Windcatcher

windtower in Souq Waqif, Doha, Qatar s, with the vertical side left open and facing directly up or down wind (often one of each per building). This design works well in areas with strong low-level winds from a consistent direction. 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.). 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 (see diagram below), 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 (evaporative cooling unit) and a (roof lantern vent). 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 (usually 4-sided), and they are typically tall towers (up to 34 meters tall) 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. (for an extreme example, see Tesla valve). 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 (see Venturi effect and Bernoulli's principle). 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. during the day. In a windless environment, a windcatcher can still function using the stack effect. 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 courtyard air will become stably stratified, the hot air floating on top of the cold air with little mixing. (it is this depth which is used for many ground-source heat pumps, often loosely referred to as "geothermal heat pumps" by laypeople). 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 (adobe, stone, brick) are common (though thinner walls with high resistance against heat transmission are more modernly sometimes used). 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 (e.g. shabestans), 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 (see lede image). At night, the windcatchers may even bring sub-freezing night air underground, helping to freeze ice. Evaporative cooling used for cooling In dry climates, the evaporative cooling effect may be used by placing water at the air intake, such that the draft draws air over water and then into the house. For this reason, it is sometimes said that the fountain, in the architecture of hot, arid climates, is like the fireplace in the architecture of cold climates. Windcatchers are used for evaporative cooling in combination with a qanat, or underground canal (which also makes use of the subterranean heat reservoir described above). In this method, the open side of the tower faces away from the direction of the prevailing wind (the tower's orientation may be adjusted by directional ports at the top). When only the leeward side is left open, air is drawn upwards using the Coandă effect. This pulls air into an intake on the other side of the building. The hot air brought down into the qanat tunnel is cooled by coming into contact with the water flow and the surrounding earth. The soil below ground level stays cool by virtue of being several meters below the surface. The insulation and heat capacity of the overlying earth maintains the same stable temperature day and night, and as nights in arid climates are quite cold, often below freezing, that stable temperature is quite cool. The air is also evaporatively cooled when some of the water in the qanat evaporates as the hot, dry surface air passes over it; the heat energy in the air is absorbed as energy of vaporization. The dry air is thus also humidified before entering the building. The cooled air is drawn up through the house and finally out the windcatcher, again by the Coandă effect. On the whole, the cool air flows through the building, decreasing the structure's overall temperature. A is a type of fountain with a thin sheet of flowing water, shaped to maximize surface area and thus evaporative cooling. Windcatchers are often used with salasabils may be used to maximize the flow of unsaturated air over the water surface and carry the cooled air to where it is needed in the building. Wetted matting can also be hung inside the windcatcher to cool incoming air. This can reduce flow, especially in weak winds. However, it can also produce a downdraft of cool air in windless conditions. The evaporative cooling within a windtower causes the air in the tower to sink, driving circulation. This is called passive downdraught evaporative cooling (PDEC). It may also be generated using spray nozzles (which have a tendency to get blocked if the water is hard) or cold-water cooling coils (like hydronic underfloor heating in reverse). == Windcatchers and climate change ==

Windcatchers can be used for mitigation of climate change as they can "reduce the building's energy consumption and carbon footprint" and for adaptation to climate change because they facilitate cooling in a warmer climate. Windcatchers can reduce temperature inside the house by in comparison to the outdoor temperature. A window windcatcher can reduce the total energy use of a building by 23.3%. == Regional use ==

Africa Egypt In Egypt windcatchers are known as , pl. . They are generally shaped as right triangular prisms with the vertical side left open and facing directly up or down wind (one of each per building). They work best if oriented within 10 degrees of wind direction; larger angles allow the wind to escape. and only started to fall out of use in the mid-20th century. Their use is now being re-examined, as air conditioning accounts for 60% of Egypt's peak electrical power demand (and thus the need for 60% of its generating capacity). Byggnadskonsten, Fornegyptiska boningshus, Nordisk familjebok.png|Dwelling house in Ancient Egypt with windcatcher. From a painting at the Pharaonic house of Neb-Ammun, Egypt, which dates from the 19th Dynasty, c. 1300 BC (British Museum). They are used in the hot, dry areas of the Central Iranian Plateau, and in the hot, humid coastal regions. Americas cricket ground in Barbados also uses a very wide aluminium windscoop. where it functions without the addition of mechanical devices in order to regulate temperature. == See also ==