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Buoyancy driven ventilation is the result of movement of air within a stack due to temperature gradient between warmer air and colder air within the stack. The warm air rises in the stack, removed from the top and is replaced by cooler air at the inlet. In tall buildings atria can be used to great effect as a thermal chimney and can be used to aid ventilation. Some of the devices and strategies to design stack ventilation in buildings are:
Thermal chimney or solar chimney uses temperature difference which creates air pressure difference for ventilation. Air that is heated rises due to natural buoyancy exits from the thermal chimney. Fresh or cool air is then drawn in at the lower end. This can be enhanced through the use of solar heat gain in the upper stack to improve the thermal difference and thus the ventilation. Haghighi & Maerefat (2010) studied a forced stack system for passive cooling of building by using an earth to air heat exchanger and a solar chimney. A ventilation pipe is buried inside the earth and the solar chimney is connected at an opposite end. Air is drawn inside the pipe; exchanges heat with the earth cools down and is drawn from the other side with the help of solar chimney, which warms up the air. They concluded that this system could potentially provide passive cooling and ventilation in stand-alone or cluster of buildings in favourable climates.
The height of the thermal chimney, temperature gradient as well as the size of the openings can determine the effectiveness of such systems. The rate of ventilation provided by stack is given by the following equation:
Qstack = Cd*A*[2gh(Ti-To)/Ti]^1/2, where
Qstack = volume of ventilation rate (m³/s)
Cd = 0.65, a discharge coefficient.
A = free area of inlet opening (m²), which equals area of outlet opening.
g =9.8 (m/s²). the acceleration due to gravity
h = vertical distance between inlet and outlet midpoints (m)
Ti = average temperature of indoor air (K), note that 27 °C = 300 K.
To = average temperature of outdoor air (K)
In hot climates thermal chimneys can also be combined with a trombe wall to increase the airflow through the chimney. Care need to be taken to insulate the chimney to the buildings interior to prevent thermal heat conduction to the interior. High ceilings allow space for high windows and stratification of the interior air, with openings at high level or on the ridge of the roof for the exhaust of hot air. Care should be taken that the upper window are shaded (e.g. through louvres) to reduce solar insolation. Fans can further enhance this.
Wind towers are designed to either extract hot air from a building through air pressure or 'suction', or to force fresh air into a building. These rely on exterior wind to create an air pressure difference to induce air movement. Wind towers can be equipped with automatic dampers to control the amount of airflow
Roof Monitors can be used to enhance stack ventilation in buildings. Warm air collects under pitched roofs and such monitors can be used to effectively remove this warm air that reduce the internal heat gain.
Double façades can be put to great effect to work as a thermal effect and works well in different seasons and climates. The double façade can also act as a buffer against outside conditions such as external noise and wind loads. However, it has risk of overheating during extreme summertime if sun-shading devices are not used in between the two skins or if the air cavity is not opened.
Buoyancy induced ventilation or stack ventilations are widely used in vernacular architecture world-wide, many can be found in middle east. Current denser built environment in the city with limited cross ventilation and the need for deep-plan buildings make the application of stack ventilation more important (Ismail & Rahman, 2012).
In buoyancy induced ventilation, high air exchange rate in cooler climates can result in higher thermal due to cooling of the building interior resulting in significantly higher heating load. Similarly care must be taken in hot climate zones in open systems to remove the internal load through ventilation. The design also has to take into consideration of the possible outdoor noise and pollution that can enter a building and minimised where possible when planning for natural ventilation. Mixed mode ventilation considers some of the limitation and allow indoor comfort environment.