Passive Cooling of Buildings by Night-time Ventilation

In modern, extensively glazed office buildings air conditioning is increasingly applied even in moderate and cold climates. Night-time ventilation is often seen as a promising passive cooling concept. However, due to uncertainties in the prediction of thermal comfort, architects and engineers are still hesitant to apply passive cooling techniques.

The basic concept involves cooling the building structure overnight in order to provide a heat sink during the occupancy period. As this requires a sufficiently high temperature difference between the ambient air and the building structure, the efficiency of night cooling is highly sensitive to climatic conditions and hence also to climate warming. A method for quantifying the climatic cooling potential (CCP) was developed and application to climatic data of 259 stations shows significant night cooling potential over the whole of Northern, Central and Eastern Europe. Based on scenarios for future emissions of greenhouse gases and aerosols changes in CCP for 1990-2100 was found to decrease substantially by the end of the 21st century. In Northern Europe, a significant passive cooling potential is likely to remain.

Because heat gains and night ventilation periods typically do not coincide in time, heat storage is essential for effective night cooling. The results show a significant impact of the heat transfer coefficient on heat storage capacity, especially for thick, thermally heavy elements and that for low air flow rates displacement ventilation is more efficient than mixing ventilation. For higher airflow rates the air jet flowing along the ceiling has a significant effect, and mixing ventilation becomes more efficient. By combining the results of the previous steps, a practicable method for the estimation of the potential for cooling by night-time ventilation during an early stage of design is proposed.