Effect of internal long wave radiation and convection on fenestration simulation

This paper presents the results of the first part of a research project. The objective of the project is to solve accurately the heat balance on a single glass window in a moderate climate. This procedure will allow the designer to calculate the main parameters of a fenestration (surface, orientation, overhangs, etc.), minimising the energy consumption for a specific enclosure (including heating, air conditioning and lighting). The main problem which most of existent models present, when trying to make the heat balance for winter conditions in a moderate climate, is that internal convection and long wave radiation are not considered efficiently. For this case, some of the existent models can predict the heat losses with an error of about 50%. The present study is mainly focused on problems related with the internal long wave radiation calculations, and secondly on the effect of internal convection coefficient on the heat balance. In the present paper, a detailed model to calculate internal radiation is proposed. A comparison between the results obtained using this model and those obtained with other current models is included. The importance of using the right internal convection coefficient on the heat balance of fenestration for this kind of application is also analysed. The proposed model calculates the internal radiation taking into account the radiation angle factor between the window and the other internal surfaces and the actual temperature of each internal surface. To calculate the temperature of the internal surfaces, the actual long wave radiation balance and the right quantity of solar radiation absorbed by each internal wall is considered.