PASSIVE_COOLING.pdf

AR6303 CLIMATE AND BUILT ENVIRONMENT

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PHILOSOPHY OF NATURAL VENTILATION

1. Reasons for using ventilation

2. Thermal comfort requirements of building occupants

3. Thermal performance of the building structure

4. Impact of the building form and its openings

5. Constraints of the local and regional climate

These are the factors to be considered at the design stage with which good ventilation system in a building can be

achieved

FUNCTIONS OF VENTILATION

1. Supply of fresh air

2. Physiological cooling

3. Removing heat from, on adding it to, the thermal mass in the building structure i.e. night time cooling

1. THE SUPPLY OF FRESH AIR

Fresh air is required in building to

Provide sufficient oxygen

Dilute odours e.g. body and food

Dilute to acceptable levels the concentration of carbondioxide produced by occupants and combustion

2. PHYSIOLOGICAL COOLING

The analysis of comfort limits with the monthly mean temperature in order to accentuate whether if there is

a need for passive heating or cooling

3. NIGHT TIME COOLING OF THE BUILDING

Night-time cooling is defined as the removal of heat from a building by natural means during the night in

order to reduce daytime cooling loads. Natural ventilation is extremely useful in drawing the heat stored in

the building during the day and venting it at night

THE WIND

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, the pressure


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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. Climatic data such as wind rose diagrams is required for the design of ventilation

A wind rose is a graphic tool used by meteorologists to give a succinct view of how wind speed and

direction are typically distributed at a particular location. Historically, wind roses were predecessors of the

compass rose (found on maps), as there was no differentiation between a cardinal direction and the wind

which blew from such a direction. Using a polar coordinate system of gridding, the frequency of winds

over a long time period is plotted by wind direction, with color bands showing wind ranges. The directions

of the rose with the longest spoke show the wind direction with the greatest frequency.

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

WIND SHADOW

Wind shadow is a phenomenon occurring when the wind air flow encounters an obstacle. After impact the

wind flow is perturbed over a certain distance creating depression zones.

Wind Rose diagram


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Air – although light – has a mass (around 1.2kg/m3), and as it moves, has a momentum, which is the

product of its mass and its velocity (kg m/s). This is a vectorial quantity which can be changed in direction

or in magnitude by another force. When moving air strikes an obstruction such as a building, this will slow

down the air flow but the air flow will exert a pressure on the obstructing surface. This pressure is

proportionate to the air velocity, as expressed in the equation

PW = 0.612 X v2

Where PW = wind pressure in N/m2

V = wind velocity in m/s

(the constant is Ns2/m4)

Positive pressure

zone / Windward

side

An evergreen windscreen breaks the

force of the wind and creates a

protective wind shadow in front and

behind. Dead air space protects the

house.

Wind shadow

diagram

Negative pressure

zone / Leeward

side

Vortex

Vortex

Airflow around

a building


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This slowing down process effects a roughly wedge shaped mass of air on the windward side (the side

which faces the wind direction) of the building, which in turn diverts the rest of the air flow upwards and

sideways. A separation layer s formed between the stagnant air and the building on the one hand and the

laminar air flow on the other hand. The laminar air flow itself may be accelerated at the obstacle, as the

area available for the flow is narrowed down by the obstacle as shown in the figure-1. As the separation

layer, due to friction, the upper surface of the stagnant air is moved forward, thus a turbulence or vortex is

developed.

Due to momentum, the laminar air flow tends to maintain a straight path after it has been diverted;

therefore it will take some time to occupy all the available, ‘cross-section’. Thus a stagnant mass of air is

formed on the leeward side (the side opposite to the wind direction or windward side), but this at a reduced

pressure. In fact it is not quite stagnant: a vortex is formed, the movement is light and variable and it is

often referred to as ‘wind shadow’.

AIR FLOW THROUGH THE BUILDING

As no satisfactory and complete theory is available, air flow pattern can only be predicted on the basis of

empirical rules derived from measurements in actual buildings or in wind tunnel studies. Such empirical

rules can give a useful guide to the designer but in critical cases it is advisable to prepare a model of the

design and test it on a wind simulator

Wind simulators may be of the open-jet type or the wind tunnel type. The former type is in use with the

Architectural Association School of Architecture which has been developed with the cooperation of the

department of Fluid Mechanics, University of Liverpool. The latter type is best represented by a

economical model developed by the Building Research Station which is described in BRS current paper

69/1968

For Qualitative studies a smoke generator can be used and the smoke traces can be photographed. This

gives a convincing picture of flow patterns, position of laminar flow and turbulences. With some practice

the wind tunnel operator can estimate velocity ratios from smoke traces with quite reasonable accuracy. For

quantitative analysis air velocity or air pressure measurements must be taken with miniature instruments at

predetermined grid points

An open jet

wind simulator

A closed wind

tunnel


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On the basis of such experimental observations the following factors can be isolated which affect the

indoor air flow (both pattern and velocities)

a. Orientation

b. External features

c. Cross-ventilation

d. Position of openings

e. Size of openings

f. Control of openings

ORIENTATION

The greatest pressure on the windward side of the building is generated when the elevation is at right angles

to the wind direction, so it seems to be obvious that the greatest indoor air velocity will be achieved in this

case. A wind incidence of 45◦ would reduce pressure by 50%

Thus the designer must ascertain the prevailing wind direction from wind frequency charts of wind roses

and must orientate the building in such a way that the largest openings are facings the wind direction

It has, however, been found by Givoni that a wind incidence at 45◦ would increase the average indoor air

velocity and would provide a better distribution of indoor air movement. The following figure shows the

relative velocities (with the free air speed taken as 100%) measured at a height of 1.2m above the floor

level.

This seems to contradict common-sense and the findings of others, but it can be explained by the following

phenomenon. The following figure shows the outline of air flow at 90◦ and at 45◦, to a building square in

plan. In the second case a greater velocity is created along the windward faces, therefore the wind shadow

will be much broader, the negative pressure (the suction effect) will be increased and an increased indoor

air flow will result. The size of the outlet opening was not varied in his experiments: it was fixed at the

maximum possible so that the suction forces had full effect. It is justified to postulate that with smaller

outlet openings this effect would be reduced, if not reversed

Effect of wind direction

and inlet opening size

on air velocity

distribution


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If often happens, that the optimum solar orientation and the optimum orientation for wind do not coincide.

In equatorial regions a north-south orientation would be preferable for sun exclusion but most often the

wind is predominantly easterly. The usefulness of the above findings is obvious for such a situation – it

may resolve the contradictory requirements

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. When planning urban

centers, specifically in heating dominated climates, having the buildings gradually increase in height will

Tall buildings improve natural

ventilation, and in lower latitudes

reduce sun exposure.

Effect of direction on

width of wind shadow

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minimize high speed winds at the pedestrian level which can influence thermal comfort. The height

difference between neighboring 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 of the given 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.

Orientation for maximum 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 cross-ventilation. 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.

EXTERNAL FEATURES

Wind shadows created by obstructions upwind, should be avoided in positioning the building on the site

and in positioning the opening in the building.

Orientation for maximum passive

ventilation

Building structures can redirect

prevailing winds to cross-

ventilation

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The wind velocity gradient is made steeper by an uneven surface, such as scattered buildings, wall fences,

trees or scrub (refer figure below) – but even with a moderate velocity gradient, such as over smooth and

the open ground, a low building can never obtain air velocities similar to a taller one. For this reason (or to

avoid specific obstructions) the building is often elevated on stilts

External features of the building itself can strongly influence the pressure build-up. For example, if the air

flow is at 45◦ to an elevation, a wing wall at the downwind end or a projecting wing of an L-shaped

building can more than double the positive pressure created. A similar funneling effect can be created by

upward projecting eaves. Any extension of the elevational area facing the wind will increase the pressure

build-up. If a gap between two buildings is closed by a solid wall, a similar effect will be produced.

The air velocity between free-standing trunk of trees with large crowns can be increased quite substantially

due to similar reasons

The opposite of the above means will produce a reduction of pressures: if a wing wall or the projecting

wing of an L-shaped building is upwind from the oepning considered, the pressure is reduced or even a

negative pressure may be created in front of the window

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.

Wind velocity

gradient


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CROSS VENTILATION

When placing ventilation openings, inlets and outlets are placed 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, cross ventilation can be increased 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.

Different amounts of ventilation and air mixing

with different windows open


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The following figure shows that in the absence of an outlet opening or with a full partition there can be no effective

air movement through a building even in a case strong winds. With a windward opening and no outlet, a pressure

similar to that in front of the building will be built up indoors, which can make conditions even worse, increasing

discomfort. In some cases oscillating pressure changes, known as ‘buffeting’ can also occur. The latter may also be

produced by an opening on the leeward side only, with no inlet.

Air flow loses much of its kinetic energy each time it is diverted around or over an obstacle. Several right-angle

bends, such as internal walls or furniture within a room can effectively stop a low velocity air flow. Where internal

partitions are unavoidable, some air flow can be ensured if partition screens are used, clear of the floor and the

ceiling

Lack of cross

ventilation

Effect of opening

position


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POSITION OF OPENINGS

To be effective, the air movement must be directed at the body surface. In building terms this means that air

movement must be ensured through the space mostly used by the occupants: through the ‘living zone’ (up to 2m

high). The figure below shows, if the opening at the inlet side is at a high level, regardless of the outlet opening

position, the air flow will take place near the ceiling and not in the living zone.

The relative magnitude of pressure build-up in front of the solid areas of the elevation (which in turn depends on the

size and position of openings) will, in fact, govern the direction of the indoor air stream and this will be independent

of the outlet opening position. The figure below shows that a larger solid surface creates a larger pressure build-up

and this pushes the air stream in an opposite direction, both in plan and in section. As a result of this, in a two storey

building the air flow on the ground floor may be satisfactory but on the upper floor it may be directed against the

ceiling. One possibilities remedy is an increased roof parapet wall.

SIZE OF OPENING

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 quickly.

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

Pressure build-up

at inlet

Air flow in a two

storey building


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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.

With a given elevational area – a given total wind force (pressure x area) – the largest air velocity will be obtained

through a small inlet opening with a large outlet. This is partly due to the total force acting on a small area, forcing

air through the opening at a high pressure and partly due to the ‘venturi effect’: in the broadening funnel (the

imaginary funnel connecting the small inlet to the large outlet) the sideways expansion of the air jet further

accelerates the particles.

Such an arrangement may be useful if the air stream is to be directed (as it were focused) at a given part of the room.

When the inlet opening is large, the air velocity through it will be less, but the total rate of air flow (volume of air

passing in unit time) will be higher. When the wind direction is not constant, or when air flow through the whole

space is required, a large inlet opening will be preferable.

The best arrangements is full wall openings on both sides, with adjustable sashes or closing devices which can assist

in channeling the air flow in the required direction, following the change of wind.

Venturi effect

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of

pipe. The Venturi effect is named after Giovanni Battista Venturi (1746–1822), an Italian physicist.

Pairing a large

outlet with a small

inlet increases

incoming wind

speed.

The pressure in the first measuring tube (1) is

higher than at the second (2), and the fluid speed

at "1" is lower than at "2", because the cross-

sectional area at "1" is greater than at "2".


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CONTROL OF OPENINGS

Sashes, canopies, louvres and other elements controlling the openings, also influence the indoor air flow pattern.

Sashes can divert the air flow upwards. Only a casement or reversible pivot sash will channel it downwards into the

living zone

Canopies can eliminate the effect of pressure build-up above the window, thus the pressure below the window will

direct the air flow upwards. A gap left between the building face and the canopy would ensure a downward pressure,

thus a flow directed into the living zone

Louvres and shading devices may alos present a problem. The position of blades in a slightly upward position would

still channel the flow into the living zone (up to 20° upwards from the horizontal)

Effect of sashes

Effect of canopies

Effect of louvres


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Fly screens or mosquito nets are an absolute necessity not only in malaria infested areas, but also if any kind of lamp

is used indoors at night. Without it thousands of insects would gather around the lamp. Such screens and nets can

substantially reduce the air flow. A cotton net can give a reduction of 70% in air velocity. A smooth nylon net is

better, with a reduction factor of only approximately 35%. The reduction is greater with higher wind velocities and

is also increased with the angle of incidence, as shown by the findings of koenigsberger et al.

AIR MOVEMENT AND RAIN

Exclusion of rain is not a difficult task and making provision for air movement does not create any particular

difficulties, but the two together and simultaneously is by no means easy. Opening of windows during periods of

wind-driven rain would admit rain and spray; while closing the windows would create intolerable conditions

indoors. The conventional tilted louvre blades are unsatisfactory on two counts:

1. Strong wind will drive the rain in, even upwards through the louvres

2. The air movement will be directed upwards from the living zone

Verandahs and large roof overhangs are perhaps the best traditional methods of protection

Koenigsberger, millar and costopoulos have carried out some experimental work, testing four types of louvres

(figure below). Only type ‘M’ was found to be capable of keeping out water at wind velocities up to 4 m/s and at the

same time ensuring a horizontal air flow into the building. The air velocity reduction varies 25 and 50%

Louvres for

rain exclusion


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STACK EFFECT 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.

Lower air pressures at

higher heights can

passively pull air through

a building.

The stack effect: hot air

rises due to buoyancy, and

its low pressure sucks in

fresh air from outside

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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.

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


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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), which 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.

Some roof vents: open stack, turbine, and gable vents

Special wind cowls in the BedZED development use the

faster winds above rooftops for passive ventilation

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The stack effect relies on thermal

forces, set up by density difference

(caused by temperature differences)

between the indoor and out-door air.

It can occur through an open

window (when the air is still): the

warmer and lighter indoor air will

flow out at the top and the cooler,

denser out-door air will flow in at

the bottom. The principle is the same

as in wing generation

Special provision can be made for it

in the form of ventilating shafts. The

highr the shaft, the larger the cross-

sectional area and the greater the

temperature difference: the greater

the motive force therefore, the more

air will be moved.

The motive force is the ‘stack

pressure’ multiplied by the cross-

sectional area (force in Newtons –

area in m2). The stack pressure can

be calculated from the equation:

Ps = 0.042 x h x ∆T

Where

Ps = stack pressure in N/m2

h = height of stack in m

∆T = temperature difference in

degC

(the constant is N/m3 degC)

Such shafts are often used for

ventilation of internal, windowless

rooms (bathrooms and toilets) in

Europe. The figure shows some duct

arrangements for multistory

buildings, with vertical or horizontal

single or double duct systems.


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SOLAR CHIMNEY

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 thermosyphons.

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.

Different solar chimney designs, from a simple black-

painted pipe to integrated Trombe roof structure

Solar chimney in a park outhouse

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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.

Thermal 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.

Solar chimney compared to a Trombe wall


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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 nighttime 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 nighttime 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 "coolth" 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 nighttime

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.

Solar chimneys can either heat

or cool a space


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During the day, thermal mass

soaks up heat; at night it is

cooled by outside air


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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.

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

http://sustainabilityworkshop.autodesk.com/sites/default/files/styles/large/public/core-page-inserted-images/thermal_cooling_evaporativecooling.jpg


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AIR FLOW AROUND THE BUILDING

When the architect’s task is the design of more than one building, a cluster of buildings or a whole settlement,

especially in a warm climate, in deciding the layout, provision for air movement must be one of the most important

considerations. After a careful analysis of site climatic conditions a design hypothesis may be produced on the basis

of general information derived from experimental findings, such as those described below. A positive confirmation

(or rejection) of this hypothesis can only be provided by model studies in a wind simulator. If the construction of

adjustable or variable layout models is feasible, alternative arrangements can be tested and the optimum can be

selected

The effect of tall blocks in mixed developments has been examined in experiments conducted by the building

research station at Garston. The following figure shows how the air stream separates on the face of a tall block, part

of it moving up and over the roof part of it down, to form a large vortex leading to a very high pressure build-up. An

increased velocity is found at ground level at the sides of the tall block. This could serve a useful purpose in hot

climates, although if the tall block is not fully closed but is permeable to wind, these effects may be reduced

A series of studies in Australia, relating to low industrial buildings, produced the surprising result that if a low

building is located in the wind shadow of a tall block, the increase in height of the obstructing block will increase

the air flow through the low building in a direction opposite to that of the wind. The lower (return –w) of a large

vortex would pass through the building.

Air stream separation at the

face of buildings

Reverse flow behind a tall block


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In Texas a series of experiments was directed at finding the downwind extent of a turbulence zone, which was found

to depend on building size, shape, type and slope of the roof, but practically unaffected by wind velocity.

Experiments at the Architectural Association Department of Tropical Studies yielded of the following results:

a. If in a rural setting in open country, single storey buildings are placed in rows in a grid-iron pattern,

stagnant air zones leeward from the first row will overlap the second row. A spacing of six times the

building height is necessary to ensure adequate air movement for the second row. Thus the ‘five times

height’ rule for spacing is not quite satisfactory

b. In a similar setting, If the buildings are staggered in a checker-board pattern, the flow field is much more

uniform, stagnant air zones are almost eliminated

HUMIDITY CONTROL

Dehumidification is only possible by mechanical means, without this, in warm-humid climates, some relief can be

provided by air movement. In hot-dry climates humidification of the air may be necessary, which can be associated

with evaporative cooling. In these climates the building is normally closed to preserve the cooler air retained within

the structure of high thermal capacity, also to exclude sand and dust carried by winds. However, some form of air

supply to the building interior is necessary.

All these functions:

Controlled air supply

Filtering out sand and dust

Evaporative cooling

Humidification

Air flow: grid-iron lay-out

Air flow: checker board

lay-out


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Are served by a device used in some parts of Egypt – the wind scoop. The following figure illustrates an example of

this. The large intake opening captures air movement above the roofs in densely built up areas. The water seeping

through the porous pottery jars evaporates, some drips down onto the charcoal placed on a grating, through which

the air is filtered. The cooled air assists the downward movement – a reversed stack effect

This device is very useful for ventilation (the above four functions), but it cannot be expected to create an air

movement strong enough for physiological cooling

WIND TOWER AND WIND CATCHER

A wind scoop

Wind catchers have been employed for

thousands of years to cool buildings in hot

climates. The wind catcher is able to chill

indoor spaces in the middle of the day in a

desert to frigid temperatures

Documents

PASSIVE_COOLING.pdf