Passive Solar Cooling-unit 4

8/10/2019 Passive Solar Cooling-unit 4

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Passive solar cooling


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Sources of heat gain

There are three major sources ofunwanted summer heat: direct solarimpacts on a building and through

windows and skylights; heat transfer and infiltration, of exterior

high temperatures, through the materialsand elements of the structure;

and the internal heat produced byappliances, equipment, and inhabitants.


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Comfort target


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Passive cooling


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Passive solar design


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HEAT GAIN CONTROL

The first step towards passive cooling is

Reducing the heat gain through openings.

a building's surroundings should be designed tominimize summer sunlight striking external surfaces,

and to prevent surrounding area heat re-radiation andreflection.

Mitigation of undesirable summer direct sun andthermal impacts is achieved through use of vegetationi.e. deciduous trees which interrupt the summer sun'sdirect path,

and ground covers which prevent ground reflection aswell as keep the earth's surface cooler therebypreventing re-radiation.


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Windows and glazing

Glazing should be minimized on the roof and the eastand west walls where summer sunlight is mostintense.

The summer sun is much higher in the sky and has a

negative impact on skylights and roof windows andlead to enormous solar heat gains. They should not beused in hot climates unless they are insulated and/orshaded

Vertical south facing glass (windows, clerestories, etc.)

with overhangs or shades, is important

The use of deciduous trees that shade the south faceand roof during the summer.


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Shading device

Proper shading

devices to keep out

the heat coming inthrough the windows

Shading devices could

be sized using this

graphic method.


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Role of vegetation in reducing heat

gain

Vegetation is perhaps the most effective way ofkeeping the intense morning and afternoon sun off theeast and west walls and windows, but care must betaken to avoid blockage of nighttime summer breezes

that can be part of the diurnal cooling strategy If vegetation is impractical, a combination of tinted or

reflecting glass and exterior shades or shade screensthat roll down over east and west windows are aneffective strategy.

Additionally, light-colored paints and materials on theroof and the walls are effective in reflecting away mostof the sunlight that makes it past your shading.


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Convective cooling

The next step in natural cooling is to take

advantage of "convective" cooling

methods - those which use the prevailing

winds and natural, gravity-inducedconvection to ventilate a house at the

appropriate times of the day.


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Natural ventilation

Locate cool air inlet vent on the buildingside which receives predominant coolsummer breezes.

The oldest, straightforwardconvective method admits coolnight air to drive out the warmair.

If breezes are predominant, highvents or open windows on the

leeward side (away fromprevailing breeze) will let thehottest air, located near theceiling, escape.

The cooler night air sweeping inthrough low open vents orwindows on the windward sidewill replace this hot air and bringrelief.

To get the best cooling rates,leeward openings should havesubstantially larger total area(50% to 100% larger) than thoseon the windward side of thehouse


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Thermal chimneys

If there are only light breezes at thesite, natural convection can still beused to ventilate and cool a house,then

The coolest air around a house isusually found on the north side,especially if this area is well shaded

by trees or shrubs and has waterfeatures.

Cool air intake vents are best locatedas low as possible on the north side.

The greater the height differencebetween the low and high vents, thefaster the flow of natural convectionand the more heat mitigation can

occur.

To aid in natural ventilation, during summer usehigh ceiling vaults, and thermal chimneys topromote rapid air changes.


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Stack induced ventilation


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Enhancing ventilation

There are two basic ways to enhance theconvective cooling rate: 1) increase thevolume of air escaping per minute, or 2) bringin cooler air.

If Delta T is the temperature differencebetween exiting indoor air and incomingoutdoor air, the overall cooling rate per hour isgiven by the simple equation:

Cooling rate = 1.08 x V x DT where V is the volume of air escaping in cubic

feet per minute.


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Turbine vents

Turbine vents at theroof peak are oneway to enhanceairflow and improvethe cooling rate.

Even gentle breezesflowing up and overthe roof peak createan upward suctionthat draws out warminterior air


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Thermal chimneys


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Thermal chimneys


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Ventilation through courtyards


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Trombe wall and ventilation

An even better approach is touse solar radiation to inducea more rapid flow.

For e.g., uses a Trombe wallvented to the outside.

Sunlight striking the concretewall will heat the air in thespace between glass andwall to temperatures above150F.

This very hot air rises quickly

and escapes, drawing cool airinto the house through lowvents on the north wall.


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Earth air cooling

Another convective coolingstrategy is the drawing ofoutdoor air is drawn throughtubes buried in the groundand dumped into the house.

Made of material that allowseasy thermal transfer, thesetubes are buried several feetdeep to avoid the warmerdaytime surfacetemperatures.

Warm outdoor air enteringthe tube gives up its heat tothe cooler earth, and coolssubstantially before enteringthe house


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Earth air tunnel


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RADIATIVE COOLING METHODS

roof ponds


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Roof pond

In the cooling cycle insulatingpanels remain closed by day toreject unwanted solar heat.

The cool ponds act as "thermalsponges", absorbing room heatconducted through the interiorceiling ceiling (metal deck)supporting them.

At night. panels are rolled back,exposing the ponds to the blackbody of the night sky and to thecooler night air and breeze.

The ponds lose heat by radiation

to the night sky and by naturalconvection to the air.

Roof pond systems areparticularly effective in regions oflow humidity and clear summernights


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EVAPORATIVE COOLING

METHODS

Evaporative methods can beused to enhance the coolingrates in convective coolingsystems. One way of doingthis is to bring the outdoor air

into the house through amoist filter or pad

The familiar evaporativecooler, is a mechanicalsystem which uses theseprinciples with a motor to

force air movement anddistribution.

Swamp cooler - drier outside air is

drawn through a moist pad. Asmoisture is picked up by the air heatis absorbed by the water, the resultis cooler air.


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Radiative & evaporative cooling


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Cooling towers

Cool towers use gravityto move cool air withoutany fans, although fansmay be used to reduce

the size of the towers. The most common cool

towers do this by havinga wet pad medium in thetop of the tower. Since

cool air is heavier thanwarm air, it will fall,creating its own airflow.

Wind is not required, butwill improve the airflow

in a cool tower.


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Cooling towers

This is the most common cooltower in use.

As wind blows through the wetpads, the water evaporates andcools the air.

Cooler pads sit at the top of a

tower with a pump re-circulatingwater over them.

As hot air passes through thepads it is cooled by theevaporation of the water.

Cool moist air is heavier than hotdry air and drops down the

tower and into your house. Inorder for the cool air to flow in,hot air must be exhausted.

Passive Solar Cooling-unit 4

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