Natural ventilation systems rely on pressure differences to
move fresh air through buildings - pressure differences can be caused by wind
or the buoyancy effect created by temperature differences or differences in
humidity.
In either case, the size and placement of openings in the
building is critical - useful to think of a natural ventilation system as a
circuit, with equal consideration given to supply and exhaust.
Wind causes a positive pressure on the windward
side and a negative pressure on the leeward side of buildings. To equalize
pressure, fresh air will enter any windward opening and be exhausted from any
leeward opening.
Buoyancy ventilation may be temperature-induced
(stack ventilation) or humidity induced (cool tower). The two can be combined
by having a cool tower deliver evaporatively cooled air low in a space, and
then rely on the increased buoyancy of the humid air as it warms to exhaust air
from the space through a stack.
NB. cool towers should be used in conjunction with stack
ventilation of the space in order to ensure stability of the flow.
Buoyancy results from the difference in air density: cool air is heavier than warm, and dry air is
heavier than humid at the same temperature.
Within a cool tower, the effect of temperature and humidity
are pulling in opposite directions (temperature down, humidity up).
Within a room, heat and humidity given off by occupants and
other internal sources both tend to make air rise. The stale, heated air
escapes from openings in the ceiling or roof and permits fresh air to enter lower
openings to replace it.
Stack effect ventilation is effective in winter, when
indoor/outdoor temperature difference is at a maximum but not so effective in
summer (wind or humidity drivers would be preferred) because it requires that
the indoors be warmer than outdoors (not desirable in summer). However, a
chimney heated by solar energy can be used to drive the stack effect without
increasing room temperature.
NB. Cool tower ventilation is only effective
where outdoor humidity is very low.
Solar Chimney (or thermal chimney)
= a way of improving the natural ventilation of buildings by
using convection of air heated by passive solar energy. A simple
description of a solar chimney is that of a vertical shaft utilizing solar
energy to enhance the natural stack ventilation through a building.
Windcatcher
(1) Downward in flow of air due to direct wind entry
The badgir = an architectural feature to cool the inside of
the dwelling, often used in combination with courtyards and domes as an overall
ventilation / heat management strategy.
The malqaf = essentially a tall, capped tower with one
face open at the top. This open side faces the prevailing wind, thus 'catching'
it, and bringing it down the tower into the heart of the building to maintain
air flow, thus cooling the interior of the building. This is the most direct
way of drawing air into the building, but importantly it does not necessarily
cool the air, but relies on a rate of air flow to provide a cooling effect.
This use of the malqaf or windcatcher has been employed in this manner for
thousands of years, as detailed by contemporary Egyptian architect Hassan
Fathy.
(2) Upward flow of air due to a wind assisted
temperature gradient
NB. qanat = underground canal. The open side of the tower
faces away from the direction of the prevailing wind so air is drawn upwards
using the Coandă effect, similar to how opening the one facing towards the wind
would pull air down into the shaft.
As there is now a pressure differential on one side of the
building, air is drawn down into the passage on the other side. This hot air is
brought down into the qanat tunnel, and is cooled by the combination of coming
into contact with the cold earth (as it is several meters below ground, the
earth stays continuously cool) as well as the cold water running through the
qanat. The air is therefore cooled significantly, and is then drawn up through
the windcatcher by the same Coandă effect. This brings cool air up through the
building, cooling the structure overall, with the water vapour from the qanat
having an added cooling effect.
(3) Upward flow of air due to a solar produced
temperature gradient
Finally, in a windless environment or waterless house, a
windcatcher functions as a solar chimney.
It creates a pressure gradient which allows less dense hot
air to travel upwards and escape out the top. This is also aided by the
day-night cycle, trapping cool air below; temperature in such an environment
cannot drop below the nightly low temperature.
When coupled with thick adobe that exhibits good heat
transmission resistance qualities, the windcatcher is able to chill lower level
spaces in mosques and houses (e.g. shabestan) in the middle of the day to
frigid temperatures (windcatchers in Persian architecture have been routinely
used as a refrigerating device for ages). The evaporative cooling effect is
strongest in the driest climates.
General design considerations
- Maximize
wind-induced ventilation by siting the ridge/long facade of a building
perpendicular to the summer winds.
- Naturally
ventilated buildings should be narrow.
- Each
room should have two separate supply and exhaust openings.
- Locate
exhaust high and inlet low - to maximize stack effect.
- Orient
windows across the room and offset from each other to maximize mixing
within the room while minimizing the obstructions to airflow within the
room.
- Window
openings should be operable by the occupants.
- Provide
ridge vents (an opening at the highest point in the roof that offers a
good outlet for both buoyancy and wind-induced ventilation).
- Consider
the use of clerestories or vented skylights.
- good
for night time thermal comfort in houses to vent heated/warm air
that rises, and allow heat to be radiated into the cold; also a good
outlet for wind driven ventilation
- Allow
for adequate internal airflow
- Consider
the use of fan-assisted cooling strategies
- Determine
if the building will benefit from an open- or closed-building ventilation
approach.
- A
closed-building approach works well in hot, dry climates where there is a
large variation in temperature from day to night. A massive building is
ventilated at night, then, closed in the morning to keep out the hot
daytime air. Occupants are then cooled by radiant exchange with the
massive walls and floor.
- An
open-building approach works well in warm and humid areas, where the
temperature does not change much from day to night. In this case, daytime
cross-ventilation is encouraged to maintain indoor temperatures close to
outdoor temperatures.
- Use
mechanical cooling in hot, humid climates.
- Try to
allow natural ventilation to cool the mass of the building at night in hot
climates.
- Open
staircases provide stack effect ventilation, but observe all fire and
smoke precautions for enclosed stairways.
- Increase
air supply intake by ensuring no outside obstruction (such as vegetation
or site objects) nor inside obstruction (such as furniture and interior
partition) obstruct inlet openings
- Have
at least a 3m height between floor and ceiling (?)
- Window
areas should not be excessive and be protected by exterior shading devices
- Reduce
the possibility of wall warming by the sun:
- light-coloured
building exteriors
- trees/shrubs
to provide shading and evaporative cooling
- grass
and other groundcover to keep ground temperatures low
- ponds
and fountains to enhance evaporative cooling
Many of the above either increase the air flow or lower the
heat gain so that the natural ventilation can effectively cool the spaces in
the building.
Mechanical cooling and ventilation systems will be used to
supplement the natural ventilation. By lowering the heat gains, the less air
flow will be required to remove the heat, thus there will be a lesser need for
mechanical cooling systems.
