Quantifying the performance of natural ventilation windcatchers

Abstract

The significant energy consumption of non- domestic buildings has led to renewed interest in natural ventilation strategies that utilise the action of the wind, and the buoyancy of hot air. One natural ventilation element is the Windcatcher, a roof mounted device that works by channelling air into a room under the action of wind pressure, whilst simultaneously drawing air out of the room by virtue of a low pressure region created downstream of the element. A significant number of Windcatchers are fitted in UK schools where good indoor air quality is essential for the health and performance of children. The performance of a ventilation system in a school classroom is determined by its ability to provide ventilation in accordance with UK government ventilation, air quality, and acoustic requirements. However, there is only limited performance data available for a Windcatcher, particularly when operating in-situ. Accordingly, this thesis investigates the performance of a Windcatcher in three ways: First, a semi-empirical model is developed that combines an envelope flow model with existing experimental data. Second, measurements of air temperature, relative humidity, carbon dioxide, and noise levels in school classrooms are assessed over summer and winter months and the results compared against UK Government requirements. Finally, air flow rates are measured in twenty four classrooms and compared against the semi-empirical predictions. The monitoring reveals that air quality in classrooms ventilated by a Windcatcher has the potential to be better than that reported for conventional natural ventilation strategies such as windows. Furthermore, an autonomous Windcatcher is shown to deliver the minimum ventilation rates specified by the UK Government, and when combined with open windows a Windcatcher is also capable of providing the required mean and purge ventilation rates. These findings are then used to develop an algorithm that will size a Windcatcher for a particular application, as well as helping to improve the ventilation strategy for a building that employs a Windcatcher.