Windcatcher
A windcatcher, wind tower, or wind scoop is a traditional architectural element used to create cross ventilation and passive cooling in buildings. Windcatchers come in various designs, depending on whether local prevailing winds are unidirectional, bidirectional, or multidirectional, on how they change with altitude, on the daily temperature cycle, on humidity, and on how much dust needs to be removed. Despite the name, windcatchers can also [|function without wind].
Neglected by modern architects in the latter half of the 20th century, the early 21st century saw them used again to increase ventilation and cut power demand for air-conditioning. Generally, the cost of construction for a windcatcher-ventilated building is less than that of a similar building with conventional heating, ventilation, and air conditioning systems. The maintenance costs are also lower. Unlike powered air-conditioning and fans, windcatchers are silent and continue to function when the electrical grid power fails.
Windcatchers rely on local weather and microclimate conditions, and not all techniques will work everywhere; local factors must be taken into account in design. Windcatchers of varying designs are widely used in North Africa, West Asia, and India. A simple, widespread idea, there is evidence that windcatchers have been in use for many millennia, and no clear evidence that they were not used into prehistory. The "place of invention" of windcatchers is thus intensely disputed; Egypt, Iran, and the United Arab Emirates all claim it.
Windcatchers vary dramatically in shape, including height, cross-sectional area, and internal sub-divisions and filters.
Windcatching has gained some ground in Western architecture, and there are several commercial products using the name windcatcher. Some modern windcatchers use sensor-controlled moving parts or even solar-powered fans to make semi-passive ventilation and semi-passive cooling systems.
Windscoops have long been used on ships, for example in the form of a dorade box. Windcatchers have also been used experimentally to cool outdoor areas in cities, with mixed results; traditional methods include narrow, walled spaces, parks and winding streets, which act as cold-air reservoirs, and takhtabush-like arrangements.
Location
The construction of a windcatcher depends on the prevailing wind direction at that specific location: if the wind tends to blow from only one side, it may have only one opening, and no internal partitions. In areas with more variable wind directions, there may also be radial internal walls, which divide the windtower into vertical sections. These sections are like parallel chimneys, but with openings to the side, pointing in multiple directions. More sections reduce the flow rate, but increase the efficiency at suboptimal wind angles. If the wind hits the opening square-on, it will go in, but if it hits it at a sufficiently oblique angle, it will tend to slip around the tower, instead.Windcatchers in areas with stronger winds will have smaller total cross-sections, and areas with very hot wind may have many smaller shafts in order to cool the incoming air. Windtowers with square horizontal cross-sections are more efficient than round ones, as the sharp angles make the flow less laminar, encouraging flow separation; suitable shaping increases suction.
Taller windcatchers catch higher winds. Higher winds blow stronger and cooler. Higher air is also usually less dusty.
If the wind is dusty or polluted, or there are insect-borne illnesses such as malaria and dengue fever, then air filtering may be necessary. Some dust can be dumped at the bottom of the windcatcher as the air slows, and more can be filtered out by suitable plantings or insect mesh. Physical filters generally reduce throughflow, unless the flow is very gusty. It may also be possible to fully or partially close the windcatcher off.
The short, wide right-triangle-prism are usually bidirectional, set in symmetrical pairs, and are often used with a and a . Wide s are more often used in damper climates, where high-volume air flow is more important compared to evaporative cooling. In hotter climates, they are narrower, and air is cooled on its way in. They are more commonly used in Africa., on the other hand, are multisided, and they are typically tall towers which can be closed in winter. They are more common in the Persian Gulf region and in areas with dust storms. Taller windcatchers also have a stronger stack effect.
Cooling methods
Night-flushing cools the house by increasing ventilation at night, when the outdoor air is cooler; windtowers can assist night flushing.A windcatcher can also cool air by drawing it over cool objects. In arid climates, the daily temperature swings are often extreme, with desert temperatures often dipping below freezing at night. The thermal inertia of the soil evens out the daily and even annual temperature swings. Even the thermal inertia of thick masonry walls will keep a building warmer at night and cooler during the day. Windcatchers can thus cool by drawing air over night- or winter-cooled materials, which act as heat reservoirs.
Windcatchers that cool by drawing air over water use the water as a heat reservoir, but if the air is dry, they are also cooling the air with evaporative cooling. The heat in the air goes into evaporating some of the water, and will not be released until the water re-condenses. This is a very effective way of cooling dry air.
Simply moving the air also has a cooling effect. Humans cool themselves using evaporative cooling when they sweat. A draft disrupts the boundary layer of body-warmed and water-saturated air clinging to the skin, so a human will feel cooler in moving air than in stagnant air of the same temperature.
Airflow forces
The windcatcher can function in two ways: directing airflow using the pressure of wind blowing into the windcatcher, or directing airflow using buoyancy forces from temperature gradients. The relative importance of these two forces has been debated. The importance of windpressure increases with increasing wind speed, and is generally more important than buoyancy under most conditions in which the windcatcher is working effectively.Airflow speed is also important, especially for evaporative cooling. It is possible for a windtower-ventilated building to have very high flow rates; 30 air changes per hour were measured in one experiment. Uniform, stable flow with no stagnant corners is important. Turbulent flow should therefore be avoided; laminar flow is more effective at maintaining human comfort.
Other elements are often used in combination with the windcatchers to cool and ventilate: courtyards, domes, walls, and fountains, for instance, as integral parts of an overall ventilation and heat-management strategy.
Wind pressure
If a windcatcher's open side faces the prevailing wind, it can "catch" it, and bring it down into the heart of the building. Suction from the lee side of a windtower is also an important driving force, usually somewhat more constant and less gusty than the pressure on the upwind side.Routing the wind through the building cools the people in the building interior. The air flows through the house, and leaves from the other side, creating a through-draft; the rate of airflow itself can provide a cooling effect. Windcatchers have been employed in this manner for thousands of years.
The windtower essentially creates a pressure gradient to draw air through the building. Windtowers topped with horizontal airfoils have been built to enhance these pressure gradients. The shape of the traditional roof also creates suction as wind blows over it.
Convection
Buoyancy is usually not the main effect driving windcatcher air circulation during the day.In a windless environment, a windcatcher can still function using the stack effect. The hot air, which is less dense, tends to travel upwards and escape out the top of the house via the windtower.
Heating of the windtower itself can heat the air inside, so that it rises and pulls air out of the top of the house, creating a draft. This effect can be enhanced with a heat source at the bottom of the windtower, but this heats the house and makes it less comfortable. A more practical technique is to cool the air as it flows down and in, using heat reservoirs and/or evaporative cooling.
A takhtabush is a space similar to the ancient Roman tablinum, opening both onto a heavily shaded courtyard and onto a rear garden court. It is designed to capture a cross-draft. The breeze is at least partly driven by convection, and may also be driven by wind pressure and evaporative cooling, so the garden and courtyard are used as windcatchers.
Buoyancy forces are used to cause night flushing.
Night flushing (colder air)
The diurnal temperature cycle means that the night air is colder than the daytime air; in arid climates, much colder. This creates appreciable buoyancy forces. Buildings may be designed to spontaneously increase ventilation at night.Courtyards in hot climates fill with cold air at night. This cold air then flows from the courtyard into adjacent rooms. The cold night air will flow in easily, as it is more dense than the rising warm air it is displacing. But in the day, the courtyard walls and awning shade it, while the air outside is heated by the sun. The cool masonry will also chill the nearby air. The courtyard air will become stably stratified, the hot air floating on top of the cold air with little mixing. The fact that the openings are at the top will trap the cool air below, though it cannot cause the temperature to drop below the nightly minimum temperature. This mechanism also works in windtowers.
Subterranean cooling
A windcatcher can also cool air by bringing it into contact with cool thermal masses. These are often found underground.Below approximately 6m of depth, soil and groundwater is always at about the annual mean-average temperature . The thermal inertia of the soil evens out the daily and even annual temperature swings. In arid climates, the daily temperature swings are often extreme, with desert temperatures often dipping below freezing at night. Even the thermal inertia of thick masonry walls will keep a building warmer at night and cooler during the day; in hot-arid climates, thick walls with high thermal mass are common. Windcatchers can thus cool by drawing air over night- or winter-cooled materials, which act as heat reservoirs.
Windcatchers are also often used to ventilate lower-level indoor spaces, which maintain frigid temperatures in the middle of the day even without windcatchers. Ice houses are traditionally used to store water frozen overnight in desert areas, or over winter in temperate areas. They may use windcatchers to circulate air into an underground or semi-underground chamber, evaporatively cooling the ice so that it melts only slowly and stays fairly dry. At night, the windcatchers may even bring sub-freezing night air underground, helping to freeze ice.