The influence of the sky radiative temperature on the building energy performance

Energy efficiency and thermal protection of buildings

Most of the current sky models correlate with local climatic conditions and specific sites, while not covering different humidity-climatic conditions. In this work, sky emissivity and sky temperature models were reviewed, taking into account the common classification that includes simplified and detailed correlations. The clear-sky and cloudy-sky temperature models were also investigated in detail, employing them for different humidity-climatic conditions (wet, normal and dry) and evaluating their influence on building energy needs. Under clear-sky conditions in winter, the maximum difference between the ambient air temperature and sky temperature is 19 K regardless of humidity-climatic conditions. Under cloudy-sky conditions, it is possible to notice dissimilarities, ranging from 5 K (wet and normal conditions) to 10 K (dry conditions). In summer, under clear-sky conditions, the maximum values are ranging from 12 K (normal conditions) to 13 K (wet and dry conditions). Under cloudy-sky conditions, the maximum values are ranging from 5 K (normal conditions) to 10 K (dry conditions). Thus, the obtained results can be applied for the investigation of the radiative heat flux between a building surface and the sky, as a simplified model. Moreover, these results can be used when the sky temperature is not available from climatic data. The obtained results specified the simplified models according to ISO 13790. Finally, taking into account the influence of different correlations in building energy simulations, it was found that heating and cooling energy demands (using the example of a translucent roof) can be affected by significant percentage differences (the rounded values), ranging from +3 % to +11 % (no heat gain) for wet climatic conditions, from –61 % to +22 % for normal climatic conditions, and, finally, from −45 % to +8 % for dry climatic conditions. The comparison among the models can be useful to address the choice of users in building energy simulations and engineering applications. Future developments will regard the longwave sky radiation measurement under field conditions in the representative cities of the world in order to propose correlations for different climatic area.