Solar heat gain through fenestration systems containing shading: Summary of procedures for estimating performance from minimal data - eScholarship

SOLAR HEAT GAIN THROUGH FENESTRATION SYSTEMS CONTAINING SHADING: SUMMARY OF PROCEDURES FOR ESTIMATING PERFORMANCE FROM MINIMAL DATA J. H. Klems Building Technologies Department Lawrence Berkeley National Laboratory Berkeley, CA 94720 Abstract The computational methods for calculating the properties of glazing systems containing shading from the properties of their components have been developed, but the measurement standards and property data bases necessary to apply them have not. It is shown that with a drastic simplifying assumption these methods can be used to calculate system solar-optical properties and solar heat gain coefficients for arbitrary glazing systems, while requiring limited data about the shading. Detailed formulas are presented, and performance multipliers are defined for the approximate treatment of simple glazings with shading. As higher accuracy is demanded, the formulas become very complicated. INTRODUCTION Several years ago ASHRAE sponsored a research project, 548-RP, to develop a method of determining the solar heat gain coefficient for complex fenestration systems, i.e., those containing non-specular (and presumably geometrically complex) sun-control or visibility elements, such as shades, venetian blinds, or translucent glazings. These systems had been characterized using the shading coefficient, a concept developed when only clear single glazing was common. The basic idea behind the shading coefficient was that for most systems the chief dependence of the solar transmission on wavelength and incident angle came from the glass layer, and the reflectance and absorptance of this layer were not large. It was therefore possible to treat shading systems as providing a modifying factor to the single glass transmittance, and this factor being assumed independent of incident angle, it could be determined by a single measurement in a calorimeter. As multiple glazings and coated glasses were introduced, the basic assumption of the shading coefficient ceased to be valid, and it was gradually replaced with the solar heat gain