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

Significance Statement

Proper utilization of daylighting in office blocks can minimize lighting internal loads, which could translate to a drop in carbon-dioxide emission. Daylighting also comes with numerous psychological and physical benefits. A comfortable daylighting in offices and schools can really improve the occupants’ performance and productivity.

A complex fenestration system containing light-redirecting micro-mirrors redirects daylight into the rear of a room, reflects direct radiation from the sun during summers but not during winters, and maintains a clear view through the glazing. The system is composed of primary one-dimensional parabolic reflective micro-mirrors and a set of secondary reflective surfaces. During summers when the incident elevation angle is high, the incoming radiation is focused on the secondary surfaces where it is reflected out. In contrast, during winters the incident radiation is focused on the parabolic surface to enter the rear of the room. Therefore, seasonal thermal control is realized through the multifunctional fenestration system with primary and secondary micro-mirrors. It is found that glazing with embedded micro-mirrors can contribute to around 20% energy saving in thermal loads compared with conventional glazing.

Researchers, Jing Gong, Dr. André Kostro, and Ali Motamed led by Dr. Andreas Schueler from Ecole Polytechnique Fédérale de Lausanne in Switzerland explored the feasible configuration of embedded micro-mirrors with panels into a glazing in a bid to enhance daylighting quantity and quality. Simulations were performed in four aspects. The impacts of micro-mirrors on the static and dynamic illuminance in an interior space were studied. Assessment of the uniformity of daylight was performed using the luminance ratio. The evaluation glare was conducted by Daylight Glare Index (DGI). Directivity of daylight through vector-to-scalar ratios was investigated. Their work is published in journal, Solar Energy.

They recorded an average annual illuminance of 550lux for the rear of the office (the region from 4m deep to 8m deep) with micro-mirrors applied to a south-facing double glazing of 2.9m (width)  1.6m (height). In simple terms, this is equivalent to about 450kWh per year of electrical consumption. When compared to an office block with low-e glazing, they recorded an average of 250lux increase in illuminance of the micro-mirrors in the whole glazing, which translates to approximately 200kWh of electricity consumption. The authors found that micro-mirrors integrated above 1.5m from the floor level gave the best glare protection.

The authors, in their work, managed to show that embedded micro-mirrors would reduce the illuminance which exceeds the useful level (2000 lux) near window and enhance the brightness in rear space. They found that the risk of glare could be lessened with a sightline parallel to the window by using micro-mirrors at a position higher than the occupant’s sitting position.

The study on daylight performance can help optimize geometrical configurations and parameters of micro-mirrors for a given locale, and gives a reasonable installation configuration.

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

About the author

Jing Gong is currently doing her PhD study in the Solar Energy and Building Physics Laboratory (LESO-PB) at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland. Her PhD project investigates the realization of multifunctional glazing with micro-mirrors for different oriented facades and various climates. Her master degree was jointly awarded by Technical University of Munich and Ludwig Maximilians University of Munich, Germany. During her master study, she exchanged at the University of Rennes 1 for a semester. She got her bachelor degree from Harbin Institute of Technology, China.

Her main academic interests include: materials processing (ablation by excimer laser and welding), materials science, thin film coating, and characterization with large-scale facilities (synchrotrons and neutron sources).

About the author

Dr. André Kostro received his MEng in computer science at the Swiss Federal Institute of technology (EPFL). He then obtained his PhD from the solar energy and building physics laboratory at EPFL, investigating encapsulated micro-mirrors for multifunctional glazing. During his thesis, he proposed a novel type of window that is transparent, protects from glare while redirecting light to provide natural lighting and offers a seasonal variation of the solar heat gain coefficient to minimize the thermal loads of buildings.

His fields of expertise are optics, ray tracing computer simulations and complex fenestration systems. He is a lecturer in building physics at EPFL and joined BASF focusing on the development of foils for light management.

About the author

Ali Motamed is currently research assistant in the Solar Energy and Building Physics Laboratory (LESO-PB) at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland. He is experienced in design, simulation and control of complex mechateronics systems. In his recent activity in LESO-PB, he has investigated the integration of novel glare rating sensors in building automation system for the purpose of electric energy mitigation and indoor comfort enhancement.

His main research interests cover mathematical modeling, multi-objective system optimization and user-centric building automations.

About the author

Dr. Andreas Schüler studied Physics at the Universities of Freiburg i.Br. (Germany), Ann Arbor, (Michigan, USA), and Basel (Switzerland), and holds a Master’s and a PhD degree. Arriving at Ecole Polytechnique Fédérale de Lausanne EPFL, he started up a research group devoted to nanotechnology for solar energy conversion. Dr. Schüler is lecturing at EPFL, and supervising PhD and Master students.

Topics of his current research include nanostructured coatings for solar glazing, selective solar absorbers, thermochromic solar collectors, and photovoltaic applications. Dr. Schüler won the Solar Energy Journal Best Technical Paper Award in 2007 and 2013.


Jing Gong, André Kostro, Ali Motamed and Andreas Schueler. Potential advantages of a multifunctional complex fenestration system with embedded micro-mirrors in daylighting. Solar Energy, volume 139 (2016), pages 412–425.

Solar Energy and Building Physics Laboratory, Station 18, École polytechnique fédérale de Lausanne (EPFL), CH1015 Lausanne, Switzerland.

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