ETFE (ethylene tetrafluoroethylene) is a lightweight material increasingly used in building applications. It has gained popularity mainly due to its daylight transmittance and the potential for energy savings. When used as cladding ETFE sheets are usually assembled into cushions, which are inflated for structural reasons. ETFE cushions can provide thermal insulation with reduced initial costs and less structural supports as compared with a conventional glazed roof. Limited research regarding the modelling of ETFE in building applications and limited availability of information on material properties led to the present study. Designers are currently facing difficulties when carrying out energy optimisation studies as part of the design process. For example, since ETFE is not entirely opaque to longwave radiation, merely treating the material as a standard glass layer can lead to errors when evaluating its thermal performance. In order to enable building designers to assess the performance of these systems, maximising performance and managing risk, it is essential to gain knowledge and develop methods to model this novel material. This study takes into account the longwave transmission properties of the ETFE material and discusses the need for a methodology for estimating surface temperatures, heat losses, and solar gains. Guidelines for integration are needed to define its properties and to evaluate performance during the building design process.
ETFE is a relatively new, lightweight material increasingly used in buildings, mainly due to its lightweight properties, its high daylight transmittance and the potentials for energy savings. When used for cladding, sheets of ETFE are usually assembled into cushions which are inflated (for structural reasons) by means of compressors. The system consists of two or more sheets of foil laid on top of each other and joined at the edges to form the cladding equivalent of an inflatable cushion. As stated, ETFE cushions can provide thermal insulation, with reduced initial cost investments and fewer supports compared with a glazed roof (Robinson, 2005). However, due to the lack of information on the material properties it becomes difficult for designers to deliver energy performance optimised designs. Additionally, since ETFE is not opaque to longwave radiation, treating it as a glass layer can lead to errors, when evaluating its performance. Therefore, it becomes essential to gain knowledge and develop methods to model this material in order to maximise performance (and minimise risk).
AIM OF THE STUDY
This paper deals with energy transmission aspects of ETFE in building design. The study focuses on thermal and optical performance of ETFE. Initially, a brief description of the ETFE material is given in order to reach a fundamental understanding of its performance. Calculation methods and approaches for existing projects are mentioned and a first evaluation of these methods is outlined. The paper presents recommendations for further studies of ETFE energy transfer modelling for building performance simulation.
ETFE in building design ETFE has approximately 95% light transmittance, but does not offer the clear visibility/transparency of glass (Robinson, 2005). As a result, ETFE solutions therefore initially found use on projects such as botanical gardens, zoological gardens, swimming pools, and exhibitions spaces. However, ETFE is increasingly finding its place in more traditional buildings as roofing for courtyards, shopping malls, atria and stores. The ETFE material has been used on prominent architectural projects such as the Eden Centre and the Water Cube and it is currently considered for a number of high profile international sports venues. Previous ETFE studies have focused mainly on structural properties and related issues, while little research has been carried out in order to determine energy transmission properties and characteristics in terms of environmental building design.
Modelling of ETFE in building simulation tools
Implementing ETFE cushions in building design is a complicated task due to the unusual transmission characteristics of the material. Since currently available commercial software tools are not developed to take into account the longwave transmittance through the ETFE layers, in practice ETFE foils are usually modelled as glazing units. Depending on the building use, the building design, the site, and geographical location of the building, this simplification may impact on the accuracy of the simulated building performance, as discussed in the following.
Shortwave and longwave radiation
This section presents a brief theoretical background, in order to gain an understanding of the particular properties of ETFE and the resulting potential shortcomings of current energy modelling tools and methods.
Electromagnetic radiation is an energy form, which comprises what we refer to as heat and light. The electromagnetic spectrum is outlined in Figure 2. The term ‘thermal radiation’ (relating to heat transfer) ranges from a wavelength of approximately 0.1μm to 100μm and includes part of the ultraviolet (UV) and all of the visible light and infrared (IR) radiation.
All bodies emit and absorb energy in the form of electromagnetic radiation. At a given temperature, the thermal radiation emitted from a surface varies for different wavelengths. The term ‘spectral’ is used to indicate this dependence. The spectral distribution depends on the characteristics and temperature of the emitting surface. In order to accurately quantify radiative heat transfer, the spectral and directional effects should be taken into account (Incropera et al., 2002).
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