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FBE03: Building Construction & Science. Lecture 6 Building Ventilation. Introduction to passive ventilation. Most decisions affecting the energy use of a building happen during schematic design. basic form and siting of a building determines most of its efficiency potential
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FBE03: Building Construction & Science Lecture 6 Building Ventilation
Introduction to passive ventilation • Most decisions affecting the energy use of a building happen during schematic design. • basic form and siting of a building determines most of its efficiency potential • trying to make changes to overall shape or location late in project development is difficult or impossible • understanding of how to work with the environment can affect the earliest stages of design
passive ventilation • Passive ventilation is one of the areas of building design most affected by siting and building configuration • most beneficial to reducing energy consumption. • The mechanics of wind flow are relatively easy to understand as are the way openings in buildings affect air movement and interior comfort
passive ventilation • So why don’t we always use these basic systems? The answer is odd – it’s largely due to efficiency • mechanical cooling systems the goal is often to keep the building envelope as tightly sealed as possible to prevent inefficient losses of conditioned air • sealed mechanical systems became the norm the passive systems became less important • sealing buildings too tightly, recycling bad air and trapping harmful air-borne illnesses
Wind movement • Wind direction and speed is affected by exterior conditions such as vegetation, adjacent buildings, and landforms • you must be able to reasonably assess how your site will be developed and what surrounding development may take place • The difficulty in determining how to place openings is deciding where the wind will be coming from and how fast it will be traveling when you want to use it
Air flow • Opening placed front and back (or windward and leeward) sides of a building will allow air to cross ventilate and form small eddies back toward the main wind direction • Placing openings in the sidewalls instead of the back will utilize the greater negative pressure at the exterior sides to draw air through the building more efficiently • mixes the air through the interior better (Fig. 6.1.1(b), (c))
Air flow • Winds that strike a building at an oblique angle flow better through buildings with openings on three sides rather than two (Fig. 6.1.1(d), (e)). • The airflow in this situation needs to be kept from eddying back out of the first opening, therefore reducing air intake (Fig. 6.1.1(h)).
functions of passive ventilation • promote health; • increase comfort with air circulation and; • remove heat from structural elements
functions of passive ventilation • Health requires enough fresh outdoor air be changed with the stale indoor air to resist air-borne germs/bacteria/mold, etc. from building up. • Comfort is achieved through evaporative and convective heat losses from adequate air movement. • Structural cooling moves heat away from surfaces where it has built before it radiates that heat into the structure
Air flow in buildings • Creating a chimney that separates lower cooler air from higher warmer air enhances stack effect • the top of the stack is allowed to heat, drawing warmer air up into it • Cool air flows into the lower space and up the chimney starting a natural convective loop of air movement • For comfort the number of changes should be about 30 times per hour.
Eastgate Centre • Passive cooling works by storing heat in the day and venting it at night as temperatures drop. • Start of day: the building is cool. • During day: machines and people generate heat, and the sun shines. Heat is absorbed by the fabric of the building, which has a high heat capacity, so that the temperature inside increases but not greatly. • Evening: temperatures outside drop. The warm internal air is vented through chimneys, assisted by fans but also rising naturally because it is less dense, and drawing in denser cool air at the bottom of the building. • Night: this process continues, cold air flowing through cavities in the floor slabs until the building's fabric has reached the ideal temperature to start the next day.
Cross Ventilation • Cross Ventilation is the most effective way to move air through a building for comfort • The effectiveness is largely dependent on the difference in temperature between the inside and outside. • Structures tend to build up heat due to people, lights, equipment, solar radiation, and other factors • if the air outside is warmer than the air inside it is not going to cool the space no matter how hard it blows through
Design of window openings for passive ventilation factor • First the wind needs to be able to get to the opening. This may mean the opening needs to occur high in the wall, low in the wall or from one side only • The air then needs to flow to where it’s needed. if it’s on the ceiling or floor no people are being touched by the air. Think about where the people will be in a room and adjust openings to run air past the core and head of their body
Design of window openings for passive ventilation factor • A variety of openings may be needed to both cool people and cool the wall/ceiling surfaces. The surfaces most needing cooling jets are typically the ceiling and west walls – the areas heated by the sun the most during the warmest part of the year. The amount of cooling needed depends on the insulation value of those surfaces, but it makes little sense to cool the floor if it’s the coolest surface in the room already.
Design of window openings for passive ventilation factor • Air supply in a mechanical system usually follows the rule of supply high/ return low, when cooling is the primary concern and the opposite when heating is the main factor. This is because cool air falls and warm air rises – therefore you might think to place inlet windows high in order to let the cool air fall and place outlet windows low. • However with airflow through a building the opposite is usually true – you want to evacuate the warm air and it’s already on the ceiling, so you need to supply low and evacuate the warm air up high. This allows greater air mixing than if you supplied high and evacuated high – which would cause an air jet at the ceiling.
Wind catcher • Wind catchers allow buildings in dense areas or where low winds are unavailable to draw wind down from higher areas • The benefit of wind catchers is that wind typically is at a higher velocity farther from the drag of the ground, so it can be directed at higher velocities down into the space.
Night cooled thermal mass system • This operates on the principle of thermal mass temperature shift – like a thermal wall that stores heat during the day and radiates it back at night • In a night cooled mass, the building structure absorbs heat during the day as a closed system and at night vents are opened using airflow evacuate the stored heat. • The mass is then cool for the next morning until it begins to store heat again.
Night cooled thermal mass system • Passive cooling systems are often hybridized with some components of a mechanical system to improve performance. • The most common of these are fans that assist in air movement when wind is not adequate.
Conclusion • Passive ventilation provide energy saving and human comfort • rising concerns over energy costs and the need to reduce the impact of building energy consumption, but there is still resistance to hybridizing passive and active systems.
