Passive Cooling in India Composite Climate

110 Int. J. Sustainable Design, Vol. 1, No. 1, 2008

Copyright © 2008 Inderscience Enterprises Ltd.

Design of passive cooling system for a building in composite climatic conditions in India

Jyotirmay Mathur* Mechanical Engineering Department, Malaviya National Institute of Technology, Jaipur, India E-mail: [email protected] *Corresponding author

Rajeev Kathpalia Vastu Shilpa Consultants, Sangath, Thaltej Road, Ahmedabad, India E-mail: [email protected]

Abstract: A combined solar chimney – wind tower concept has been developed for a typical urban institution building in Delhi. Due to site limitations, the concept of solar chimney based ventilation has been introduced through roof mounted thermo-siphon air panels of 70 m2 absorber area. A wind tower provided on the suction side of fresh air has potential for cooling incoming air by 5°C by using 250 m2 of stone cladding. Openings for airflow inside the buildings are designed to suit smooth natural ventilation. Evolution of the final design through various constraints is explained in this paper.

Keywords: solar chimney; wind tower; sustainable building; energy efficiency; solar radiation; passive ventilation.

Reference to this paper should be made as follows: Mathur, J. and Kathpalia, R. (2008) ‘Design of passive cooling system for a building in composite climatic conditions in India’, Int. J. Sustainable Design, Vol. 1, No. 1, pp.110–126.

Biographical notes: Jyotirmay Mathur is Coordinator for Postgraduate Program in Energy Engineering and Reader in Mechanical Engineering Department at the Malaviya National Institute of Technology, Jaipur, India. He has experience of 15 years research and teaching of engineering, particularly, energy related subjects. He works in the fields of energy efficiency in buildings, passive systems for cooling and ventilation, solar thermal systems, and energy policy modelling. He has published 18 research papers in referred journals, presented 35 papers on international forums, authored/edited five books in the field of energy efficiency and renewable energy besides successfully completing several nationally and internally funded research projects.

Rajeev Kathpalia is a Postgraduate in Architecture and Urban Design. He is partner of Vastu Shilpa Consultants and Director, Vastu Shilpa Foundation for Studies and Research in Environmental Design, a non-profit research organisation at Ahmedabad, India. The organisation and Mr. Kathpalia are well known names in the field of sustainable building design in India. In his professional career of 28 years, he has received several awards for sustainable building design. He is visiting faculty at the CEPT, Ahmedabad,

Design of passive cooling system for a building 111

and acts as core faculty in international architectural studios organised in collaboration with several universities of international repute such as University of Stuttgart, Berne, Arhus, Washington and Michigan. He has been member of various national level committees such as CII National Committee for Housing.

1 Introduction

Over the past few years, concern towards energy efficiency and incorporation of passive cooling systems in buildings has increased. In earlier times, when energy was not available in its most popular form, i.e., electricity, building designers had to rely on natural ways and means for maximising comfort inside the building envelope. A major difference between ancient times and today’s situation is that today, none of the major resources namely, land, money and time are available in abundance. This has led to a situation where passive features must be designed more scientifically and technically as compared to the past. Literature related to contemporary building design reveals that there are numerous examples displaying success stories in different climatic conditions, worldwide. In India, there are several showcase buildings such as a solar passive house in hot arid zones in India (Bansal and Minke, 1995) and a passive-cooled building for semi-arid zones (Srivastava et al., 1984) using wind tower, earth berm and evaporative cooling systems. Attempts have been made to design sustainable buildings for hot and dry climates using optimal building form and layout. On the other hand, there are buildings like MLA hostels in Simla, for cold climatic conditions using passive heating concepts (Majumdar, 2002). The first platinum rated LEED accredited building of India also displays use of passive cooling systems in building design (CII-GBC, 2004). There are also some examples of unsuccessful passive cooling systems. Bakiwala (2002) has prepared a report on non-functional passive features concluding that the failure was primarily due to two reasons:

• inadequate design calculations or design by gut-feel

• over-expectation from the passive feature.

It also suggests that, as compared to the success stories, failure stories spread faster and are acting as indirect barriers to popularisation of passive cooling or heating concepts.

As compared to design of passive features for hot or cold climatic conditions, design for composite climates is little more typical and may lead to failure of concept if not done properly. The present paper shows various steps that have come in the way of designing passive systems for the climatic conditions of Delhi in India. In particular, it discusses various feasibility related issues that govern, or sometimes dominate in, the process of decision making. This paper additionally describes the situations or limitations that sometimes compel adoption of a design that is not corresponding to best performance of a passive system. Many detailed calculations have been conducted for every decision that required understanding of the transient heat transfer process. However, for simplicity and understanding of the main effects, only simplified versions of calculations are being presented here.

112 J. Mathur and R. Kathpalia

2 Description of building

2.1 Building location

The building to be studied in the present paper is located in New Delhi, the capital city of India on a plot measuring 999.00 sqm. As per the existing laws, the maximum permissible ground coverage is 249.8 sqm i.e., 25% of plot area. The maximum permissible building height is 26 m with setbacks of, 9 m in the front, 6 m in the rear and 4.5 m on each side. These details were extremely important for calculations related to shading by surroundings (Figure 1).

Figure 1 Site plan showing the existing and proposed CSE building

2.2 Building usage

Meant for use as an institutional building, the building has to house institutional offices, classrooms for holding training programmes, library and dormitories for accommodating participants of training programmes. The typical feature of such a combination is that offices would operate daily from 0900 h to 1700 h, classrooms would be used from 0900 h to 1700 h, only when the training programmes are organised and the dormitories would be occupied before 0900 h and after 1700 h during the days of training programmes. The library is likely to be used extensively during the training programmes. However, it has to remain open for access of office staff and visitors on all working days.

2.3 Building layout and plan

Since the site is oriented in North-South direction, a conventional layout would have longer sides facing East-West. The building scheme of B + G + 3, i.e., five floor building including basement, ground and three floors over the ground floor, was adopted.


A stepping-in of the building profile towards the west provides mutual shading and cutting down the fenestration on E-W face (Figure 2). Openings on north wall for improved day-lighting and use of terrace gardens were major steps for reducing the heat gain (Figure 3). The basement is designed for use as a library, the ground floor for housing the classrooms, first and second floors for office purpose, and the top floor for having the dormitories.

Figure 2 Stepped profile of building towards west

Figure 3 Photograph showing series of water bodies, plants and terrace gardens at different levels


3 Passive architectural concepts

3.1 Solar chimney

The concept of solar chimney utilises the classical concept of buoyancy. Air, on becoming hot, also becomes lighter due to reduction in density. This lighter air has a natural tendency to rise upwards. If one creates and maintains a higher temperature in a duct than the ambient, then the warm air in the duct rises upwards. If suitable openings are provided for exit of arm air into atmosphere and entry of room cooler into the duct, the concept can be utilised for creating ventilation in buildings (Figure 4).

Figure 4 The principle of stack effect of solar chimney based ventilation

Before starting the design calculations, it is most important to collect and study the relevant climatic data. For the purpose of solar chimney, availability of solar radiation and ambient temperature are the two most important data. The following are the important details that should be considered carefully:

• availability of solar radiation on the North, South, East, West and horizontal surfaces

• month-wise and time-wise variation of incident solar radiation on each of the above surfaces

• month-wise variation in time when incident solar radiation reaches its peak on these surfaces

• month-wise and time-wise variation in ambient temperature during sunshine-hours.

3.1.1 Step-1

Initially, a solar chimney was proposed at the South-East corner of the building to serve a dual purpose, as a source of light as well as to induce ventilation in the building (Figure 5). The following factors played important roles in the decision regarding the south-east orientation of the vertical absorber:


• A vertical absorber and chimney can easily be integrated into the building façade.

• During summer months, i.e., April–September, ambient temperature at 0900 h is high enough and simple ventilation through ambient air can not bring the indoor climate to a comfortable level.

• During the months of March and October, the ambient temperature between 0900 h and 1300 h is within the acceptable range of temperature and similarly between 1600 h and 1800 h it is again in the acceptable range.

• A solar chimney can, therefore, be used for ensuring natural ventilation for 4 h duration over four months every year.

• If the absorber of the solar chimney is oriented to capture the maximum solar radiation during the morning hours for functioning at maximum effectiveness, during evening hours, it does not receive sufficient radiation. Therefore, either there haveto be two parts of the absorber for working during morning and evening hours, or there should be a solar tracking system. Since both of the above options tend to increase the initial cost it was decided to drop the evening utility of the system and to focus on morning time utility only.

• A study of solar radiation revealed that a South–East oriented absorber would receive maximum solar radiation between 0800 h and 1300 h during the months of March and October. Hence, the decision was taken to work on the South-East oriented solar chimney with a vertical absorber.

Figure 5 Location of solar chimney with absorber in South-East corner

It was observed that, in Delhi, on average, 500 W/sqm solar radiation falls on a south-east oriented vertical surface between 0800 h and 1300 h in the month of March. The ambient temperature at this time is in the range of 20–30°C.

Taking the results of the experimental studies conducted at MNIT, Jaipur (Bansal et al., 2006), at the solar radiation intensity of 500 W/sqm and 24°C room temperature, 1 sqm absorber area of solar chimney can induce 0.03 kg/s of ventilation rate.


As per the BIS norms for ventilation, 20 cubic feet per minute of fresh air are required in an office building.

For 200 occupants, the total requirement of fresh air: 20 × 200 cubic feet per minute. (approx. 120 m3/min).

Therefore, area of absorber that is required to meet the ventilation requirement: (120 × 1.2)/(0.03 × 60) = 80 sqm (approx.)

The identified chimney area had dimensions 3.5 m × 6 m with 15 m height. If a South-East oriented absorber is to be placed diagonally, the maximum possible area of the absorber can be approximately 98 sqm, which is higher than the required area of 80 sqm. Some of this 98 sqm area will be taken up by openings at each floor for inlet of air into the chimney. Hence, the available area is almost equal to the required area of absorber.

While designing openings for entry of air into the chimney from different floors, special care is to be taken to avoid short-circuiting of air from lower floors to higher floors. Air, coming out of lower floors, after rising to the level of higher floors, may get sucked into the higher floors if the pressure of air coming out of the higher openings is lower than the pressure of rising air (Figure 6). To prevent this possibility, openings at higher levels are kept smaller as compared to openings at lower levels.

Figure 6 Section of building through solar chimney

3.1.2 Step-2

It was felt that if an opaque absorber is placed on the South-East corner of the building, there would be loss of daylight that otherwise would have entered through the windows provided on that side. The following alternatives were explored to strike a balance between the performance of the solar chimney and to have daylight:

• Leaving a 2.5 m wide slit on the east face for bringing in the daylight, the absorber area gets reduced to 75 sqm (Figure 7).

• To use a perforated absorber with several holes. This would have served a dual purpose; one, it would have facilitated entry of daylight into the building, two, it would have provided a restricted outside view to the person sitting close to that area. The sizes of, and spacing between, the holes could even be designed from the point of view of aesthetics.


• To leave horizontal slits in the absorber for entry of sunlight. This would have created a view similar to the black and transparent fringes that appear in spectral analysis.

Figure 7 Plan showing absorber

Both of the ideas have the disadvantage that the effective area of the absorber is decreased, which, in turn, decreases the ventilation rate, which was the prime objective.

3.1.3 Step-3

To increase the area of the absorber and to have the provision of sunlight, another alternative was developed. It was proposed to have a fringed absorber on the Southern as well as the Eastern wall of the chimney area, instead of having a South-East oriented absorber. The disadvantage of doing this was that the gap between the absorber surface and glass cover increases extensively as compared to the previous case. This has two negative impacts on the performance of the solar chimney. Firstly, it increases the amount of air trapped between the glass cover and the absorber. If the amount of air were more, it would be heated to a lower temperature by the same amount of heat. Second, much of the absorber area is ineffective since, when radiation falls on the South face, the East does not receive much radiation and vice-versa. Therefore, this idea was not encouraged.

3.1.4 Step-4

It is a well-known fact that capture of solar radiation can be increased by properly inclining the absorbing surfaces. Utilising this knowledge, which was technically supported by Bansal et al. (2005) and other researchers Aboulanga and Abdrabboh (2000) for application in a solar chimney, attempts were made to improve the performance of the solar chimney within the restrictions related to absorber area. Extending the work by Adnan et al. (2002) for optimising tilt of solar collectors, it has been found by Bansal et al. (2006) that a 45° inclination from the horizontal gives the maximum ventilation rates as compared to other inclinations. This is due to two factors: as we make the absorber flatter from vertical, incident solar radiation increases due to more solar rays falling normal to the absorber plane. On the other hand, the effective


height of absorber or the ‘stack height’, which is the cosine of its inclination with vertical, reduces. These are two conflicting effects related to the performance of solar chimneys and at 45° inclination there is an optimal combination of these two effects.

The problem with inclined absorbers is that, due to inclined absorbers, they have to be protruding out of the building façade else they can only be mounted on the terrace. The first option may not be allowed as per building bye-laws and it even makes the appearance quite awkward. Therefore, the only option left for design was the use of inclined surfaces on the terrace (Figure 8).

Figure 8 Terrace mounted solar chimney panel details

Since the solar incidence on inclined surfaces increases as compared to a vertical one, the area requirement needs re-calculation. For the same time of use, it was seen that the expected mass flow rate would be 0.03 m3/s/sqm of absorber leading to about 70 sqm of absorber area requirement.

Since, the rules do not allow anything higher than 26 m from the ground level, the obvious option for having 70 sqm area was to have more than one absorber within the height limitation. While designing the inclined absorbers, care is to be taken regarding:

• Keeping sufficient spacing between two adjacent absorbers to avoid shading of one on the other.

• Orientation of absorbers has to be in tune with the time-of-use. As discussed earlier, South-East orientation suits the purpose of chimney to a greater extent as compared to true south orientation as in case of solar water heaters.

• Unless proper provisions are made, a terrace mounted chimney tends to create a differential ventilation effect in a multi-story building. Opening sizes are to be arranged very carefully to ensure that almost equal amounts of ventilation take place in all the floors. Due to this reason only, tall buildings require a different type of arrangement for solar ventilation that was not possible with the layout of this building.

Attempts were made to fit absorbers having a total area of 70 sqm and height within the building laws. The place close to the East wall was identified for this purpose mainly due to absence of any other alternative, otherwise, the absorbers could be distributed all over the building area to have a direct connection with the space to be ventilated. The layout of different absorbers of solar chimney has been shown in Figure 9.


Figure 9 Terrace plan showing the position of solar chimney panels

It should be clearly stated that a solar chimney only serves the purpose of creating ventilation in a building. The temperature inside the building would depend upon the temperature of ambient air and can not be less than the same. For cooling the incoming air, other passive concepts have to be added to the path of ventilation.

For entry of air into the chimney, openings from all the different floors were made using collapsible window openings in the stair well. For suction of air, dual provision was given. The common solution evolved by providing direct suction of air from the stair well. To enhance the suction, to avoid short-circuiting between floors, an additional air-path is provided through the East facing cavity wall (Figure 10). These cavities or additional paths are connected to the inclined absorbers for suction of air.

Figure 10 Part section showing the provision for dual suction of hot air


3.1.5 Step-5

In view of the above mentioned limitation of the solar chimney, it was felt that the utility of the designed system could be improved by creating a wind tower in the path of incoming fresh air. The concept of wind tower is almost reverse of the solar chimney. Several examples of wind towers can be seen in ancient buildings and especially in the Iranian type of architecture. A tall tower, through which atmospheric air is sucked, pre-cools the air before entry to the living space. Such towers are characterised by internal surfaces having high heat-capacity, e.g., massive stone walls. The working of a wind tower is in a daily cyclic manner and suits a place where the difference in day and night temperatures is high (such as in desert areas that involve similar climatic conditions).

The major design parameters in this case are: surface area that would extract heat from warm incoming air, and weight of the heat absorbing material, stone per se. Provisions should be ensured for losing the heat collected by the stone during the day time either by natural flow of air or through some artificial mechanism such as spraying water or using an exhaust fan.

Another important task while integrating a wind tower in a building is to ensure optimum path of air flow on the suction side, through the building and on the exhaust side. The suction side should provide an inlet opening in such a way that it harnesses wind at a height and direction where the free flowing velocity of air is maximum. In this case, again, the maximum height restriction plays an important role.

With the integrated wind tower- solar chimney, the system is expected to be useful for a longer duration as compared to the solar chimney alone. Instead of serving during March and October, the system is likely to be useful during March–April, and September–October up to 1300 h, instead of utility only up to 1100 h as in case of a solar chimney alone.

3.1.6 Step-6

As indicated above, the most important part in the design of the conceived system is the calculation of surface area requirement, the thermal mass requirement and opening sizes to ensure proper air flow through all targeted parts of the building.

• First of all, the building plan was again studied to identify a possible location for the wind tower. Converting the soft-courtyard near the West-wall offered the only possible feasible location. This courtyard was earlier designed to serve the purpose of carrying gas-pipes from toilets and for service purposes (Figure 11).

• Conversion of this space has been done keeping in mind that the above mentioned two basic necessities are not compromised with. Care should be taken to ensure that the wind tower does not capture foul air due to the proximity of toilets.

• Due to the time taken in heat transfer between the downward moving, incoming air and the cooler surfaces of wind tower, as a rule of thumb, a 1 m high wind tower corresponds to 1°C temperature drop in air, provided proper surface to volume ratio is maintained.


• The height of each floor in this building is approximately 3 m; therefore, it was felt that the wind tower would not be able to produce sufficient cooling before entry into the third or second floor. Its effect could be best felt in the lowermost floor i.e., basement. Since the utilisation level of the basement is not very high and also due to the fact that basement would remain cooler than the upper floors anyway, it was decided to provide openings of cool air from the wind tower into the ground floor and first floor. By doing this, we get a height of about 15 m before entry to the ground floor and 12 m before entry to the first floor. For the situation when the ambient air temperature is around 40°C, if designed properly, the system would be able to supply air at 30°C.

Figure 11 Plan showing the position of the wind tower

3.1.7 Step-7

The required area of opening for air flow into the building from the wind tower can be expressed as:

Aopening = V/u and V = n × v

where

Aopening is area of the opening through which air enters the building ‘V’ is desired volumetric flow rate ‘u’ is velocity of air flow ‘n’ is number of persons occupying the room ‘v’ recommended air flow rate per person (20 ft3/min) No. of persons on ground floor: 100 Volume of air required: 20 × 100 = 2000 ft3/min = 33.33 ft3/sVelocity of air coming out of wind tower: 4 ft/s Therefore, area required to have 50 ft3/s flow at 4 ft/sec velocity is 33.33/4 = 8.33 ft2

In the case of two openings, one each for left and right wings, the area per opening is 4.17 ft2.


Therefore, a window having a size of 1 m × 0.4 m (clear area) would be sufficient for this purpose.

For the two openings on the first floor, and one opening on the second floor, since the incoming air would be less cool, larger openings are required to ensure the same order of air volume flow. Therefore, on the first floor, windows of 1 m × 0.4 m and on the second floor, openings of 1 m × 0.45 m size are recommended.

All the openings are to be provided close to the floor level.

3.1.8 Step-8

For calculating the thermal mass required to store heat from air, the following approach can be followed:

Designing the tower for 5°C drop in temperature of air, the following energy balance can be written.

The total volume of air to be cooled:

V’ = operating duration × volumetric flow rate (V).

Total heat to be extracted from air:

Q = V’ × ρ × Cp,air × (Ti,air – Tf,air)

where

Q is heat lost be air during entire day ρ is density of air Cp,air is specific heat of air Tiair,Tfair are initial and final temperatures of air at entry and exit of tower, respectively.

From the energy conservation law, it is clear that: Heat lost by air = heat stored by stone Heat storage capacity of stone: Q’ = mCp,stone(Ti,stone – Tf,stone)

wherem is mass of stone cladding Cp,stone is specific heat of stone Ti,stone, Tf,stone are initial and final temperatures of stone.

From the above two equations, we get: 1700 h

,stone ,stone ,stone ,air ,air ,air0900 h( ) { ( )}d .p i f p i fmC T T V C T T tρ− = −

Since the weather data available for Delhi is discrete, i.e., average for every hour, the above equation can also be solved after discretisation and solving it for every hour.

The following data have been used for using the above equation and for finding the required mass of stone cladding:

Area available for air to flow: 4 sqm (in the wind tower) Velocity of flow: 4 ft/s (1m/sec approx.) Volume of air to be cooled: 4 m3/s


Usage timing: 8 h usage (0800 h to 1700 h) Specific heat of stone is: 0.9 kJ/kgoK.

The mass of stone required for cladding was calculated to be 153,600 kg (say 150 Ton). The weight of the stone is dependent upon the specific heat. For example if marble is

used, much less weight would be sufficient but it would simultaneously become a costly option.

For the calculation of heat exchange, the available area has been considered using the surface area to volume ratio (S/V) of 5.

For wind towers, a surface to volume ratio of at least five should be maintained. The higher the ratio the better the performance.

For a 4 sqm flow area and 15 m height of tower, the volume of chimney is: 60 sqm. For an S/V ratio of 5, the area required is: 5 × 60, i.e., 300 sqm. The surface available within the tower is about 90 sqm. Due to the openings at the

second floor, first floor and ground floor, effectively, it is further reduced. Therefore, it was decided to have additional surfaces with stone cladding on partitions or channels created inside the wind tower section. Since the entire amount of air does not come to the ground floor, the area requirement was modified to 250 sqm instead of 300 sqm (Figure 12).

Figure 12 Plan showing the wind tower with stone cladding

3.2 Flow contours for coupling of wind tower and solar chimney

Besides ensuring the required amount of surface and capacity for extraction of heat, utilisation does not take place unless a proper and complete path is provided for air movement within the building from the entrance in the wind tower to the exit through the solar chimney. Details of the anticipated flow contours on the three floors are shown in Figures 13–17.


Figure 13 Ground floor plan

Figure 14 First floor plan

Figure 15 Second floor plan


Figure 16 Third floor plan

Figure 17 Sectional view of all floors

While planning for the path of air within the building, care should be taken regarding avoiding abrupt or sharp turns and any excessive length of flow path. At the same time, it is to be ensured that flowing air covers most parts of the building for observing effective ventilation and drop in indoor temperature.

4 Conclusion

The concept of a roof mounted solar chimney with an inclined absorber has evolved as a final solution for typical urban buildings. The innovative approach of providing an additional opening on the lower side of the absorber for use as an in-built skylight


improves utility of the entire structure. The conventional use of the solar chimney for ventilation has been extended to integrate with a passive cooling system of wind tower and the use of skylights to improve the daylight levels. All the three concepts considered aim to reduce the use of conventional electric devices in the building.

The combination of wind tower and roof solar chimney facilitated natural ventilation and cooling of incoming air without the use of active devices. For inducing natural ventilation, an inclined, blackened metallic surface area of 70 m2 has been recommended. Due to space limitations, this entire area was split into four different absorbers. Further, for cooling of air, a wind tower having stone cladding over an area of 250 m2 and weighing 150 ton was required to obtain a drop of 5°C in temperature. The combination of solar chimney and wind tower is found to be a good design option for urban buildings.

The paper demonstrates how natural resources can be utilised to design sustainable buildings. In urban areas, designing truly sustainable buildings is extremely difficult. However, the building design explained in this paper can be considered to be more sustainable compared to conventional urban buildings. Different approaches for addressing various limitations, such as availability of space and effects of neighbouring buildings have been explained in detail. The results achieved indicate that adoption of such concepts represents a step forward in the design of sustainable buildings for the type of climatic conditions considered in this work.

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Passive Cooling in India Composite Climate

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