Abstract
This research focuses on the influence of outdoor convective heat transfer coefficient on thermal load calculation in Building Energy Simulation programs. In building energy is commonly used the overall heat transfer coefficient value (U-value) to characterize external walls and windows in terms of heat gain or loss through them. Thus, the higher the U-value the lower the thermal resistance and the larger the heat transfer through the enclosure element and vice versa. Calculating the mentioned value involves not only the layers making up the building element, but also the internal and external surface resistances. The external surface resistance has the greatest level of uncertainty. This resistance is calculated as the inverse of the sum of the outdoor convective and radiant heat transfer coefficients. Regulations and Building Energy Simulation programs in Spain (LIDER and CALENER) use a constant outdoor convective heat transfer coefficient value. This study analyses the importance of a detailed calculation of the outdoor convective heat transfer coefficients on the thermal loads of buildings, distinguishing between two types of envelope elements: facade walls and windows. The extrapolation of the dimensionless numbers quantifying the convective heat transfer to outdoor air velocity and temperature difference between the external surface and outdoor air has been done. Therefore, the corresponding correlation can be selected in order to determine the outdoor convective heat transfer coefficient as a function of easily measurable parameters. Once the outdoor convective heat transfer coefficient has been calculated by using the appropriate correlation, the relative error on the thermal load calculation due to assuming a constant reference outdoor convective heat transfer coefficient value is estimated. Firstly, the maximum possible relative error on the thermal load calculation has been delimited. Then, the estimation of the relative error for different representative cases has been developed for facade walls and windows. The results show that above a certain U-value of reference, the relative error on the thermal load calculation due to assuming a constant outdoor convective heat transfer coefficient can reach 34.66% by overestimation. Keywords: Building Energy Simulation programs, Overall Heat Transfer Coefficient, Outdoor Convective Heat Transfer Coefficient, Thermal Loads.
Published Version
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