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Natural ventilation is the preferred means of ventilation in most parts of the world. Designing effective natural ventilation strategies in conjunction with mechanical ventilation systems (such as mixed mode ventilation) creates avenues for potential energy savings. Studies show that effective use of natural ventilation as opposed to buildings with air conditioning and mechanical ventilation reduces the carbon emissions by half.
Adequate ventilation is first and foremost required to provide sufficient oxygen and to dilute indoor pollutants to an acceptable level. Ventilation also creates a sense of thermal comfort to the building’s occupants and aids in passive cooling by regulating body temperature and also interior temperature of the building. Cooling through ventilation is based on the fundamental heat-transfer mode of convection, where the air flowing next to a surface carries away heat, provided it is at a lower temperature than the surface. When it passes over the human body, it increases the evaporation rate from the skin and en-hances heat extraction. Thermal comfort can be achieved even when the incoming air is at a higher temperature than the mean radiant temperature of the indoor air. For example, an increase of air veloci-ty by 0.15 m/s compensates a temperature increase of 1 °C at a relative humidity of 75%. Perceived temperature differences up to 2 °C may occur when air movement is increased by 0.8 - 1.6 m/s.
Adequate ventilation is required to maintain air quality in the space preventing odours and concentration of indoor pollutants and the build-up of carbon dioxide. The required air change rate (expressed in terms of air change per hour (ach) or m3/hr) for different spaces depend on the activity being carried out in the space and are specified by national and international building and ventilation codes such as CIBSE, ASHRAE etc.
Beyond providing adequate ventilation levels natural ventilation within certain parameters provides a sensation of cooling. Building’s structure can be cooled down when ambient air at lower temperature sweeps the building structure and carries away the accumulated heat within the thermal mass. This technique is often referred to as night-flush ventilation and is effective in places with considerable difference in diurnal temperature range.
The thermal comfort in a building can be improved through appreciable wind speeds within certain limits. Where this cooling effect is desired care should be taken to locate and orient the building to best catch the cool breezes. The design objectives should be to obtain a continuous airflow throughout the building, to direct the airflow through the occupied zones and to achieve appreciable air velocities at the occupants’ level. Such airflow can provide a perceived cooling effect even at a room temperature as high as 34 °C. The higher air speed increases the rate of sweat evaporation from the skin, minimising the discomfort from the feeling of wet skin. Thus, it is especially beneficial when the relative humidity of the air is high.
Key parameters that need to be considered for achieving cooling sensation with by means of natural ventilation are rate of ventilation, air temperature, mean radiant temperature, humidity levels and wind speed within the space. All the parameters are interrelated to each other and several empirical relations have been formulated for their calculation.
Using natural ventilation for cooling is most effective when the ambient air temperature is under 35 - 36 °C and the wind velocity in the interior spaces is in the range of 0.4 – 3 m/s. This of course is always a function of the humidity of the air and depends on the activity in the space. However, air speed beyond a certain point, typically 1.6 m/s is not desirable as it causes unwanted turbulence. Moreover, thermal comfort is a complex phenomenon involving multiple factors and also being specific to geographical region and personal tolerances. More information on thermal comfort can be found in the section “thermal comfort”.
Natural ventilation is predominant means of ventilation as well as passive cooling in most parts of the world. The effectiveness of natural ventilation however, depends on the climate and function of the space. Spaces with high internal heat gains smoke and fume generating spaces such as kitchen and laboratories, infectious wards in hospitals etc., however, require enhanced ventilation systems.
For all general purposes residential buildings around the world with exception of few Scandinavian countries and the USA use natural ventilation irrespective of climate and heating or cooling systems. However, the trend of moving towards mechanical ventilation systems is increasing especially with the increasing rate of air conditioning systems that call for closed and airtight spaces.
Most of the newly built office buildings are designed to be air-conditioned with mechanical ventilation considering the high internal heat loads due to equipment and lighting. For buildings with low to moderate heat gains mechanical ventilation could be designed in such a way to enable natural ventilation during the periods of favourable outdoor weather conditions. Such systems are typically referred to as mixed-mode or hybrid systems.
Air movement through buildings can be wind-induced or caused by differences in pressure due to a temperature gradient (stack effect). Temperature differences will cause the air to have different densities and air will flow naturally between the colder (denser) part of the building and the warmer (less denser) part. For wind induced ventilation shallow floor plans with windows or openings at either side work well, as do internal courtyards. Stack effect causes the hot air to rise up, which can then be extracted at the top, drawing fresh air into the building through an opening at the lower level.
Please note that the focus of this section is on building design with respect to natural ventilation. For information on siting, urban and microclimate effects on wind patterns and ventilation refer to Passive options “site” and “micro climate”. In addition please refer to recommendations to find in which cases natural ventilation is required. For example, cross ventilation for thermal comfort is highly recommended in hot-humid climate while it is undesirable in hot and dry climate.
In wind induced ventilation the wind blowing against a building will cause a pressure difference on the buildings surface and airflow will be created between the areas high pressure in the windward side and areas of low pressure in the leeward side. Wind gushes through the openings in the building flowing form high-pressure areas to low pressure areas. Wind driven ventilation is effective, however, difficult to control.
Buoyancy driven ventilation is the result of movement of air within a stack due to temperature gradient between warmer air and colder air within the stack. The warm air rises in the stack, removed from the top and is replaced by cooler air at the inlet. In tall buildings atria can be used to great effect as a thermal chimney and can be used to aid ventilation.
Mixed-mode ventilation strategy combines both natural and mechanical ventilation. The heating, cooling and ventilation systems are designed in such a way that to allow for natural ventilation when the ambient weather conditions are suitable. The control strategies to enable mixed mode strategies such as window opening etc. can be both automated or individually controlled by the occupants.
For the effective natural ventilation, building design need to consider opening’s detail, climate and microclimate of the site and building’s functional use.
Proper type and location of the openings provide adequate ventilation rates under design varying weather and occupancy conditions. Selection of suitable openings (e.g. windows, doors, vents, solar chimneys) allow desired control of airflow in the room/building, depending on ventilation requirement and relevant climatic conditions (WHO, 2009).
For wind driven ventilation narrow plan depth with linear building form provide comfort (e.g. in single residential buildings). Deeper floor plan works well in buoyancy driven ventilation as air flows from the punctures in the plan with chimneys for inlets and outlets (e.g. in multi-residential buildings and service sectors). In order to drive pressure with buoyancy the vertical distance between the inlet and the outlet is significant (Kleiven, 2003). The wrong positioning of inlet and outlet openings create patches of stagnant air.
The performance or the thermal comfort through the natural ventilation types depends on the local climate (WHO, 2009). Studies show that mixed mode and hybrid ventilation is best suited for most of the climate zones, comparatively less in cold climates. For moderate climate all type of natural ventilation is suitable, while for hot and dry climate wind driven ventilation works well.
Wind direction as well as wind speed influence the amount of natural ventilation. As local microclimate conditions depend on natural and man-made obstructions on topography, vegetation, neighbouring buildings, wind data, they are required to be interpreted carefully (Walker, 2014). It reduces overalpping wind shadows. The positioning of fresh air inlet and air exhaust taking into account both dominant and prevailing wind direction and unusual period by the time of day and season increase effectiveness of natural ventilation (WHO, 2009). A staggering of buildings with at least 50 m distance can improve the wind flow.
In some cases due to extreme climate conditions, security issue and cultural issues, openings are remained close which lower the ventilation rate. The possible outdoor noise, dust and pollution must be taken into account and minimised where possible when planning for natural ventilation. Mixed mode and hybrid ventilation is suitable to overcome such situation (WHO, 2009). For natural ventilation control and to increase its effectiveness, modeling tools help to analyse and check the indoor thermal comfort at differing operating conditions and outdoor environment.