Download Passive Cooling Just like passive heating, cooling your building

Passive Cooling
Just like passive heating, cooling your building using passive strategies is important for
reducing energy usage in your building. Specifically, utilizing passive cooling strategies like
natural ventilation, air cooling, and shades can reduce your demand for mechanical cooling
while maintaining thermal comfort.
[Natural Ventilation]
Natural ventilation, also called passive ventilation, uses natural outside air movement and
pressure differences to both passively cool and ventilate a building.
Natural ventilation is important because it can provide and move fresh air without fans. For
warm and hot climates, it can help meet a building's cooling loads without using mechanical
air conditioning systems. This can be a large fraction of a building's total energy use.
Successful natural ventilation is determined by having high thermal comfort and adequate
fresh air for the ventilated spaces, while having little or no energy use for active HVAC
cooling and ventilation.
You can choose the right strategy based on the temperature and humidity of your site. The
following chart shows how much these different strategies can extend the comfortable
climate range for people.
When not to use natural ventilation:
Sites with high levels of acoustic noise, such as near heavy traffic zones, may be less suitable
for natural ventilation because large openings in the building envelope can make it difficult
to block outside noise. This can sometimes be solved by using acoustical ventilation louvers.
Also, sites with poor air quality, such as adjacent to busy freeways, may also be less
desirable for natural ventilation. Such sites may overcome poor outdoor air quality with
filters and ducting, though this usually requires some mechanical fan systems.
Quantifying Ventilation Effectiveness:
To measure the effectiveness of your ventilation strategies, you can measure both the
volume and speed of the airflow.
The volume of the airflow is important because it dictates the rate at which stale air can be
replaced by fresh air, and determines how much heat the space gains or losses as a result.
The volume of airflow due to wind is:
Q_wind = K • A • V
Q_wind = airflow volumetric rate (m³/h)
K = coefficient of effectiveness (unit less, see below)
A = opening area, of smaller opening (m²)
V = outdoor uninterrupted wind speed (m/h)
The coefficient of effectiveness is a number from 0 to 1, adjusting for the angle of the wind
and other fluid dynamics factors, such as the relative size of inlet and outlet openings. Wind
hitting an open window at a 45° angle of incidence would have a coefficient of effectiveness
of roughly 0.4, while wind hitting an open window directly at a 90° angle would have a
coefficient of roughly 0.8.
When placing ventilation openings, you need to place both air inlets and air outlets; often
they do not have the same area. The opening area used in this equation is the smaller of
the two.
Air Speed and Temperature in Buildings:
In addition to volume, you should design for the wind speed inside your building. Wind
speed is a component of human comfort, and the speed you want depends on the climate.
Higher velocity air causes more effective cooling, because it pulls heated air away faster,
and because it helps sweating be more effective by evaporating it faster. Even a moderate
wind speed can cool perceived temperatures 5°C (9°F) compared to still air. This is how fans
make people feel cooler even though they do not change the temperature of the air.
However, the ability of air movement to cool people depends on whether it is the air itself
that is hot, or if the radiant temperatures of the room’s surfaces are hot. The hotter the air
itself is, the less it helps to move it. If people are primarily hot from surrounding radiant
temperatures, however, moving air helps more. The ASHRAE 55 standard provides
guidelines for how much cooling is possible with air movement at different speeds, for
different mean radiant temperatures. A 3°C temperature rise can be nullified by a 0.8 m/s
increase in air speed when air temperatures are 5°C below radiant temperatures, but if air
temperatures are 5°C warmer than radiant temperature, it would require a 1.6 m/s increase
in air speed. This is far above what is acceptable wind conditions for light office work.
You’ll need to make sure that wind speeds inside the building aren’t so high that they
disturb the occupants. Fast winds can blow papers around on desks, blow people's hair
around, etc (refer back to the Beaufort Wind Scale). Referring to ASHRAE 55 for thermal
comfort guidelines regarding air speeds for interior spaces, the standard suggests that air
speeds appropriate for indoor environments do not exceed 0.2 m/s or 0.447 mph. ASHRAE
also accounts for elevated air speeds that will increase the acceptable temperature. The
maximum allowable elevated airspeed is 1.5 m/s or 3.579 mph.
It is also important to consider how often the air in a room is replaced, as an important
feature of natural ventilation is that it supplies occupants with fresh air. The number of
times the air in a room is replaced is known as air changes per hour, ACH, or the air change
rate. It is determined by both the size of the room and the volumetric flowrate of air (Q).
Q_wind, referenced above, is a component of this overall flow rate.
There are standards and recommendations for how much fresh outside air should be
delivered to different building spaces, and to people within the building. For example,
ASHRAE 62.2001 specifies 0.35 air changes per hour for residential living areas, but also
specifies a minimum volumetric flowrate of 15 ft3/min (CFM) per person.
The equation is:
ACH = (Q / V) * (conversion factor)
Q = volumetric flow rate of fresh air
V = Volume of room or space
Conversion Factor = If the volumetric flow rate, time scales, and volumes are incongruous
units. For example, if Q is in cubic feet per minute (CFM) and volume is in ft2, you’d need to
multiply by 60 to get it in terms of hours. If Q is in cubic meters per second, or Liters per
second, the conversion factor would be different.
Thermal Mass:
Thermal mass can also have an impact on natural ventilation. Sometimes a space can get
too hot for natural ventilation to have an impact on thermal comfort. However, you can use
thermal mass to help maintain a consistent temperature and avoid big jumps. By stabilizing
the temperature swings, you have a better chance of using natural ventilation effectively.
Best practice design strategies for enhancing natural ventilation with thermal mass is
explained further through night flushing.
[Wind Ventilation]
Wind ventilation is a kind of passive ventilation that uses the force of the wind to pull air
through the building.
Wind ventilation is the easiest, most common, and often least expensive form of passive
cooling and ventilation. Successful wind ventilation is determined by having high thermal
comfort and adequate fresh air for the ventilated spaces, while having little or no energy
use for active HVAC cooling and ventilation.
Strategies for wind ventilation include operable windows, ventilation louvers, and rooftop
vents, as well as structures to aim or funnel breezes. Windows are the most common tool.
Advanced systems can have automated windows or louvers actuated by thermostats.
If air moves through openings that are intentional as a result of wind ventilation, then the
building has natural ventilation. If air moves through openings that are not intentional as a
result of wind ventilation, then the building has infiltration, or unwanted ventilation (air
leaking in).
Strategies for Wind Ventilation:
The keys to good wind ventilation design are the building orientation and massing, as well as
sizing and placing openings appropriately for the climate. In order to maximize wind
ventilation, you’ll want the pressure difference between the windward (inlet) and leeward
(outlet) to be maximized. In almost all cases, high pressures occur on the windward side of a
building and low pressures occur on the leeward side.
The local climate may have strong prevailing winds in a certain direction, or light variable
breezes, or may have very different wind conditions at different times. Often a great deal of
adjustability by occupants is required. Consult climate data for wind rose diagrams.
The local climate may also have very hot times of the day or year, while other times are
quite cold (particularly desert regions). In summer, wind is usually used to supply as much
fresh air as possible while in winter, wind ventilation is normally reduced to levels sufficient
only to remove excess moisture and pollutants.
Site, Massing, and Orientation for Wind Ventilation:
Massing and orientation are important because building height and depth play a huge role
in the structure's ability to effectively pull outside air through occupied spaces. The massing
and orientation pages discuss how to optimize them for passive ventilation. In a nutshell,
upper floors and roofs are exposed to more wind than lower floors, and buildings with thin
profiles facing into the path of prevailing winds are easiest to ventilate. Atria and open-plan
spaces also help wind ventilation be more effective.
Cross Ventilation:
When placing ventilation openings, you are placing inlets and outlets to optimize the path
air follows through the building. Windows or vents placed on opposite sides of the building
give natural breezes a pathway through the structure. This is called cross-ventilation.
Cross-ventilation is generally the most effective form of wind ventilation.
It is generally best not to place openings exactly across from each other in a space. While
this does give effective ventilation, it can cause some parts of the room to be well-cooled
and ventilated while other parts are not. Placing openings across from, but not directly
opposite, each other causes the room's air to mix, better distributing the cooling and fresh
air. Also, you can increase cross ventilation by having larger openings on the leeward faces
of the building that the windward faces and placing inlets at higher pressure zones and
outlets at lower pressure zones.
Placing inlets low in the room and outlets high in the room can cool spaces more effectively,
because they leverage the natural convection of air. Cooler air sinks lower, while hot air
rises; therefore, locating the opening down low helps push cooler air through the space,
while locating the exhaust up high helps pull warmer air out of the space. This strategy is
covered more on the stack ventilation page.
Steering Breezes:
Not all parts of buildings can be oriented for cross-ventilation. But wind can be steered by
architectural features, such as casement windows, wing walls, fences, or even strategicallyplanted vegetation.
Architectural features can scoop air into a room. Such structures facing opposite directions
on opposite walls can heighten this effect. These features can range from casement
windows or baffles to large-scale structures such as fences, walls, or hedgerows.
Wing Walls:
Wing walls project outward next to a window, so that even a slight breeze against the wall
creates a high pressure zone on one side and low on the other. The pressure differential
draws outdoor air in through one open window and out the adjacent one. Wing walls are
especially effective on sites with low outdoor air velocity and variable wind directions.
[Stack Ventilation and Bernoulli's Principle]
Stack ventilation and Bernoulli's principle are two kinds of passive ventilation that use air
pressure differences due to height to pull air through the building. Lower pressures higher
in the building help pull air upward. The difference between stack ventilation and Bernoulli's
principle is where the pressure difference comes from.
Stack ventilation uses temperature differences to move air. Hot air rises because it is lower
pressure. For this reason, it is sometimes called buoyancy ventilation.
Bernoulli's principle uses wind speed differences to move air. It is a general principle of fluid
dynamics, saying that the faster air moves, the lower its pressure. Architecturally speaking,
outdoor air farther from the ground is less obstructed, so it moves faster than lower air, and
thus has lower pressure. This lower pressure can help suck fresh air through the building. A
building's surroundings can greatly affect this strategy, by causing more or less obstruction.
The advantage of Bernoulli’s principle over the stack effect is that it multiplies the
effectiveness of wind ventilation. The advantage of stack ventilation over Bernoulli's
principle is that it does not need wind: it works just as well on still, breezeless days when it
may be most needed. In many cases, designing for one effectively designs for both, but
some strategies can be employed to emphasize one or the other. For instance, a simple
chimney optimizes for the stack effect, while wind scoops optimize for Bernoulli’s principle.
For example, the specially-designed wind cowls in the Bed ZED development use the faster
winds above rooftops for passive ventilation. They have both intake and outlet, so that fast
rooftop winds get scooped into the buildings, and the larger outlets create lower pressures
to naturally suck air out. The stack effect also helps pull air out through the same exhaust
vent.
After wind ventilation, stack ventilation is the most commonly used form of passive
ventilation. It and Bernoulli's principle can be extremely effective and inexpensive to
implement. Typically, at night, wind speeds are slower, so ventilation strategies driven by
wind is less effective. Therefore, stack ventilation is an important strategy.
Successful passive ventilation using these strategies is measured by having high thermal
comfort and adequate fresh air for the ventilated spaces, while having little or no energy
use for active HVAC cooling and ventilation.
Strategies for Stack Ventilation and Bernoulli’s Principle:
Designing for stack ventilation and Bernoulli's principle are similar, and a structure built for
one will generally have both phenomena at work. In both strategies, cool air is sucked in
through low inlet openings and hotter exhaust air escapes through high outlet openings.
The ventilation rate is proportional to the area of the openings. Placing openings at the
bottom and top of an open space will encourage natural ventilation through stack effect.
The warm air will exhaust through the top openings, resulting in cooler air being pulled into
the building from the outside through the openings at the bottom. Openings at the top and
bottom should be roughly the same size to encourage even air flow through the vertical
space.
To design for these effects, the most important consideration is to have a large difference in
height between air inlets and outlets. The bigger the difference, the better.
Towers and chimneys can be useful to carry air up and out, or skylights or clerestories in
more modest buildings. For these strategies to work, air must be able to flow between
levels. Multi-story buildings should have vertical atria or shafts connecting the airflows of
different floors.
Solar radiation can be used to enhance stack ventilation in tall open spaces. By allowing
solar radiation into the space (by using equator facing glazing for example), you can heat up
the interior surfaces and increase the temperature that will accelerate stack ventilation
between the top and bottom openings.
Installing weatherproof vents to passively ventilate attic spaces in hot climates is an
important design strategy that is often overlooked. In addition to simply preventing
overheating1, ventilated attics can use these principles to actually help cool a building.
There are several styles of passive roof vents: Open stack, turbine, gable, and ridge vents, to
name a few.
To allow adjustability in the amount of cooling and fresh air provided by stack ventilation
and Bernoulli systems, the inlet openings should be adjustable with operable windows or
ventilation louvers. Such systems can be mechanized and controlled by thermostats to
optimize performance.
Stack ventilation and the Bernoulli Effect can be combined with cross-ventilation as well.
This matrix shows how multiple different horizontal and vertical air pathways can be
combined.
Solar Chimneys A solar chimney uses the sun's heat to provide cooling, using the stack
effect. Solar heat gain warms a column of air, which then rises, pulling new outside air
through the building. They are also called thermal chimneys, thermosiphons, or
thermosiphons.
The simplest solar chimney is merely a chimney painted black. Many outhouses in parks use
such chimneys to provide passive ventilation. Solar chimneys need their exhaust higher
than roof level, and need generous sun exposure. They are generally most effective for
climates with a lot of sun and little wind; climates with more wind on hot days can usually
get more ventilation using the wind itself.
Advanced solar chimneys can involve Trobe walls or other means of absorbing and storing
heat in the chimney to maximize the sun's effect, and keep it working after sunset. Unlike a
Trobe wall, solar chimneys are generally best when insulated from occupied spaces, so they
do not transfer the sun's heat to those spaces but only provide cooling.
Herma chimneys can also be combined with means of cooling the incoming air, such as
evaporative cooling or geothermal cooling.
Solar chimneys can also be used for heating, much like a Trobe wall is. If the top exterior
vents are closed, the heated air is not exhausted out the top; at the same time, if high
interior vents are opened to let the heated air into occupied spaces, it will provide
convective air heating.
This works even on cold and relatively cloudy days. It can be useful for locations with hot
summers and cold winters, switching between cooling and heating by adjusting which vents
are open and closed.
[Night-Purge Ventilation]
Night-Purge Ventilation (or "night flushing") keeps windows and other passive ventilation
openings closed during the day, but open at night to flush warm air out of the building and
cool thermal mass for the next day.
Night-purge ventilation is useful when daytime air temperatures are so high that bringing
unconditioned air into the building would not cool people down, but where night-time air is
cool or cold. This strategy can provide passive ventilation in weather that might normally be
considered too hot for it.
Successful night-purge ventilation is determined by how much heat energy is removed from
a building by bringing in night-time air, without using active HVAC cooling and ventilation.
Night flushing works by opening up pathways for wind ventilation and stack ventilation
throughout the night, to cool down the thermal mass in a building by convection. Early in
the morning, the building is closed and kept sealed throughout the day to prevent warm
outside air from entering. During the day, the cool mass absorbs heat from occupants and
other internal loads. This is done largely by radiation, but convection and conduction also
play roles.
Because the "colt" of night-purge ventilation is stored in thermal mass, it requires a building
with large areas of exposed internal thermal mass. This means not obscuring floors with
carpets and coverings, walls with cupboards and panels, or ceilings with acoustic tiles and
drop-panels. Using natural ventilation for the cooling also requires a relatively unobstructed
interior to promote air flow.
Limitations:
These systems have some limitations due to climate, security concerns, and usability factors.
Climatically, night flushing is only suitable for climates with a relatively large temperature
range from day to night, where night-time temperatures are below 20 or 22°C (68 or 71°F).
If the building is occupied at night, like residences, the ventilation should not be so cold as
to be uncomfortable for occupants. In addition, the location should be one with adequate
wind at night to provide the cooling.
Usability can be a concern, as the opening and closing of all the openings every day can be
tiresome for occupants or maintenance staff, and they may not always open and close
everything at the optimal times. This can be solved with mechanized windows or ventilation
louvers, controlled by either a timer or a thermostat-driven control system.
Another usability issue is the possibility of rain coming in at night, damaging property or
interior finishes. While rain is not a common occurrence in climates where night flushing
works best, it can be addressed with overhangs, ventilation louvers with steep angles, and
other structural measures.
Security can be a concern, especially in buildings that are unoccupied at night. This can be
overcome with adequate security structures, such as bars or screens, or more sophisticated
electronic systems.
[Air Cooling]
In very hot climates it's often necessary to prevent outdoor air from getting into the building
un-conditioned during the heat of the day. However, natural ventilation can still be an
option even in hot climates, particularly in hot dry climates. Two techniques can be used:
faster air movement, and passively cooling incoming air.
Faster air movement on people's skin helps because it encourages evaporation of sweat,
making them feel cooler at higher temperatures than normal.
Passively cooling incoming air before it is drawn into the building can be achieved by
evaporative cooling and/or geothermal cooling.
Evaporative Cooling:
If the inlet air is taken from the side of the building facing away from the sun, and is drawn
over a cooling pond or spray of mist or through large areas of vegetation, it can end up
several degrees cooler than outside air temperature by the time it enters occupied spaces.
Geothermal Cooling:
Inlet air can also be cooled by drawing it through underground pipes or through an
underground plenum (air space). The air loses some of its heat to the surfaces over which it
passes. Underground, these surfaces tend to be at roughly the annual average
temperature, providing cooling in summer and warming in winter. This strategy is best for
dry climates, as moisture in dark cool places can lead to poor indoor air quality.
Many early versions of geothermal cooling used rock stores or gravel beds for their thermal
storage capacity; however, the additional resistance to air flow was quite high, often
requiring a powered fan or pump. Large open plenums can provide almost as much cooling
or warming with only minimal obstruction.
[Massing & Orientation for Cooling]
Massing and orientation are important design factors to consider for passive cooling,
specifically, natural ventilation. As a general rule, thin tall buildings will encourage natural
ventilation and utilize prevailing winds, cross ventilation, and stack effect.
Massing Strategies for Passive Cooling:
Thinner buildings increase the ratio of surface area to volume. This will make utilizing
natural ventilation for passive cooling easy. Conversely, a deep floor plan will make natural
ventilation difficult-especially getting air into the core of the building and may require
mechanical ventilation.
Tall buildings also increase the effectiveness of natural ventilation, because wind speeds are
faster at greater heights. This improves not only cross ventilation but also stack effect
ventilation.
While thin and tall buildings can improve the effectiveness of natural ventilation to cool
buildings, they also increase the exposed area for heat transfer through the building
envelope. Sometimes this is good, sometimes not. See Massing & Orientation for Passive
Heating.
When planning urban centres, specifically in heating dominated climates, having the
buildings gradually increase in height will minimize high speed winds at the pedestrian level
which can influence thermal comfort. The height difference between neighbouring buildings
should not exceed 100%.
Orientation Strategies for Passive Cooling:
Buildings should be oriented to maximize benefits from cooling breezes in hot weather and
shelter from undesirable winds in cold weather. Look at the prevailing winds for your site
throughout the year, using a wind rose diagram, to see which winds to take advantage of or
avoid.
Generally, orienting the building so that its shorter axis aligns with prevailing winds will
provide the most wind ventilation, while orienting it perpendicular to prevailing winds will
provide the least passive ventilation.
The effectiveness of this strategy and aperture placement can be estimated. Here are some
rules of thumb for two scenarios in which windows are facing the direction of the prevailing
wind:
For spaces with windows on only one side, natural ventilation will not reach farther than
two times the floor to ceiling height into the building.
For spaces with windows on opposite sides, the natural ventilation effectiveness limit will be
less than five times the floor to ceiling height into the building.
However, buildings do not have to face directly into the wind to achieve good crossventilation. Internal spaces and structural elements can be designed to channel air through
the building in different directions. In addition, the prevailing wind directions listed by
weather data may not be the actual prevailing wind directions, depending on local site
obstructions, such as trees or other buildings.
For buildings that feature a courtyard and are located in climates where cooling is desired,
orienting the courtyard 45 degrees from the prevailing wind maximizes wind in the
courtyard and cross ventilation through the building.
[Apertures for Cooling]
The simple act of opening a window can often provide immediate cooling effects. But how
do the size and placement of that window impact the effect you feel? Window design and
ventilation louver design greatly affects passive cooling potential, specifically natural
ventilation. Be sure to visit the wind, stack, and night-purge ventilation pages to learn more
about more specific opening strategies.
Opening Shape:
Opening shape matters and can influence airflow effectiveness. Long horizontal strip
windows can ventilate a space more evenly. Tall windows with openings at top and bottom
can use convection as well as outside breezes to pull hot air out the top of the room while
supplying cool air at the bottom.
Opening Size:
Window or louver size can affect both the amount of air and its speed. For an adequate
amount of air, one rule of thumb states that the area of operable windows or louvers should
be 20% or more of the floor area, with the area of inlet openings roughly matching the area
of outlets.
However, to increase cooling effectiveness, a smaller inlet can be paired with a larger outlet
opening. With this configuration, inlet air can have a higher velocity. Because the same
amount of air must pass through both the bigger and smaller openings in the same period of
time, it must pass through the smaller opening more quickly1.
Note that a small air inlet and large outlet does not increase the amount of fresh air per
minute any more than large openings on both sides would; it only increases the incoming air
velocity. Basic physics says that air cannot be created or destroyed as it moves through the
building, so in order for the same amount of air to pass through a smaller opening, it must
be moving faster.
Air flows from areas of high pressure to low pressure. Air can be steered by producing
localized areas of high or low pressure. Anything that changes the air's path will impede its
flow, causing slightly higher air pressure on the windward side of the building and a negative
pressure on the leeward side. To equalize this pressure, outside air will enter any windward
openings and be drawn out of leeward openings.
Because of pressure differences at different altitudes, this impedance to airflow is
significantly higher if the air is forced to move upward or downward to navigate a barrier
without any corresponding increase or decrease in temperature.
Opening Types:
Windows that only open halfway, such as double-hung and sliding windows, are only half as
effective for ventilation as they are for daylight. Some casement windows and Jalousie
windows, however, can open so wide that effectively their entire area is useful for
ventilation.
Casement windows can deflect breezes, or can act as a scoop to bring them in, depending
on wind direction. Jalousie windows (horizontal louvered glazing) can catch breezes while
keeping out rain.
You can also use ventilation louvers instead of windows for your openings. Their
coefficients of effectiveness will be the same as windows of the same geometry, such as
Jalousie windows. Ventilation louvers often open so wide that nearly all their area is useful
for ventilation. They are typically oriented horizontally to prevent rain from entering; this is
an advantage over most windows. Ventilation louvers also provide visual privacy, and can
even provide acoustic damping.
(http://sustainabilityworkshop.autodesk.com/buildings/wind-ventilation)