PASSIVE COOLING

Reporter: Aira Rowie S. Altovar

Topics: PASSIVE COOLING

What is Passive Cooling?What’s the purpose of Passive Cooling?Some techniques of Passive CoolingoSHADINGoVENTILATIONS AND COOLING

☺Natural Ventilation☺Stack Ventilation☺Cross Ventilation☺Thermal Comfort☺Night Ventilation

OTHER TECHNIQUES:

☺Evaporative Cooling

☺Desiccant Cooling

☺Time Lag Cooling

☺Earth Cooling

Definition of terms:Passive House

- refers to the rigorous, voluntary, Passivhaus standard for energy efficiency in a building, reducing its ecological footprint.

Ecological Footprint- The ecological footprint is a measure of

human demand on the Earth's ecosystems.Conduction - The transfer of energy, such as heat or an

electric charge, through a substance.

Convection- The act or process of conveying; transmission.

Radiation- is energy that comes from a source and travels through some material or through space.

PASSIVE COOLING

Passive Cooling- Passive cooling refers to technologies or design features

used to cool buildings without power consumption, such as those technologies discussed in the Passive house project.

The term "passive" implies that energy-consuming mechanical components like pumps and fans are not used. Passive cooling building design attempts to integrate

principles of physics into the building exterior envelope to:Slow heat transfer into a building. This involves an

understanding of the mechanisms of heat transfer: heat conduction, convective heat transfer, and thermal radiation (primarily from the sun).

Remove unwanted heat from a building. In mild climates with cool dry nights this can be done with ventilating. In hot humid climates with uncomfortable warm / humid nights, ventilation is counterproductive, and some type of solar air conditioning may be cost effective.

Purpose of passive cooling:

Provides indoor comfort Low maintenance Zero/ Low energy consumption Low running cost Promotes healthy environment

☺Saves the Earth

Some passive cooling techniques:

SHADING Shading a building from solar radiation can be

achieved in many ways.

Buildings can be orientated to take advantage of winter sun (longer in the East / West dimension), while shading walls and windows from direct hot summer sun. This can be achieved by designing location-specific wide eaves or overhangs above the Equator-side vertical windows (South side in the Northern hemisphere, North side in the Southern hemisphere).

Overhang is use for shading

VENTILATION› The mechanical system or equipment used

to circulate air or to replace stale air with fresh air.

Ventilation in buildings has three main purposes:

1. To maintain a minimum air quality

2. To remove heat (or other pollutant)3. To provide perceptible air movement to

enhance thermal comfort

Natural Ventilation

oStack Ventilation

oCross Ventilation

oNight Ventilation

Stack Ventilation

Stack ventilation is where air is driven through the building by vertical pressure differences developed by thermal buoyancy. The warm air inside the building is less dense than cooler air outside, and thus will try to escape from openings high up in the building envelope; cooler denser air will enter openings lower down. The process will continue if the air entering the building is continuously heated, typically by casual or solar gains.

Chimney Effect

The most effective application of this natural law (stack effect) is a "thermal chimney," a solar-exposed enclosure tall enough to generate maximum air flow and massive enough to retain heat and power the system into the evening hours.

When to use stack ventilation? Stack ventilation, can operate when no

wind pressure is available. A building can be designed to induce its own ventilation by duplicating the temperature stratifications that are the source of wind itself.

It must be born in mind that the stack effect can only take place when the average temperature in the stack is greater than the outside air.

What to remember when using Stack Ventilation?

Typically the stack effect is quite weak, and therefore openings and ducts must be large, to minimize resistance.

The pressure difference within the stack varies with height resulting in diminishing air flows from spaces opening on to the stack, as their height above ground floor increases.

In tall spaces (multi-room height) the temperature of the air may be hotter in the upper zone. This is referred to as stratification. For a given average temperature, this means that there is a cooler zone at the bottom, which is good news if this is the only occupied space. However it means that rooms facing the upper zone may experience unwanted heat gains, as well as reduced stack effect due to their smaller stack height.

Disadvantage

Due to the weakness of the driving pressures generated by thermal buoyancy, openings have to be large and unobstructed. This means that they will readily transmit noise. Noise attenuating techniques, often used in ductwork of mechanical systems, involve labyrinthine pathways, lined with acoustic absorber. This principle can be applied here but has to be on a large scale in order to cause a minimum flow resistance.

Cross Ventilation Wind-induced ventilation uses

pressures generated on the building by the wind, to drive air through openings in the building. It is most commonly realised as cross-ventilation, where air enters on one side of the building, and leaves on the opposite side, but can also drive single sided ventilation, and vertical ventilation flows.

Example of a cross ventilation:

What to remember when using Cross Ventilation?

Wind speed and direction is very variable. Openings must be controllable to cover the wide range of required ventilation rates and the wide range of wind speeds.

As with stack ventilation, the internal flow path inside the building must be considered.

For cross-ventilation, bear in mind that the leeward space will have air that has picked up heat or pollution from the windward space. This may limit the depth of plan for cross-ventilation.

Disadvantage

As with stack ventilation, the requirement for large openings may present problems with noise control. Also, the need to provide flow paths within the building may conflict with acoustic separation beteen internal spaces. However, the provision of by-pass ducts can help reduce this.

Thermal Comfort

the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation.

Night Ventilation

Night ventilation is the use of the cold night air to cool down the structure of a building so that it can absorb heat gains in the daytime. This reduces the daytime temperature rise.

An overheating prevention strategy which uses little or no fossil energy, and together with other passive strategies such as natural ventilation and shading , can avoid the use of air-conditioning. This saves energy (and CO2 emissions), and once set-up would require lower maintenance than mechanical systems.

Other Techniques:

EVAPORATIVE COOLING› Swamp coolers, fountain courts, and

atrium pools are all applications of evaporative cooling, a particularly powerful technique in climates of low relative humidity. When a body of water is placed in a hot and relatively dry space, the water evaporates into the air and increases humidity.

Example of an evaporative cooling

Desiccant Cooling System combines induced ventilation

to bring air from underground over an activated charcoal desiccant and cool the interior with dry air. As the air warms and exits high on the south wall, it passes over the saturated desiccant plate, spurring the evaporative process

Example of desiccant evaporation

Time Lag Cooling

The principle is that the transmission of heat through mass—stone, concrete, adobe-—is both delayed and attenuated over time. Depending on the material and the thickness of a massive wall, the delay can stretch from two to 12 hours, and the greater the lag the greater the attenuation of heat transmitted.

Earth Cooling

Another method for taking advantage of Mother Earth is to pre-condition air by running it through subterranean cool pipes before it enters the building, or by storing it in a below-grade rock storage chamber before use

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