Clean Indoor Environment
This article provides a holistic alternative of traditional cooling system through natural ventilation and clean air circulation in Indian scenario. Combined wind and stack ventilation with filtering incoming air may be an eco-friendly alternative…
- Dr. J. Sarkar, Dr. A. Sarkar
Natural ventilation is the process of supplying and removing air through an indoor space by natural means, meaning without the use of a fan or other mechanical system. It uses outdoor air flow caused by pressure differences between the building and its surrounding to provide ventilation and space cooling. The use of natural ventilation is definitely an advantage with the raising concerns regarding the cost and environmental impact of energy use. Not only does natural ventilation provide ventilation (outdoor air) to ensure safe healthy and comfortable conditions for building occupants without the use of fans, it also provides free cooling without the use of mechanical systems. When carefully designed, natural ventilation can reduce building construction costs and operation costs and reduce the energy consumption for air-conditioning and circulating fans. An additional bonus is that no longer will any noisy fan be of your concern.
Natural ventilation is very common in India. Even in the Mughal period when gross domestic product of India was one-fourth of the world economy, natural ventilations were widely used. It can be well understood from the design of Hawa Mahals where corridors are set in such a way to allow a high rate of air infiltration inside the building and ensure passive cooling in the same. However, with the advancement of technology, cooling systems were confined to refrigeration and air-conditioning. The underlying principal in these cooling systems is the Clausius statement of 2nd law of thermodynamics. According to the law, power input is required to run these cooling systems since in these systems heat is transferred from low temperature to high temperature body. For the sake of convenience, a heat engine can be coupled with refrigeration system for power input. In that heat engine heat is mostly supplied by burning fossil fuel which causes disturbance to the environmental harmony. Not only this, it is very difficult to find eco-friendly refrigerant. Commonly used refrigerants are being phased out as they are highly responsible for global warming (greenhouse gas) and ozone layer depletion. Considering these problems in active cooling, passive cooling through natural ventilation of wind is highly required to maintain harmony with nature.
There are basically two types of natural ventilation that can be employed in a building: wind driven ventilation and stack ventilation. Both of which are caused by naturally occurring pressure differences. However, the pressure difference that causes wind driven ventilation uses the natural forces of the wind whereas stack ventilation is caused by pressures generated by buoyancy as a result in the differences in temperature and humidity. Hence, there are different strategies in the optimization of the two types of natural ventilation.
Buoyancy ventilation can be induced by temperature (known as stack ventilation) or by humidity (known as cool tower). Most commonly used is the stack driven ventilation. For stack ventilation to work properly there must be a temperature difference. As the warm air (usually given off by the occupants and their computers), which is less dense, in the building rises, the cooler air is sucked from the openings below. Design considerations for stack ventilation are (i) inlets should supply air low in the room. Outlets should be located across the room and at high level, (ii) the vertical distance between the inlet and exhaust openings should take advantage of the stack effect, (iii) use skylights or ridge vents and (iv) the function as fire exits of enclosed staircases should not be compromised if stack ventilation is incorporated into the design. With stack ventilation, it does not rely on the wind. On hot summer days with no wind, the naturally occurring stack effect can take place with relatively stable air flow. Moreover, because it does not rely on the pressure and direction of the wind, there is a greater control on locating the air intake. However, stack driven ventilation is limited to a lower magnitude than wind driven ventilation. It is also very dependent on the inside and outside temperature differences.
Figure 1: Map of India with three different wind roses in coastal regions
Wind Driven Ventilation
As naturally occurring wind blows across a building, the wind hits the windward wall causing a direct positive pressure. The wind moves around the building and leaves the leeward wall with a negative pressure, also known as a sucking effect. If there are any openings on the windward and leeward walls of the building, fresh air will rush in the windward wall opening and exit the leeward wall opening to balance and relieve the pressures on the windward and leeward walls. Capturing the wind and bring ventilation to the building depend on the building shape, building orientation and location, building form and dimensions, window typologies and operations, types, shape and size of openings, construction methods and detailing, external elements, urban planning consideration, etc.
Wind Speed and Availability
India is a tropical subcontinent which has larger coastal regions. In these regions, wind flow is highly induced due to differential heating of land and water. In these coastal regions, lots of villages are also situated especially in the states of Orissa, Andhra Pradesh, Tamil Nadu and Kerala. In these villages there are many hut like structures where natural ventilation can be provided by the proper setting of the window. Obviously, wind speed plays a major role in natural ventilation. From the view of human comfort wind speed should neither be very large nor very low. Best wind speed ranges for passive cooling is the wind speed for breeze which varies from 2-8 m/s. Now the question is the availability of this wind speed range in that particular location. Wind speed is highly location specific. Even in a particular location wind speed greatly varies in different months. To determine availability of a particular wind speed range, an availability factor can be defined which can be equated to the probability of occurrence of wind speed in the particular range for that location. The prerequisite of determining availability factor is the measurement of wind speed in each hour. India has a large network of meteorological stations controlled by Indian Meteorological Department (IMD) which measures hourly mean wind speed data in each of twenty-four hours of a day and 3 sec gust wind speed data. However, wind zone map of India is based on 3 sec gust wind speed data in which India is divided into six zones and basic wind speeds on the basis of peak gust wind speeds have been determined which vary from 33 to 55 m/s. The map is very much useful for specification and codification of design wind load on structures under extreme wind conditions. However, for wind induced natural ventilation parent wind climate modeling is of greater importance. For this purpose it is required to understand the measurement and analysis of hourly mean wind speed data. Hourly mean wind speed is measured at a height of 10 m by dyne pressure tube anemograph. It is measured in the last 10 minutes before the completion of the full hour and the last 10-min mean is considered as hourly mean wind speed. Though wind speed data is non-stationary, they can be converted into independent and identically distributed data (iid) by carrying out time series analysis and removing trend as well as white noise from the data. After converting wind speed data into random data, they can be modeled by a suitable probability distribution. It has been observed that wind speed probability density distributions, especially for the coastal regions, are highly skewed. Hence, normal distribution may not be an appropriate choice for wind speed data modeling. For this purpose, Weibull, Log Normal and Exponential distributions can be used. The availability factor can be determined by integrating probability density distribution within the appropriate wind speed range.
Figure 2: Wind rose of Ahmadabad
Wind Direction and Wind Rose
Apart from wind speed, wind direction also plays a crucial role especially in corridor setting. Wind rose is required for this analysis. Wind rose can be defined as the pictorial representation of wind direction from which fraction of time for a particular wind direction can be determined. Three typical wind roses are depicted in Fig. 1. For better clarity, the wind rose of Ahmadabad is depicted in Fig. 2 below. It helps in determining the most favorable wind direction of a particular location and accordingly, corridors can be oriented. It is also equally necessary to fit wind direction data into suitable statistical models such as two-component and four-component von-Mises distributions.
Wind Circulation and Corridor Setting
After orienting the corridor according to wind direction, inside circulation can also be provided. When wind flows over a bluff body, at the back of the same vortices are formed due to adverse pressure gradient and subsequent boundary layer separation. At the sharp edge also where the gauge pressure is negative, circulation occurs widely and the same also occurs at the front of the object mildly due to distortion of the stream line. This phenomenon of fluid mechanics can be used to provide natural circulation which is depicted in Fig. 3.
Figure 3: Wind Induced Circulation
Now-a-days due to global warming and high level of pollution, atmospheric boundary layer changes which would lead to the change of the wind climate. The aerosols in the air also increase drastically which also absorb solar radiation. As a result, temperature increases but the irradiation on the surface of the earth becomes gradually less which would in turn affect the pressure gradient and wind flow. Hence, it may not be possible to predict wind speed and direction from probability distribution without considering the effects of climate change. Therefore, it is also required to determine the long term trend of wind speed and direction for proper predictions of the same.
Design Strategies for Natural Ventilation
As shown in Fig. 4, wind and stack action combined yields best circulation and hence the design for natural ventilation should incorporate maximizing both the wind and stack driven ventilation design concepts. General design considerations include: (i) Increase air supply intake by ensuring no outside obstruction (such as vegetation or site objects) nor inside obstruction (such as furniture and interior partition) obstruct inlet openings; (ii) Rooms should have inlet and outlet openings located in opposing pressure zones. This can include openings on the windward and leeward walls or on the windward wall and roof; (iii) All occupied spaces should have an inlet and outlet opening in which at least a minimum of one opening should be an operable window to control flow; (iv) Inlets should supply air at a location low in the room. Outlets should be located across the room and at a higher level; (v) The long facade of the building and the majority of the openings should be should be directed so that the windward wall is perpendicular to the summer wind; (vi) Use skylights or ridge vents. They are very desirable for night time thermal comfort in houses to vent heated/warm air that rises, and allow heat to be radiated into the cold. It is also can be a good outlet for wind driven ventilation; (vii) At least 3m allowance for the floor to ceiling; (viii) window areas should not be excessive and be protected by exterior shading devices; (ix) Design for high thermal capacity and exposed ceilings for night cooling; (x) Reduce the possibility of wall warming by the sun through use of light-coloured building exteriors, trees/shrubs to provide shading and evaporative cooling, grass and other groundcover to keep ground temperatures low, and ponds and fountains to enhance evaporative cooling; and (xi) Internal loading should be kept low. Many of the considerations taken above is to either increase the air flow or lower the heat gain so that the natural ventilation can effective cool the spaces in the building.
Figure 4: Distribution of Inside and Outside Pressures over Height of Building
Unique system of “natural ventilation” in Lotus Temple of New Delhi is based on the principles of ancient buildings ventilation. Image for representation purpose only
Clean Indoor Environment
Outside air may contains unacceptably high level of pollutant including ozone, pollen, carbon monoxide, particulate matter, odors, toxic agent, etc. Hence, the filtering of incoming air is required to get clean environment, which is very essential for comfort and health. The type and number of filters are generally dependent on clean room applications. The main filter groups include: (1) filters for removing solid and liquid particles (viscous impingement and dry screen available in renewable, cleanable and throwaway variations, fibrous, electrostatic and air washers); (2) filters for removing gases and vapors (e.g. activated carbon and chemical filters); (3) filters for removing bacteria (e.g. ultraviolet or germicidal lamps). Apart from air inlet, the pollutants may enter in the air conditioning space with occupancy also and hence they are required to enter and exit the cleanroom through airlocks, air showers and/or gowning rooms, and they must wear special clothing designed to trap contaminants that are naturally generated by skin and the body.
Efficient cooling systems are highly required but in harmony with nature. In most of the cases, traditional cooling systems cause harm to the environment and also human health. Hence, the passive cooling system through the natural ventilation of wind can be considered as a holistic solution in place of traditional cooling system. However, large uncertainties are involved in the availability of the desired wind speed range. Climate change can also influence both wind speed and direction. Accordingly, corridors should be oriented for passive cooling and many design strategies discussed in this article have to follow. As the outside air contains large amount of dust and pollutants, the proper filtering system is needed to get fresh air.
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