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

Research Article

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.

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, Lighting, Building Thermal, 40% of total energy consumption in terms of primary energy Transparent selective films represent an interesting Acousticsand Modeling (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,

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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 Single sheet, 5 mm Silver Finally, many researchers studied multilayer window systems, also using ray tracing techniques to model multi- SS5SP Single sheet, 5 mm Sputtered reflection and multi-transmission phenomena (Kuhn et al. SS5UV Single sheet, 5 mm UV control 2000; Maamari et al. 2006; Chow et al. 2007). DS4 Double sheet, 4-6-4 mm None DS4SI Double sheet, 4-6-4 mm Silver 3 Description of the samples DS4SP Double sheet, 4-6-4 mm Sputtered DS4UV Double sheet, 4-6-4 mm UV control The various simulations and the experimental campaign DS5 Double sheet, 5-6-5 mm None considered different single and double sheet glass samples DS5SI Double sheet, 5-6-5 mm Silver commonly used in buildings; three different films were applied DS5SP Double sheet, 5-6-5 mm Sputtered to the glass samples, to obtain different combinations. In DS5UV Double sheet, 5-6-5 mm UV control particular, 4, 5 and 6 mm thick St. Gobain float glass was DS6 Double sheet, 6-6-6 mm None considered for the single sheet glass samples, while St. Gobain Climalit of different thicknesses (4-6-4, 5-6-5 and DS6SI Double sheet, 6-6-6 mm Silver 6-6-6 mm) was used for the double sheet glass samples. DS6SP Double sheet, 6-6-6 mm Sputtered Silver, sputtered and UV control films, manufactured by DS6UV Double sheet, 6-6-6 mm UV control Intelligence Solar, were applied to the various glass samples, according to the combinations in Table 1. For the sake of brevity, among the three single sheet glass samples considered, only the results for 5 mm thick St. Gobain float glass are reported in the paper.

4 Instrumentation and experimental results

The measurements used to validate the simulation codes were carried out with a VARIAN Cary 2300 spectrophotometer, working in the 185− 3152 nm wavelength range with an accuracy up to 0.2 nm in the UV and visible range and 0.8 nm in the infrared range. When a measure of reflection is required, it is possible to use the integrating sphere, within the 300− 2000 nm wavelength range. An accurate description of the instrument and of its calibration is presented in (Asdrubali and Bisegna 2004). As far as the instrumentation accuracy, previous studies by the Authors (Asdrubali and Bisegna 2004) demonstrated that the mean Fig. 1 Goniometer used inside the spectrophotometer for angular square root of transmittance and reflectance values measured, measurements

78 Asdrubali and Baldinelli / Building Simulation / Vol. 2, No. 2 varying the wavelength in the 300− 2500 nm range, with a Figures 2 and 3 present the influence of the film on the step of 10 nm, according to the International Standard transmittance and reflectance (%) as a function of (ISO 9050 2003). Each measurement is repeated three times wavelength for a double sheet glass sample. and the average over the three series of measurements is Sputtered and silver films significantly reduce the calculated (Nostell 2000). transmittance in all the wavelengths measured (the maximum Optical data has been measured as a function of value of the transmittance is respectively about 40% and wavelength and of the angle of incidence of light. For the 25% in the visible range) and correspondently increase the sake of brevity, only a few examples of measurement results reflectance (up to 50% in the visible range for the silver film, are reported here. which demonstrates the so-called “mirror effect”). UV control

Fig. 2 Transmittance as a function of wavelength for a 5-6-5 mm glazing with different films

Fig. 3 Reflectance as a function of wavelength for a 5-6-5 mm glazing with different films Asdrubali and Baldinelli / Building Simulation / Vol. 2, No. 2 79

films, on the contrary, modify the optical properties of the V(λ) is the spectral luminous efficiency for photopic vision glass only as far as the transmittance in the UV region, while defining the standard observer for photometry, in the other parts of the solar spectrum the two curves are Δλ is the wavelength interval. practically overlapping. Table 2 gives the results for all glazings samples tested The results concerning silver films coincide with similar with the spectrophotometer. measurements carried out by Leftheriotis. It is clear that the influence of glass thickness is almost Figure 4 plots the transmittance (%) as a wavelength negligible, compared to the effect of different films, which function for a 6-6-6 glazing sample without film, for instead change significantly both the light transmittance and different angles of incidence. As can be seen, there are the external light reflectance. This data has been used to significant differences only for high values of the angle of compare the experimental results with the predicted values. incidence (>50°). For this reason, in this work the analysis will be limited to the normal incident radiation, even if Table 2 Light transmittance and external light reflectance results there are films whose properties are more angle-dependant. for the samples tested by spectrophotometry Starting from the experimental data collected, it is Code τv ρv possible to evaluate the light transmittance and the external light reflectance as defined by ISO 9050 (2003): SS5 91 % 9 % SS5SI 19 % 61 % 780 SS5SP 39 % 16 % ∑ Dτλ ()() λV λ Δλ SS5UV 87 % 10 % λ=380 τv = 780 (1) DS4 82 % 16 % ∑ DVλ () λ Δλ DS4SI 19 % 55 % λ=380 DS4SP 38 % 22 % DS4UV 79 % 16 % 780 DS5 82 % 14 % ∑ Dρλ ()() λV λΔλ λ=380 DS5SI 17 % 49 % ρv = 780 (2) DS5SP 34 % 22 % ∑ DVλ () λ Δλ λ=380 DS5UV 79 % 15 % DS6 77 % 14 % where: DS6SI 16 % 48 %

Dλ is the relative spectral distribution of illuminant D65, DS6SP 33 % 20 % τ(λ) is the spectral transmittance of glazing, DS6UV 79 % 15 %

Fig. 4 Transmittance as a function of wavelength and of the angle of incidence for a 6-6-6 mm glazing without film

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5 Theoretical model ρ'1 is the internal reflectance coefficient of layer 1,

τ2 is the transmittance coefficient of layer 2,

Possible combinations of glass thicknesses, number of layers, ρ2 is the external reflectance coefficient of layer 2. type and position of films and space filling gases increase As a consequence, the spectral absorbance α of the the amount of experimental data to be acquired. glazing system is With the aim of limiting the measurement campaign, it ()=+= () ()−− () () is useful to refer to a theoretical model to predict optical αλ α12 λ α λ1 ρλ τλ (5) characteristics of complex transparent materials, starting Therefore, by taking the spectrophotometric characteristics from the properties of single layer components. of the external layer with the applied film in combination Actually, the behaviour of gases used to fill the space with the characteristics of the internal layer characteristics, between the glass panes should also be taken into account, it is possible to obtain the properties of the entire glazing but the thickness of the spacers normally installed is sample. A datasheet was originally developed to implement limited to a few millimetres, therefore, the influence of the the theoretical model described in Fig. 5. gap may be neglected. The case of a double layer glazing sample is studied 6 Ray tracing technique with the two parallel radiation screens (ISO 9050 2003), with one side subjected to incident radiation (Fig. 5). A ray tracing model—available from one of the commercial Layer 1 consists of a glass substratum with a film tools available (Lambda Research Corporation 2008)—was applied on the internal side: it is considered as a unique also used to predict the properties of double layer glass layer since all experimental measurements were executed samples with applied films. with the single glass sample already equipped with the film. The code contains a ray tracing program for optical Layer 2 is simply a glass pane. analysis of solid models and it traces rays using the The spectral transmittance τ that passes through the so-called “Generalized Raytracing”. This technique allows double sheet glass sample with the applied film is given by the launching of rays into a model without making any the following equation: assumptions as far as the order in which objects and surfaces will be intersected. At each intersection, individual τ12()λ τ ()λ τλ()= (3) rays may be absorbed, reflected, refracted, diffracted and 1− ρλρ'12() ()λ scattered. While the spectral external reflectance ρ is Single coated glass spectral absorbance can be measured through the spectrophotometer, thus becoming an input of 2 τλρ12() ()λ the code; with this data, the ray tracing model calculates the ρλ()=+ ρ1 (4) 1− ρλρ'12() ()λ extinction coefficient k with the following equation:

where: α()λ λ k = (6) τ1 is the transmittance coefficient of layer 1, 4π

ρ1 is the external reflectance coefficient of layer 1, The refractive index n used in the model is evaluated through the Sellmeier equation (Sellmeier 1871):

222 2 aλⅠⅡⅢ a λ a λ nλ()−1 =++222 (7) λa−−−ⅣⅤⅥ λa λa

where coefficients aⅠ, aⅡ,…, aⅥ depend on the type of glass and are available in the simulation code database for a certain number of materials. If coefficients are not available, measured transmittance and reflectance data for the specific glass can be used to calibrate the software, changing the

coefficients aⅠ, aⅡ,…, aⅥ until a satisfactory correlation between experimental data and program output is achieved. For the examined glass samples without applied films, Fig. 5 Theoretical model scheme for the evaluation of optical good results were found with the Schott-Ultr 30 glass, which properties of a double sheet glazing with one film was therefore used for all simulations (Table 3). Asdrubali and Baldinelli / Building Simulation / Vol. 2, No. 2 81

Table 3 Schott-Ultr 30 glass coefficients for Sellmeier equation Table 4 Light transmittance and external light reflectance comparison results for double glazings without film and with three different Coefficient Value types of film aⅠ 0.730713411 Experimental Theoretical Ray tracing aⅡ 0.636951703 data model simulation aⅢ 0.856177259 Sample Property Value Value Difference Value Difference −3 2 aⅣ 3.14818243×10 μm DS4 τv 82% 83% –1% 82% 0% −2 2 aⅤ 1.29910178×10 μm ρv 16% 16% 0% 17% –1% 2 2 aⅥ 1.43604779×10 μm DS4SI τv 19% 18% +1% 19% 0%

As for films, the more effective way to insert the spectral ρv 55% 61% –6% 58% –3% properties in the simulation code consists of gathering their DS5SP τv 34% 36% –2% 37% –3% extinction coefficients and the refractive indexes, for example, ρv 22% 17% +5% 13% –9% by manufacturer data sheets. Unfortunately, in most cases DS6UV τv 79% 78% +1% 83% –4%

the film composition is unknown, therefore, a different ρv 15% 17% –2% 13% +2% approach has to be followed. properties were included in the model starting from 7 Models validation experimental measurements on single glass samples with applied films. With the ray tracing code it is possible to Figures 6 shows the comparison among the theoretical model, change surface characteristics of materials, therefore, one the ray tracing simulation and the experimental results for face of the glass with applied films is given a surface the spectral transmittance and reflectance of a 4-6-4 glass property in terms of absorption and reflection to take into sample without films (sample DS4), in the visible wavelength account of the presence of the film. Once the surface range. optical constants for glass samples with applied films are An almost perfect match of the curves is shown, as obtained, it is possible to create all glass-film combinations confirmed by the evaluation of the single-number indexes within the simulation code.

τv and ρv (Table 4). Figures 7, 8 and 9 show the comparison results for a The lack of spectral data for the films makes it more silver film (sample DS4SI), a 5-6-5 glass sample with a difficult to implement the ray tracing simulations, so, their sputtered film (sample DS5SP), and a 6-6-6 glass sample

Fig. 6 Spectral transmittance and reflectance comparison among experimental data, ray tracing simulation, and theoretical model for a 4-6-4 glazing without films

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Fig. 7 Spectral transmittance and reflectance comparison among experimental data, ray tracing simulation, and theoretical model for a 4-6-4 glazing with a silver film

Fig. 8 Spectral transmittance and reflectance comparison experimental data, ray tracing simulation, and theoretical model for a 5-6-5 glazing with a sputtered film Asdrubali and Baldinelli / Building Simulation / Vol. 2, No. 2 83

Fig. 9 Spectral transmittance and reflectance comparison among experimental data, ray tracing simulation, and theoretical model for a 6-6-6 glazing with a UV control film with a UV control film (sample DS6UV). It is evident that, compared with a commercial simulation code which uses a apart from slight differences in the reflectance data, the ray tracing technique. An experimental campaign was curves practically overlap, as well as the single-number carried out for various glazings with different films in order indexes τv and ρv (Table 4). to measure the optical properties (transmittance and The comparative analysis shows therefore that, after an reflectance) and to evaluate the main lighting indexes to be appropriate calibration, both the theoretical model and the used in the validation of the codes. ray tracing simulations almost coincide with the experimental Results show that after a brief calibration process with data, at least regarding the light transmittance. As far as experimental data, both ray tracing simulations and the reflectance curves, the match is less precise, especially for theoretical model provide good estimations of optical films presenting reflection values comparable with the properties of glass samples with applied films, especially in transmittance values; this phenomenon is probably due to terms of spectral transmittance (the maximum difference the use of an indirect method for the evaluation of film between simulated and measured data being ±4%). The spectral properties both in the simulation code and in the reflectance results obtained from the simulation models show theoretical model. slight differences from experimental data. The mismatching could be probably reduced if the film spectral properties 8 Conclusions were obtained directly from the manufacturer. It is quite onerous to obtain experimental measurements, Among the different types of innovative transparent materials but once the simulation models are validated, all the various and lighting control systems which have been recently combinations of glazing-films can be tested without much proposed, transparent selective coatings and films represent effort. This allows the possibility of changing many variables an interesting option, in order to minimise the energy such as glass thickness, type of film, type of glass and film demand of buildings and to guarantee visual comfort and position, so the desired day lighting performance can be desired day lighting performance. obtained. In order to predict the optical properties of multilayered The validated models are therefore particularly useful glazing systems, including films, starting from the properties to predict the performance of an existing window or of single sheets, a theoretical model was implemented and transparent façade treated with selective films.

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