A simple step heating approach for wall surface temperature estimation in the SOlar and LongWave Environmental Irradiance Geometry (SOLWEIG) model

Mean radiant temperature (T_mrt) is one of the most important meteorological variables for human thermal comfort on clear, calm and hot days. It is the sum of all short- and longwave radiation that a human is exposed to. In urban environments, a large part of the radiation received by a person consist of emitted longwave radiation from vertical sunlit facades, especially when standing in close vicinity to such surfaces. This requires accurate estimations of wall surface temperatures. Here, we present a new simple approach based on a step heating equation to calculate wall surface temperatures in the SOlar and LongWave Environmental Irradiance Geometry (SOLWEIG) model. Wall surface temperatures are calculated based on short- and longwave radiation that the wall surface is exposed to together with air temperature and information of thermal conductivity, specific heat capacity, density, wall thickness, albedo and emissivity. Moreover, wall surfaces are divided into unique voxels that enable partial shading of a wall. The approach presented here is fast, as opposed to often commonly used conduction models that require detailed information on different layers of a wall to estimate heat transfer and the resulting surface temperature. Its implementation into the SOLWEIG model enables differentiation of buildings materials into e.g. wood, concrete and brick, which was previously not possible. The minimal input data required on wall surface characteristics also makes it easier to incorporate new wall materials.