Passive Cooling of Buildings

Abstract

The external climate (temperature, radiation, humidity, and wind) determines the heating and cooling requirements of a building. Increased cooling load due to solar gain is generally the main problem being faced by designers, but this can usually be taken care of by good innovative design. Besides shading of windows from direct sun, especially on the east and west facades, protection from diffused sunlight and reflected sunlight is also important. Solar heat ingress through walls and roof can be reduced by using white or light colours, selective paints etc on the exterior and by providing appropriate insulation. Shading by vegetation is particularly effective because it also reduces heat island effect and the surrounding air is cooled by transpiration. Several natural systems of creating an indoor heat sink by cooling the structure by night sky radiation or by evaporation of water can be used to cool buildings with negligible energy consumption.  These cooling systems will be briefly described in relation to their applicability in different climates.  

Paper describes a variety of natural cooling techniques and various options available for low energy cooling of buildings like natural ventilation, evaporative cooling, thermal mass, nocturnal ventilation, phase change materials, or mechanical air conditioning etc. Salient features of various studies on passive cooling techniques are discussed in this paper.

Keywords: Temperature, humidity, comfort, cooling, evaporative cooling, passive features

1.   Introduction

With growing appreciation of energy conservation, efforts are being made all over the world to evolve techniques of designing more energy efficient buildings having an inherent healthy and comfortable indoor environment. Reduction in energy consumption in buildings can be achieved in two different ways;

(i)     by alleviating wastage in energy by the traditional inefficient and improperly designed heating, cooling, ventilating and lighting appliances and

(ii)   by optimum utilization of non-conventional renewable sources of energy like solar and wind energy and by the integration of passive devices in the design of buildings for comfort conditioning’.

These two approaches jointly contribute towards the overall energy efficiency of buildings.

Appropriate architectural design such as orientation of buildings, the location of fenestrations  with appropriate shading devices, creating a green belt of trees and vegetation around the buildings for reducing heat island effect and cooling of ambient air appreciably improves  thermal performance by reducing heat load. Massive walls and high ceiling were common passive features used to reduce the swing in temperature. Such proven energy conservation features have now been forgotten by the building designers.

In view of the fact that a large population in developing countries like India, living in hot-dry and in hot-humid conditions suffers serious thermal discomfort for a considerable part of the year. The condition is worsened by the already inadequate and depleting sources of conventional energy leading to ever increasing emphasis on the search for effective solutions for low-cost cooling targeting innovative S&T concepts, materials and designs.

2.   Comfort conditioning - basic techniques

Air-conditioning is a system of conditioning air by reducing or increasing its temperature and humidity but it does not deal with the effect of excessively hot or cold structure on human comfort. Comfort conditioning deals with both the building envelop as well as the ambientb air.  Thermal comfort is directly related to the conditions under which human body can give off surplus metabolic heat. Human body gives off this heat by evaporation of sweat, radiation and convection. At air 23^oC temperature and structure also at 23^oC, the heat loss due to these avenues is 17% by evaporation 13% by radiation to cool structure and 70% by convection[1]. With air temperature at about 30^oC and structure at 52^oC heat is lost by evaporation 78%, radiation 0% and convection 22%. But when indoor air temperature reaches above 37^oC and structure is at 50^oC (common condition in Indian houses in peak summer) body can lose heat only by evaporation, aided by fan as long as humidity is low, and gains heat both from hot air and hot structure.

This is a highly uncomfortable situation. Now, as the humidity goes above 65% sweat does not evaporate fully and flows on body surface. Such running sweat does not help in heat removal as such it becomes almost impossible for the body to give off heat. However, heat loss by radiation towards a cool structure is not affected by ambient air temperature. Under these situations if the structure is cooled down to wet bulb temperature (about 25^oC) a large amount of body heat is lost by radiation towards the cool structure. Thus  cooling of  structure plays very important role both in hot dry and hot humid conditions in summer, even under high humidity, while the heat loss by radiation remains unaffected.  

Human body has 98% absorptivity and emissivity in the infrared long wave region. Therefore, it has a tremendous potential of absorbing heat from hot walls and ceiling and also of radiating heat to cool walls and ceiling.

Table1. Percentage of actual heat loss to the environment by various avenues

Air Temperature (oC)	Wall  Temperature (oC)	Percentage Heat Loss due to
		Evaporation	Radiation	Convection
17.1 16.0 22.8 29.4 35.4	19.1 49.1 22.8 52.4 36.6	10 21 17 78 100	40 13	50 79 70 22

Several natural processes or a combination thereof, can be used to provide thermal comfort  in buildings which may include cooling by long-wave radiation towards night sky or a heat sink, slowing the rate of heat flow, exchanging unwanted heat of a building by induced ventilation through stored ‘çoolth’ of mild climates, cool dry nights or underground earth or water which maintains a constant temperature throughout the year. In hot humid climates with uncomfortable warm / humid nights, such ventilation can be counterproductive, and there some type of solar air conditioning may be more cost effective

Although there are many strategies of natural cooling of buildings, the primary cooling systems in relation to their applicability in different climate types are briefly described.

2.1  Convective cooling

Convective cooling implies cooling the structural mass of the building by air at night and utilizing the cooled mass during the following day as a sink, to absorb heat entering into a building or generated inside the building. During the day, interior ventilation is deliberately kept as low as possible to avoid bringing in hot daytime air. As a result, the indoor temperatures remain lower than those in a similar building without such convective cooling. Convective cooling is being applied successfully in many regions with suitable climate conditions. It is a proven cooling technique, but it is essential that the required building design characteristics are achieved.

2.2 Nocturnal ventilation

                 

Theoretical as well as experimental studies on assessment of contribution of night ventilation towards cooling of buildings during the following day have been carried out by several investigators for different types of structures under different climatic conditions. In 1992 Givoni [2] derived relationships for estimating the reduction caused in indoor minimum (eq.1)  and maximum temperatures (eq.2) caused by the provision of night ventilation for medium and high mass buildings with light external colour and fully shaded windows.                It is also reported that potential of night ventilation for lowering the day time temperature indoors below the outdoor temperature is proportional to the ambient diurnal temperature range. It was observed that night ventilation has only a very small effect on the indoor maxima of the low-mass buildings. However, it is very effective in lowering the indoor maximum temperature for high mass buildings below the outdoor maximum. In case of the  particular buildings covered in the above studies, the maximum temperature in the low mass building was 2⁰ C above the outdoor maxima whereas in a high mass building the maximum temperature was 2⁰ C below the corresponding outdoor maxima when there was no night ventilation in the buildings. The provision of night ventilation in low mass buildings resulted in indoor maximum temperature which was very close to the outdoor maxima. In case of high mass building, the night ventilation at the rate of 45 ach lowered the indoor maximum temperature by 3.5⁰ C when outdoor maximum temperature was 38⁰ C .  Extensive studies on the effect of these parameters on the availability of indoor air motion have been carried out in  CBRI.

This type of cooling is applicable in regions with vapour pressure below approximately 17 mm HG and a large diurnal temperature range (above approximately 10° C), where the day-time temperature is above the comfort limit and the minimum temperature is below approximately 20°C. Under these conditions, daytime ventilation is not desired.  In order to secure effective convective cooling, the envelope of the building should be well-insulated (average U value below approximately 0.67 W/m²°C) and have sufficient thermal mass.

2.3  Radiant cooling

Building envelope which gets heated during sunshine hours emits long wave radiation to sky during night due to the fact that the mean radiant temperature of the clear sky falls below the ambient air temperature. Thus heat in the building's structure is discharged by long-wave radiation to the night sky, as a result of this; exposed building surfaces lose heat which ultimately causes cooling of building.  Roof having maximum exposure to sky is the most effective radiative cooling part of the building envelope.

There are two primary types of radiant cooling systems. The first type is delivers cooling through the building structure, usually slabs, these systems are also named thermally activated building systems (TABS). The second type is systems that deliver cooling through specialized panels. Systems using concrete slabs are generally cheaper than panel systems and offer the advantage of thermal mass while panel systems offer faster temperature control and flexibility. Radiant cooling systems are usually hydronic, cooling using circulating water running in pipes in thermal contact with the surface. Typically the circulating water only needs to be 2-4°C below the desired indoor air temperature [3]. Once having been absorbed by the actively cooled surface, heat is removed by water flowing through a hydronic circuit, replacing the warmed water with cooler water. Underground water which remains at about 23^oC (Roorkee) throughout the year can be used as the coolant in summer and as source of heat in winters.

Radiant nocturnal cooling of a massive roof can provide effective cooling in almost all climates, except in humid cloudy regions. This system can be applied only to one-storey buildings, with a roof of high mass and high conductivity.

2.4 Roof surface evaporative cooling

It is well established fact that 60% to 80% of incident heat on buildings enters through the roofs and only 20% to 40% heat enters either through the fenestration on exposed walls of a single story building. Cooling caused due to evaporation of water has been used in a number of ways for cooling the interior of buildings. It was revealed that evaporative roof cooling could reduce the cooling load by 40 per cent in hot dry climates. Results of computation of the interior surface temperature of a typical concrete roof treated by a controlled water spray and an untreated roof indicate significant decrease in ceiling temperature due to roof spraying. Further, controlled spraying was found to be more effective as compared to the continuous spraying.

Roof-Surface-Evaporative technology developed at CBRI Roorkee [4] has been observed to possess the potential of complete neutralization of  total incident heat on roofs and the roof so cooled act as a heat sink to the heat that enters through fenestration /exposed walls.

Figure 3. Effect of RSEC on exposed RCC roof and reversal of temperature gradient by RSEC

 

Figure 4. Effect of RSEC on indoor air temperature

The technology consists of laying a thin uniform organic material lining (double layered empty jute cement bags or 6 mm thick Coir matting in its natural colour) on either especially designed or treated roof terraces, in their close contact. The lining is constantly maintained in a just moist condition by electronically controlled spray of water, day and night throughout the hot dry/ hot humid periods for continuous and quick evaporation of water from the roof surface by absorbing solar radiation in the day time and by sucking out heat from indoor air at night. Necessary gadgets both for manual or automatic operations have been developed to accomplish the above requirements. The water trapped in the jute bags/Coir matting evaporates continuously at all temperatures. This water is evaporated by solar heat absorbed by roof aided by summer air movements. The incident heat due to Sun’s rays on roofs is also consumed from the evaporation of the water present in wet mattings and therefore, the same cannot add to the heat content of roof. Higher the incident heat of the sun and wind speed on roofs higher will be the quantity of water evaporated and higher will be the cooling effect.

The process is ideal for all types of buildings viz low cost, multi-storied, industrial, conditioned or thermally uncomfortable buildings to bring appreciable saving in energy, running & Capital cost of air-conditioning and increase human welfare, efficiency & productivity. Water requirement varies from buildings to building depending on the actual heat content and also on the climate conditions. The average requirement for a normal single story building is 6 to 9 lit of water per sq meter of roof area per day under hot humid to hot dry climates. It has been calculated, based on experimental results, that 1000 gallons of water can produce cooling equivalent to nearly 900 tons of refrigeration with the proper installation & utilization of the technology.

A complete know-how of the above system has been developed after successfully implementing the cooling technology in more than two hundred buildings (Unconditioned/Conditioned) single and up to five storied, industrial sheds of AC/G.I. Sheet roofing including folded plate roofs at different parts of the country under different water supply conditions. Among the various installations, the one on the roofs of Solar Energy Centre, is the largest for which advanced automatic spraying system was designed by CSIR-CBRI.

2.5  Earth air tunnel cooling

Near constancy of temperature at a few metre depth inside the ground has been in use for cooling of buildings in hot climates in different parts of the world for centuries. It is reported that temperature at a depth of 4 metre below the surface of the earth remains nearly constant round the year. In summer the outdoor air is at a temperature higher than the temperature inside the ground. Hence the outdoor hot air during its passage through a tunnel constructed at a depth of about 4 metre below the earth’s surface gets cooled. This cool air is passed through the building and thus the indoor environment is cooled.

It is well established that the cooling efficiency of the aforesaid system depends, interalia, upon the length, size and material of the pipe and its depth below the ground surface, local climatic conditions and the rate of air supply through the pipe. A study on assessment of heating and cooling performance of earth tube heat exchanger for moderate climates was carried out by Trombe et al [5]. The heat exchanger consisted of a PVC tube with external and internal diameter equal to 0.20 and 0.19 m respectively. This was buried at an average depth of 2.5 m below the earth’s surface. The air flow rate through the pipe was varied from 306 to 405 m³ / h. It was observed that the temperature at the exit of the pipe was higher for the higher flow rates. The increase was higher when the air blower was run round the clock as compared to the increase observed with intermittent running of the fan. Hence a proper management of running the fan is necessary for adequate functioning of the system. It was found that air from the unconditioned room is effectively cooled by 3⁰C as it passes through the earth coupled heat exchanger tube. 

3  Other passive cooling features

Louvers, overhangs or awnings provided on windows help control direct entry of sun into the room especially during summer months. Shading a building from solar radiation can be achieved in many ways. Buildings can be orientated to take advantage of winter sun (longer in the East / West dimension), while shading walls and windows from direct hot summer sun. This can be achieved by designing location-specific wide eaves or overhangs above the Equator-side vertical windows (South side in the Northern hemisphere, North side in the Southern hemisphere). The walls exposed to sun shaded by providing appropriate texture thereon or by erecting some external screen in front of the wall are effective to prevent direct sun entry into buildings. Shading of roof which receives the maximum solar radiation as compared to walls oriented in different directions has been recognized as an important step in achieving reduction of external heat entry in buildings. One of the very effective method of lowering the external surface temperature of the roof is to paint it with a coating which has minimum absorption for solar radiation and high emission for long wave radiations. Few passive features are described below:

3.1  Shading of roof

                  Roof  receives the maximum solar radiation as compared to walls oriented in different directions. Therefore, shading of roof has been recognized as an important step in achieving reduction of external heat entry in buildings. Several methods have been attempted  to exploit this aspect of passive cooling. Planting of deciduous plants or creepers atop the roof is the simplest method for shading the roof and lowering its temperature. It has been shown that a well-designed green roof with a covering of  plants having thick foliage and horizontal leaf distribution acts as a high quality insulation and reduces the heat entry through roof in summer. Experimental observations indicated that surface temperature of roof top without vegetation were more than 15⁰ C higher than that of the roof covered with vegetation.

Computation of heat flux through the roof also revealed that heat flux of 200 W/ m² entered the room through the uncovered roof whereas at the roof section covered with vegetation, about 10 W / m² was transferred upward from inside of the room[6]. Roof shaded by Roof Surface Evaporative Cooling (para 2.3) can reduce roof top temperature of RCC slabs from 60^oC to nearly 30^oC in hot dry summers. A study conducted by Gupta  [7] in Jodhpur showed that a layer of  closely packed small inverted earthen pots is very effective in reducing the heat transmission through the roof in hot-dry climate. Investigations carried out in CBRI [8] on heat flow through roofs  demonstrated through field measurements that removable insulation which can be rolled during night also contributes significantly towards reduction in heat flow through roofs.

 

3.2 Shading of walls

The walls exposed to sun are shaded by providing appropriate texture thereon or by erecting some external screen in front of the wall. These provisions cut the direct solar radiation on the walls and may also add to the aesthetics of the building. Experimental study carried out revealed that shading of houses by nearby trees reduces the cooling load and causes about 30 per cent reduction in seasonal cooling energy.

Hayano [9] also reported that heat gain through a wall is reduced by 75 per cent simply by growing a thick layer of wines on the wall. A row of trees is reported to produce 50 per cent reduction in heat gain through an adjoining West facing wall.  

 

3.3  Shading  of windows

Extensive studies on development of shading devices for windows have been carried out in CBRI. Based on these studies, simple method for designing  appropriate shading devices for windows with different orientations  have been evolved. This data has been included in  BIS code and also published in the form of a  Building Digest [ 10 ]. With this information in hand,  it is possible to design vertical louver or horizontal louver or their combination to achieve complete shading of a window at any desired station in the country. Apart from louvers mounted externally on windows, venetian blinds mounted thereon or deciduous trees may also be used to prevent the direct entry of sun through windows. 

Optimum dimensions of the louver depend on the duration of sunshine on the window façade. Windows of the same dimensions but oriented differently should have different dimensions of louvers to be effective. A simple box type of louver  may be suitable on an eastern façade, a slightly more complicated vertical and horizontal louver system on the southern façade and egg crate type on the western façade. The northern façade receives only very early morning or late afternoon sunshine and hence no elaborate systems are needed and only rain shade is sufficient. It is reported [11] that overhang with optimum dimensions can produce cooling load reduction of 12.7 per cent in summer without causing any sufficient change in sunshine hours received in winter. It is worth mentioning that an overshadowing of the windows must be avoided as it reduces availability of daylight indoors, which in turn results in increased consumption of energy for artificial lighting.

3.4 Shading from blinds and trees

West-facing rooms are especially prone to overheating because the low afternoon sun that penetrates deeper into rooms during the hottest part of the day. Methods of shading against low East and West sun are deciduous planting and vertical shutters or blinds. West-facing windows should be minimized or eliminated in passive solar design. Methods of shading against low East and West sun are deciduous planting and vertical shutters or blinds. West-facing windows should be minimized or eliminated in passive solar design.

3.5 Painting of roof

One of the very effective method of lowering the external surface temperature of the roof is to paint it with a coating which has minimum absorption for solar radiation and high emission for long wave radiations. Studies carried out in Delhi  [12] also showed that depending on the level of ventilation, air temperatures within the white coloured buildings were 4⁰ to 8⁰C lower than the dark buildings during mid summer conditions. Study at CSIR-CBRI of roofs treated with white glazed tiles showed a reduction of roof surface temperature by 25^oC, due to their high reflectivity of solar heat. Estimation  of reduction in energy consumed for cooling is

also a basis to assess the thermal performance of heat reflective coatings. A series of tests

carried out by Parker et al [13] in occupied houses in Florida revealed that the drop in consumption in air conditioning energy after the application of reflective roof coatings was around 19 per cent of the pre treated situation. In a similar study carried out by Akbari et al [14], it was found that changing the roof albedo from 0.18 to 0.73 would save about 23 to 80 per cent of cooling energy during the entire cooling season.  Studies carried out in CBRI [15] have also demonstrated considerable reduction in energy consumption in cold storage buildings by treating the roof and walls with white glazed tiles and heat reflective coatings.

3.6 Proper insulation of roof & walls and use of Phase change materials

In a climate that is cool at night and hot in the day, phase change materials can be  strategically placed inside the building structure to absorb latent heat of phase change to liquid in the day time and gradually give it off at night as the material changes its phase back to solid and gives off its latent heat of condensation which may be removed by radiation to the cool night sky if needed. The process thus acts as an insulation which slows the heating of the building when the sun is hot. Phase change materials can be designed to extract unwanted heat during the day, and release it at night.

4. Conclusion

In developing countries of the world situated in/near the tropical zone more than 80% population is forced to live and work in horrible thermal conditions. Thermally uncomfortable conditions are not only harmful to the physical and mental development but are a main cause of lower work output. The limited financial and energy resources of such countries cannot provide comfortable living and working environment to their people without the use of passive cooling techniques. The techniques can be integrated in the design of buildings in an effective and acceptable manner without hampering the aesthetics of the buildings. Natural cooling and cooling-load avoidance will not only help to conserve energy and help reduce the adverse environmental impacts of fossil-fuel use but also satisfy bioclimatic comfort requirements within the buildings. To promote the use of these technologies nationwide awareness, dissemination and training activities are required to be organized on priority. Sets of minimal meteorological data as well as information on thermal properties of indigenous building materials are required. The adaptation and improvement of traditional passive cooling and storage systems may offer useful solutions if adequate research and development effort is put into the study of the subject. In this scenario hybrid passive system emerges a viable option that may provide greater reliability and attract wider application of passive techniques in the design of buildings in hot climate.

 

5. Acknowledgement

The study forms a part of the research programme of Central Building   Research Institute, Roorkee and the paper is published with the kind   permission of the Director.          

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