Passive design maximises the use of ‘natural’ sources of heating, cooling and ventilation to create comfortable conditions inside buildings. It harness environmental conditions such as solar radiation, cool night air and air pressure differences to drive the internal environment. Passive measures do not involve mechanical or electrical systems.
This is as opposed to ‘active’ design which makes use of active building services systems to create comfortable conditions, such as boilers and chillers, mechanical ventilation, electric lighting and so on. Buildings will generally include both active and passive measures.
Solar chimneys are generally tall, wide structures constructed, facing the sun, with a dark coloured, matt surface designed to absorb solar radiation. As the chimney becomes hot, so it heats the air inside it. The hot air rises up the chimney and is vented out of the top, and in so doing it draws more air in at the bottom of the chimney. This can be used to drive passive ventilation in buildings where cross ventilation or stack ventilation may not be sufficient, and where designers wish to avoid using energy-consuming mechanical ventilation.
Solar chimneys are particularly effective in climates that are humid and hot. They are most efficient when they are tall and wide, but not very deep. This maximises the surface area that can absorb solar radiation and maximises the surface area in contact with the air inside the chimney. Variations in design can incorporate multiple chambers to further increase surface area and may use materials with high solar absorption such as metal to maximise the temperatures achieved within the chimney. Low emissivity coatings, and glazing can also be used to reduce heat losses back to the outside, similar to the design of trombe walls. It is important that the chimney is insulated from the building itself so that heat gains do not transmit into occupied spaces.
In cooler conditions, the chimney can be used to direct absorbed heat back into the building by closing it at the top.
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