Natural Ventilation systems rely on natural driving forces, such as wind and temperature difference between a building and its environment, to drive the flow of fresh air through a building. Both work on the principle of air moving from a high pressure to a low pressure zone. Natural ventilation systems are usually integrated into building systems where there is some mechanical support; these are called mixed mode or hybrid ventilation buildings. The main benefit of some augmentation by mechanical systems is that there is less unpredictability with indoor environment conditions, though it will result in greater energy use.
Natural ventilation can be an appropriate choice when compared to air conditioning in the temperate climate of Victoria, particularly as the nights are cool and this can be used to pre-cool the building (see CH2 for case study of natural ventilation used for night purge www.ch2.com.au). It can save substantial amounts of energy by decreasing or eliminating the need for mechanical cooling. It may also improve the building’s indoor air quality. Buildings with well-designed natural ventilation systems often provide very comfortable and pleasant environments for the occupants.
There are two fundamental approaches to designing for natural ventilation that will be effective in most Victorian situations:
> Cross ventilation which uses air-pressure differentials caused by wind
> Stack ventilation which uses the increased buoyancy of air as it warms up
Depending on the approach when using natural ventilation, a conscious choice may need to be made by the building users to have greater fluctuations in indoor thermal conditions. The temperature will not always be predictable to plus or minus half a degree, as the use of external air will, if it is not tempered using mechanical systems, mean that temperature and humidity vary.
Design issues to consider with cross ventilation
Using cross ventilation will have a strong influence on building aesthetics and site planning. To maximize the effectiveness of openings, narrow buildings with open plans and well placed openings work best (particularly if the longest faces of the building are perpendicular to the typical wind direction). Furthermore, single-loaded corridors (rooms only on one side of a corridor) will provide better airflow than double-loaded ones as it makes it easier to provide openings on opposite walls. Building elements like fins, wing walls, parapets and balconies may be designed to enhance wind speeds and should be an integral part of cross-ventilation design though caution needs to be taken that they do not cause turbulence and block air flow)4
In one of my high rise project, we are determine to apply bioclimatic approach.
At the attic -crown, we attempt to provide natural cross ventilation, for the hot air which reheated under the glass room.
these ventilation will drawn the hot air out, to be replaced by fresh air.
Although the outside natural air is not cool at midday, but its flowing and changes effects the temperature of the glass or the building skin.
apendix
1. http://www.geocities.com/ResearchTriangle/Facility/8776/Pag01I.htm
the bio climatic approach in high rise building, is one of some important aspect to save energy.
2. idem
3. http://www.ellipsis.com/yeang/theory/040.000.html#top
4. http://www.resourcesmart.vic.gov.au/documents/Natural_Ventilation_Systems.pdf
