Simulations of local heat islands in Zürich with coupled CFD and building energy models

Due to the urban heat island (UHI) effect the air temperatures in urban areas are most of the time higher compared to the temperatures in rural areas. Further the wind speeds are lower due to wind sheltering. In the past decades cities and therefore also the urban heat islands have been continuously growing. The local microclimate has a strong impact on the energy demand of buildings and the human comfort and health. Only few guidelines exist for urban planners to mitigate UHI effects or its impacts. This study aims at simulating the effect of new buildings on the local urban microclimate for a building site in Zürich (Switzerland). One-way coupled CFD and building energy simulations are conducted to determine the increase in air and surface temperatures due the presences of new buildings. The daytime local air temperatures for weather conditions with high ambient air temperatures, which are considered to be most critical for thermal comfort, are studied. The results show that the formation of local hot spots strongly depend on the building geometries, building materials, the strength of buoyancy and the wind directions and wind speeds.