Chapter 10. Passive Cooling Contents 10.1 Introduction to Cooling 10.2 Historical and Indigenous Use of Passive Cooling 10.3 Passive Cooling Systems 10.4 Comfort Ventilation VS Night Flush Cooling 10.5 Basic Principles of Air Flow 10.6 Air Flow Through Buildings 10.7 Examples of Ventilation Design 10.8 Comfort Ventilation 10.9 Night-Flush Cooling 10.10 Smart Facades and Roofs 10.11 Radiant Cooling 10.12 Evaporative Cooling 10.13 Cool Towers 10.14 Earth Cooling 10.15 Dehumidification with a Desiccant 10.16 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 There are 5 methods of passive cooling I. Cooling with Ventilation 1) Comfort ventilation 2) Night flush cooling II. Radiant Cooling 1) Direct radiant cooling 2) Indirect radiant cooling III. Evaporative Cooling 1) Direct evaporation 2) Indirect evaporative cooling IV. Earth Cooling 1) Direct coupling 2) Indirect coupling V. Dehumidification with a Desiccant
10.4 COMPORT VENTILATION VS NIGHT-FLUSH 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. 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. inlet (high) inlet (mid) inlet (low) outlet (high) outlet (mid) outlet (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 the opening is always on the leeward side to maximize the negative pressure (Fig. 10.6v). The simple open-stack ventilators in BRE office buildings achieve significant ventilation 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 his Unite d Habitation at Marseilles, France. Unité d'habitation, Marseille, France, 1945 10.7 EXAMPLE OF VENTILATION DESIGN 1) Wind Tunnel Test Plan Scale model of an urban district Control Room Visualization of air flow by smoke
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 10.11 RADIANT COOLING 1) Direct radiant cooling A building s roof structure cools by radiation to the night sky.
2) Indirect radiant cooling Radiation to the night sky cools a heat-transfer fluid, which then cools the building.
10.12 EVAPORATIVE COOLING When water evaporates, it draws a large amount of sensible heat from its surroundings 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 be not only cooled but also humidified. This method is called direct evaporative cooling. If, however, the building or indoor air is cooled by evaporation without humidifying the indoor air, the method is called indirect evaporative cooling. 1) Direct Evaporative Cooler
2) Indirect Evaporative Cooler 10.13 COOL TOWERS - 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 the top 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.
10.14 EARTH COOLING Earth, especially wet earth, is both a good conductor and storer of heat (i.e., it has 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 these curves during a year. Due to large heat capacity, the soil temperature fluctuates less and less as the soil depth increases. At about 6 m in depth, the summer/winter fluctuations becomes very small and a year-round steady-state temperature exists. 1) Cooling the Earth.
2) Direct Earth Coupling - When earth-sheltered buildings have their walls in direct contact with the grou nd (i.e., there is little or no insulation in the walls), we say that there is direct e arth coupling. - Directly coupled earth cooling works well when the steady-state earth tempera ture 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. The earth 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 thermal comfort 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 heat avoidance.