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

Mixed-Dimensionality Approach for Advanced Ray Tracing of Lamellar Structures for Daylighting and Thermal Control

The appropriate choice of the type of glazing and glazed area in a façade 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. All requirements cannot be fulfilled at all times and priorities have to be set to find a compromise between occupant comfort, design objective, cost and energetic efficiency. An innovative glazing system combining daylighting, glare protection, seasonal thermal control and clear view was developed [1] and patented by the authors. This design was developed using a novel ray tracing approach to obtain a strongly angular dependent transmission with specific angular distribution. Taking advantage of the changing elevation of the sun between seasons, a seasonal variation is created by a strongly angular dependent transmittance. In this paper we present the mixed dimensionality approach used to achieve a very fast and accurate ray tracing of any lamellar structure that has a two dimensional profile. The originality of the presented Monte Carlo algorithm is the separation of intersection and interaction. Intersections are computed using only the two dimensions of the profile thereby increasing significantly computational speed. Interactions are computed using vector calculus in three dimensions and provide accurate results with very little computational load. With such optimizations, the user interface could be designed to give an instantaneous idea of the light path in the modelled system. The model also calculates an accurate bidirectional transmittance distribution function that is used in a Radiance simulation to obtain a rendering of the daylighting distribution in an office space. Hereby we can compare the daylighting performances of the novel design based on optical microstructures with those of other CFSs. Finally the combination of simulated angular dependent transmittance and Meteonorm data provides an estimate of transmitted energy over the year and proves the efficiency of the presented optical microstructures for dynamic thermal control. The proposed working principles of redirection and angular dependent transmittance are thereby demonstrated. The software provides all the mentioned results in the user interface where the performances of different designs can also be compared, making the optimization process of a profile with a defined objective very intuitive

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

Towards novel glazing with seasonal dynamics based on micro compound parabolic concentrators

Thermal loss and overheating caused by glazing is a key issue in reducing CO2 emission and energy consumption of buildings. A novel glazing based on micro compound parabolic concentrators (CPCs) is proposed. The glazing consists of a polymer layer with embedded micro CPCs, which is attached to a glass pane of glazing. Thanks to the geometry and the micrometric size of the embedded CPCs, the proposed novel glazing can reduce energy consumption in cooling, improve visual comfort and maintain clear view through glazing. In the present work, the potential benefits of such glazing are preliminary estimated for Dubai. The seasonal dynamics are investigated by analyzing the direct solar transmittance for the working hours on the spring equinox and the winter solstice. The improvement of visual comfort is studied based on the assessment of glare. First samples with micro CPCs are fabricated, and the high transparency is achieved. The optical characterization using goniophotometer confirms the seasonal dynamics of the novel glazing

Location Based Study of the Annual Thermal Loads with Microstructured Windows in European Climates

Glazed envelopes can cause significant thermal energy gains or losses. The installation of a novel design of Complex Fenestration Systems (CFS), such as embedded mirrors, could significantly contribute to reduce the energy consumption. In order to determine the influence of this glazing technology on thermal loads, a parametric study considering twenty-two European locations has been carried out. Simulations were performed for each location, to evaluate the range of latitudes for which the installation of the microstructures is advantageous. Optical microstructures found to be a valid solution to increase energy savings up to 20% when compared to a sun protective glazing

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