A framework for rapid diagnosis of natural ventilation effect during early design stage using Thermal Autonomy

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ABSTRACT Natural ventilation is one of the most important passive strategies for optimizing building energy saving and environmental performance. However, it is difficult for natural ventilation design to realize the utmost utilization of ambient energy based on the conventional design process due to the design performance of natural ventilation is significantly influenced by the schematic plan of a building. Therefore, this study introduces a design process using a concise model of Thermal Autonomy as an evaluation metric for the early design stage. To apply the proposed metric to modern office buildings, which are equipped with effective HVAC systems, a practical and common thermal comfort criterion of 21°C–27°C is adopted to replace the adaptive comfort standard in the original Thermal Autonomy quantification. The availability of the proposed metric for rapid diagnosis is evaluated through parametric simulation based on a typical office building in Shanghai. The evaluation result and the visual map of Thermal Autonomy are verified to determine the proper air change rate and to help architects to understand the design performance of their natural ventilation scheme. In the case study, the proper air change rate is estimated at 8 ac/h corresponding to a comfort hour of 1314 h. This comfort hour value accounts for the maximum value of 50% during the occupied hour. In addition, the effect of Thermal Autonomy in decision-making regarding envelope insulation and initial operation conditions is examined. It is beneficial to maintain the indoor thermal environment by setting the lower outdoor temperature limit for opening windows at 17°C under a higher feasible ventilation volume. A relatively high envelope insulation level with 50 mm or 75 mm insulation layer is advantageous for improving Thermal Autonomy under a larger ventilation volume condition. The method proposed in this study is practical for objectively evaluating the design performance of natural ventilation during the early design stage.