Natural Ventilation for Sustainable Building DesignBy Eng. (Prof.) R A Attalage & Eng. P H Viraj Nimarshana
In building designs, great attention needs to be paid to the indoor environment of buildings since people spend 80-90% of their time inside the buildings, and their comfort, health, and productivity are directly affected by the indoor environmental quality (IEQ). Thermal comfort has been identified as the most influencing parameter on IEQ among others of visual comfort, acoustics and indoor air quality,. Use of mechanical ventilation and air conditioning (MVAC) systems to achieve indoor thermal comfort in buildings is continuously increasing day by day. In EU, 40% of the total energy consumption is utilized for buildings operation and 60% of that is consumed by HVAC systems,,. Since a major fraction of the building energy is applied for indoor conditioning and building services, even a fractional amount of energy saving will make a big impact on total energy consumption. In addition, poorly designed or maintained mechanical ventilation systems have been reported to be associated with health issues such as sick building syndrome (SBS).
Impact of natural ventilation
Natural ventilation driving fresh air into building interiors is one of the most effective passive cooling strategies. This employs natural forces due to wind and buoyancy aspects. Thus, it helps to remove aged air from the enclosed space with pollutants and generated/accumulated internal heat. In the absence of mechanical ventilation systems, natural ventilation could be an effective and attractive energy saving mechanism for the building’s ventilation, provided that it ensures both acceptable indoor air quality and thermal comfort levels to its occupants. Furthermore, 30%–40% less energy consumption is reported in naturally ventilated buildings compared to those of mechanically ventilated buildings,. Thus, natural ventilation appears to be attracting considerable interest among building designers. Fig 1. shows some popular buildings which use natural ventilation as passive cooling strategy.
Natural ventilation techniques:
Natural ventilation has been judiciously used by many ancient civilizations and been utilized in traditional and modern buildings either with wind driven ventilation (Fig. 2), buoyancy driven ventilation (Fig. 3(a)) or combination of both (Fig. 3(b)). It is noted that most natural ventilation system designs rely more on buoyancy compared to wind driven design. Wind may assist, or destruct ventilation flow in the interior of the building.
Two key different patterns could be identified dealing with wind driven natural ventilation, notably single-sided ventilation (Fig. 4 (a), (b)) and cross ventilation (Fig. 3 (c), (d)). It is found that cross ventilation can provide effective ventilation with high flow rates compared to that obtained by single-sided ventilation.
In the contrary, buoyancy driven ventilation is employed assisted by the stack effect and solar-induced effect as shown in Fig. 5.
Challenges due to uncertainties and limitations:
Even though natural ventilation is conceptually simple, its detailed design could be really challenging due to the random nature of the micro-climatic conditions. As, the rate of ventilation depends on the direction and magnitude of pressure forces and flow path resistance, this incorporates an additional complexity. Several factors such as the shape and orientation of the building, features of the building’s surroundings, and the position, size and shape of openings have a significant influence. Therefore, designers need to be able to quantitively incorporate the effects of the said factors on the indoor climate in view of establishing the indoor airflow and temperature field that can be achieved for acceptable human thermal comfort.
Decision making options at design stage:
There are several commonly employed approaches to predict the performance of naturally ventilated buildings. They could be classified as network model, zonal model, wind tunnel test with scaled down model, and computational fluid dynamics (CFD) among which the later takes priority. Network models are employed to predict the airflow rate through the openings of a building but not the internal flow fields thus insensitive to prediction of pollutants. Wind tunnel approaches are still limited and call for large investments and human intervention but with a better accuracy.
Best possible approach to produce promising results:
In contrast to other methods, CFD is an alternative approach which can be employed to establish detailed airflow and temperature fields in and around buildings. Due to the recent developments in turbulence modeling and computer speed/capacity, CFD is becoming popular by providing informative results along with low labor and equipment costs. With the said developments, CFD modeling of a domain containing buildings, their surroundings and its interior spaces becomes the obvious choice (Fig. 6).
Indices for thermal comfort evaluation:
Predicted Mean Vote (PMV) model is often regarded as the most widely used in thermal comfort standards. The extended PMV model has been developed taking the adaptive behavior of occupants in to account by incorporating a reduction of metabolic rate and an expectancy factor to the original PMV model. The Standard Effective Temperature (SET*) index also accounts for the prediction of thermal comfort in naturally ventilated buildings. SET* is the model recommended by ASHRAE’ for the prediction of thermal comfort conditions where airspeeds higher than 0.2 m/s. SET* accounts for the combined effect of temperature, airspeed, humidity, activity level, and clothing insulation similar to the PMV model. Moreover, the output of the model is a temperature that humans perceive rather than a thermal sensation vote. Fig. 7 depicts CFD simulation carried out using OpenFOAM software to predict airflow and PMV distribution of a single-story residential building in Colombo, Sri Lanka.
Due to the uncertainties and inherent limitations of natural ventilation, it is not widely utilized nowadays. Furthermore, the unacceptable pollution levels in the micro-climatic conditions on the indoor environment quality imposes an addition barrier to the consideration of natural ventilation. However, with the rising cost for electricity and fuel, mandates for decarbonizing and net-zero carbon initiatives paying attention towards natural ventilation would be essential. The use of CFD modelling and accompanied validation would enable to better understand the performance of building interiors under diverse natural ventilation strategies in view of optimized building designs and related research.
P.H.V. Nimarshana conducts research activities related to the aforementioned area in the Department of Mechanical Engineering, University of Moratuwa under the supervision of Prof. R.A. Attalage. National Research Council (NRC Grant No. 14-139) has provided financial supports to carry out these research activities. The aim of the research is to propose a performance-based approach to evaluate the indoor thermal comfort of naturally ventilated buildings in Sri Lanka.
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Eng. P H Viraj Nimarshana
BSc. Eng. (Hons), MSc
Lecturer, Department of Mechanical Engineering
University of Moratuwa.
Eng. (Prof.) R A Attalage
BSc Eng, M.Eng, DEA, PhD
Dean, Graduate Studies & Research / Professor in Mechanical Engineering,
Sri Lanka Institute of Information Technology
Member IESL, Corporate Member SLEMA, Member ASHRAE,
Fellow Lanka Association of Building services Engineers (LABSE),
Member SLAAS- Section C, Fellow National Academy of Sciences Sri Lanka