Passive Cooling

1. Passive Cooling

2. Natural Ventilation • Air Cooling • Shades

3. Natural Ventilation • Also called passive ventilation • Uses natural outside air movement/pressure differences to cool/ventilate the building • Provide and move fresh air without fans

4. When Not to Use • Sites with high levels of acoustic noise • Sites with poor air quality

5. Air Speed and Temp in Buildings • Higher velocity is more effective for cooling • Pulls heat away faster • Helps sweat evaporate quicker

6. Opening Design

7. Shape • Long horizontal strip windows ventilate a space more evenly • Tall windows with openings at top and bottom can use convection and outside breezes • Pulls out hot air from the top of the room and brings in cool air at the bottom

8. Size • Rule of thumb: operable windows/louvers should be 20% of the floor area with the inlet size roughly matching the outlet • A smaller inlet paired with a larger outlet can increase velocity

9. Wind Ventilation • A type of passive ventilation • Uses the force of the wind to pull air through a building • Easiest, most common, and typically least expensive form • Successful ventilation is determined by having a high thermal comfort and adequate fresh air for ventilated spaces

10. Wind Ventilation • Strategies include • Operable windows • Ventilation louvers • Rooftop vents • Structures to aim or funnel the breeze • Pressure difference should be maximized between windward (inlet) and leeward (outlet) • High pressure on the windward side

11. Site, Massing, Orientation • Upper floors and roofs are exposed to more wind than lower floors • Buildings with thin profiles facing into the path are easiest to ventilate • Openings create inlets and outlets to maximize air flow

12. Cross Ventilation

13. Cross Ventilation • Not best to place openings directly across from each other • Some areas are well cooled and others are not • Place openings across from each other to distribute cool and fresh air • Have large openings on both windward and leeward faces and place inlets at higher pressure zones • Place inlets low and outlets high

14. Cross Ventilation

16. Steering Breezes • Not all areas of the building will be oriented for cross breezes • Use architectural features such as casement windows, wing walls, fences or planted vegetation

17. Wing Walls • Project outward next to a window so that any breeze against the wall creates a high pressure zone on one side and a low pressure side on the other • Effective on sites with low outdoor air velocity and variable wind directions

19. Stack Ventilation and Bernoulli • Use air pressure differences due to height to pull air through a building • Low pressures higher in the building help pull air upward

20. Stack Ventilation • Uses temp differences to move air • Hot air rises because it is lower pressure • Sometimes called buoyancy ventilation

21. Bernoulli • Uses wind speed differences to move air • The faster the air moves the lower the pressure • Outdoor air farther from the ground has less obstructions and moves faster than the lower air • This creates a lower pressure which can suck fresh air through the building

22. Bernoulli • Advantage over stack effect is that it multiplies the effectiveness of wind ventilation • Stack ventilation does not need wind though • Designing for usually means designing for both • Can emphasize one over the other

23. Chimney will optimize the stack effect • Wind scoops optimize Bernoulli’s principle • Besides wind ventilation, stack ventilation is the most commonly used form of passive ventilation • Typically at night wind speeds are slower so ventilation is less effective

24. Strategies • Cold air is sucked in through a low inlet opening and hotter air escapes through high outlets (roughly the same size openings) • Have large differences in height between inlets and outlets • Towers, skylights, clerestories, and chimneys can be useful. Air must be able to move between floors

26. Strategies • Solar radiation can be used to enhance the effect in tall spaces • Heat up interior surface/temp which will accelerate ventilation • Weatherproof vents in hot climates

27. Strategies • To allow adjustability in cooling and fresh air, inlets should be adjustable with operable windows or louvers • Combine with cross ventilation

29. Solar ChimneyThermal chimney, Thermosiphon,Thermosyphon • Uses the sun’s heat to provide cooling with the stack effect • Warms a column of air which rises pulling new air up/in

30. Thermal Chimney • Simplest ones are chimneys painted black • Need exhaust higher than the roof level and sun exposure

31. Advanced Chimney • Involve a Trombe wall, but are better insulated from occupied spaces so they don’t heat the space

32. Or can be used to aid with heating

33. Night Purge Ventilation • Keeps windows and other passive vents closed during the day but open at night to flush warm air out • Useful when daytime air temp is so high that bringing unconditioned air into the building doesn’t cool people down but nighttime air is cool/cold • During the day thermal mass soaks up heat and at night is cooled by the outside air

35. Night Purge • Need a large area of exposed internal thermal mass • Also need a relatively unobstructed interior

36. Air Cooling • Useful in hot dry climates • Faster air movement • Helps encourage evaporation of sweat • Passively cooling • Evaporative cooling • Geothermal cooling

37. Evaporative Cooling • If the air inlet is taken from the side of the building facing away from the sun and is drawn over a cooling pond or some mist or even through large areas of vegetation it can end up several degrees cooler than the outside air temp by the time it enters the building

39. Geothermal Cooling • Draw the air through underground pipes or air space • Air loses some heat to the surfaces as it passes over them • Underground the surfaces tend to have roughly the avg annual temp • Provides cooling in summer and heating in winter • Best for dry climates