Chapter 10.Passive Cooling

Contents

10.1 Introduction to Cooling10.2 Historical and Indigenous Use of Passive Cooling10.3 Passive Cooling Systems10.4 Comfort Ventilation VS Night Flush Cooling10.5 Basic Principles of Air Flow10.6 Air Flow Through Buildings10.7 Examples of Ventilation Design10.8 Comfort Ventilation10.9 Night-Flush Cooling10.10 Smart Facades and Roofs10.11 Radiant Cooling10.12 Evaporative Cooling10.13 Cool Towers10.14 Earth Cooling10.15 Dehumidification with a Desiccant10.16 Conclusion

10.1 INTRODUCTION TO COOLING• The three-tier design approach to sustainable cooling

Tier 1: Heat avoidance: shading, orientation, color, vegetation, insulation, daylighting, control of internal heat sources to minimize heat gain in the buildings.

Tier 2: Passive cooling: techniques and systems to lower temperature, and ventilation to shift the comfort zone to higher temperature.

Tier 3: Mechanical cooling: required when the combined effort of heat avoidance and passive cooling is not sufficient to maintain thermal comfort.

10.2 HISTORICAL AND INDIGENOUS USE OF PASSIVE COOLING• Passive cooling is much more dependent on climate than passive heating. Passive

cooling strategies for hot and dry climates are very different from those for hot and humid climates.

1) Passive Cooling in Hot and Dry Climates:

- Buildings usually have few and small windows, light surface colors, and massive construction, such as adobe, brick, or stone.

- In urban settings with little wind, wind scoops are sometimes used to maximize ventilation. When there is a strong prevailing wind direction, as in Hyderabad, Pakistan, the scoops are all aimed in the same direction.

- In other areas, where there is no prevailing wind direction, such as Dubai, wind towers with many openings are used.

- Where the humidity is low, evaporative cooling is very effective. Fountains, pools, water trickling down walls, and transpiration from plants can all be used for evaporative cooling.

- Massive domed structures are an appropriate strategy in hot and dry regions. During the day, the sun see little more than the horizontal footprint of the dome, while at night almost all full hemisphere sees the night sky. Thus, radiant heating is minimized while radiant cooling is maximized. Domes also have high spaces where stratification will enable the occupants to inhabit the cooler lower levels.

- A large quantity of earth or rock is an effective barrier to the extreme temperatures in hot and dry climate

2) Passive Cooling in Hot and Humid Climates:

- Buildings usually have large windows, large overhangs, and lightweight structures to maximize natural ventilation and to minimize solar heat gain.

10.3 PASSIVE COOLING SYSTEMS

I. Cooling with Ventilation1) Comfort ventilation2) Night flush cooling

II. Radiant Cooling1) Direct radiant cooling2) Indirect radiant cooling

III. Evaporative Cooling1) Direct evaporation2) Indirect evaporative cooling

IV. Earth Cooling1) Direct coupling2) Indirect coupling

V. Dehumidification with a Desiccant

• There are 5 methods of passive cooling

1) Comfort Ventilation

- Ventilation during the day and night to increase evaporation and convective heat transfer from the skin and thereby increasing thermal comfort.

- The air is passed directly over people.

10.4 COMPORT VENTILATION VS NIGHT-FLUSH COOLING

2) Night Flush Cooling

- Ventilation at night to precool the building for the next day.

- Natural ventilation or fans bring cool outdoor air into the building to make contact with and cool the indoor mass.

10.5 BASIC PRINCIPLES OF AIR FLOW

1) Reason for the flow of air: Air flows either because of natural convection currents, caused by differences in temperature, or because of differences in pressure (Fig. 10.5a).

2) Types of air flow: There are 4 basic types of air flow: laminar, separated, turbulent, and eddy currents (Fig. 10.5b, Fig. 10.5e).

3) Inertia: Since air has some mass, moving air tends to go in a straight line. When forced to change direction, air streams will follow curves but never right angles.

4) Conservation of air: Since air is neither created nor destroyed at the building site, the air approaching a building must equal the air leaving the building. Thus, lines representing air streams should by drawn as continuous.

5) High- and low-pressure areas: As air hits the windward side of a building, it compresses and creates positive pressure (+). At the same time, air is sucked away from the leeward side, creating negative pressure (-) (Figs. 10.5c, 10.5d, 10.5e)

6) Bernoulli effect: In the Bernoulli effect, an increase in the velocity of a fluid decreases its static pressure. (Figs. 10.5f, 10.5g, 10.5h, 10.5i).

The velocity of air increases rapidly with height above ground. Thus, the pressure at the ridge of a roof will be lower than that of windows at ground level. Consequently the Bernoulli effect will exhaust air through roof openings. (Fig. 10.5j).

7) Stack effect: The stack effect can exhaust air from a building by the action of natural convection (Figs. 10.5k, 10.5l, 10.5m, 10.5n, 10.5o).

Solar Chimney and Earth Tube

10.6 AIR FLOW THROUGH BUILDINGS

1) Site Conditions

Adjacent buildings, walls, and vegetation on the site will greatly affect the air flow through a building.

2) Window Orientation and Wind Direction

3) Window Locations

4) Fin Walls

5) Horizontal Overhangs and Air Flow

6) Vertical Placement of Windows

The purpose of the air flow will determine the vertical placement and height of windows. For comfort ventilation, the windows should be low, at the level of the people in the room. High openings are important for night-flush cooling where air must pass over the structure of the building (Fig. 10.6o).

The vertical location of inlet windows greatly affect the direction of air stream in the room, while outlet windows have no effect on the air direction.

outlet (high) outlet (mid) outlet (low)

inlet (high) inlet (mid) inlet (low)

7) Inlet and Outlet Sizes and Locations

8) Insect Screens

Air flow is decreased about 50 % by an insect screen. To compensate for the effect of the screen, larger window openings are required. A screened-in porch is especially effective because of the very large screen area that it provides (Fig. 10.6t)

9) Roof Vents

Passive roof ventilators are typically used to lower attic temperatures. The common wind turbine enhances ventilation about 30 % over an open stack and deflectors can enhance the air flow as much as 120 % (Fig. 10.6u).

The BedZED housing project uses rotating vents with wind vanes so that theopening is always on the leeward side to maximize the negative pressure (Fig.10.6v).

The simple open-stack ventilators in BRE office buildings achieve significantventilation by the height of the opening (Fig. 10.6w).

Although monitors and roof vents are often a part of traditional architecture,they can also be integrated very successfully into modern architecture (Figs.10.6x and y).

10) Fans

In most climates, wind is not always present in sufficient quantity when needed, and usually there is less wind at night than during the day. Thus, fans are often required to augment the wind.

11) Partitions and Interior Planning

Open plans are preferable because partitions increase the resistance to air flow, thereby decreasing total ventilation. Before air conditioning became available transoms (windows above doors) allowed for some cross ventilation.

Le Corbusier came up with an ingenious solution for cross ventilation in hisUnite d’Habitation at Marseilles, France.

Unité d'habitation, Marseille, France, 1945

1) Wind Tunnel Test

PlanScale model of an urban district

Control Room Visualization of air flow by smoke

10.7 EXAMPLE OF VENTILATION DESIGN

2) CFD(Computational Fluid Dynamics) Modeling

Visualized wind flow around a building

Visualized air flow vectors in an atrium

10.8 COMFORT VENTILATION

- Air passing over the skin creates a physiological cooling effect by evaporating

moisture from the surface of the skin. It also increase sensible heat release from the

skin by a forced convection.

- The term comfort ventilation is used for this technique of using air motion across

the skin to promote thermal comfort.

- For comfort ventilation, the air flow techniques should be used to maximize the air

flow across the occupants of the building.

10.9 NIGHT-FLUSH COOLING

- In all but the most humid climates, the night air is significantly cooler than the daytime air.

This cool night air can be used to flush out the heat from a building’s mass.

- The precooled mass can then act as a heat sink during the following day by absorbing heat.

10.10 SMART FACADES AND ROOFS

1) Direct radiant cooling

A building’s roof structure cools by radiation to the night sky.

10.11 RADIANT COOLING

2) Indirect radiant cooling

Radiation to the night sky cools a heat-transfer fluid, which then cools the building.

• When water evaporates, it draws a large amount of sensible heat from itssurroundings and converts this heat into latent heat in the form of water vapor.As sensible heat is converted to latent heat, the temperature drops.

• If the water evaporates in the building or in the fresh-air intake, the air will benot only cooled but also humidified. This method is called direct evaporativecooling.

• If, however, the building or indoor air is cooled by evaporation withouthumidifying the indoor air, the method is called indirect evaporative cooling.

10.12 EVAPORATIVE COOLING

1) Direct Evaporative Cooler

2) Indirect Evaporative Cooler

- Cool towers are passive evaporative coolers that act like reverse chimneys.

- At the top of the tower, water is sprayed on absorbent pads. As air enters thetop of the tower, it is cooled, becomes denser, and sinks.

- Thus, instead of hot air flowing up, cool air flows down inside the cool towers,filling the building with cool air without the help of fans.

10.13 COOL TOWERS

• Earth, especially wet earth, is both a good conductor and storer of heat (i.e., ithas high heat capacity).

• The curves in Figure 10.14a show the earth temperatures as a function of depth.The ground temperatures at any depth fluctuate left and right between thesecurves during a year. Due to large heat capacity, the soil temperature fluctuatesless and less as the soil depth increases.

• At about 6 m in depth, the summer/winter fluctuations becomes very small anda year-round steady-state temperature exists.

10.14 EARTH COOLING

1) Cooling the Earth.

2) Direct Earth Coupling

- When earth-sheltered buildings have their walls in direct contact with the ground (i.e., there is little or no insulation in the walls), we say that there is direct earth coupling.

- Directly coupled earth cooling works well when the steady-state earth temperature is a little below 15 ℃

- If the earth is much colder, the building must be insulated from the ground.

3) Indirect Earth Coupling

- A building can be indirectly coupled to the earth by means of earth tubes.

- When cooling is desired, air is drawn through the tubes in the building. Theearth acts as a heat sink to cool the air.

10.15 DEHUMIDIFICATION WITH A DESICCANT

• In humid regions, dehumidifying the air in summer is very desirable for thermalcomfort and control of mildew.

• Two methods to remove moisture from the air:1) Condensing moisture in the air by reducing the temperature below the dew

point temperature by air conditioner. This method requires mechanical work using electric power.

2) A desiccant (drying agent) such as silica gel, natural zeolite, activated alumina, and calcium chloride is used to absorb large amount of water vapor from the air. This method requires to regenerate the desiccant saturated with water by boiling off the water using heat.

10.16 CONCLUSION

• Passive cooling strategies have the greatest potential in hot and dry climates.

• In very humid regions only comfort ventilation will be very helpful.

• However, in every climate, the first and best strategy for summer comfort is heatavoidance.