Microstructured glazing for daylighting, glare protection, seasonal thermal control and clear view

The appropriate choice of glazing in a facade depends on many factors. They include amongst other criteria: location, orientation, climatic condition, energetic efficiency, usage of the building, required user comfort, and the architectural concept. On the south facade of high-rise buildings in particular, it is a challenge to have simultaneously large glazed area, no glare, no excessive cooling loads, a clear view and sufficient natural light flux. In Switzerland, electric lighting, heating and air conditioning account for about 74% of the total energy demand in private housing and 32% of the overall Swiss electricity usage. This energy consumption can be strongly influenced by using the most appropriate fenestration system. A software was developed during this thesis to engineer new complex fenestration system (CFS) that have a two dimensional profile. The originality of the implemented Monte Carlo ray tracing algorithm is the separation of intersection and interaction. The model also calculates an accurate bidirectional transmission distribution function that is used in combination with Radiance to obtain a rendering of the daylighting distribution in an office space or dynamic daylight metrics such as the daylight factor and daylight autonomy. Finally, to estimate the thermal performances, a simple nodal thermal model was added to simulate the temperature evolution and the thermal loads in a given office. This tool was validated. A glazing combining several functions and that can contribute to significantly reduce energy consumption in buildings was developed using this novel ray tracing approach. It was designed to obtain a strongly angular dependent transmission and a specific angular distribution of transmitted light. The engineered geometry provides elevated daylight illuminance by redirecting the incoming light towards the depth of the room. This redirection simultaneously reduces the glare risk. For an optimised usage of available solar radiation, the transmission of direct sunlight is maximised in winter and minimised in summer. Taking advantage of the changing elevation of the sun between seasons, such a seasonal variation can be created by a strongly angular dependent transmittance. A fabrication process was identified and samples of embedded micromirrors were produced to demonstrate the feasibility. The fabrication of such structures required several steps. The fabrication of a metallic mould with a high aspect ratio and mirror polished surfaces is followed by the production of an intermediate polydimethylsiloxane mould that was subsequently used to replicate the structure with a ultraviolet (UV) curable polymer. Selected facets of these samples were then coated with a thin film of reflective material. Finally, the structures were filled with the same polymer to integrated the mirrors. The blocking effect can be obtained by a combination with well placed reflective stripes, those were fabricated by lift-off lithography. The samples were characterised during the various fabrication steps using various microscopy techniques, energy-dispersive X-ray spectroscopy, profilometry and optical measurements. A setup was built for the measures of angular dependent transmittance. The final samples redirect up to 70% of the light flux and are very transparent when looking through at normal incidence

CFSPro: Ray Tracing for Profile Optimisation of Complex Fenestration Systems using mixed dimensionality approach

An advanced optical ray tracing software was developed for the extensive study of Complex Fenestration Systems. Using an algorithm mixing two and three-dimensional approaches, very fast and accurate computation of large number of rays in complex geometries could be performed. In this paper it is described how the software was extended to study the impact of such systems on daylighting and thermal properties in a space. The simulation was made location dependent and an estimate of illumination values and temperatures in a space was added. For accurate and rapid results, diffuse and direct radiation were separated and a matrix multiplication approach was used to derive daylight availability and hourly thermal loads. A novel glazing that was engineered with this simulation tool and combines the functions of daylighting, glare protection, and seasonal thermal control while conserving a clear view will illustrate the performance study

Potential advantages of a multifunctional complex fenestration system with embedded micro-mirrors in daylighting

Proper daylighting designs in interior spaces may reduce the energy costs of electric lighting and improve the productivity of occupants. This paper reports the potential advantages of a multifunctional complex fenestration system with embedded micro-mirrors in daylighting by simulation. The system consists of a polymer layer with an array of embedded parabolic micro-mirrors, which was attached to a glass pane of glazing. By appropriate geometrical design, the embedded micro-mirrors may combine the function of keeping a clear view through the glazing due to the microscopic size of the embedded mirrors, and improving daylighting quality and quantity because of the redirection of daylight by micro-mirrors. In order to fully explore the potential advantages of the daylighting system with embedded micro-mirrors, three configurations were investigated: micro-mirrors in the upper third of a double glazing, micro-mirrors in the upper two-thirds of a double glazing and micro-mirrors in the whole glazing. Metrics based on illuminance, uniformity, glare and directivity were used to assess daylight performance for a south-facing façade in Lausanne (46°30′N and longitude 6°37′E), Switzerland. Compared with a standard low-e double glazing, the daylighting system with micro-mirrors could induce more daylight in the rear of a room and improve the uniformity of the daylight distribution. By integrating micro-mirrors in the upper two-thirds of the glazing, the risk of glare was reduced for a sightline parallel to the glazing on the spring equinox and on the winter solstice. For the case with micro-mirrors in the whole glazing, the directivity of daylight along the depth of the room was significantly improved from the harsh range to the preferred range at noon, suggesting a comfortable visual situation

Towards microstructured glazing for daylighting and thermal control

Glass is a central element to modern architecture and can cover up to 100% of a building façade. The main purpose of large glazed areas is to create bright, comfortable and healthy spaces. It was shown that increasing natural light in offices reduces sickness but high visual transmittance (τv) and excessive energetic transmittance (τe) can have opposite consequences: a high τv can cause glare and visual discomfort for occupants while a high τe induces overheating which has to be balanced with air conditioning in summer. The energetic and daylighting performances of a fenestration system are central and important issues for architects and the right compromise between good lighting levels, electrical savings, solar gains in winter and overheating in summer is not easy to find. Over the past decades, progress was made and some solutions to these problems were found. Various types of blinds and shadings have been introduced to prevent glare, achieve a good daylight factor even far from the window and permit to adapt to conditions all along the year. Sun protection glazings on the other side are static systems with a selective coating to limit the transmitted part of the solar spectrum: traditionally a step function with maximum values in the visible range and minimal values in the infra-red and ultraviolet range cuts down excessive solar gains. Recent research show that the transmitted spectrum can be refined and applying a 'M' shaped transmittance distribution, a ratio of τe / τv = 0.33 can theoretically be reached [1]. A market study on complex fenestration systems integrating daylighting functions and thermal control shows that apart from blinds and coatings which can be found in many variations, few products exist. Cutting edge elements such as laser cut panel, prismatic sheets and other micro-structures were studied. The study showed that there is no existing static complex fenestration system (CFS) combining the advantages for both daylight and energetic aspects with a seasonal behaviour. We are investigating a novel micro structure combining functions of daylighting, glare protection, overheating protection in summer and thermal insulation in winter. The optical performances of envisaged structures were evaluated with a simple two dimensional ray tracing program developed specially for the study of laminar structures. This tool permits to optimize parameters and search for new solutions

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

Light diffusion in GFRP laminates for building construction

The light diffusion of hand lay-up glass fiber-reinforced polymer (GFRP) laminates was investigated with a spectrophotometric set-up using an integrating sphere. Reinforcement weights of the laminates ranged from 410 to 3280 g/m2 and fiber volume fractions were between 0.20 to 0.35. The influence of resin gel times on light diffusion was also studied. Experimental results showed that blue light was more diffused than other colors, particularly in laminates with longer gel times. The refractive indices of resin and glass fibers were investigated and a significant mismatch below 500-nm wavelength was observed. Numerical ray-tracing simulations were performed and the trend of the experimental total and diffuse transmittances were successfully modeled. These analyses confirmed that the spectral mismatch of refractive indices was responsible for the wavelength dependency of light diffusion in GFRP laminates

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