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Difference between revisions of "Cooling homes in the hot, humid tropics"
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Revision as of 18:02, 8 October 2011
The cooling of homes in hot, humid climates is a problem. Let's say that electric powered air conditioning is not an accepted possibility. Evaporative cooling is not really feasible because of the humidity of the air; little evaporation will take place. This leaves the removal of the hot air, especially near the ceiling and forced air circulation.
Creating a breeze
Temperature differences created by the sun can be used to create a breeze, which gives a cooler feeling. This due to the wind chill effect. The breeze causes sweat to evaporate more quickly from your skin. Evaporation consumes energy and this energy is taken from the sweat left behind so your perceived skin temperature drops. The lowest temperature this can achieve is the air dew point - the temperature at which moisture starts to condense out of the air. This temperature is determined by the humidity of the air. The drier the air the colder it can get before condensation starts so the greater the cooling effect it can acieve.
A solar chimney is a proven way of doing this, which has the advantage of being most effective on days of strong sunshine, when it is most needed.
A Windcatcher is a traditional Persian architectural feature. An opening at high level facing the wind direction, is connected to a vertical shaft, catching the wind and directing it downwards. This can be combined with an opening facing away from the wind as an exhaust path.
Wind Powered ventilators
There is an article entitled Natural Cooling of Malaysian Urban Homes by Free Wind which deals with this question. First it uses standard, unpowered turbine ventilators to remove hot air in the building. Then it suggests an, as yet untried, system to circulate the air in the home using a wind powered rotating fan. On a hot day the air may well be dead still at ground level but only a few meters above the building, there is often a light breeze. The concept involves building a simple tower on the roof with an anemometer device on the top. This turns a shaft going straight down, through the roof to a standard ceiling fan in the house. See an overly simple diagram in the article itself at the link given below. An imaginative worker could better this design considerably.
If their is a significant difference in temperature between day time and night time then increasing the thermal mass of a building will mean that it takes longer to heat up during the day and building temperature will be colder than outside air temperature during the day. This effect is what causes caves and massive fortifications to be colder inside than the day time temperature outside. For caves the thermal mass is so great in some cases that the temperature varies little during the year so you can use the annual temperature cycle to keep the space cool in summer.
To maximise this effect you need to maximise the ventilation through the thermal mass element during the night, using the cold night time aire to cool the structure, then minimise the ventilation during the day so the cooling effect lasts as long as possible.
Systems using this effect:
If the air into the building is drawn in through an underground covered trench then, if the trench is long enough, the air will be cooled down to the same temperature as the ground by the time it enters the building.
Buildings can be covered in earth to increase their thermal mass.
Adobe (mud) is a traditional construction material. It is not as strong as concrete so thicker walls are required and so building using this construction method have greater thermal mass.
Exposed concrete soffit
Modern low energy buildings use concrete rather than steel for their structure to take advantage of the greater thermal mass. Instead of having false ceilings hiding the building services these buildings have the concrete soffit (the underside of the floor slab) on view. During the summer night time the windows open automatically to increase ventilation and make sure the slab is cooled to the night time outside air temperature. This can mean that in the height of summer the indoor temperature is below the outside air temperature for most of the day.
Improved thermal insulation will not cool a building however it is effective in maximising the effect of increased thermal mass. Where a building has air conditioning or Comfort cooling then improved thermal insulation will reduce the amount of cooling needed to maintain the required conditions.
Where a building has high thermal mass construction then thermal insulation on the outside will stop the heat outside from warming up the massive walls, ensuring the 'coolth' stored in the thermal mass is all used to cool the occupied space rather than cooling the outside.
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