Natural Ventilation in Built Environment - Springer Link

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Natural Ventilation in Built Environment

Natural Ventilation in Built Environment TONG YANG1, DEREK J. CLEMENTS-CROOME2 1 Department of Civil and Building Engineering, Loughborough University, Leicestershire, UK 2 School of Construction Management and Engineering, University of Reading, Reading, UK

Article Outline Glossary Definition of the Subject Introduction Vernacular Architecture Natural Ventilation Principles Natural Ventilation Design Requirements Design Guidelines Selection of Ventilation Strategies Case Studies Future Directions Acknowledgments Bibliography

Glossary Air changes per hour (ACH) The volumetric flow rate of supply air, divided by the volume of the ventilated space. Advanced natural ventilation system (ANV) Integration of basic natural ventilation strategies such as cross ventilation and stack effect with smart controls. BEMS Building energy management system. BREEAM Building research establishment environmental assessment method – UK origin. Exfiltration/infiltration Air flow through unintended leakages out/into buildings. Hybrid ventilation Combined natural and mechanical ventilation (also called mixed-mode ventilation). Indoor air quality (IAQ) Indoor Air Quality – broadly defined by the purity of the air but often CO2 is used as an indicator. Mixed-mode ventilation See hybrid ventilation. Natural ventilation Use of natural forces, i.e., pressure differences generated by wind or air temperature, to

introduce and distribute outdoor air into or out of a buildings. Night cooling The use of night air to cool the building using wind towers or a fan to circulate the air. PAQ Perceived air quality. Thermal comfort The state of mind that expresses satisfaction with the surrounding thermal environment. Ventilation Provides fresh air into a building to ensure good air quality for occupant health and well-being. Ventilation effectiveness The ability of a ventilation system to exchange the air in the room and also the ability to remove airborne contaminants. Ventilation flow rate The amount of air per unit time into the ventilated space (liter per second or l/s, cubic meters per hour or m3/h). Well-being Healthy mind and body. Definition of the Subject Natural ventilation uses the natural forces of wind and buoyancy to introduce fresh air and distribute it effectively in buildings for the benefit of the occupants. Fresh air is required to achieve a healthy, fresh, and comfortable indoor environment for people to work and live in. Natural ventilation can ensure or support the supply of adequate breathing air, adequate ventilation of contaminants, adequate thermal conditioning and moisture dissipation, and contribute to well-being through a connection to the dynamics of nature. For natural ventilation to be effective, there has to be a close relationship between the architecture and the air circulation system. This includes the relationship between the built form, the site environment in a particular location, and the layout within the building. The Natural History Museum in London, designed by Alfred Waterhouse in the Victorian age, is an excellent example of design for natural ventilation. The architect designed the built form to encourage the flow of air through each space in the building by the use of two ventilation towers at the back of the building to induce air flow through stack ventilation [1]. Buildings should be designed to take full advantage of the prevailing natural forces such as wind, outdoor temperature, sunlight, incorporating building elements such as towers, atria, and thermal mass to ventilate and

V. Loftness, D. Haase (eds.), Sustainable Built Environments, DOI 10.1007/978-1-4614-5828-9, # Springer Science+Business Media New York 2013 Originally published in Robert A. Meyers (ed.) Encyclopedia of Sustainability Science and Technology,

#

2012, DOI 10.1007/978-1-4419-0851-3


cool occupied spaces. In many climates there is a growing proportion of naturally ventilated buildings using natural features and forces to reduce a building’s environmental or carbon footprint. Introduction The reasons for ventilating a space with air are as follows: 1. Ventilation air provides oxygen that is needed for human life processes; it takes about 4 s for inhaled air to pass through the respiratory system and transfer oxygen to the blood and then to the brain; poor-quality air deficient in oxygen with consequent high CO2 levels impedes clear thinking and concentration. 2. Ventilation air dilutes; the contaminants may be CO2 from respiration, odors secreted through the human skin, cigarette smoke, or emissions from other process such as dust, allergens, aerosols, toxic gases, and particulates in general. 3. Ventilation promotes and directs air movement in the space, removing excessive heat and/or moisture essential for comfort and well-being. Traditional vernacular architecture has taught us the best of sustainable architecture and ecologically sensitive adaptation, using passive features ranging from building orientation and form, appropriately sized and oriented openings linked with vertical forms, the benefits of local materials and mass for night cooling, and the relationship of buildings in context to ensure effective air flows. Vernacular Architecture Vernacular architecture blends buildings into their specific settings, so that there is a natural harmony between the climate, architecture, and people. Vernacular architecture learned from the environmental variations of place relating to local variations in temperature, humidity, sun, wind, rain, earthquakes, and storms. In climates where the diurnal range may be 17 C, vernacular buildings allow a variation in indoor temperature of only some 4 C through time-lag and night cooling. In climates where humidity may be 90%, vernacular buildings support human comfort by

allowing air to flow over the many thermo-receptors on the human body. Vernacular architecture also adapted to ensure indoor air quality through natural ventilation through the careful design and placement of indoor pollutant generators from stoves to commodes. Four vernacular solutions are further described: wind towers and courtyards, termite mounds, and igloos, each integrating the conditioning power of natural ventilation in unique responses to local climate. Wind Towers The wind towers called bagdirs are a distinctive and ancient feature of Islamic architecture. It has been used for centuries to create natural ventilation in buildings. Examples of wind towers (Fig. 1) can be found throughout the Middle East, Pakistan, and Afghanistan and now are sometimes incorporated into Western architecture. Wind flowing around a building causes separation of flow which creates a positive pressure on the windward side and a negative pressure on the leeward side of the building. Due to its height the wind tower enhances the positive pressure on the windward side; it is then directed through the tower into the building. Airflow follows the pressure gradients within the structure and exits through purposely designed openings and as well as through the leeward side of the tower. The size and location of openings (e.g., windows, doors, etc.) and distribution of internal party walls have a great impact on encouraging cross flow and mixing of the indoor air. The principal factor is the buoyancy which depends on the temperature difference and the height. During the day the sun heats up the structure warming the internal air which then rises through the wind tower, as illustrated in Fig. 1. At night the cool night air lowers the temperature of the structure and the internal air and the heavier air then flows downward cooling the internal spaces after the heat of the day. Figure 2 shows how wind towers can also provide natural cooling for underground water cisterns. Courtyards Courtyards are one of the oldest plan forms for dwellings going back thousands of years and appearing

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1 Air Flow(Day)

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Hall 5 Ground Level 6 Air Flow (Night:No Wind) Basement

Natural Ventilation in Built Environment. Figure 1 Bagdir in Dubai, in United Arab Emirates [2]

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Natural Ventilation in Built Environment. Figure 2 Wind towers in Yazd, Iran to ventilate houses, are also constructed to cool underground cisterns (water reservoir) [3]

as a distinctive form in many regions in the world. Examples exist in Latin America, China, the Middle East, Mediterranean, and in Europe. Preserving the basic typology of the courtyard, local climate and culture has created a unique style for each region.

The Courtyard House (Siheyuan) is a typical form in ancient Chinese architecture, especially in northern China. It offers space, comfort, quiet, and privacy. A Siheyuan consists of a rectangle with a row of houses bordering each side around a courtyard, normally with


a southern orientation and having the only gate usually situated in the southeast side. Walls protect the houses from the harsh winter winds and the spring dust storms that frequently occur in Northern China from the Gobi desert in Mongolia. The house’s deep eaves allow the winter sun’s warmth to be directed into the rooms, while they also provide cooling shade and protection from the summer rains. Their design reflects the traditions of China, following the rules of Feng Shui and Confucian tenets of order and hierarchy. All the rooms around the courtyard have doors and large windows facing onto the yard and small windows high up on the back wall facing out onto the street. Ridged roof tops provide shade in the summer and retain warmth in the winter. The verandah divides the courtyard into several big and small spaces that are closely connected, providing a common place for people to enjoy whatever the weather. The courtyard is an open-air living room and garden with plants, rocks, and flowers, for family members to chat and gather. In cold northern China, courtyards are built broad and large to increase the exposure to sunlight, and there are more open areas inside the courtyard walls for daylight, fresh air, rainwater for plants and gardens to

Natural Ventilation in Built Environment. Figure 3 A typical courtyard house in southern China

be harnessed. In hot southern China, the courtyard houses (Fig. 3) are built with multiple stories to encourage cross ventilation flow incorporating natural cooling effects. The orientation of houses is not strictly north–south aligned, but follows the local topology of hills and easy access to water sources. Lessons from Nature: Termitaries Termites are an outstanding example in the animal kingdom of ingenious animal architects in the sense of master builders. Over 2,000 species live in tropical and subtropical regions and have shown us by analogy the art of designing for living in a variety of dwelling styles with natural ventilation. Termites build their nest so as to achieve automatic ventilation to regulate the internal temperature, as well as constantly managing control of gas exchange and moisture level. They do not keep a set temperature, but allow a gradual change between the seasons determined by the external environmental temperature. In Australia, compass termites build large-sized mounds in the form of huge, flat chisel-shaped blades, with their long axis pointing north–south. This arrangement exposes the minimum possible area to the midday sun but allows the mounds to catch the rays of the early morning and late evening sun, when the termites need warmth, especially in the cold season; peak temperatures can be lowered by about 7 C with N–S orientation and thus maintain a preferred temperature of 30–32 C [4]. There are two main types of termite mounds: (1) The open ventilation mounds which let air flow into or out through chimneys or holes built into the mounds; (2) The completely enclosed mounds in which gases are exchanged through the porous thin-walled tunnels. The nest of a termite species Apicotermes gurgulifex is shown in Fig. 4a. It is embedded in the soil but clothed by a mantle of air; the nest is constructed from the excrement of the termites so is well insulated. Its outer wall has a pattern of raised, ring-shaped configurations which surround an array of precisely spaced and shaped ventilation slits. These slits link the external and the internal spaces. The termitary of the fungus cultivating termites, Macrotermes bellicosus in Fig. 4b, may reach a height of 3 or 4 m and contain more than two million termites.

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Air space 2. above nest

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Royal cell 4.

Air 6. Brood chambers

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Supporting ridge with air ducts

CO2 O2

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5. Fungus chambers 1. Cellar

Natural Ventilation in Built Environment. Figure 4 Ventilation of termite molds (a) Nest of a termite species Apicotermes gurgulifex [4]; (b) Longitudinal section through the nest of Macrotermes bellicosus from Ivory Coast showing the air being circulated by buoyancy [4]

Airholes

Cold sink

Sleeping platform

Natural Ventilation in Built Environment. Figure 5 Illustration of an igloo

The fungus chambers are built into complex sponge-like structures with numerous supporting ridges with air ducts. The air in the fungus chamber is heated by fermentation processes and the metabolic heat generated by the termites. The hot air rises and enters the duct systems in the ridges, the walls of which are porous allowing carbon dioxide to escape from and oxygen to enter the dwelling. The cooler air flows down to the cellar and replaces the rising warmer air. Igloos Inuit people build igloos as shelters from the extreme weather conditions in the Arctic. The igloo (Fig. 5) has

an excellent thermal performance without mechanical equipment. The hemispherical shape of the igloo provides the maximum resistance to winter gales from all directions, which at the same time exposing the minimum surface area to heat loss. The dome uses packed snow blocks, some 500 mm thick, 1,000 mm long, and 150 mm wide, which are laid in a continuous in sloping pile. Effectively, the shape encloses the largest volume with the least material, so it can be heated by a blubber lamp. Coated by a glaze of ice on the interior surface, the finished dome is made stronger and windproof. The interior surface is also draped with animal skins and furs to prevent radiant and convective heat loss between the cold floor and the walls. Measurements


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Natural Ventilation in Built Environment. Figure 6 Wind speed variation with height and terrain conditions [7]

have shown that with no heat source apart from the small blubber lamp, internal air temperatures are held at levels of 6 to 4 C with external temperatures of 24 and 40 C [5]. General Traditional building technologies have evolved and been adapted over time by people and animals in all climates to meet specific needs, accommodating the values, economies, and the culture inspired ways of life. However versatile they are all reflect the basic principles described in the next section. Natural Ventilation Principles Natural forces to drive ventilation can be wind pressures or pressure generated by the density difference between indoor and outdoor air. Wind-Driven Ventilation Wind is caused by pressure differences in the atmosphere. The general flow of wind close to the Earth’s surface is subjected to boundary layer effects, so called

the atmospheric boundary layer, in which wind speed is influenced by surface friction of the ground. The variation of wind speed in height on different terrains is illustrated in Fig. 6. Wind speed correction coefficients for different terrain conditions in the UK are listed in BS 5925 [6]. When the path of the wind is checked by obstacles, such as trees and buildings, then an energy conversion takes place. Velocity pressure is converted to static pressure, so that on the windward side an overpressure is produced (about 0.5–0.8 times the wind velocity), whereas on the leeward side an under-pressure results (about 0.3–0.4 times the wind velocity). The pressure distribution on the roof varies according to pitch. Figure 7 shows areas of positive and negative pressures generated by wind normal to building front: wind-driven flow through inlets on positive pressure faces and outlets on negative pressure faces [8]. The pressure differentials arising across a building cause infiltration of air through window cracks and other openings. Relative to the static pressure of the free wind, the pressure on any point on the surface of a building fac¸ade pw can be approximated by the equation:

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Natural Ventilation in Built Environment Direction of wind Direction of wind –

–

Roof pitch >30°

– Roof pitch