Nov 21 2019
Scientists from the University of Cambridge have assessed the effectiveness of non-mechanical, low-energy techniques for controlling temperature and humidity in a series of experiments.
They learned that a difference in temperature between inside and outside has very little impact on how well a room is ventilated when ventilation is mainly powered by wind.
On the other hand, wind can boost ventilation rates by nearly 40% above that which is induced by a temperature difference between a room and the outdoors. The precise rate of ventilation will be subject to the room’s geometry.
The study outcomes have been published in the journal Building and Environment, and could be applied to assist designers and urban planners to integrate natural ventilation principles into their designs, to enable buildings to be maintained at a comfortable temperature while using less energy.
Heating and cooling constitute a major proportion of energy consumption in buildings: in the United States, this is as high as 50%. Furthermore, as temperatures worldwide continue to increase, the need for air-conditioning—which releases greenhouse gases—also increases, forming a damaging feedback loop.
Natural ventilation, which regulates indoor temperature without using any mechanical systems, is a substitute for conventional heating and cooling techniques, which decreases energy usage and greenhouse gas emissions.
Natural ventilation is a low-energy way to keep buildings at a comfortable temperature, but in order to increase its use, we need simple, accurate models that can respond quickly to changing conditions.
Dr Megan Davies Wykes, Study Lead Author, Department of Engineering, University of Cambridge
There are two key types of natural cross-ventilation: buoyancy-driven and wind-driven. Cross-ventilation happens in rooms that have windows situated on opposite sides. Wind that blows on a building can cause a high pressure on the windward side and a low pressure at the leeward side, which forces flow across a room, supplying fresh air in from outside and ventilating a room.
Ventilation can also be induced by differences in temperature between the outside and inside of a room, since incoming air is heated by equipment or people, leading to a buoyancy-induced flow at a window.
We’ve all gotten used to having a well-controlled, narrow temperature range in our homes and offices. Controlling natural ventilation methods is much more challenging than switching on the heat or the air conditioning, as you need to account for all the variables in a room, like the number of people, the number of computers or other heat-generating equipment, or the strength of the wind.
Dr Megan Davies Wykes, Study Lead Author, Department of Engineering, University of Cambridge
In this study, the scientists used a tiny model room positioned inside a flume to recreate the movements of air within a room when windows are kept open in different wind and temperature conditions.
Davies Wykes and her colleagues applied the results from lab-based experiments to design mathematical models to predict how the difference in temperature between outside and inside impacts how well a room is ventilated.
The scientists discovered that the rate of ventilation relies less on temperature and more on wind. Anyone who has opened a window to try to cool down a room on a hot night will no doubt be aware of how ineffective this is when wind is not present.
This is because, in a number of rooms, windows are built halfway up the wall, and when they are opened, the warm air near the ceiling cannot easily escape. Without the “mixing” effect of the wind, the warm air will remain at the ceiling, unless there is a different way for it to escape at the top of the room.
It was surprising that although temperature differences do not have a strong effect on the flow of air through a window, even small temperature differences can matter when trying to ventilate a room. If there are no openings near the ceiling of a room, warm indoor air can become trapped near the ceiling and wind is not effective at removing the trapped air.
Dr Megan Davies Wykes, Study Lead Author, Department of Engineering, University of Cambridge
The future steps will be to integrate the results into building design, rendering it easier to develop well-ventilated, low-energy buildings.
The research was conducted as a part of the Managing Air for Green Inner Cities (MAGIC) project, which is building computer models for natural ventilation, so that designers can integrate natural ventilation into city design, decreasing the demand for energy. The MAGIC project is sponsored by the Engineering and Physical Sciences Research Council (EPSRC).
Natural ventilation experiment
False color corresponds to intensity normalized by background intensity. Red corresponds to dyed warm fluid, while blue is undyed cooler ambient fluid. The external flow is from left to right. Video Credit: Megan Davies Wykes and El Khansaa Chahour.