Badgir

Badgir (wind-tower) or wind catcher is a traditional structure used for passive air-conditioning. Wind catchers are found throughout the Middle East, from Pakistan to North Africa. They are built in many regions of Iran, predominantly on houses in areas with a hot arid climate. Wind catchers are mostly brick towers which generally rise from 30 cm to 5 m above the roof (the tallest badgir in the world, built in Yazd, Iran is 33.35 m above the roof).

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Wind catchers may have different functions due to different circumstances however, in all these rolls, wind catchers generate air circulation. These functions may be directing airflow downward using direct wind entry, directing airflow upwards using a wind-assisted temperature gradient, or directing airflow upwards due to temperature gradient cause by sunlight as in solar chimney. 

  

When there is a wind through the open face of wind tower, literally it is caught and forced to flow down the tower to cool the interior parts. The air is not necessarily a cool air yet it provides cooling effect due to the circulation. This cooling effect can be maximized with exposure to water evaporation. Therefore, wind towers are usually equipped with a fountain or a pool of water. The wind blow on the water surface leads to temperature reduction. 

Scientifically speaking, the electromagnetic radiation emitted by the sun is made of photons of different energies. The absorption of photons by air molecules causes a gain in kinetic energy and hence molecules will move faster and get warmer. The motion of air molecules combined with Earth rotation causes the wind. Wind is an important factor in evaporating water and thus cooling the environment. Badgirs are designed to tolerate the heat using the effect of wind and evaporation. In hot weather as the temperature increases, the average kinetic energy of the particles increases. This can be seen in the Maxwell-Boltzmann equation:   

Where m is molar mass of the molecules, v is its velocity, T is the temperature and K is a constant known as Boltzmann Constant. Hence, temperature is defined as the measure of the average kinetic energy of the particles. By increasing the kinetic energy the speed of inter-molecular movements increases. Furthermore, the second law of Thermodynamics states that the total entropy of the universe is always increasing. Therefore, due to the high kinetic energy, evaporation takes place and the fastest moving particles leave the surface. If vapor steam formed inside the Badgir is allowed to expire into the air taking away energy and entropy, the system will work at high efficiency. This is why the system works better in the arid climate with low humidity. In hot dry climate, some water molecules have high kinetic energy so they are bouncing up at high speed to escape into the air as gas (water vapour). This involves a change of state from liquid to gas and can take place all the time at any temperature. During evaporation, warm wind does work on water molecules by giving them kinetic energy to overcome the inter-molecular attraction exerted on them by molecules under them.  The amount of energy absorbed by water is given by the following equation:

Q = m . L

Where Q is the amount of energy needed to change the state of water, m is the mass of water and L is the latent heat of vaporization of water (2.27 × 10^6 Joules per kg). The energy is taken from the surroundings into water as latent heat to keep water temperature constant. Therefore, evaporation is an endothermic process that cools down the surroundings. Since evaporation is an endothermic process, Badgir can cool down buildings by evaporating great amounts of water.