Wind towers: passive cooling attributes
Wind towers, also known as wind catchers or baud-geers, are one form of passive cooling for buildings. They were developed in hot, dry desert areas of the Middle East, and work best in areas with similar conditions.
They work primarily by creating flows of air through the building they are attached to. They do this both by utilizing the buoyancy of air, and by exploiting differences of air pressure in and close to the tower. Their design lets them use either wind or temperature change to produce air pressures within the tower which are different to those in the attached building. It is the difference in air pressure that generates air flow, as the pressure seek to equalize.
Moving air strips heat from objects, in this case from the building and people inside. This is, of course, the well known wind-chill effect. From this, it is clear why wind towers are used during summer, but closed off at building level in winter.
Wind towers are most effective if the air flow they generate flows over a wet or damp surface, which allows for evaporative cooling of the air stream. This colder, damper air cools buildings and people further. However, this effect is lost in humid climates, where the air already has a high water content and cannot be cooled by more water.
Although they look similar, wind towers are structurally and functionally different to solar chimneys. Solar chimneys provide an escape route for hot air to rise out of a building, but do not pull air into a building as a wind tower does.
Components
The head of the tower: unlike a chimney, a wind tower has openings on its side, where they can face directly into the wind. The number of openings depends on local wind patterns. If there is only one prevailing wind, the openings are on one side only. If winds can come from several directions, more than one side has openings accordingly.
Traditionally, the inner surfaces of the head are curved to 'scoop' the wind down the shaft. While this does improve performance a little, it can be expensive or troublesome to construct. Little performance is lost if the internal shape of the head is simply a straight-sided box.
The tower functions more efficiently if the opening are fitted with wind-operated dampers or curtains that allow the windward opening to be open to the wind, but seal off the other openings.
Screens of moderate gauge mesh prevent birds and large insects getting in the openings without too much loss of air pressure. Fine screens block too much air flow.
The upper part of the tower, above the roof line: needs high thermal mass and high heat exchange area to facilitate temperature differences that help 'drive' the tower.
Traditionally, this section of the tower has thick external walls and solid internal divisions, which create several vertical internal channels. The internal divisions add to the heat exchange area. The internal and external walls are constructed of materials with good thermal storage capacity (U-factor), and are thick to further increase thermal storage.
A modern variant is to fill the upper section of the tower with vertically-positioned unglazed ceramic pipes, around 4 inches in diameter. A pump is used to spray water on the pipes, allowing evaporative cooling of the incoming air.
The lower part of the tower, below the roof line: has openings into each level of the building. These openings must have doors or shutters to control air flow under different conditions.
Evaporative sections: as mentioned above, in hot dry climates the cooling ability of wind towers is significantly increased if the air flows over a wet or damp surface somewhere along its track. By evaporating water, the air is cooled more than by flow alone: it takes a relatively large amount of heat to vaporize water, which efficiently 'strips' heat from the air. Evaporation also increases the humidity of the air, which adds to human comfort in very dry desert areas.
Some ways to achieve evaporative cooling with a wind tower are:
- Extending the tower into a basement. This utilizes the dampness inherent in the deeper soil, even in desert areas, seeping through porous wall materials.
- Having a pool or fountain at the base of the wind tower, or just outside it in the hallways leading off it, is aesthetic as well as practical.
- A wind tower in conjunction with an underground body of water, which can be a stream or a reservoir. This system needs two shafts down to where the water lies. One is situated outside the building, the other inside near the opening in the base of the wind tower. As air flows down the wind tower, it sucks cool air out of the inside shaft. This suction in turn pulls warm outside air through the outside shaft. As this outside air flows over the water towards the inside shaft, it is cooled. An underground body of water can service several wind towers in this way.
- A wind tower sited a little distance from the building it serves, with an underground passageway connecting the two. The passageway runs beneath a garden, where water seeps through the soil into it. Here, the walls of the passageway must be made of porous material so water can seep through them.
- As described above, water can be pumped up into the tower and sprayed on its internal surfaces. This is very effective, but requires some source of power to work. This means the tower is no longer a purely passive system of cooling.
How wind towers work
Cooling occurs when heat is transferred from a hot object to its environment. Wind towers cool buildings and the people in them by generating air flows, which strip heat by convection.
How the towers generate air flow depends on several natural mechanisms, both in the structure of the tower itself and the environment around it. Good wind tower function depends on how effectively its design utilizes these mechanisms.
- Wind. Wind is basically air under pressure, moving from an area of high pressure to a lower one. Wind moving some yards above the ground generally moves faster (and therefore at higher pressure) than close to the ground, where obstructions and friction impede it. Wind towers therefore extend above the roof line, to catch these faster breezes.
- Low pressure at building doors and windows. It is the difference between the high pressure of the air 'caught' by the tower and lower air pressure at the openings of the windows that generates air flow through the building. The presence of the building itself creates pockets of lower pressure as the air moves around its outside, with higher pressure on the openings facing the wind and lower pressure at openings in the lee of the wind. Here, opening and closing openings depending on which way the wind is blowing enhances tower function. Air pressure around the building can be further lowered by a courtyard, which protects the building from wind, and thus lowers air pressure at any doors and windows opening into it.
- Buoyancy of air. The buoyancy of air depends on its temperature: hot air rises, cold air sinks. When there is no wind, the tower offers a route for cold air to sink into the building, or hot air in the building to rise out of it. This movement of air creates a pressure difference, sucking more air in behind it, creating an air flow.
- Cold, clear nights. The cool, cloudless nights of the desert allow a lot of heat transfer by radiation. Warm structures lose heat to the sky on cool nights. Cloud cover limits this heat loss, much like a blanket does. Deserts, with little or no cloud, continue radiating heat all night. This means that heat accumulated in the walls of the tower during the day is lost during the night. In areas where nights are warm and/or cloudy, this night time cooling is much more limited.
- Thermal storage or U-factor. This works two ways in wind towers. Night-time coolness stored in heavy walls and internal partitions of a tower cools morning air, making it heavier so it sinks down through the tower into the building. Naturally, as the tower cools the air, it heats up itself. Once the tower reaches the same temperature as the surrounding air, this aspect of its cooling effect is lost. The amount of thermal mass as tower has controls how much coolness it can store, and how rapidly it heats up the next day. Towers with low thermal mass cannot store much coolness and heat up very quickly, so stop cooling morning air quickly.
- Evaporation: as noted above, hot dry air passing over water leads to evaporation, which strips heat from the air. It is not just the temperature difference that enables this, but also the dryness of the air. Humid air, even if it is hot, is limited in its ability to contain more water vapor, so prevents much evaporation.
Wind towers reach best performance where all these factors combine. They are an ingenious response to conditions in a hot desert summer with dry air and cool clear nights. They simply do not work as well in areas with milder summer temperatures, warm nights and/or moderate to high humidity.
Wind tower function under different conditions
Traditional Middle Eastern wind towers work in different ways depending on whether it is day or night, still or windy. They can either draw air in and funnel it through the connected building. Or they can act as a chimney, pulling air up through the building.
Wind direction and speed, solar radiation and air temperature affect the functioning of the wind tower. As a result, its effectiveness fluctuates through out the day.
On a windy day: The wind enters the openings at the top of the tower. Pressure differences between the tower, the building and the space outside the openings of the building create a flow of air through the building.
On a still day: The mass of the tower has cooled during the night. In the morning, as the hotter daytime air hits the cool tower, it is cooled, and so becomes heavier, sinking down through the tower, into and through the building. This draws more air in behind it, which cools in its turn. As the tower itself warms up, this effect is lost. However, the opposite effect kicks in: the tower starts acting as a chimney. As the air inside the building and chimney gets hotter, it starts to rise. The tower gives the air an escape route, and as it rises it grows hotter as it travels through the now-heated chimney. This creates an up-draft, pulling warm air in the building up and out. This chimney effect is also known as the 'stack effect'.
On a windy night, the operation of the tower is similar to that on a windy day: it funnels airflow into the building. Although the tower itself needs to lose the heat it accumulated during the day to be most effective, the wind combined with the cool night temperatures and clear night skies of the desert gradually accomplish this.
On a still night, the tower functions again like a chimney. The daytime heat stored in its massive walls heats the air inside it, creating the stack effect and pulling air up through the building. Leaving windows open at night enhances this effect, as the cool night air is sucked into and through the building by the chimney action.
Air pressure effects on the leeward side of a wind tower
An additional interesting effect is created when the wind is blowing. On the lee of the tower, the side away from the wind, air is at lower pressure and creates a suction effect. If the tower has openings on the leeward side, air will be sucked up through the channels on that side of the tower, pulling air through the building.
This effect can be very useful if wind from a particular direction is usually dusty. Rather than allowing wind full of dust to enter a building, the windward side of the tower can be blocked, with openings on the leeward side. By using suction on the lee of the tower, cooling airflow can still be generated in the building, minus the dust.
But the same effect can be a problem under other circumstances. In a multi-channel wind tower, air enters the tower on the windward side but is sucked out on the lee, reducing the available air flow. One simple way to avoid this is to have wind-operated baffles on all openings in the head of the tower. On the windward side, the wind blows the baffles open, getting into the tower. But on all other sides, the wind pressure keeps the baffles shut, preventing suction on the lee. This significantly improves tower performance. Note that where the building is surrounded by a courtyard, shutting the leeward side of the tower increases air flow, but less dramatically than in a building without a courtyard.
Design features to deal with dust
Deserts and other hot, dry areas tend to be dusty places. One of the problems with wind towers is they let dust in along with air. There are several design modifications that can at least reduce this problem:
- Taller towers. Air closer to the ground is dustier, so tall towers pull in relatively dust-free air from higher up.
- Leeward openings only, as discussed above. This is most useful in areas where one prevailing wind is dustier than others.
- 'Dust shelves' in the base of the tower. When air moves from a small space to a larger one, it slows down and drops a lot of the dust it is carrying. One traditional way to exploit this was to build the base of the tower broader than the upper shaft, and line the walls with 'shelves' (actually more like pockets) in which dust could settle.
- A similar effect is gained by not having doorways between the tower and the upper floors of a building, but routing all the air to the basement, with vents in the basement ceiling to carry the air to the floor above. As air flows out into the larger basement, it drops its dust on the floor, then moves up through the ceiling grilles to the rest of the building.
Other design considerations
Wind towers should be designed to cope with peak cooling load (ie the hottest summer weather), with adjustable doors to moderate their effect in periods that are not so hot. Height, cross sectional area and the internal configuration of the wind tower need to be designed to suit local climate conditions. A series of formulae to help do this can be found in a paper by Dr Mehdi Bahadori, An improved design of wind towers for natural ventilation and passive cooling, Solar Energy Vol.35 No.2, 1985, pp.119–129.
Note that when conditions dictate a large cross-sectional areas for the tower, it may produce better distribution of air through the building to split the area between two or more wind towers, instead of one large one.
Wind towers are capable of providing cooling all year round, whatever the weather conditions. While this is desirable in hot summers, it is a problem in winter, when it drains of building of heat. It is therefore vital that openings between the tower and the building can be sealed as completely as possible to avoid gaps.
References:
Bahadori, Mehdi N., Passive Cooling Systems in Iranian Architecture, Scientific American, Vol.238, No. 2, Feb. 1978, pp. 144–154
Bahadori, Mehdi N., Natural Cooling in Hot Arid Regions, in Solar Energy Application in Buildings, ed. Sayigh, A. A. M., Academic Press, New York, 1979, pp. 195–225
Bahadori, Mehdi N., Natural Air-Conditioning Systems, in Advances in Solar Energy, Vol. 3, ed. Boer, K.W., American Solar energy Society Inc, Boulder Colorado, and Plenum Press, New York, 1986, pp. 283–356
Karakatsanis, C., Bahadori, Mehdi N. and Vickery, B. J., Evaluation of Pressure Coefficients and Estimation of Air Flow Rates in Buildings Employing Wind Towers, Solar Energy Vol. 37 No. 5, 1986, pp. 363–374
Bahadori, Mehdi N., An Improved Design of Wind Towers for Natural Ventilation and Passive Cooling, Solar Energy Vol. 35 No. 2, 1985, pp. 119–129