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Passive cooling systems overview[edit]

Passive cooling covers all natural processes and techniques of heat dissipation and modulation without the use

of energy.[1] Some authors consider that minor and simple mechanical systems (e.g. pumps and economizers)

can e integrated in passive cooling techniques! as long they are used to enhance the effectiveness of the

natural cooling process.

["]

 Such applications are also called #hyrid cooling systems$.

[1]

 %he techniques forpassive cooling can e grouped in two main categories&

• Preventative techniques that aims to provide protection and'or prevention of eternal and internal heat

gains.

• Modulation and heat dissipation techniques allow the uilding to store and dissipate heat gain

through the transfer of heat from heat sins to the climate. %his technique can e the result of  thermal

mass or natural cooling.

Preventative %echniques[edit]

Protection from or prevention of heat gains encompasses all the design techniques that minimizes the impact

of solar heat gains through the uilding$s envelope and of internal heat gains that is generated inside the uilding

due occupancy and equipment. *t includes the following design techniques&[1]

• Microclimate and site design + ,y taing into account the local climate and the site contet! specific

cooling strategies can e selected to apply which are the most appropriate for preventing overheating

through the envelope of the uilding. %he microclimate can play a huge role in determining the most

favorale uilding location y analyzing the comined availaility of sun and wind. %he ioclimatic chart! the

solar diagram and the wind rose are relevant analysis tools in the application of this technique.[-]

• Solar control + properly designed shading system can effectively contriute to minimizing the solar

heat gains. Shading oth transparent and opaque surfaces of the uilding envelope will minimize the

amount of solar radiation that induces overheating in oth indoor spaces and uilding$s structure. ,y

shading the uilding structure! the heat gain captured through the windows and envelope will e reduced.

• Building form and layout + ,uilding orientation and an optimized distriution of interior spaces can

prevent overheating. /ooms can e zoned within the uildings in order to re0ect sources of internal heat gain

and'or allocating heat gains where they can e useful! considering the different activities of the uilding. or

eample! creating a flat! horizontal plan will increase the effectiveness of cross+ventilation across the plan.

2ocating the zones vertically can tae advantage of temperature stratification. %ypically! uilding zones in the

upper levels are warmer than the lower zones due to stratification. 3ertical zoning of spaces and activities

uses this temperature stratification to accommodate zone uses according to their temperature requirements.[-]orm factor (i.e. the ratio etween volume and surface) also plays a ma0or role in the uilding$s energy and

thermal profile. %his ratio can e used to shape the uilding form to the specific local climate. or eample!more compact forms tend to preserve more heat than less compact forms ecause the ratio of the internal

loads to envelope area is significant.[4][5]

• Thermal insulation + *nsulation in the uilding$s envelope will decrease the amount of heat transferred

y radiation through the facades. %his principle applies oth to the opaque (walls and roof) and transparent

surfaces (windows) of the envelope. Since roofs could e a larger contriutor to the interior heat load!

especially in lighter constructions (e.g. uilding and worshops with roof made out of metal structures)!

providing thermal insulation can effectively decrease heat transfer from the roof.

• Behavioral and occupancy patterns + Some uilding management policies such as limiting the

amount of people in a given area of the uilding can also contriute effectively to the minimization of heatgains inside a uilding. ,uilding occupants can also contriute to indoor overheating prevention y& shutting

off the lights and equipment of unoccupied spaces! operating shading when necessary to reduce solar heat

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gains through windows! or dress lighter in order to adapt etter to the indoor environment y increasing their 

thermal comfort tolerance.

• Internal gain control + 6ore energy+efficient lighting and electronic equipment tend to release less

energy thus contriuting to less internal heat loads inside the space.

6odulation and heat dissipation techniques[edit]

%he modulation and heat dissipation techniques rely on natural heat sins to store and remove the internal heat

gains. 7amples of natural sins are night sy! earth soil! and uilding mass.[8]%herefore passive cooling

techniques that use heat sins can act to either modulate heat gain with thermal mass or dissipate heat through

natural cooling strategies.[1]

• Thermal mass - 9eat gain modulation of an indoor space can e achieved y the proper use of the

uilding$s thermal mass as a heat sin. %he thermal mass will asor and store heat during daytime hours

and return it to the space at a later time.[1] %hermal mass can e coupled with night ventilation natural

cooling strategy if the stored heat that will e delivered to the space during the evening'night is not

desirale.

• Natural cooling + :atural cooling refers to the use of ventilation or natural heat sins for heat

dissipation from indoor spaces. :atural cooling can e separated into four different categories& cooling and

ventilation! radiative cooling! evaporative cooling! and earth coupling.

3entilation[edit]

3entilation as a natural cooling strategy uses the physical properties of air to remove heat or provide cooling to

occupants. *n select cases! ventilation can e used to cool the uilding structure! which susequently may serve

as a heat sin.

• Cross Ventilation + %he strategy of cross ventilation relies on wind to pass through the uilding for the

purpose of cooling the occupants. ;ross ventilation requires openings on two sides of the space! called the

inlet and outlet. %he sizing and placement of the ventilation inlets and outlets will determine the direction and

velocity of cross ventilation through the uilding. %he uilding structure acts as a sin through the day and asors internal heat

gains and solar radiation. 9eat can e dissipated from the structure y convective heat loss y allowingcooler air to pass through the uilding at night. %he flow of outdoor air can e induced naturally or

mechanically. %he net day! the uilding will perform as a heat sin! maintaining indoor temperatures elow

the outdoor temperature. %his strategy is most effective in climates with a large diurnal swing so the typical

maimum indoor temperature is elow the outdoor maimum temperature during the hottest months.[1?] %hermal mass is a necessary component to dissipate heat at night.

/adiative ;ooling[edit]

 ll o0ects constantly emit and asor radiant energy. n o0ect will cool y radiation if the net flow is outward!

which is the case during the night. t night! the long+wave radiation from the clear sy is less than the long+wave

infrared radiation emitted from a uilding! thus there is a net flow to the sy. Since the roof provides the greatest

surface visile to the night sy! designing the roof to act as a radiator is an effective strategy. %here are two typesof radiative cooling strategies that utilize the roof surface& direct and indirect.[8]

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• "irect #adiant Cooling + *n a uilding designed to optimize direct radiative cooling! the uilding roof

acts as a heat sin to asor the daily internal loads. %he roof acts as the est heat sin ecause it is the

greatest surface eposed to the night sy. /adiative heat transfer with the night sy will remove heat from

the uilding roof! thus cooling the uilding structure. /oof ponds are an eample of this strategy. %he roof

pond design ecame popular with the development of the Sytherm system designed y 9arold 9ay in

1=55. %here are various designs and configurations for the roof pond system ut the concept is the same for all designs. %he roof uses water! either plastic ags filled with water or an open pond! as the heat sin while

a system of movale insulation panels regulate the mode of heating or cooling. @uring daytime in the

summer! the water on the roof is protected from the solar radiation and amient air temperature y movale

insulation! which allows it to serve as a heat sin and asor! though the ceiling! the heat generated inside.

 t night! the panels are retracted to allow nocturnal radiation etween the roof pond and the night sy! thus

removing the stored heat from the day$s internal loads. *n winter! the process is reversed so that the roof

pond is allowed to asor solar radiation during the day and release it during the night into the space elow.["]

• Indirect #adiant cooling + heat transfer fluid removes heat from the uilding structure through

radiative heat transfer with the night sy. common design for this strategy involves a plenum etween the

uilding roof and the radiator surface. ir is drawn into the uilding through the plenum! cooled from the

radiator! and cools the mass of the uilding structure. @uring the day! the uilding mass acts as a heat sin.

7vaporative ;ooling[edit]

$vaporative cooling. %he design relies on the evaporative process of water to cool the incoming air while

simultaneously increasing the relative humidity. saturated filter is placed at the supply inlet so the natural

process of evaporation can cool the supply air. part from the energy to drive the fans! water is the only other

resource required to provide conditioning to indoor spaces. study of field performance results in Auwait

revealed that power requirements for an evaporative cooler are approimately 5-B less than the power

requirements for a conventional pacaged unit air+conditioner .[11]  s for interior comfort! a study found that

evaporative cooling reduced inside air temperature y =.4C; compared to outdoor temperature..[1D]

7arth ;oupling[edit]

7arth ;oupling uses the moderate and consistent temperature of the soil to act as a heat sin to cool a uilding

through conduction. %his passive cooling strategy is most effective when earth temperatures are cooler than

amient air temperature! such as hot climates.

• "irect coupling + @irect coupling! or  earth sheltering! occurs when a uilding uses earth as a uffer for

the walls. %he earth is an endless heat sin and can effectively mitigate temperature etremes. 7arth

sheltering improves the performance of uilding envelope assemlies y reducing the magnitude of

conductive and convective heat loss and gains y reducing infiltration.[1E]

• Indirect Coupling. uilding can e indirectly coupled with the earth y means of earth ducts. n earth

ducts is a uried tue that acts as avenue for supply air to travel through efore entering the uilding. Supply

air is cooled y way of conductive heat transfer etween the concrete tues and soil. %herefore! earth ducts

will not perform well as a source of cooling unless the soil temperature is lower than the desired room air

temperature.[1E] 7arth ducts typically require long tues to cool the supply air to an appropriate temperature

efore entering the uilding. fan is required to draw the cool air from the earth duct into the uilding. Some

of the factors that effect the performance of an earth duct are& duct length! numer of ends! thicness of

duct! depth of duct! diameter of the duct! and air velocity.

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