Passive Cooling

Natural Ventilation Air Cooling Shades

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

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

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

Opening Design

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

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

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

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

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

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

Cross Ventilation

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

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

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

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

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

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

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

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

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

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

Solar Chimney Thermal 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

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

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

Or can be used to aid with heating

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

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

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

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

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