André Kostro

Embedded microstructures for daylighting and seasonal thermal control

A novel concept for an advanced fenestration system was studied and samples were produced to demonstrate the feasibility. The resulting novel glazing will combine the functions of daylighting, glare protection, and seasonal thermal control. Coated microstructures provide redirection of the incident solar radiation, thus simultaneously reducing glare and projecting daylight deep into the room in the same manner as an anidolic mirror-based system.The solar gains are reduced for chosen angles corresponding to a estival elevations of the sun, thereby minimising heating loads in winter and cooling loads in summer. A ray-tracing program developed especially for the study of laminar structures was used for the optimisation of structures with the above mentioned goals. The chosen solution is based on reflective surfaces embedded in a polymer film that can be combined with a standard doubled glazed window. The fabrication of such structures required several steps. The fabrication of a metallic mould with a relative high aspect ratio and mirror polished surfaces is followed by the production of an intermediate Polydimethylsiloxane moulds that was subsequently used to replicate the structure with a UV curable polymer. Selected facets of these samples were then coated with a thin film of highly reflective material in a physical vapour deposition process. Finally, the structures were filled with the same polymer to integrated the mirrors. The samples were characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), confocal microscopy and laser profilometry. A miniature goniophotometer was built to assess the performance of the structured glazing. The daylighting behaviour was successfully demonstrated.

Total light transmittance of glass fiber-reinforced polymer laminates for multifunctional load-bearing structures

The total light transmittance of hand lay-up glass fiber-reinforced polymer laminates for building construction was investigated with a view to two architectural applications: translucent load-bearing structures and the encapsulation of photovoltaic cells into glass fiber-reinforced polymer building skins of sandwich structures. Spectrophotometric experiments on unidirectional and cross-ply glass fiber-reinforced polymer specimens in the range from 0.20 to 0.45 fiber volume fraction and artificial sunlight exposure experiments on encapsulated amorphous silicon photovoltaic cells were performed. Analytical models have been developed to predict light transmittance through glass fiber-reinforced polymer structures and the percentage of solar radiation reaching encapsulated photovoltaic cells. The total amount of fibers in the laminates was the major parameter influencing light transmittance, with fiber architecture having little effect and regardless of fiber volume fraction. Eight-three percent of solar irradiance in the band of 300–800 nm reached the surface of amorphous silicon photovoltaic cells encapsulated below structural glass fiber-reinforced polymer laminates with a fiber reinforcement weight of 820 g/m2, demonstrating the feasibility of conceiving multifunctional glass fiber-reinforced polymer structures.

Monte Carlo simulations of quantum dot solar concentrators: ray tracing based on fluorescence mapping

One promising application of semiconductor nanostructures in the field of photovoltaics might be quantum dot solar concentrators. Quantum dot containing nanocomposite thin films are synthesized at EPFL-LESO by a low cost sol-gel process. In order to study the potential of the novel planar photoluminescent concentrators, reliable computer simulations are needed. A computer code for ray tracing simulations of quantum dot solar concentrators has been developed at EPFL-LESO on the basis of Monte Carlo methods that are applied to polarization-dependent reflection/transmission at interfaces, photon absorption by the semiconductor nanocrystals and photoluminescent reemission. The software allows importing measured or theoretical absorption/reemission spectra describing the photoluminescent properties of the quantum dots. Hereby the properties of photoluminescent reemission are described by a set of emission spectra depending on the energy of the incoming photon, allowing to simulate the photoluminescent emission using the inverse function method. By our simulations, the importance of two main factors is revealed, an emission spectrum matched to the spectral efficiency curve of the photovoltaic cell, and a large Stokes shift, which is advantageous for the lateral energy transport. No significant energy losses are implied when the quantum dots are contained within a nanocomposite coating instead of being dispersed in the entire volume of the pane. Together with the knowledge on the optoelectronical properties of suitable photovoltaic cells, the simulations allow to predict the total efficiency of the envisaged concentrating PV systems, and to optimize photoluminescent emission frequencies, optical densities, and pane dimensions.

CFSpro: ray tracing for design and optimization of complex fenestration systems using mixed dimensionality approach

Advanced optical ray tracing software, CFSpro, was developed for the study and optimization of complex fenestration systems (CFSs). Using an algorithm mixing 2D and 3D approaches, accurate computation of large numbers of rays in extruded geometries can be performed and visualized in real time. A thin film model was included to assess the spectral control provided by coatings. In this paper, the ray tracing model is described and validated. A novel glazing, engineered with this simulation tool, is presented. It combines the functions of daylight provision, glare protection, and seasonal thermal control while conserving a view to the outside at near normal incidence.

Microfabrication of curved sidewall grooves using scanning nanosecond excimer laser ablation

Novel glazing with embedded micro-mirrors can significantly reduce the energy consumption due to cooling and lighting in buildings. Especially promising are large arrays of periodic micro compound-parabolic-concentrators (CPCs) with angular-selected transmittance. For the production of micro CPCs, curved sidewall grooves with a controlled optical surface and an aspect ratio of about 2.3 are fabricated on polycarbonate substrates by scanning nanosecond 248-nm excimer laser ablation. The likewise obtained microstructures can be used as master mold for replication. The cross-sections of the micro grooves are characterized by confocal microscopy, and the extracted morphologies are used for the ray-tracing simulation of the optical devices. Prior to the scanning ablation using a suitable mask in the optical path, the depth profiles under static ablation are investigated to identify ablation rate, imaging resolution and produced surface. Interestingly for the width of the mask opening being less than 6 μm, the ablation rate is increased due to optical interference and /or less shielding by debris. Concerning the scanning ablation, the depth of the curved sidewall grooves ranges from 48 μm to 114 μm, corresponding to the width of the groove opening being in the range from 20 μm to 50 μm. The observed final shapes in cross-sections are in good agreement with the design of the mask. For both theoretical and fabricated groove shapes, the angular-selected transmittance profiles predicted from ray-tracing simulations are highly similar. Scanning nanosecond excimer laser ablation is therefore a promising approach for the realization of high-quality micro CPCs.

Feasibility study on a novel daylighting system with embedded micro compound parabolic concentrators (CPCs)

A novel glazing system consisting of a polymer layer with embedded micro compound parabolic concentrators (CPCs), which is attached to a glass pane of glazing, is proposed. It aims to reduce the energy consumption due to cooling in buildings, provide daylighting, and maintain the transparent view. In the present work, the daylighting system is modelled for ray-tracing simulation, and the angular-dependent transmittance at the azimuth angle of 0° is calculated. Structural characterization is conducted using optical microscope for the microstructures which serve as support for the reflective thin films of a micro CPC. Based on self-shading effect of a microstructure, facet-selective deposition of Aluminum with various thickness has been achieved by physical vapor deposition. Spectral measurement has been used to characterize the optical properties of the Aluminum thin films. Diffraction effect with respect to the thin film thickness on the transmission of linear micro-CPCs arrays is investigated by a monochromatic laser beam and visual observation. The results of the present work provides the reference for the optimization of the transmittance of the deposited thin film for a micro CPC, in order to achieve the desired optical property.

Optically-derived mechanical properties of glass fiber-reinforced polymer laminates for multifunctional load-bearing structures:

This paper demonstrates how the translucency of glass fiber-reinforced polymer (GFRP) laminates allows the derivation of their mechanical properties through optical measurements. Spectrophotometric, goniophotometric and tensile experiments were performed on unidirectional and cross-ply hand lay-up GFRP laminates with fiber volume fractions ranging from 0.15 to 0.45. An analytical model to predict the directional fiber volume fractions—and thus the mechanical properties of GFRP laminates—has been developed based on the total and diffuse transmittance and directional light scattering of the laminates. It is demonstrated that structurally optimized GFRP laminates can meet the requirements for GFRP skylights and the encapsulation of photovoltaic cells into translucent GFRP laminates.