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Benoit Cushman‐RoisinENGS 44
8 May 2019
Building Ventilationin hot climates
Rule #1: Priorities
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Types of passive cooling techniques – Summary
1. Cooling with ventilation‐ Night flush (using cold night air to pre‐cool building for next day)‐ Comfort ventilation (day or night, to promote evaporation from skin)
2. Radiant cooling‐ Direct radiant cooling (through roof structure, out to night sky)‐ Indirect radiant cooling (heat loss to night sky
transferred to a fluid that in turn cools the structure)
3. Evaporative cooling‐ Direct evapora on (water spray at air inlet, T↓ but RH ↑)‐ Indirect evaporative cooling (water sprayed on roof to cool roof)
4. Earth cooling‐ Direct coupling (earth embankment)‐ Indirect coupling (air entering structure by way of buried tubes)
5. Dehumidification with a desiccant(for removal of latent heat in the humidity)
Recall the psychrometric chart
If it’s cold, humidity is not a problem. But, if it’s hot, a
distinction must be made between dry and humid.
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Ventilation rules for hot climates
Hot & dry climate → Night flush ven la on (in combina on with good thermal mass)
Hot & humid climate → Comfort ven la on (and as li le thermal mass as possible)
The benefit of ventilation: With increased wind speed, the air feels cooler.
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Mind the microclimates on a hill, and take advantage of them.
South slopes offer more solar heat and shorter shadows.
Using the wind effectivelyPart 1: Orientation of the structure & pitch of roof
Prevailing wind direction (not necessarily west to east)
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Using the wind effectivelyPart 2: Using the Venturi effect (lower air pressure where air flows faster)
https://www.researchgate.net/profile/Chin_Haw_Lim/publication/233982406/figure/fig2/AS:300104650117123@1448561932282/Fig‐2‐Venturi‐shaped‐roof‐geometry‐and‐experimental‐house‐with‐wind‐induced‐natural.png
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Using the wind effectivelyPart 3: Stack effect (hot air rising by buoyancy)
Examples of solar chimneys:
From small to large
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Using the wind effectivelyPart 4: Placement of operable windows
If inlet and outlet do not have the same size
NOT SO GOOD MUCH BETTER
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Small windows and lots of thermal mass for hot and dry places
Greece
Turkey
More examples of historical and indigenous use of passive cooling
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Courtyards are self‐shading and may be combined with evaporative cooling.
Replica of a Roman Villa at the Getty Museum in California
Large pools of water frequently helped cool Roman villas in sunny and dry Italy.
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House of Loreio Tiburtino in Pompeii, Italy
As it is now As it probably was in Roman times
Dining terrace was cooled by an indoor water canal.A grapevine pergola provided shade.The terrace is oriented to the south for winter heating.
Massive dome structures for hot and dry climate:
Sun shines from near top, seeing only the footprint of the structure, but night time cooling is through the broader surface of the dome.
The dome offers the additional benefit of much thermal mass.
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A well designed mansion in Virginia, making use of the stack effect (hot air rises into the belvedere over the open staircase)
Traditional construction in the Southeast USA (hot & humid) before the invention of air conditioning
All the structures have operable windows.
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NIGHT
DAY
Radiant Cooling:Another form of natural ventilation combined with thermal mass
Using the ground temperature for passive cooling
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Can you see the drawbacks of these techniques?