External environmental temperature: proposed new formulation

Abstract

Heat transfer calculation across the external envelope of a building is, generally, made by assuming a hypothetical temperature for the outside environment called ‘sol-air temperature’. The sol-air temperature model has obvious limitations since it neglects the impact of factors of great significance such as the emissivity and configuration factors of the surrounding obstructions, the ground and the sky. Attempts made to include long-wave radiation exchange between building surfaces and the surrounding environment, have been over-simplified by assuming the surroundings to be at air temperature. However, without detailed information concerning the nature of such surroundings this assumption will remain unjustifiable and may lead to unreliable calculations of heat transfer. The thermal performance of a wall or a roof will vary with the effect upon its exposure to the sun, the ground, the sky and the proximity of obstructions and the air movement, as these may influence long-wave radiation and convection heat transfers. A new formulation for sol-air temperature prediction which allows for these factors is introduced and is presented in this paper. Numerical calculations were performed using monthly mean hourly data of the outside air temperature, wind speed and solar radiation for El-Oued in southern Algeria and for Bracknell in southern England. Results showed little difference between the present model and the sol-air temperature model for high wind speeds, whereas for low wind speeds the difference is significant.