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.
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.
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 aestival elevations of the sun, thereby minimizing 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 optimization 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.
Overheating is a common problem both with the use of active and passive solar energy in thermal solar energy
systems and in highly glazed buildings. In solar thermal collectors, the elevated temperatures occurring during
stagnation result in reduced lifetime of the collector materials. Highly glazed building facades provide high solar
gains in winter, but imply in most cases high energy needs for air conditioning in summer. A solution to such
problems might be provided by "smart" thermochromic coatings. A durable inorganic thermochromic material is
vanadium dioxide. At 68°C, VO<sub>2</sub> undergoes a reversible crystal structural phase transition accompanied by a
strong variation in optical properties. By doping the material with tungsten, it is possible to lower the transition
temperature making it suitable as a window coating. In order to simulate the optical behaviour of multilayered
solar coatings, precise knowledge on the optical material properties is necessary. Experimental data reported in
the literature are rare and controversial. We determined the complex dielectric function for VO<sub>2</sub>:W by
spectroscopic UV-VIS-NIR ellipsometry above and below the transition temperature and subsequent point-by-point
analysis of the ellipsometric psi/delta data. For a validation, the solar reflectance, absorptance and
transmittance were measured by spectrophotometry in the visible range and in the near infrared range up to 2500
nm. The experimental reflectance spectra have been compared with the computer simulations based on the
determined optical material properties. Finally, we collected optical data in a more extended wavelength range by
digital infrared imaging to detect the switch in thermal emissivity of VO<sub>2</sub>:W at around 45°C.
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.