Simulated Natural Ventilation – The following examples appear to have been developed from the vernacular principles of ventilative cooling, but use components to further enhance the cooling effects, either by controlling or making reliable the wind flow direction and velocity. This is particularly useful when there is little ventilation available.
1. Direct Ground Cooling:
Even in a hot, humid climate, the earth surrounding the building can act as a direct passive cooling source. If shaded by the underside of the raised home and cooled by summer rains (causing evaporation), a 10-12ºK difference in maximum air and earth temperatures can be achieved (Givoni B. 1994 Florida Study).
This coolth can significantly reduce the temperature of ventilation, and by installing PVC air pipes and simple mechanical devices (fans), the effects can be made more efficient. The cooled ventilation can either be introduced into the building (see fig. 1a) or the warm internal air can be recirculated into the ground, acting as a natural heat sink (see fig. 1b).
2. Indirect Evaporative Cooling:
Ventilation can also be cooled by indirect evaporation before entering a building, using a water reservoir (tank, roof pond), heat exchanger (matting, concrete slab) and a simple fan to control this cooling technique; in essence, a very simplified air conditioner. Via convection currents and long wave radiation, the cooled heat exchanger reduces temperatures of the interior space by cooling the ventilation, without effecting levels of humidity (see fig. 2).
3. Roof Ventilation Devices:
A solar chimney is a device that enhances the stack effect (see fig. 3). Although it cannot independently generate enough air movement to cool the body, it expels hot unwanted air from an interior space, which is as critical. By painting the metal chimney black, the air temperature inside increases, causing a suction effect, and draws the internal air from the space, up and out the chimney.
Mechanical Cooling:
We need to be reminded that “forced ventilation should only be used to compensate for climatic impacts rather than poor buildings design” (Koch-Nielsen H. 2002). The above principles should be applied and, if thermal comfort is still not achieved, only then should mechanical cooling devices be used. This may be the only solution for residences in a dense urban situation where the Heat Island Effect results in higher temperatures, or ventilation is limited. Issues such as safety, pollution, space restrictions and cost can reduce the effectiveness of cooling, as the openness required to optimize ventilation cannot always be achieved. Adjacent high-rise buildings may block or divert airflow and high land values may dissuade developers from creating a microclimate (vertical gardens, for example), which occupy valuable square footage. Low-energy buildings can be more costly during design phase, particularly if Computer Fluid Dynamics are used to predict climate behavior, but this is recovered during running costs. Unfortunately, this frequently happens to be overlooked by clients.
The Malaysian architect, Ken Yeang, stated in the Far Eastern Review that AC cannot be abandoned in the tropics but passive techniques can reduce the need for mechanical cooling devices. Initially, this appears ideal; however, combining ventilative cooling with AC is problematic, as the appropriate design principles for each set of circumstances are quite opposing. Naturally ventilated buildings require maximum ventilation, large openings and surface area, whereas AC is most efficient when ventilation is minimized, openings are small, and the building is compact. If both techniques are combined, the architect runs into the danger of neither the passive nor mechanical cooling techniques achieving maximum efficiency. Instead, they should design for the most intended use.
Conclusion
The vernacular has been developed over many centuries and is therefore particularly fine-tuned to our climatic needs. Not only can it assist with modern passive techniques, but it can and should be applied directly at any available opportunity. Comfort ventilation and the stack effect form the bases of most modern ventilation cooling techniques when considering ground cooling, indirect evaporative cooling and solar chimneys. With the implementation of modern materials and applications, the ventilative cooling technique can be made more effective.
It appears forgotten knowledge is fortunately being relearnt. With this, we need to recognize the potential of passive cooling techniques whilst reducing our reliance on AC. Technology is here, it can work to enhance the vernacular, and the combination should be embraced to achieve the optimum. “This can be and has to be done by not copying/faking the old designs but drawing lessons recognizing the values and reinterpreting them in the present context” (Vernacular Architecture in Queensland, Australia). However, in some situations, some compromises need to be made, and if these mean the implementation of mechanical devices, the occupant should ensure these are powered by renewables and designed to maximize efficiency.
Vic Bezemer
Currently Studying Masters Architecture: Advanced Environmental Energy Studies
Interior Architect: Roger Downing & Partner co. ltd. BVI
Contact: [email protected]
