Transcript Passive Cooling - Hatboro

Passive Cooling
• Natural Ventilation
• Air Cooling
• Shades
Natural Ventilation
• Also called passive ventilation
• Uses natural outside air movement/pressure
differences to cool/ventilate the building
• Provide and move fresh air without fans
When Not to Use
• Sites with high levels of acoustic noise
• Sites with poor air quality
Air Speed and Temp in Buildings
• Higher velocity is more effective for cooling
– Pulls heat away faster
– Helps sweat evaporate quicker
Opening Design
Shape
• Long horizontal strip windows ventilate a
space more evenly
• Tall windows with openings at top and bottom
can use convection and outside breezes
– Pulls out hot air from the top of the room and
brings in cool air at the bottom
Size
• Rule of thumb: operable windows/louvers
should be 20% of the floor area with the inlet
size roughly matching the outlet
• A smaller inlet paired with a larger outlet can
increase velocity
Wind Ventilation
• A type of passive ventilation
• Uses the force of the wind to pull air through
a building
• Easiest, most common, and typically least
expensive form
• Successful ventilation is determined by having
a high thermal comfort and adequate fresh air
for ventilated spaces
Wind Ventilation
• Strategies include
– Operable windows
– Ventilation louvers
– Rooftop vents
– Structures to aim or funnel the breeze
• Pressure difference should be maximized
between windward (inlet) and leeward
(outlet)
• High pressure on the windward side
Site, Massing, Orientation
• Upper floors and roofs are exposed to more
wind than lower floors
• Buildings with thin profiles facing into the
path are easiest to ventilate
• Openings create inlets and outlets to
maximize air flow
Cross Ventilation
Cross Ventilation
• Not best to place openings directly across
from each other
– Some areas are well cooled and others are not
– Place openings across from each other to
distribute cool and fresh air
– Have large openings on both windward and
leeward faces and place inlets at higher pressure
zones
– Place inlets low and outlets high
Cross Ventilation
Steering Breezes
• Not all areas of the building will be oriented
for cross breezes
• Use architectural features such as casement
windows, wing walls, fences or planted
vegetation
Wing Walls
• Project outward next to a window so that any
breeze against the wall creates a high pressure
zone on one side and a low pressure side on
the other
• Effective on sites with low outdoor air velocity
and variable wind directions
Stack Ventilation and Bernoulli
• Use air pressure differences due to height to
pull air through a building
• Low pressures higher in the building help pull
air upward
Stack Ventilation
• Uses temp differences to move air
• Hot air rises because it is lower pressure
• Sometimes called buoyancy ventilation
Bernoulli
• Uses wind speed differences to move air
• The faster the air moves the lower the
pressure
• Outdoor air farther from the ground has less
obstructions and moves faster than the lower
air
– This creates a lower pressure which can suck fresh
air through the building
Bernoulli
• Advantage over stack effect is that it multiplies
the effectiveness of wind ventilation
• Stack ventilation does not need wind though
• Designing for usually means designing for
both
• Can emphasize one over the other
• Chimney will optimize the stack effect
• Wind scoops optimize Bernoulli’s principle
• Besides wind ventilation, stack ventilation is
the most commonly used form of passive
ventilation
• Typically at night wind speeds are slower so
ventilation is less effective
Strategies
• Cold air is sucked in through a low inlet
opening and hotter air escapes through high
outlets (roughly the same size openings)
• Have large differences in height between
inlets and outlets
• Towers, skylights, clerestories, and chimneys
can be useful. Air must be able to move
between floors
Strategies
• Solar radiation can be used to enhance the
effect in tall spaces
• Heat up interior surface/temp which will
accelerate ventilation
• Weatherproof vents in hot climates
Strategies
• To allow adjustability in cooling and fresh air,
inlets should be adjustable with operable
windows or louvers
• Combine with cross ventilation
Solar Chimney
Thermal chimney,
Thermosiphon,Thermosyphon
• Uses the sun’s heat to provide cooling with
the stack effect
• Warms a column of air which rises pulling new
air up/in
Thermal Chimney
• Simplest ones are chimneys painted black
• Need exhaust higher than the roof level and
sun exposure
Advanced Chimney
• Involve a Trombe wall, but are better insulated
from occupied spaces so they don’t heat the
space
• Or can be used to aid with heating
Night Purge Ventilation
• Keeps windows and other passive vents closed
during the day but open at night to flush
warm air out
• Useful when daytime air temp is so high that
bringing unconditioned air into the building
doesn’t cool people down but nighttime air is
cool/cold
• During the day thermal mass soaks up heat
and at night is cooled by the outside air
Night Purge
• Need a large area of exposed internal thermal
mass
• Also need a relatively unobstructed interior
Air Cooling
• Useful in hot dry climates
• Faster air movement
– Helps encourage evaporation of sweat
• Passively cooling
– Evaporative cooling
– Geothermal cooling
Evaporative Cooling
• If the air inlet is taken from the side of the
building facing away from the sun and is
drawn over a cooling pond or some mist or
even through large areas of vegetation it can
end up several degrees cooler than the
outside air temp by the time it enters the
building
Geothermal Cooling
• Draw the air through underground pipes or air
space
• Air loses some heat to the surfaces as it passes
over them
• Underground the surfaces tend to have
roughly the avg annual temp
– Provides cooling in summer and heating in winter
• Best for dry climates