Daylight illumination system by vertical Transparent Prismatic Lightguide for an office building

Transcription

1 360 Daylight illumination system by vertical Transparent Prismatic Lightguide for an office building Antonio ALVAREZ FERNANDEZ-BALBUENA* Researcher, Daniel VAZQUEZ-MOLINÍ* Professor, Berta GARCÍA-FERNANDEZ* Researcher, Lucas GARCÍA-RODRÍGUEZ ** Director, Teresa GALÁN-CAÑESTRO ** Researcher I+D, *Dept. of Optics, School of Optics, University Complutense of Madrid, c/ Arcos de Jalón nº118., Madrid, 28037, Spain, Phone **Optics Department, Lledó Iluminación S.A., C/ Cid Campeador, 14, Móstoles, Madrid, 28935, Spain, Phone ABSTRACT The daylight in buildings is a fundamental tool for the improvement of quality and comfort, also for the control of cost and power consumption. The use of daylight as a primary source of lighting is nowadays a basic requirement in sustainable architecture. Reducing dependence on -non-renewable energy-sources is a priority for government authorities and policy. The principal objective of this paper is to develop an efficient prismatic hollow light pipe that directs daylight into interior spaces applied in office buildings. It will be applied in a building in Sevilla (Spain). The extraction system is obtained by means of two different ways: first one is a very focusing system and the second one is for diffuse light. The first one is done by a dielectric prismatic structure with specific design, which extracts a perpendicular light path and second to diffuse light of the pipe due to the rounding and defects on the main prismatic sheet. This design allows to obtain a transparent simple and beautiful pipe which can be well integrated in building design. To obtain a uniform diffusion along the light guide, the spatial distribution of the extraction sheets is variable and can be adapted to the requirements of each space. The optical model makes it possible to determine the light distribution and the efficiency of the light pipe used for different solar positions. Keywords: daylight, dielectric prismatic lightguide, light pipe, total internal reflection. 1. INTRODUCTION Daylight provides high quality lighting, reduces energy use and it has numerous beneficial physical and psychological effects on people. Light pipes can transfer light from a building s roof into the depths of the building [1]. The overall wattage of installed lights is reduced and the consumption of electricity decreases due to lesser use of artificial light-

2 361 Colour and Light in Architecture_First International Conference 2010_Proceedings Fig. 1: Elevation of the office building configuration Fig. 2 Example of refraction (a) and TIR (b) in prismatic film. Fig. 3: Circular heliostats mirrors (0.9 meter diameter). ing. The most common light pipes are mirror reflective guides, prismatic light guides[2], lens guides and fiber optic guides each can be combined with numerous lights collecting and distributing systems. Lightguide performance varies with the length of the guide, the maintenance condition, the collecting system, the distributing apertures, and the direction from which light is directed. The principal objective of this paper is to show the development of a new light pipe extraction concept and its simulation model using TracePro ray-trace software [3], which shows a daylight system used to redirect incoming sunlight or skylight to areas where it is required. The system controls daylight levels and redirects sunlight distribution to different floors of the office building (fig.1). The prism light guide described later transmits light by total internal reflection, which gives higher efficiency and homogeneous light distribution through the guide. 2. SIMULATION MODEL 2.1 Office building The office building consists of four floors with identical areas (21 m 21 m m) and the sill height is 0.3 m above the interior floor level. In the simulation, the walls, floors and ceilings are furnished with perfect absorbing properties surfaces. The light guide is installed in the center of the office building and the sunlight is distributed thought the workspace using the light guidance structure. The light guide has a diameter of 2 m and it goes through first, second and third floors. The total length is 16,7 m. In the vertical hollow light guide proposal five basics elements are used: two heliostats, a prismatic lightguide, a diffuser film, a mirror and a prismatic extractors film [4]. Several designed models were used in this study, but only two will be presented. Model 1, shown in Fig. 4, considers the structure as a simple prismatic hollow light guide and Model 2 considers the incorporation of two prismatic band films in first and second floor light guide. 2.2 Prismatic Lightguide The lightguide structure is a prismatic hollow tube with a thin polycarbonate film with right angle prism sections. Light travels mainly in the hollow air space inside the guide and bounces off by total internal reflections (TIR). This

3 362 Colour and Light in Architecture_First International Conference 2010_Proceedings configuration has an angular acceptance cone determined by the refractive index of the prismatic film with an at least half- angle of about 30º, although it can be up to 90º. If this angular criterion is not met, the light will instead be rejected out of the light pipe through the prismatic material. In this project two types of commercial prismatic sheets have been used, one of them with 90º prisms that transport the light through the guide and the other is a 70º prismatic film perpendicular to the previous one that works extracting the light in different floors. The fraction of the light that escapes from the guide is controlled by the width of a 70º prismatic film called the extractor, which alters the angular characteristics of the reflected light in order to cause a portion of it to escape. Figure 1 (a) is an example of light entering in a prismatic film through prism apex side, in this case light is divided in two main arms (refracted light), rest of light than is guided in the prismatic structure is Fresnel light with low flux energy. In figure 1 (b) the prismatic film produces a TIR, in this case the light is returned in the direction from which it came Optical simulation model With the aim to get a better uniformity factor at the entrance of the building guide a soft diffuser film spreads the light with high-transmission efficiency through the guide. One mirror design of specularly reflecting aluminum material (Miro 20) was located at the bottom of the light guide, hence the luminous flux was passing in a reverse direction with respect to solar light increasing the transmitted light in the intermediate floors. Luminance of the light guide system was examined under different elevation angles of the sun. The flux contribution of the sun and the two heliostats (Fig.3) that enters the guide is presented in Table 1 and they produce a uniform collimated output beam to simulate the sun light. Table 1: Luminance and illuminance used for different elevation angle conditions. The opticals models enabled us to determine the light distribution in two different simulations to evaluate the profit of the proposed model called Model 2. The first system showed for transporting and distributing solar light is a single long vertical prismatic hollow light guide (Model 1). In the second system we propose to incorporate two 70º prismatic film of 0.5 meters on the top and the lower part of the second and first floors to increase the flux extracted in that intermediate floors (Model 2).

4 363 Colour and Light in Architecture_First International Conference 2010_Proceedings Comparisons in both models (with and without extractor sheet) were performed under a homogeneous parallel light source over a range of incident angles Chromatic behaviour of light pipes A theoretical calculation has been done to analyze the relative difference in spectral radiance and chromatic content between the light transmitted through an aluminum cylindrical guide and a hollow prismatic light guide working in total internal reflection [5]. In order to evaluate these changes, the Cielab displacement has been calculated for each one of the cases associated with the light source and those obtained for the resultant spectral power distribution across light guide. Considering these displacements, the chromatic coordinates CIE 1931 have been calculated for different lengths of guides. 3. RESULTS Fig. 4 (a) Raytrace of the suggested systems called model 1.The structure is a simple prismatic hollow light guide. (b) Raytrace of the suggested systems called model 2. It comprises two heliostats, 90º prismatic hollow light guide and 70º prismatic band sheets in first and second floor light guide (extractor system). Fig. 5: Model 1, graphical presentation of the results from the simulated luminous flux levels for the 15º, 40º y 70º directions of incidence in the building floors. Fig. 6: Model 2, graphical presentation of the results from the simulated Luminous flux levels for the 15º, 40º y 70º 3.1 Light distribution analysis To analyze the light distribution property of the guides, the raytrace showed in Figure 4, was repeated for different directions of incidence in both proposed simulations. The data is shown in Figure 5 and 6 generating flux graphics to characterize the system. Figures 5 and 6 show that daylight with a single prismatic light guide provides the greater illuminance at the ground and third floor. When we analyze the efficiency obtained in both systems adding the extraction system, we observe the improvement with regard to the simple prismatic light guide, reaching a higher efficiency flux (η) (Eq. (1)) between 22 and 64 times more flux in the first and the second floor (Table 2). Total flux obtained at the exit of the office building is showed in table 3. (1) f h = f Model2 Model1

5 364 Colour and Light in Architecture_First International Conference 2010_Proceedings Table 2: Output flux Model 2 vs output flux Model 1. Table 3: Total flux obtained at the exit of the office building. Fig. 7: Spectral representative distributions of both types of guide for different guide s lengths Fig. 8: Chromatic diagram CIE31 of the color to the exit for several guides lengths. Three consecutive points (D5m, D10m and D15m) with three lengths in meters represent the aluminum guide. The results obtained with the illuminant D65 and the prismatic guides (P) are superposed. 3.2 Chromatic behaviour The prismatic guide presents greatest transmission across the whole spectrum (fig.7) and the spectral relative power is constant. In case of the aluminum spectrum, the zone of the short lengths of wave in which it diminishes the energy due to the low spectral reflectance of the aluminum. The chromaticity of the color is specified by the chromatic coordinates CIE 1931 (fig.8). If there are increases in the length of the aluminum guide, the coordinates move towards the zone of the yellow ones, not existing changes in case of the prismatic guide (P). The prismatic guide coordinates mach-up with the illuminant D65 coordinates as a matter of the reflectance is constant for the whole spectrum.

6 365 Colour and Light in Architecture_First International Conference 2010_Proceedings 4. CONCLUSIONS In this work we have showed that it is possible to use an efficient system for getting daylight inside buildings. It is possible to usefully functional daylight systems that also are architecturally beautiful, since it is not necessary to work with opaque pipes as it would be waterpipes o electrics pipes. Light transmittance through light guides depends on optical properties of light guide components as specular reflectance of light pipe and also on the aspect ratio between length and diameter of the light guide. We have developed an efficient system for directing the sunlight into the light pipe per a wide range of sun positions. With this system, a prismatic extraction system is used to provide useful illumination levels inside the building. This design of the prismatic lightguide is an angle-dependent specular reflection model and provides good enough uniformity and no glare extraction. The system obtains a better color behavior in the prismatic guide due to the fact that chromatic displacement does not exist in the exit spectrum. REFERENCES [1] Swift P.D., Smith G.B.. Cylindrical mirror light pipes Solar Energy Materials and Solar Cells, Volume 36, Issue 2, February 1995, Pages [2] Kneipp, K.G Use of prismatic films to control light distribution, p International Lighting in Controlled Environments Workshop, NASA-CP [3] TracePro Opto-Mechanical Design Software [4] Vázquez-Moliní D., Álvarez Fernández-Balbuena A., González-Montes M., Bernabeu E., García-Botella A., García-Rodríguez L., and Pohl W., Guiding daylight into a building for energy-saving illumination. SPIE Newsroom, DOI: / , San Diego [5] García-Fernandez B., Vazquez-Moliní D., Fernandez-Balbuena A.A., Zoido J., Alteraciones cromáticas en diversos tipos de guías de luz, IX Congreso Nacional de Color, Madrid 2010.