The Performance of Natural Ventilation Windcatchers in Schools - A Comparison between Prediction and Measurement

Windcatchers are roof mounted devices that use the action of the wind to provide top down natural ventilation to a room. Here, fresh air is channelled into a room while, at the same time, stale air is drawn out. This provides a simple but attractive natural ventilation methodology that is increasing in popularity in U.K. schools. However, an analysis of system performance has largely been limited to laboratory based measurements and the use of CFD to generate predictions. Moreover, analysis is normally restricted to the operation of an autonomous Windcatcher whereas, in reality, it is likely to operate in a building in which other sources of ventilation are present (an open window for example) which can significantly alter the performance. The aim of this paper is to provide a tool for estimating the performance of a Windcatcher from basic data that is typically available to the engineer in the building design phase. Accordingly, the methodology uses data that one could reasonably be expected to have for a building's ventilation performance. This paper also reviews in situ performance based on measurements in U.K. schools both with and without open windows. Predictions generated by a semi-empirical model are then compared against measurement data and this is shown to deliver generally good agreement between the two, both with and without open windows, provided the theoretical predictions are presented in terms of an upper and lower performance limit. Furthermore, both experiment and theory clearly demonstrate that a large increase in the ventilation rate is possible if one combines the operation of a Windcatcher with, say, an open window, and that this ventilation rate is greater than that which would be achievable from a window operating on its own.