On the sky temperature models and their influence on buildings energy performance: A critical review

Since the early 20th century, many studies have been conducted on sky temperature models and sky emissivity. These models are related to local weather conditions and specific sites. Therefore, the existing variations among the models have been an important factor for the development of new models for different locations. The existing models do not cover the whole planet even though sky temperature assessment is necessary to evaluate the net radiative heat transfers between surfaces and sky vault. So, considering building energy performance, radiative cooling and other engineering purposes, the evaluation of sky temperature is fundamental and needs to be properly accounted. This study aims at providing a critical review about the existing correlations for the calculation of sky emissivity and sky temperature, referring to different climatic conditions. Firstly, available correlations were classified and described. Some models were then applied to various significant locations all over the world to highlight differences as far as sky emissivity and sky temperatures. Subsequently, some correlations were implemented in a dynamic simulation code, to assess their influence on building annual energy needs. For this reason, a sample building was considered and the influence of different correlations on heating and cooling energy demands was investigated. The results were compared with the sky temperature values obtained by applying the standard ISO 13790, which suggests simplified correlations when the sky temperature is not available from climatic data. Comparing the analyzed models, significant deviations were found: from 10.8 °C to 19.7 °C under clear sky conditions and from 7 °C to 17.2 °C under cloudy sky conditions