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Theoretical Modelling and Experimental Evaluation of the Optical Properties of Glazing Systems with Selective Films

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Theoretical Modelling and Experimental Evaluation of the Optical Properties of Glazing Systems with Selective Films BUILD SIMUL (2009) 2: 75–84 DOI 10.1007/S12273-009-9112-5 Theoretical modelling and experimental evaluation of the optical properties of glazing systems with selective films ArticleResearch Francesco Asdrubali( ), Giorgio Baldinelli Department of Industrial Engineering, University of Perugia, Via Duranti, 67 — 06125 — Perugia — Italy Abstract Keywords Transparent spectrally selective coatings and films on glass or polymeric substrates have become glazing, quite common in energy-efficient buildings, though their experimental and theoretical characterization selective films, is still not complete. A simplified theoretical model was implemented to predict the optical optical properties, properties of multilayered glazing systems, including coating films, starting from the properties of spectrophotometry, the single components. The results of the simulations were compared with the predictions of a ray tracing simulations commercial simulation code which uses a ray tracing technique. Both models were validated thanks to several measurements carried out with a spectrophotometer on single and double Article History Received: 24 March 2009 sheet glazings with different films. Results show that both ray tracing simulations and the Revised: 14 May 2009 theoretical model provide good estimations of optical properties of glazings with applied films, Accepted: 26 May 2009 especially in terms of spectral transmittance. © Tsinghua University Press and Springer-Verlag 2009 1 Introduction Research in the field of glazing systems technology received a boost passing from single pane to low-emittance The problem of energy efficiency in buildings has been at window systems, and again to low thermal transmittance, the centre of a broad scientific and technical debate in recent vacuum glazings, electrochromic windows, thermotropic years (Asdrubali et al. 2008; Hamza and Greenwood materials, silica aerogels and transparent insulation materials 2009; Perez-Lombard et al. 2009). Currently, the energy (TIM) (Seeboth et al. 2000; Gugliermetti and Bisegna 2003; consumption in buildings in the European Union constitutes Kaushika and Sumathy 2003). Building Thermal, Building Thermal, Lighting, 40% of total energy consumption in terms of primary energy Transparent selective films represent an interesting Modeling and Acoustics (EEA Annual report 2007 and Environmental statement option for the control of solar heat gain, to be used to treat 2008), and in this sense the publication of the European windows or façades especially in existing buildings, to improve Directive 91 (EU Directive 2002) represents a great occasion the performance of windows and transparent façades. A for obtaining a definitive answer in terms of reducing recent study (Bakker and Visser 2007) demonstrated that a energy consumption in buildings. larger use of solar control glazings in residential buildings In the Mediterranean region the problem of energy in European Union countries could avoid the emission of consumption is more complex because the air-conditioning up to 80 million tons of CO2, which represents 25% of the load is as important as the heating load. Many different target established by the European Commission for energy types of innovative transparent materials and lighting savings in the residential sector in 2020. control systems have been developed in recent years, in Although glazing systems are extremely important for order to maximise energy savings by reducing solar heat building energy efficiency, International and Italian Standards gain in summer and exploiting solar energy in winter, (UNI EN ISO 13790 2008; Italian Legislative Decree n° 311 allowing at the same time good day lighting performance. 2006) consider the problem in a very simplified way, E-mail: [email protected] 76 Asdrubali and Baldinelli / Building Simulation / Vol. 2, No. 2 List of symbols a Sellmeier coefficient (m2) Subscripts D distribution of illuminant D65 1 layer 1 k extinction coefficient 2 layer 2 n refractive index Ⅰ first coefficient of Sellmeier equation V luminous efficiency for photopic vision Ⅱ second coefficient of Sellmeier equation Ⅲ third coefficient of Sellmeier equation Greek symbols Ⅳ fourth coefficient of Sellmeier equation Ⅴ fifth coefficient of Sellmeier equation α spectral absorbance Ⅵ sixth coefficient of Sellmeier equation Δ interval v visible λ wavelength (m) λ spectral ρ spectral reflectance τ spectral transmittance Superscripts ' internal providing the mean transmittance coefficient only for a few treatments may include void metallization or sputtering standard systems, and referring then to the certifications of (which consists of bombing the polyester layer with metallic the specific materials for other values. ions). In order to predict the performance of multilayer glazings, The most common applications are: low-emittance obtained by combining different panes and films whose films, which combine high solar and visible transmittance properties are known, a theoretical prediction model of the with low thermal emittance; reflective films, which are lighting transmittance and reflectance of complex glazing designed to pass a high proportion of visible radiation was implemented and then compared with a commercial whilst reflecting most of the near infrared component of code, based on the ray tracing technique. Finally, both incident solar radiation, thus reducing solar heat gain in models were validated by means of experimental results, summertime; and UV control films, which may be used to carried out on single and double sheet glazings with different preserve works of art and more in general materials which selective films. Spectral transmittance and reflectance are sensitive to light. measurements were carried out and single number indexes Films have a thin layer of glue on one face to stick them were calculated from experimental data to characterize the onto the glass or the surface which is to be treated; to protect lighting performance of the glazing systems. the metallic surface of the film, a further layer used as an anti-scratch protection is laminated and coupled with the 2 Transparent selective films film (Hutchins and Platezer 1996). Many researchers have investigated the optical properties Transparent selective coatings and films are being of selective coatings and films for window applications. manufactured nowadays by all major glass and glazing Roos et al. (2000) investigated the effect of the angle of companies all over the world. They represent quite an incidence of solar radiation on the optical properties of advanced technology and are being increasingly used in solar control windows; Nostell (2000) presented the results double and even triple glazing systems to improve window of a wide experimental campaign on various coatings, while performance. There are many different available coatings Durrani et al. (2004) measured the optical properties of and the applications are correspondingly various: optical three-layer systems on glass substrates. filters, heat mirrors, low-emittance films, protective and The modelling of complex fenestration systems (CFS), decorative coatings (Leftheriotis et al. 2000). including multi-layer glass panes, solar control films, Films are generally made of thin layers of polyester, translucent materials and shading devices, has been done stuck together with an extremely even and thin layer of by various researchers. Rubin et al. (1998) presented various glue, for a total thickness which may vary from 0.025 to equations to model the optics of composite systems. Alvarez 0.350 mm (from 25 to 350 μm). The different layers may et al. (2005) modelled the heat transfer of multiple-layer contain coloured materials or may be treated superficially glazing with selective coatings, while Li et al. (2008) evaluated so as to obtain the desired optical properties. The surface the benefits with regards to lighting and cooling energy Asdrubali and Baldinelli / Building Simulation / Vol. 2, No. 2 77 consumption in an office building using solar control films. calculated according to (CEI UNI ENV 13005 2005), is Maestre et al. (2007) developed a new model for the angle- lower than 0.5%. dependent optical properties of coated glazings, while Laouadi The spectrophotometer has also a goniometer, which is and Parekh (2007a, b) developed optical models of complex used to perform transmittance measurements varying the fenestration systems based on the bidirectional optical angle of incidence of radiation (Fig. 1). property distribution functions. The approach is extremely According to the Italian Standard for glass tests (UNI rigorous, since it allows the predicting of the effects of 7885 1978), the sample is cut in adequate dimensions, so as complex glazing on the view-through, window luminance to completely cover the opening through which the luminous and visualization of indoor objects illuminated by the flux passes; after measuring the thickness, which must be window. However, for simple thermal or lighting calculations within the range of tolerance, it is polished before being to be used for building design or for window product inserted into the measuring cell. Measurements are developed ratings, the approach suggested by Maestre and Laouadi and Parekh appears to be too sophisticated—since a large Table 1 Description of the samples analysed amount of experimental data must be collected and complex Code Glass Film calculations must be carried out—and actually not necessary SS5 Single sheet, 5 mm None for the required level of detail. SS5SI
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