Top-down precooled natural ventilation

This paper studies the summertime regime of precooled natural ventilation of an auditorium or other occupied open-plan space equipped with a high-level vent and a low-level vent. A chiller unit is connected to the high-level vent to provide precooling. Fresh air from the exterior comes into the room through the high-level vent passing through the precooling system. Precooled air then produces negative (downward) buoyancy, which overcomes positive (upward) buoyancy produced by the occupants, and displaces original air in the room downwards and out through the low-level vent. This leads to an equilibrium in which a steady downflow is maintained, and in which the room becomes thermally uniform at a temperature below that of the exterior. A quantitative model is developed to describe these conditions and successfully tested with analogue laboratory experiments. The model shows that for a given room geometry and chiller, there is a maximum heat load which can be accommodated while maintaining ventilation and thermal comfort through downward ventilation. We show how effective and energy-efficient ventilation may be achieved through coordinated adjustment of the vent area and the amount of cooling. Practical application: Top-down precooled natural ventilation can be an effective and energy efficient technique for providing thermal and ventilation comfort in a wide range of modern buildings during high summer or in warm climates. The present work describes how the system works, and how it may be controlled to achieve satisfactory results in terms of comfort and energy efficiency.