Wind Towers  - passive cooling - Iran

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Wind Towers - passive cooling - Iran

source  visit by Earth Science Australia
             http://www.solaripedia.com/13/205/2089/wind_tower_yazd_iran.html
             http://www.solaripedia.com/13/205/2085/wind_tower_convection_illustration.html
             http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.466.2985&rep=rep1&type=pdf

What are wind towers?
Functions of a wind tower
The wind tower function based on several principles
Cooling water in cisterns
      see also Qanats - re-directing underground water in Iran...
      see also Vertical shaft windmills in Iran...


wind towers in Yazd Iran

What are wind towers?

Wind tower is a key element in traditional architecture of Iran. It is seen in settlements in hot, hot-dry and hot-humid climates. They look like big chimneys in the sky line of ancient cities of Iran. They are vertical shafts with vents on top to lead desired wind to the interior spaces and provide thermal comfort. This architectural element shows the compatibility of architectural design with natural environment. It conserves energy and functions on the basis of sustainability principles.
A wind tower is an architectural device used for many centuries to create natural ventilation in buildings.

internal pool on lower floor in a house

Functions of a wind tower

The function of a wind tower tower is to catch cooler breeze that prevail at a higher level above the ground and to direct it into the interior of the buildings. It is not known who first invented the wind tower, although some claim it originated in Iran and it can be seen in. wind towers come in various designs, such as the uni-directional, bi-directional, and multi-directional.

wind towers in Kerman Iran

A wind tower is a formal structural element in Iranian architecture that is used to convey the wind current to the interior spaces of buildings in order to provide living comfort for occupants. In Iranian architecture a wind tower is a combi- nation of inlet and outlet openings. The tunnel provides cool air for the building while serving as a conduit through which the stuffiness within the building is conveyed through its shaft. There were wind towers in Bam which were destroyed by earthquakes; they weren't directly connected to the living hall. They were built away from the house. An additional underground tunnel links the base of the wind tower to the basement.

wind tower design incorporating passive cooling from a water supply

In most wind towers, especially the four sided types, the tower is divided by partitions. One of the shafts operates all the time to receive the breeze and the other three shafts work as outlet air passages. They convey the stuffiness out of the living space through the “flue” (chimney) effect. The chimney effect is based on the principle that the air density increases with the increase in temperature. The difference in temperature between the interior and exterior parts of a building and between different regions creates different pressures and result in air cur- rents. The average relative humidity in moisture in hot and dry regions is low and it is necessary more humidity there for wind towers are used to provide living comfort through the use of the air current and evaporation. Through the wind tower, the air current first passes over a stone pond and fountain after entering a building, thereby bringing humidity to the other spaces in the building
 
urban wind towers

The wind tower function based on several principles:

  1. First, a wind tower is capped and has several directional ports at the top (traditionally four). By closing all but the one facing away from the incoming wind, air is drawn upwards using the Coanda effect, similar to how opening one facing the wind would push air down the shaft. This generates significant cooling ventilation within the structure below, but is not enough to bring the temperature below ambient alone - it would simply draw hot air in through any cracks or windows in the structure below.
  2. Therefore, the key to generating frigid temperatures seems to be that there are very few cracks at the base of the thick structure below, but there is a significant air gap above the qanat (a water management system used to provide a reliable supply of water to human settlements or for irrigation in hot, arid and semi-arid climates). A qanat has quite a lot of water inside, because there are frequent well-like reservoirs along its path. Completely shaded from the sun, a qanat also aggregates the cold, sinking air of the night, which is then trapped within, unable to rise up to the less dense surface air. A wind tower, however, can create a pressure gradient which sucks at least a small amount of air upwards through a house. This cool, dry night air, being pulled over a long passage of water, evaporates some of it and is cooled down further.
  3. Finally, in a windless environment or waterless house, a wind tower functions as a Solar or thermal chimney that uses convection of air heated by passive solar energy. It creates a pressure gradient which allows less dense hot air to travel upwards and escape out the top. This is also compounded significantly by the day-night cycle mentioned above, trapping cool air below. The temperature in such an environment can't drop below the nightly low temperature. These last two functions have gained some ground in Western architecture, and there are several commercial products using the name wind tower.

Cooling water in cisterns

When coupled with thick mud brick, the wind tower is able to chill lower-level spaces in mosques and houses in the middle of the day to cooler temperatures.
So effective has been the wind tower in Persian architecture that it has also been routinely used as a refrigerating device.

wind towers to cool water in a cistern

Many traditional water reservoirs, or ab anbars, are built with wind towers that are capable of storing water at near freezing temperatures for months in summer. High humidity environments destroy the evaporative cooling effect enjoyed in the dry conditions seen on the Iranian plateau; hence the ubiquitous use of these devices in drier areas such as Yazd, Kashan, Nain, and Bam.

passive water cooling of a cistern