Tuesday, May 22, 2012

Wind ventilation

Wind ventilation is a kind of passive ventilation, using the force of the wind (or local air pressure differences) 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.
Using the wind for passive cooling and fresh air

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.   

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 loses 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.

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.  The graph below shows how wind speed cools people without changing air temperature.  Even a moderate wind speed can keep people comfortable at 5°C (9°F) warmer temperatures than in still air.

Comfortable air temperature vs. wind speed for people dressed in light summer clothing

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. 

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.  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 page discusses 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.

Cross-ventilation (bottom) is more effective
than ventilation that does not pass through the whole space (top).  

Window placement for cross ventilation

It is generally best not to place openings directly across from each other in a space--this could 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.
Different amounts of ventilation and air mixing with different windows open

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. 

Opening Design

Window design and ventilation louver design greatly affects the airflow.  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.

Opening Shape

Opening shape matters as well.  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.  

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.

Building structures can redirect prevailing winds to cross-ventilation

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.  

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.  

Cooling Incoming Air

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.  (See graph on wind speed vs. comfortable temperatures, above.)
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.

A courtyard fountain in the Alhambra cools air before it enters the building 

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.

1 See the physics of the Venturi effect.

[Source " co-past from http://sustainabilityworkshop.autodesk.com/]

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