Long-wave radiationĪ fictive sky temperature, dependent on ambient temperature, emissivity, and cloudiness, is introduced to account for the long-wave radiative heat exchange between the building envelope and the sky. Normally, thermal building-dynamics simulation programs allow for the consideration of such shading. Also, certain wings or parts of the building itself may shade the part under investigation permanently or over certain time periods. Since insolation often has a very significant effect on the heat balance of a building, shading by buildings or other objects in the surroundings of the building must be taken into account. Normally, for energy analysis, this is not critical, but it may have a significant effect on the natural ventilation of multistory buildings. Neither effect normally is accounted for in building simulation programs. This changes the convective heat transfer and leads to increased temperatures of supply air for natural ventilation. In low-wind conditions, free convective flows drift up the warm external wall surface. The solar radiation absorbed on external building surfaces increases the wall surface temperature, thus leading to a change in the heat conducted through the component. Depending on the model used, discrepancies for the boundary conditions may occur with the same basic set of solar radiation data, thus leading to differences in the simulation results. Several diffuse sky models are available. ![]() It is composed of three parts, referred to as isotropic, circumsolar, and horizontal brightening. ![]() The diffuse sky radiation is not uniform. For nonhorizontal surfaces, the diffuse radiation is composed of the contribution from the diffuse sky and reflections from the ground. In the simulation, solar radiation input values must be converted to radiation values for each surface of the building. The total or global solar radiation has a direct part (beam radiation) and a diffuse part (see Fig. Wind affects the convective heat transfer on external walls and is a driving force for natural ventilation. Year-round outdoor air humidity must be considered when studying condensation conditions. Outdoor air humidity strongly affects the latent cooling load and energy requirements during the summer season. Moreover, outdoor air temperature is a driving force for natural ventilation, as the difference between indoor and outdoor air temperature causes the stack effect. Outdoor air temperature affects the heat transfer through external walls and roofs and the heat transfer by ventilation. Outdoor air temperature is an important factor regarding the building energy balance. Yang Yang, in Industrial Ventilation Design Guidebook (Second Edition), 2021 3.4.2.4 Outdoor conditions Outdoor air temperature
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