Passive Cooling & Natural Ventilation PDF

Cooling Passive Active natural ventilation mechanical ventilation Natural ventilation provided by: Openings Doors Windows Shafts Advantages: Not expensive Energy efficient healthy Mechanical ventilation provided by: Air conditioning Fans Air coolers Exhaust Fans...

Cooling Passive Active natural ventilation mechanical ventilation Natural ventilation provided by: Openings Doors Windows Shafts Advantages: Not expensive Energy efficient healthy Mechanical ventilation provided by: Air conditioning Fans Air coolers Exhaust Fans Disadvantages: Expensive Require energy What is natural ventilation? Ventilating a building results in an air exchange between the inside and outside at various rates. When natural ventilation could be used in Bahrain? Hourly ET 35 JAN FEB 30 MAR 25 APR ET 20 MAY JUN 15 JUL 10 AUG 5 SEP OCT 0 :00 :00 :00 :00 :00 :00 :00 :00 :00 :00 :00 :00 0:0 NOV 06 08 10 12 14 16 18 20 22 02 04 06 0:0 DEC Min H Max  Hot period Jun, Jul, Aug, Sep cold period Jun, Feb, Mar, Nov, Dec Comfort zone Apr, May, Oct (22- 27) So we can depend on natural ventilation for cooling buildings from October till April and require mechanical cooling only from May to September.  Basic principles of airflow 1) Air will always flow from high pressure to lower pressure. 2) Air has mass and it will tend to continue in its direction until altered by an obstruction. 3) Airflow is smooth with adjacent air moving in similar direction and speed - Two currents of air traveling in opposite direction will be separated by eddies. - Slow, gentle alterations will preserve laminar flow. - Abrupt alterations results in turbulent flow. 4) Bernoulli effect: causes a decrease in pressure when air is accelerated in order to cover a greater distance than adjacent airflow. 5) venturi effect: causes an acceleration when laminar airflow is constricted in order to pass through an opening. 6) The stack effect results when air in the building warms, density decrease and air rises to escape out from high openings. 7) Cross ventilation requires an outlet as well as an inlet.  Factors affects the airflow Low pressure zones occur along the sides parallel to the wind and on the leeward side of the building A low building placed in Multiple low pressure zones the wind ward path of a are caused by linear tall building produces a arrangement of building large amount of resulting in wind shadow turbulence between them. with minimum ventilation potential. Raising a tall building reduces the high pressure on the windward side by allowing airflow under the building. An opening on windward side only result in poor ventilation, an additional leeward opening is essential to complete the connection between high and low pressure regions.  Window placement Cross ventilation where openings are on opposite walls creates best airflow. Windows on adjacent walls, ventilation can be improved by wing walls. Effective open area of various window types  Maximum interior airspeed is created when the inlet is smaller than the outlet. If the inlet is larger than the outlet, velocity in the room is reduced. This has potential for cooling a localized exterior area. Double roof strategies Roof is a major source of unwanted heat gain in buildings To get rid of it, introduce a ventilated air space between the roof and the ceiling below An inlet centered in the Opening located at the wall restricts airflow to a corners of the building side outlet due to an abrupt allow the inertia to change in direction, flow is continue the motion in the increased by repositioning same direction in a smooth the inlet to a more diagonal curve until the outlet is location and adding a reached. baffle. The vertical position of the inlet window is important in maximizing the airflow through the lower occupied portion of the room. The low inlet is best for cooling. The outlet location has little effect on flow within the room. Interior Partitions Depending on the size of interior partition openings the velocity of incoming airflow near the inlet window can exceed that of the outside.  Case Study Center Zion National Park Visitor (ZNP) Location: in Utah 150 miles from lass Vegas.  Most of the cooling is done with natural ventilation via the operable clerestory windows. Additional evaporative cooling is provided with passive downdraft cool towers. The cool towers designed to operate on natural convection driven primary by buoyancy.  Water is pumped to four sets of pads on the top of the tower. This cool, dense air “falls” through the tower and exit through large openings at the bottom of the tower. The cool air drawn into the building then the hot air that already inside the space rise and exit through the open clerestory windows. The only energy required for each tower is (249w) for water pumps.  Passive cooling Natural ventilation Done by: Tasneem Sami 20063498 Sara Ahmed 20064725