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enhancing thermal comfort in Buildings for arid regions: by using passive cooling strategies

By: Ridha Alquahmed 201316170ABSTRACT

The purpose of this research is to identify passive cooling design strategies in buildings to reduce the use of mechanical systems and save energy, which will enhance thermal comfort in arid climate regions buildings.

ABSTRACT

2

1. INTRODUCTION

4

2. Historical Passive cooing

2

3. AirFlow through buildings

2

4. passive cooling strategies 

2

I. cooling with ventilation

2

ii. radiant cooling

2

III. evaporative cooling

2

IV. Earth cooling

2

v. Dehumidification 

2

5. Conclusion

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6. References

2

INTRODUCTION

To accomplish thermal comfort in the summer the focus must be on 3 different strategies (three-tier design approach) Fig(1), the most important and most recommended is heat avoidance, the design should as much as possible minimize gaining heat by providing shading systems, appropriate insulation, orientation, color, vegetation, and controlling daylight. However, this report will focus on the second-tire approach  which is passive cooling, this approach can be accomplished by controlling air movement which creates different strategies and that will be the main ideas of this report, and to control air one must understand how it moves as this is what will be discussed in the next chapters. However, many countries have climates that when the previous two approaches ( heat avoidance and passive cooling) are not enough to achieve thermal comfort then one must rely on the least recommended strategy third-tire approach mechanical cooling, however this approach should only be rayed on whatever the previous two could not accomplish, this approach will minimize the use and the size of mechanical systems.  Historical passive cooling

Examples are every now and then are much clearer than definitions in explaining principles. the following examples of ancient and indigenous buildings will illustrate what is passive cooling and how is works. Passive cooling is a lot more dependent on climate than passive heating. therefore, passive cooling techniques for dry climates are very different from the ones for hot and humid climates. In warm and dry climates, one typically reveals buildings with few and small windows, mild surface colors, and big construction, which include adobe, brick, or stone Fig (2). hot and dry climates have excessive diurnal temperature degrees, the nights generally tend to be cool. night air flow can then be used to chill the indoor mass, which will then act as a heat sink the following day. and to not overload heat sinks the designs usually have light colors, closed shutters during daytime while allowing air flow.  
Wind scoops were used thousands of years ago in Egypt Fig (3), when there is a prevailing wind direction all wind scoops are facing it, while other places like the Middle East wind towers with many openings are used for not having a prevailing wind direction Fig (4). These rectangular towers are divided by diagonal walls, which create four separate air wells facing four different directions. 
   
Wind towers have shutters to keep out unwanted ventilation. In dry climates, they also have a means of evaporating water to cool the incoming air. Some wind towers have porous jugs of water at their base, while others use fountains or  trickling water Fig (5).
 Also in the Middle East there is a popular wind catcher called mashrabiya Fig (6), extruded windows comfortable to sit and sleep, getting daylight into the building and ventilation also using jugs of water to cool the house by evaporative cooling which is a very effective method, in some houses other features serves evaporative cooling like fountains and pools or even vegetation. Combining all     these features and adding them into a house with indoor courtyard with a atriums are very effective because the courtyard would be self-shaded and blocking hot air and sucking cool air in Fig (7).

Much of Japan has very hot and humid summers. To maximize natural ventilation, the traditional Japanese house uses post-and-beam construction, which allows the lightweight paper wall panels to be moved out of the way in the summer Fig(8). Large overhanging roofs protect these panels and also create an outdoor space called an engawa. Also large vents increase the ventilation through the building Fig (9).    
AIRFLOW THROUGH BUILDINGS

To maximize the effectiveness of the strategies that will be discussed in the next chapters, one must understand how airflow though buildings and the basic principles of airflow, the next 7 points are quoted from air movement manual:

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).  

Types of airflow. There are four types of airflow: laminar, separated, turbulent, and eddy currents. Fig (11) shows the four types by lines representing airstrems.  

Inertia. Since air has some mass, moving air tends to go in a straight line. When forced to change direction, airstreams will follow curves but never right angles.
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.
High- and low-pressure areas. As air hits the windward side of a building, it compresses and creates positive pressure Fig (12). At the same time, air is sucked away from the leeward side, creating negative pressure. Air deflected around the sides will generally also create negative pressure. Note that these pressures are not uniformly distributed. The type of pressure created over the roof depends on the slope of the roof Fig (13).  

  

Bernoulli effect. In the Bernoulli effect, an increase in the velocity of a fluid decreases its static pressure. Because of this phenomenon, there is negative pressure at the constriction of a venturi tube Fig (14).The venturi tube illustrates the Bernoulli effect: As the velocity of air increases, its static pressure decreases. Thus, an opening at the constriction would suck in air.  

There is another phenomenon at work here. 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, even without the help of the geometry of a venturi tube, the Bernoulli effect will exhaust air through roof openings Fig (15)  

Stack effect. The stack effect can exhaust air from a building by the action of natural convection. The stack effect will exhaust air only if the indoor-temperature difference between two vertical openings is greater than the outdoor-temperature difference between the same two openings Fig (16). To maximize this basically weak effect, the openings should be as large and as far apart vertically as possible. The air should be able to flow freely from the lower to the higher opening to minimize obstructions.  

after explaining the principles, the factors affecting airflow though a building, the pressure distribution around the building, factors like the direction of air entering windows, size, location and details of windows will be discussed in the next points.
Window orientation and wind direction
Winds apply maximum pressure when they are 90 degrees to the surges they are hitting, while the pressure is reduced to 50% when the degree is about 45 degrees. However, indoor ventilation is better when the maximum pressure is applied because that leads to more motion of air indoor Fig (17). In most climates,   
because of summer requires shading systems and winters need of sun designs usually are oriented with a long axis in the east-west direction, Fig (18) shows the range of wind in direction that work well with the orientation, and even when wind are long from east to west, the solar orientation is far more important because winds are more easily controlled and rerouted then the sun Fig (19).    

Window Locations
The location of windows is very important, cross ventilation is very effective because air is both pushed and pulled through the building by a positive pressure on the windward side and a negative pressure on the leeward side, Fig (20) showing opposite locations of windows is very effective. While ventilation from   windows on adjacent walls can be either good or bad, depending on the pressure distribution, which varies with wind direction Fig (21). However, ventilation from   windows on one side of a building can vary from fair to poor, depending on the location of windows. Since the pressure is greater at the center of the windward wall than at the edges, there is some pressure difference in the asymmetric placement of windows, while there is no pressure difference in the symmetric scheme Fig (22). 

 

Fin Walls
Fin walls can increase ventilation though windows on the same side of a building by changing the pressure distribution 

PASSIVE COOLING strategies 

This chapter will discuss five main strategies, that combines traditional methods used in the past and sophisticated modern techniques. achieving thermal comfort can be done by either cooling buildings by removing heat and storing it in other heat sinks, or raising the comfort zone to include the high indoor temperature in the process by controlling other factors like air speed, humidity or mean radiant temperature (MRT), this will make people feel more comfortable even though the building is not being cooled.

Passive cooling strategies include cooling with ventilation, radiant cooling, evaporative cooling, earth cooling and dehumidification. 

Cooling with ventilation

1. Comfort Ventilation: Increasing evaporation by exposing the skin to airflow through day and night, which will increase thermal comfort.
2. Night-flush Cooling: Night breeze cooling the building for the next day.

Radiant Cooling

1. Direct: a building roof structure cools by radiation to the night sky 
2. indirect: Radiation to the night sky cools a heat-transfer fluid, which cools the building in the process.

Evaporative Cooling

Earth Cooling

Dehumidification Conclusion

References 

Boutet, T. S. Controlling Air Movement: a Manual for Architects and Builders. New York: McGraw-Hill Book Company1987.

Konya, Allan, and Maritz Vandenberg. Design primer for hot climates. Reading, enk.: Archimedia Press Limited, 2011.

Olgyay, Victor, Aladar Olgyay, Donlyn Lyndon, Victor W. Olgyay, John Reynolds, and Ken Yeang. Design with climate: bioclimatic approach to architectural regionalism. Princeton: Princeton University Press, 2015.