Download Passive Design Summary 4 - Passive Cooling Natural Ventilation

Passive Design Summary 4 - Passive Cooling
Natural Ventilation:
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Natural ventilation is able to provide and move fresh air without fans.
It can also help meet a building’s cooling loads without using mechanical air
conditioning systems for warm and hot climates.
A successful natural ventilation has high thermal comfort and adequate fresh air for
the ventilated spaces while having little or no energy use for active HVAC cooling and
ventilation.
When not to use natural ventilation:
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Sites with high levels of acoustic noise.
Sites with poor air quality.
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 (unitless, 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.
Thermal Mass:
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Thermal mass can have an impact on natural ventilation.
Thermal mass is able to be used to help maintain a consistent temperature and avoid
big jumps.
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
strategically-planted 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 Trombe 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
Trombe 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 Trombe 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:
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Night-Purge ventilation keeps windows and other passive ventilation openings
closed during the day but open at night to flush warm air out of the building and
cooling thermal mass for the next day.
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.
Air Cooling:
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.
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.