Thermal Resistance of a Window with an Enclosed Venetian Blind: A Simplified Model

Abstract

Solar gain has a strong influence on building energy consumption and peak cooling load. Venetian blinds are routinely used to control solar gain. Software based on 1-D models is available to accurately predict the thermal performance of glazing systems but the development of models for shading devices is at a very early stage. An accurate model has been formulated to quantify the thermal resistance of a glazing system with an enclosed venetian blind. It is possible to account for pane spacing, slat angle, alternate fill gases and the presence of a low-emissivity coating. Effective longwave optical properties are assigned to the blind layer in order to calculate radiant heat transfer. An exceptionally simple model for convective heat transfer, the reduced slat length (RSL) model, has been developed on the basis of guarded heater plate measurements. CFD results reveal reasons for the very close agreement between measurement and the RSL model. The new simulation capability can be applied to the quantification of U-factor and Solar Heat Gain Coefficient. The simplicity of the RSL model is particularly valuable in the context of building energy simulation where CPU time must be used sparingly.