program ANR HABISOL. Authors would like to thank the European Community that supported the SOPHIA project with the funding of FP7-SOPHIA grant agreement no. 262533

Accelerated aging test of solar reflectors according to the new AENOR standard – results of a round Robin test

Durability tests of reflector materials for concentrating solar applications are crucial to guarantee the profitability of the plants and to ensure a proper efficiency during their lifetime. A standard including a set of five accelerated aging tests is close to be published by the Spanish standardization entity AENOR, within the sub-committee AEN/CTN 206/SC117. Under the framework of the STAGE-STE project, a Round Robin Test was organized by six partners to evaluate the comparability of results obtained in their respective laboratories after performing these durability tests. According to the results, in general a good agreement among the partners was found, with negligible to slight reflectance losses. In addition, it was noticed that the reference standard used in the reflectance measurements is of high importance. The conclusions of this work will help to improve the standard in future versions.The research leading to this work has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 609837 (STAGE-STE). The authors would like to thank the participants of the working group for solar reflectors of WG 2 of the Spanish sub-committee AEN/CTN 206/SC 117

Achievements of EU funded project BFIRST on BIPV technology

EU funded “Building-integrated fibre reinforced solar technology” (BFIRST) project (Grant Agreement number 296016) was launched in 2012 by a consortium of EU companies, research institutes and universities, led by Tecnalia. The project, which will end in early 2017, is focused on the design, development, fabrication and demonstration of a set of standardized multifunctional photovoltaic products for building integration using an innovative manufacturing solution based on glass fibre-reinforced composite materials. This novel encapsulation technology is the basis for a wide range of new BIPV (building-integrated photovoltaic) products with enhanced building integration possibilities. The resulting modules present advanced characteristics in terms of flexibility of design, adaptability to non-planar geometries, structural properties and lightweight, among others. They provide additional advantages related to cost reduction in transport, manipulation, assembly and installatio