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.

Embedded microstructures for daylighting and seasonal thermal control

Mario Geiger, Nicolas Jolissaint, André Gabriel Kostro, Yves Leterrier, Jean-Louis Scartezzini, Andreas Schueler

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.

Spie-Int Soc Optical Engineering2012

On advanced daylighting simulations and integrated performance assessment of complex fenestration systems for sunny climates

Chantal Basurto Dávila

The inclusion of daylight in buildings represents several benefits: its use not only signifies a reduction in the building energy consumption through the compensation of electric lighting, it also has positive effects in the execution of human activities. Through its spectral composition, it contributes to create a better interior atmosphere for visual comfort, leading to a better performance on working tasks. It also influences biological human cycles, which has an impact on human alertness, mood and well-being. Compared with electric light, its intensity and dynamic variations have a stimulating effect by providing a connection with the exterior environment. However, for buildings located at low latitudes (between 23°N and S), the inclusion of daylight implies the admission of sunrays, which affect the visual comfort and perception of the indoor environment by altering the interior luminance distribution increasing the risk of glare. An additional unfavourable effect is the increment of the interior thermal loads which represent a risk of overheating for the occupants. In those regions, common practices such as: the reduction of the window size, the use of tinted glazing and solar protection are usually applied to buildings in the search of overcoming such problems. However, the use of such strategies also implies a reduction of the admission of daylight inducing the use of electric light, which is incongruent in countries with large amounts of this natural resource. The use of Complex Fenestration Systems (CFS) may represent a solution given their characteristics: daylight redirection and direct solar rays protection. The latter would contribute to control the admission of solar gains while maintaining a good visual interior environment. However, the appropriate selection of CFS requires a careful evaluation of their features and performance regarding their suitability to the building location. This thesis explores the potentiality of using CFS to improve the interior daylight distribution in buildings located at low latitudes while maintaining a satisfactory visual and thermal interior environment for the occupants. In order to do this, the existing daylight situation of two office rooms located in the central-north of México (Zacatecas 22° 783' N., 102° 583' W, Altitude: 2543m) were monitored from 2011 to 2013. Illuminance and luminance were measured on periods of the year that are crucial for the interior luminous environment (summer and winter solstices together with spring equinox), in order to characterize the existing daylighting situation and to study the dispersion of results obtained using a virtual model which reproduces the features of both offices. The performance of five CFS was then tested using computer simulations in order to assess their suitability to the local sky conditions. For this, three main factors were taken into account: the improvement of the interior daylight distribution, the risk of glare and overheating. The effects of the CFS in a room regarding such conditions were simulated using RADIANCE and Energy Plus using BTDF data (Bidirectional Transmission Distribution Function that characterizes the CFS’s lighting transmission properties) assessed by the use of a gonio-photometer. The assessment was performed first during the winter and summer solstices as well as spring equinox and secondly then on annual basis. The latter was done in order to take into account the daylight variation characteristics of two locations with prevailing clear sky conditions. The results obtained allow determining the CFS that better contributes to a better interior luminous environment in each building without compromising the thermal and visual comfort of the occupants.

EPFL2014

On advanced daylighting simulations and integrated performance assessment of complex fenestration systems for sunny climates

Chantal Basurto Dávila

EPFL2014