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.