Lecture 11 Natural Ventilation (Passive Cooling)

Transcription

1 Islamic University-Gaza Faculty of Engineering Architecture Department Principles of Architectural and Environmental Design EARC 2417 Lecture 11 Natural Ventilation (Passive Cooling) Instructor: Dr. Suheir Ammar

2 Natural Ventilation (Passive Cooling) 1. Principles of Natural Ventilation NV 3. Vegetation and ventilation 2

3 Natural Ventilation (Passive Cooling) 3 Natural ventilation is necessary for: Fresh air supply into the building, Increasing, through air movement, the rate of perspiration evaporation from the bodies of those in the building, Cooling the interior of the building by exchanging relatively warm hot internal air with cooler outside air 1. Principles of Natural Ventilation For air to move into and out of a building, a pressure difference between the inside and outside of the building is required. The pressure difference is caused by: wind (or wind effect) difference in air density due to temperature difference between indoor and outdoor air (stack or chimney effect) combination of both wind and stack effects.

4 1.1. Understanding air flow Principle 1: Air will always flow from region of high pressure to lower pressure high pressure low pressure Principle 2: Air has mass and it will tend to continue in its direction until altered by an barrier or an adjacent air flow. 4 Copyright barrier Wondershare Software

5 1.1. Understanding air flow Principle 3: The overall effect of wind at a site is so large that locally deflected airflow (by trees or buildings) will tend to return to the direction and speed of the site wind. 5

6 1.1. Understanding air flow Principle 4: Airflow is smooth with adjacent air moving in similar direction and speed. 1. Slow, gentle alterations of flow direction will preserve laminar flow, 2. Sudden alterations results in turbulent flow whereby adjacent air currents separate suddenly into swirling, unpredictable directions. 3. When two currents of air traveling in opposite directions, they will always be separated by eddies because adjacent particles of air always move in the same direction. gentle alterations 1 6 Sudden alterations 2

7 1.1. Understanding air flow Principle 5: when laminar airflow is constricted in order to pass through an opening, the same volume of air must pass through a smaller area, so it causes an acceleration of air flow. an acceleration air flow 7

8 1.1. Understanding air flow Principle 6: Cross-ventilation requires an outlet as well as an inlet. Analogy: water cannot be put into a bottle that already full unless some old water is removed first. 8

9 1.1. Understanding air flow Principle 7: The stack effect Stack effect is the result of air density decreasing as temperature increase. Warm air rises through upper openings and replaced by cooler through low openings. The stack effect 9

10 1.2. Stack effect Stack effect is the result of air density decreasing as temperature increase. Warm air rises through upper openings and replaced by cooler through low openings. If the wind shifts, the air circulation may reverse, causing the solar-heated air to be blown down into the living space, effectively heating the building rather than cooling it. For this reason, any solar chimney should have a wind-activated ventilator cap to ensure that airflow is always positive. 10 Warm air rises Copyright heated Wondershare air to be blown Software down

11 1.1. Understanding air flow The Wind effect

12 Stack and wind effect 12

13 Wind effect When air flow is due to wind, air enters through openings in the windward walls, and leaves through openings in the leeward walls. Wind pressures are generally high/positive on the windward side of a building and low/ negative on the leeward side. The occurrence and change of wind pressures on building surfaces depend on: wind speed and wind direction relative to the building. the location and surrounding environment of the building. shape of the building.

14 1.3. Wind effect The following diagrams describe principles of ventilation airflow around buildings and the pressure distribution patterns due to wind. 14 Plan Low pressure zones occur along the sides parallel to the wind and on the leeward side of the building.

15 1.3. Wind effect A low building placed in the windward path of a tall building produce a large amount turbulence instability between the two buildings. 15 An opening on windward side only results in poor ventilation.

16 1.3. Wind effect Raising a tall building reduce the high pressure on the windward side by allowing airflow under the building. 16

17 Wind & building arrangement³ Staggered building arrangements result in reduced wind shadows. Airflow patterns through a) a normal, b) a scattered c) a diagonal layout of buildings.

18 Wind & building arrangement

19 Single Sided Ventilation: This kind of ventilation is normal and generally practiced, but it is only useful up to a certain room depth Single Sided Double Opening: An advancement of the single sided principle provides a double opening, which is considerably more efficient. 19 Cross Ventilation: In order to obtain the optimal airflow with minimal draught, the windows on the windward side are opened less than on the lee side.

20 20 Passive Cooling: As the external temperature drops at night, the building can be cooled by partially opening the vents around the building - often called night-cooling.

21 21

22 22 A solar chimney uses the sun's heat to provide cooling, using the stack effect. Solar heat gain warms a column of air, which then rises, pulling new outside air through the building. Solar chimneys can also be used for heating. If the top exterior vents are closed, the heated air is not exhausted out the top; at the same time, if high interior vents are opened to let the heated air into occupied spaces, it will provide convective air heating

23 Different solar chimney designs 23

24 Solar chimney in buildings 24

25 2.2. Window placement 1. On opposite walls in rooms, crossventilation creates the best airflow. 2. On adjacent walls in a room, crossventilation can be improved by positioning of wing walls adjacent to the window openings. 3. On one exterior wall in a room, with only wing walls adjacent to window have been found to increase ventilation. 25

26 2.2. Window placement³ Air flow patterns around a building and impact of opening location on ventilation 26

27 2.2. Window placement on the same or adjacent walls wing wall design patterns for two windows on the same or adjacent walls showing probable airflow patterns and wind directions for improved ventila1on performance due to wing walls: excellent good good poor excellent good 27 Copyright poor Wondershare Software poor

28 2.2. Window placement Maximum air exchange is created when the inlet and outlet areas are equal, making the optimum configuration when building cooling is the goals. 28 lf the inlet is larger than the outlet, velocity in the room is reduced (although velocity outside just to-leeward of the outlet is increased). This has potential for cooling a localized exterior area such as a patio

29 2.2. Window placement 29 Maximum interior air speed is created when the inlet is smaller than the outlet, making the optimum configuration when people cooling is the goal.

30 2.2. Window placement 30 An inlet centered in the wall restricts airflow to a side outlet due to an abrupt change in direction, flow is increased by repositioning the inlet to a more diagonal location and adding a baffle directs entering air diagonally in the direction of the outlet.

31 2.2. Window placement 31 Openings located at the corner of the building allow the inertia to continue the motion in the same direction in a smooth curve until the outlet is reached.

32 2.2. Window placement The vertical position of the inlet window is important in maximizing the airflow through the lower, occupied portion of the room. The outlet location has little effect on flow within the room. 32 Copyright The low inlet Wondershare is better for Software cooling.

33 2.2. Window placement An overhang above the inlet window directs the interior airflow along the ceiling out of the occupied zone. The addition of a slot separating the overhang from the building redirects the flow down into the 33 room, increasing the useful cooling effect.

34 2.2. Window placement An overhang or an awning increases air velocity through a window below them 34 Sashes configuration may have a significant effect on the flow of air through an opening.

35 The location and orientation of interior partitions can affect the velocity and direction. The partition placed parallel to the direction of airflow have least effect. 35 The partition placed perpendicular to the direction of airflow redirects the pattern and reduces the velocity.

36 Partitions placed outside of the main airflow path have little effect. Partitions placed blocking the path create a barrier effect, creating motionless areas. 36

37 3. Vegetation and ventilation³ Vegetation increasing, decreasing and directing airflow 37 Vegetation causes pressure differences which shifts the air path

38 3. Vegetation and ventilation 2 good ventilation poor ventilation 38

39 3. Vegetation and ventilation 2 39

40 3. Vegetation and ventilation a low hedge Effect on ventilation of a low hedge (less than 3 ft high) located parallel to the windward windows at different 40 distances.

41 3. Vegetation and ventilation a medium hedge Effect on ventilation of a medium hedge located parallel to the windward windows at different distances. 41

42 3. Vegetation and ventilation a high hedge 42 Effect on ventilation of a high hedge located parallel to the windward windows at different distances.

43 3. Vegetation and ventilation Effect on ventilation of a 30-ft tree on the windward sides at different distances. 43

44 Reference: 1. Moore, Fuller, Environmental control systems: heating, cooling, lighting. New York : McGraw-Hill 2. Santamouris, M., Allard, F., Energy, E. C. D.-G. f., & Programme, A. (1998). Natural Ventilation in Buildings: A Design Handbook: James and James (Science Publishers) Limited 3. Heiselberg, P. (2006). Design of Natural and Hybrid Ventilation. Aalborg: Department of Civil Engineering, Aalborg University. (DCE Lecture Notes; No. 5). Thank you 44