Energetic restoration of buildings can make a significant contribution to the reduction of CO2-emissions in Germany. The improvement of the thermal performance of building envelopes is an important factor in this context. The demand of heating energy can be lowered to 50 % by applying appropriate thermal insulations.
Innovative, light and flexible membrane constructions have increasingly attracted the attention of architects in recent times. For example, they can be used to cover large areas, permitting increased use of daylight by using transparent or translucent materials while at the same time avoiding direct glare effects. The glass or textile fabrics used can create impressive optical effects, but do call for new concepts in order to optimise the energy requirements of buildings for heating and climate control. Highly developed materials can play a role here: when equipped with so-called low-e coatings, they behave like mirrors with regard to thermal radiation (Fig. 1). In summer, they reduce the heat influx into the building, thus reducing the cooling load, while in winter they reflect thermal radiation from inside the room and transmit only a small fraction of this heat to the outside. At the Bavarian Center for Applied Energy Research (ZAE Bayern) in Würzburg, Germany, research activities are carried out to develop such low-e coatings based on pigments.
Absorption and scattering of incoming thermal radiation at different pigments can be described among others by Mie-theory physically and mathematically (Fig. 2). With this knowledge it is possible to optimize the applied materials and therefore the complex refractive index as well as the particle dimensions. The experimental aim is an increase of the back-scattering and thus a rise of the reflection of infrared radiation which leads to a decrease of the thermal radiation.
The resulting spectral emittances of the developed low-e fabrics are depicted in Fig. 3 in comparison to an uncoated fabric. The low-e coating with a user-definable color has a thermal emittance of 0.3, whereas the silver-colored coating reaches a thermal emittance of 0.1. This is an enormous reduction of the emittance in contrast to the thermal emittance of an uncoated fabric of 0.95. For quantifying these parameters adequate metrological know-how and excellent measurement equipment are necessary.
Fig. 1: Reflection of thermal radiation by a low-e fabric (left) in comparison to an uncoated fabric (right).
Fig. 2: Scattering of electromagnetic radiation at a particle with known complex refractive index.
Fig. 3: Spectral emittance of low-e fabrics in comparison to uncoated fabrics.
For more information please contact:
Dr. Hans-Peter EbertHead of Division: Functional Materials for Energy TechnologyBavarian Center for Applied Energy Research (ZAE Bayern)Am Hubland, 97074 Würzburg, GermanyPhone: +49 931/ 705 64-34Fax: +49 931/ 705 64-60Email: email@example.com://www.zae-bayern.de