A SIMPLE CHART TO DESIGN SHADING DEVICES CONSIDERING THE WINDOW SOLAR ANGLE DEPENDENT PROPERTIES Marie-Claude Dubois Department of Construction & Architecture, Lund University P.O. Box 118, 221 00 Lund, Sweden, tel. +46 46 222 7347, fax +46 46 222 4719, marie-claude.dubois@bkl.lth.se Abstract – A simple chart useful to design shading devices is presented. The chart, which is complementary to existing solar path diagrams, provides additional information about the window’s solar angle dependent properties and its geometrical relationship to the sunbeam. This information allows to make meaningful hypotheses about the optimum geometry of the shading device. Two examples are provided where the chart is used to define the geometry of an awning on a south- and west-oriented office room in Stockholm. The examples show that the chart is useful to restrict the early design hypotheses and identify the optimum awning geometry at an early design stage. 1. INTRODUCTION Solar shading devices can substantially reduce the cooling load of buildings. According to a recent literature review (Dubois, 1997), this reduction is between 23-89% depending on the type of shading device used, the building orientation, the climate, etc. In order to save energy, shading devices should be integrated to a building’s facade at an early design stage. This can be achieved using ”traditional” design tools like solar path diagrams and shading masks or special computer programs that automatically ”generate” the optimum shading device geometry as a function of a set of input parameters (e.g. orientation, latitude). 1.1 Traditional tools Although there exist numerous design methods based on solar path diagrams (Dourgnon, 1965; Van den Eijk, 1965; Markus & Morris, 1980; Etzion, 1992), the Olgyays’ (1957) and Mazria’s (1979) methods are probably the most popular ones. In both the Olgyays and Mazria’s design methods the building’s overheating period is plotted onto the solar path diagram and a shading ”mask” that avoids direct sun during the overheating period is defined. The main difference between the two methods is the kind of solar projection used. The Olgyays used a projection of the sun onto a horizontal plane parallel to the ground (Fig. 1) while Mazria used a projection onto a vertical cylinder (with the long axis perpendicular to the ground). By “unfolding” the cylinder, a two-dimensional diagram is obtained, where the abscissa and ordinate represent the solar azimuths and altitudes and where the curves radiating away from the south represent the solar time (Fig. 2). This projection is advantageous for studies of facade elements like windows and shading devices since the sun’s projection is viewed “parallel” to the building facade. Traditional methods have some limitations: their accuracy is limited by the size of the charts and they yield shading devices that are larger than necessary since they are only capable of returning a “binary” answer (Etzion, 1992). This is due to the fact that they indicate an “unshaded” condition even when a small area of the opening is lit by direct sun and a “shaded” condition the rest of the time. Fig. 1 Solar path diagram used by the Olgyays (1957). Fig. 2 Solar path diagram used by Mazria (1979).