Green Buildings-passive cooling techniques

Notes on ARC 306 GREEN BUILDINGS : Unit 3

Compiled by CT.Lakshmanan B.Arch., M.C.P. Page 1

UNIT-3 PASSIVE AND ACTIVE COOLING CONCEPTS

Passive Cooling techniques : General principles – Evaporative cooling, Nocturnal radiation cooling, Passive Dessicant cooling, induced ventilation, earth sheltering, Berming, Wind Towers, earth – Air tunnels, Curved Roofs & Air Vents, Insulation , etc. Active Cooling techniques : Air coolers.

Case studies on buildings designed with passive cooling techniques.

Natural Ventilation Natural ventilation, also called passive ventilation, uses natural outside air movement and pressure differences to both passively cool and ventilate a building. Natural ventilation is important because it can provide and move fresh air without fans. For warm and hot climates, it can help meet a building's cooling loads without using mechanical air conditioning systems. This can be a large fraction of a building's total energy use. Successful natural ventilation is determined by having high thermal comfort and adequate fresh air for the ventilated spaces, while having little or no energy use for active HVAC cooling and ventilation. When not to use natural ventilation Sites with high levels of acoustic noise, such as near heavy traffic zones, may be less suitable for natural ventilation because large openings in the building envelope can make it difficult to block outside noise. This can sometimes be solved by using acoustical ventilation louvers. Also, sites with poor air quality, such as adjacent to busy freeways, may also be less desirable for natural ventilation. Such sites may overcome poor outdoor air quality with filters and ducting, though this usually requires some mechanical fan systems. Cross Ventilation When placing ventilation openings, you are placing inlets and outlets to optimize the path air follows through the building. Windows or vents placed on opposite sides of the building give natural breezes a pathway through the structure. This is called cross-ventilation. Cross-ventilation is generally the most effective form of wind ventilation.



Stack Effect Air moves through a structure in response to pressure differences generated by either the thermal buoyancy (stack effect) or wind. Buoyancy pressures are generated by air warmer than its surroundings as the warmer air is of lower density than the cooler air. The pressure generated is dependent upon the average temperature difference between inside and outside and the height of the 'stack' or column of warmer air. Where there are openings at the top and bottom of the stack, the cooler heavier air will enter the lower openings and displace the warmer lighter air at the top. This is known as 'displacement ventilation', and if there is a source of heat which maintains the stack, the flow will continue. It is important to note that in these conditions air temperatures low down will be close to outdoor temperatures and those higher up will be warmer. Traditionally, this concept was used very commonly by having high ceilings in conjunction with ventilators and low level openings, courtyards and atria.

Courtyards The central courtyard of a typical traditional house was a very common passive solar device and was often considered the lung of the house. The success of the cooling principle of courtyards depends on a combination of climate, building shape and wall materials and varied across the region depending on location, size and affluence. In densely populated urban areas, the central courtyard of large houses was used for cooling by trapping cool nighttime air, which sank into the shaded bottom of the courtyard. The principle of using a central courtyard as a means for keeping adjacent rooms cool was based on the pressure difference be-tween cool air and warm air. Cool night time air from high above the courtyard, heavier than warm air, sank into the courtyard and replaced the slightly warmer air that had accumulated there during the day and which was unable to escape because the ambient daytime air temperature above the courtyard was warmer. In order for the cool air to stay in the base of the yard, the external perimeter walls of surround-ing rooms were built of materials, which provided insulation from outside heat because of their thermal mass, and with few external openings that were kept closed during the hot season. This was important as a breeze blowing through the rooms into the courtyard would replace the cool air. This same thermal mass in walls next to the courtyard also conserved the ambient coolness of the yard and adjacent rooms, but it was the volume of air in the yard and the rooms, which acted as the main 'reservoir' of coolness.



In order to achieve the cooling effect, the courtyard was kept sufficiently deep compared to the height of surrounding walls to ensure that the base of the courtyard remained in the shade for most, if not all, of the day. If the courtyard was either too large, or the walls too low, the cool air 'sink' effect was not achieved. In larger houses, the same use of pressure differences between cool and warm air were used with twin courtyards. In the sunny large courtyard hot air raised to be replaced by cooler air drawn from the adjacent shaded small courtyard. In the night, cooler air from high up above the build-ing was drawn down into the small courtyard. The space between the two courtyards had a con-stant breeze and was used as the living area in the house. In densely laid out cluster housing, courtyards were used to decrease solar gain through building aggregation and through shading. The courtyard buildings were organized around central outdoor spaces that were joined at their external perimeter minimizing the overall building area exposed to the Sun. The space of the courtyard was shaded by overhead canopies such as trellises, or by design proportions of the courtyard. Courtyards that were taller than their width generally ex-cluded sunlight on the ground plane for most of the day. By decreasing solar gain, the courtyard was made comfortable and habitable throughout the day and night. In conjunction to this, massive building materials such as brick and stone with large heat capaci-ties were used to build the walls and roofs of the spaces surrounding the courtyard. They pro-vided thermal lag, i.e., the materials absorbed heat during the day and radiated heat to the cooler sky at night diminishing the overall diurnal temperature swing and providing cooling during the day. In rural areas with more space around the houses, the guiding principle in Vaastu Shastra was more prevalent and effected cross-ventilation through a series of openings from the entrance door, through a central courtyard and out of an opening at the rear. This was the 'air funnel' of the house. The entrance lobby acted as a wind funnel focusing the incident wind into the court-yard that lay on this air funnel, which in turn ventilated the living areas grouped around it. The courtyard was the main source of air exchange from inside the built space to the outer free space. In some houses, water bodies were placed in the courtyards to cool and humidify the incoming air.



Verandahs The main function of verandahs found in vernacular houses in Hyderabad was to prevent the di-rect heat and glare of the sun from entering the house. The external verandas were provided with seats and head rests to form cool and comfortable places for informal gatherings. Similar internal walkways adjoining the courtyards kept the heat out and provided for air movement to the inte-rior built spaces. Solar shading Among all other solar passive cooling techniques solar shading is relevant to thermal cooling of buildings especially in a developing country owing to their cost effectiveness and easy to implement. Rural India and developing countries in Middle-east region has witnessed a steep rise masonry houses with RCC roofs. However the availability of electric power in the villages especially during summer is limited. These RCC roofs tend to make the indoor temperature very high around 41°C: This is due to high roof top temperature of around 65°C in arid regions. Solar shading with locally available materials like terracotta tiles, hay, inverted earthen pots, date palm branches etc. can reduce this temperature significantly. Shading with tree reduces ambient temperature near outer wall by 2°C to 2.5°C. On an average a depression of six degree centigrade in room temperature has been observed when solar shading techniques are adopted. Kumar, Garg and Kaushik evaluated the performance of solar passive cooling techniques such as solar shading, insulation of building components and air exchange rate. In their study they found that a decrease in the indoor temperature by about 2.5°C to 4.5°C is noticed for solar shading. Results modified with insulation and controlled air exchange rate showed a further decrease of 4.4°C to 6.8°C in room temperature. The analysis suggested that solar shading is quite useful to development of passive cooling system to maintain indoor room air temperature lower than the conventional building without shade Shading by overhangs, louvers and awnings etc. Well-designed sun control and shading devices, either as parts of a building or separately placed from a building facade, can dramatically reduce building peak heat gain and cooling requirements and improve the natural lighting quality of building interiors. The design of effective shading devices will depend on the solar orientation of a particular building facade. For example, simple fixed overhangs are very effective at shading south-facing windows in the summer when sun angles are high.



Shading of roof Shading the roof is a very important method of reducing heat gain. Roofs can be shaded by providing roof cover of concrete or plants or canvas or earthen pots etc. Shading provided by external means should not interfere with night-time cooling. A cover over the roof, made of concrete or galvanized iron sheets, provides protection from direct radiation. Disadvantage of this system is that it does not permit escaping of heat to the sky at night-time

A cover of deciduous plants and creepers is a better alternative. Evaporation from the leaf surfaces brings down the temperature of the roof to a level than that of the daytime air temperature. At night, it is even lower than the sky temperature

Covering of the entire surface area with the closely packed inverted earthen pots, as was being done in traditional buildings, increases the surface area for radiative emission. Insulating cover over the roof impedes heat flow into the building. However, it renders the roof unusable and maintenance difficult. Broken china mosaic or ceramic tiles can also be used as top most layer in roof for reflection of incident radiation.



Another inexpensive and effective device is a removable canvas cover mounted close to the roof. During daytime it prevents entry of heat and its removal at night, radiative cooling. Fig. shows the working principle of removable roof shades. Painting of the canvas white minimizes the radiative and conductive heat gain

Shading by trees and vegetation Proper Landscaping can be one of the important factors for energy conservation in buildings. Vegetation and trees in particular, very effectively shade and reduce heat gain. Trees can be used with advantage to shade roof, walls and windows. Shading and evapotranspiration (the process by which a plant actively release water vapor) from trees can reduce surrounding air temperatures as much as 5°C. Different types of plants (trees, shrubs, vines) can be selected on the basis of their growth habit (tall, low, dense, light permeable) to provide the desired degree of shading for various window orientations and situations. The following points should be considered for summer shading 1. Deciduous trees and shrubs provide summer shade yet allow winter access. The best locations for deciduous trees are on the south and southwest side of the building. When these trees drop their leaves in the winter, sunlight can reach inside to heat the interiors.



2. Trees with heavy foliage are very effective in obstructing the sun’s rays and casting a dense shadow. Dense shade is cooler than filtered sunlight. High branching canopy trees can be used to shade the roof, walls and windows. 3. Evergreen trees on the south and west sides afford the best protection from the setting summer sun and cold winter winds. 4. Vertical shading is best for east and west walls and windows in summer, to protect from intense sun at low angles, e.g. screening by dense shrubs, trees, deciduous vines supported on a frame, shrubs used in combination with trees. 5. Shading and insulation for walls can be provided by plants that adhere to the wall, such as English ivy, or by plants supported by the wall, such as jasmine. 6. Horizontal shading is best for south-facing windows, e.g. deciduous vines (which lose foliage in the winter) such as ornamental grape or wisteria can be grown over a pergola for summer shading. Shading by textured surfaces Surface shading can be provided as an integral part of the building element also. Highly textured walls have a portion of their surface in shade as shown in Figure 5. The increased surface area of such a wall results in an increased outer surface coefficient, which permits the sunlit surface to stay cooler as well as to cool down faster at night

Wing Walls

Wing walls project outward next to a window, so that even a slight breeze against the wall creates a high pressure zone on one side and low on the other. The pressure differential draws outdoor air in through one open window and out the adjacent one. Wing walls are especially effective on sites with low outdoor air velocity and variable wind directions.



Different wing walls of better and worse effectiveness, on same wall and adjacent walls.











THE MICROCLIMATE If the best offense is a good defense, the best passive cooling strategy pays as much attention to the

microclimate surrounding a building as to the building itself. Landscaping and vegetation can have a tremendous impact on natural comfort inside a building,

affecting both summer cooling and winter heating loads, both of which should be considered in the physical design of a site.

Massed earth is particulary effective as a windbreak. Earth berming can be used to deflect winter winds, to channel summer breezes into interior spaces,

and to shape the air circulation pattern around a building. Vegetation is an asset of greater worth. Evergreens make excellent windbreaks, and deciduous plants,

shedding their leaves to allow the penetration of solar radiation in winter, in summer become complex cooling mechanisms.

Deciduous leaves reflect infrared, heat-bearing energy and filter cool, green light to the ground. Combined with air movement, the evaporation of water into the air that occurs in plant transpiration has

an evaporative cooling influence. The general effect in an area of massed vegetation is to keep temperatures in the shade a good 10-

15°F lower than ambient—a particularly valuable phenomenon if that cooler air can be sent into the building.



EVAPORATIVE COOLING Evaporative cooling is a passive cooling technique in which outdoor air is cooled by evaporating water before it is introduced in the building. Its physical principle lies in the fact that the sensible heat of air is used to evaporate water, thus cooling the air, which in turn cools the living space in the building. Evaporation occurs at the water-air interface. An increase in the proportion of the contact area between water and air enhances the rate of evaporation and thereby the potential for cooling. The presence of a waterbody such as a pond, lake or sea near the building, or a fountain in the courtyard can provide a cooling effect. Cisterns or wetted surfaces can also be placed in the incoming ventilation stream. Such direct systems typically use little or no auxiliary power, are simple and can avoid the need for large surfaces of water and movement of large volumes of air. They are, therefore, particularly suited to hot and dry regions. The airflow in these systems can be induced mechanically or passively – for example, evaporative cooling towers that humidify the ambient air can be used. This is direct evaporative cooling. The main disadvantage of direct systems is in the increased moisture content of the ventilation air supplied to the indoor spaces. High evaporation may result in discomfort due to high humidity. However, passive evaporative cooling can also be indirect − the roof can be cooled with a pond, wetted pads or spray, and the ceiling transformed into a cooling element that cools the space below by convection and radiation without raising the indoor humidity. The efficiency of the evaporation process depends on the temperatures of the air and water, the vapour content of the air, and the rate of airflow past the water surface. The provision of shading and the supply of cool, dry air will enhance evaporation. A comprehensive discussion on evaporation has been reported by Bansal et al. The most commonly used evaporative cooling system in north India is the desert cooler consisting of water, evaporative pads, a fan and a pump. It is a hybrid type of direct evaporative cooling system. Watt has proposed the following guidelines for using evaporative cooling: Direct evaporative coolers should have an average saturation efficiency of 70% or more, and the

cooled air should enter the indoor space without any additional heat gain. The maximum indoor air velocity induced by the cooled air must be 1 m/s. The room temperature should be reduced by at least 3°C before the cool air is discharged out of the

room. The temperature of the cooled space should be about 4 °C below the outdoor dry bulb temperature.

This is necessary to counteract the incoming radiant heat. Passive Downdraft Evaporative Cooling (PDEC) Evaporative cooling was extensively used in the vernacular architecture of Pakistan, Iran, Turkey and Egypt. Wind catchers called 'malqafs' captured wind and directed it over porous water pots, thus cooling the air as a result of latent heat of vaporisation. This system maintained a balance between two important parameters of passive cooling – thermal performance and ventilation effectiveness. Contemporary passive downdraft evaporative cooling systems consist of a downdraft tower with wetted cellulose pads at the top of the tower. Water is distributed on the top of the pads, collected at the bottom into a sump and recirculated by a pump. Certain designs exclude the re-circulation pump and use the pressure in the supply water line to periodically surge water over the pads, eliminating the requirement for



any electrical energy input. In some designs, water is sprayed using micronisers or nozzles in place of pads, in others, water is made to drip. Thus, the towers are equipped with evaporative cooling devices at the top to provide cool air by gravity flow. These towers are often described as reverse chimneys. While the column of warm air rises in a chimney, in this case the column of cool air falls. The air flow rate depends on the efficiency of the evaporative cooling device, tower height and cross section, as well as the resistance to air flow in the cooling device, tower and structure (if any) into which it discharges. This system depends on two basic factors that determine its effectiveness: (1) amount of cooling of the ambient air achieved, and (2) the rate at which this conditioned ambient air replaces the stale air within the building. The former can be easily achieved by increased airwater contact zone. This factor usually dictates the height of the tower and in turn, influences the massing of the building design. The second factor, however, requires a complex interplay of different variables to achieve an effective performance. These variables dictate the configuration of the tower termination, the positioning of multiple towers within the building, the circulation pattern within the building, and even the configuration of openings between adjacent spaces served by these towers. The temperature of the incoming ambient air drops while crossing the pads. Therefore, the height of the tower and the area of the wetted pads are not expected to have any appreciable effect on the temperature of the air in the tower in a given combination of ambient dry and wet bulb temperatures. However, these two system design factors affect the airflow rate, and hence the total cooling effect generated by the system. Performance Analysis PDEC systems have been used with various types of cooling devices such as, spray devices (pressure and ultrasonic nozzles), aspen fibre pads, corrugated cellulose pads, etc. The performance analysis would thus vary depending on the evaporating cooling facilities provided in the tower. Aspen pads cause a high pressure drop relative to sprays and corrugated media, but they are low in cost. Spray devices may require efficient mist eliminators for removing fine droplets from the air because mist impedes air flow.

NOCTURNAL COOLING Buildings may be cooled indirectly by ventilating at night, if the ambient air is cooler than the room air. This cools the interior mass of the building and on the following day, the cooled mass reduces the rate of indoor temperature rise and thus provides a cooling effect. This strategy is termed as nocturnal ventilative cooling. Because buildings are usually occupied during the day, nocturnal ventilative cooling can be effective only to the extent that it can lower indoor temperatures the following day. It is particularly important that the indoor maximum temperature the next day be lowered. Such lower indoor temperatures can be achieved only if the building is kept closed and unventilated during the daytime hours. In this respect, daytime comfort ventilation and nocturnal cooling are mutually exclusive. At any given place on any given day, one or the other should be considered as the best approach to provide daytime comfort. There are several design options to provide the thermal mass that will serve as the nocturnal cold storage: structural mass of the building such as walls, partitions, floors, etc., cooled by whole space ventilation embedded air spaces (passages) within floors, ceilings and/ or walls through which outdoor air is

circulated



specialised storage such as a rock bed or a water tank with embedded air tubes, cooled at night by outdoor air

The applicability of nocturnal ventilation cooling is limited to a certain range of conditions. Limitations on applicability are posed on the one hand by climatic conditions, and on the other by the comfort and functional needs of occupants. These limitations particularly affect the decision to leave windows open throughout the night to obtain effective nocturnal ventilation cooling.

During the day, thermal mass soaks up heat; at night it is cooled by outside air

NOCTURNAL RADIATION COOLING Nocturnal radiation cooling refers to cooling by exposure of any element of the external envelope of the building to a cool night sky. Warm objects directly exposed to the sky radiate their heat out to it at night. Heat loss occurs by emission of long wavelength radiation, and hence surfaces should ideally have high emissivity. The presence of clouds at night limit the amount of heat that can be radiated to the outer space, but on a clear night, the effective sky temperature can be significantly lower than the ambient air temperature. The heat accumulated during the day is lost by radiation to the cool night air, thereby cooling the envelope. The envelope thus acts as cold storage during the day, drawing the heat away from the living space. The method works efficiently in arid climates, where ambient temperatures in the night are significantly lower than the day temperatures. Nocturnal radiation cooling works without consuming any water, unlike evaporative cooling systems. Its operation is illustrated in Fig. The roof, being the part exposed to the sky, is the most effective long wave radiator. The rate of heat exchange depends on the temperature difference between the emitting surface and the surrounding



atmosphere. Regions with large diurnal temperature variations will have higher nocturnal radiation cooling. Vapour pressure and the presence of clouds in the sky also affect the heat exchange. For effective radiant cooling, the thermal link between the emitting surface and the living space has to be good. Otherwise, the cooling resulting from radiation exchange will only serve to cool the ambient air, rather than the living space. The roof pond is an example of the concept of nocturnal radiation cooling . In this system, a mass of water is stored on the roof of the building. During summer days, the pond is protected and insulated by an external, movable and reflective insulation. The insulation prevents solar radiation from reaching the water mass and keeps it cool. The cool water then absorbs heat from the rooms below and cools the indoor air. At night, the insulation is removed and the water cools by convection and radiation. The effectiveness of the roof pond may be gauged from the fact that an indoor temperature of 21oC can be maintained when the outside temperature is as high as 35oC.

Fig. 3.30 Nocturnal radiation cooling: working principle In winter, the panel positions are reversed. During the day, the insulation is removed so that heat is absorbed by water for heating the interior. At night, the insulation cover reduces the heat loss. The effectiveness of the roof pond in winter is no less than that in summer: the indoor temperature can be maintained at about 21ºC while the outside is as low as –1.1oC [10]. The principles involved in this technique are schematically represented in Fig.



Working principle of a roof pond Water in transparent bags or in metal / fibreglass tanks is kept on the roof, the depth ranging from 150 to 300 mm. The top of the container/bag must be transparent to solar radiation whereas its bottom (inside surface) should be of a dark colour. If both sides of the container are transparent, then the top surface of the roof needs to be blackened for absorbing solar radiation. A clear top and black bottom helps in minimising temperature stratification in the pond water. Otherwise, hot water at the top would lose its heat to the exterior, and the cold water at the bottom would inhibit the heat transfer to the interior of the building. The movable insulation is usually of 50 mm thick polyurethane foam, reinforced with fiberglass strands and sandwiched between aluminium skins. The water-proofing layer of the roof should not inhibit the heat transfer from the pond to the interior [10]. The details of a roof pond are shown in Fig. 3.32. Radiation is responsible for the thermal interaction between the roof and the living space. Therefore, the ceiling of the room must not be very high, as the intensity of the radiation reduces with height or distance. This technique is effective for one or two storeyed buildings.



Detail of a roof pond Variations may be achieved by altering the ratios of heat transfer surfaces to thermal mass. The larger the storage volume, the greater and longer the heat storage. Smaller containers provide greater heat exchange as the surface area increases, resulting in faster distribution. During winter, a transparent cover may be provided over the water bags, leaving a gap. Air is blown through these gaps, forming an insulation cover to reduce heat loss. During summer the gaps are flooded with water and the transparent cover is removed. Another way of using nocturnal radiation cooling is to expose lightweight radiators to the night sky. Through these radiators, a fluid is circulated which gets cooled. The cooled fluid can be used to cool a thermal storage system at night. The cold storage can be used the following day for space cooling. Radiative cooling is effective in the hot and dry climate where nights are clear. In the case of humid atmosphere, condensation may occur on the radiating surface due to a decrease in its temperature to below dew point. The condensate transfers the latent heat of vaporization to the surface, keeping it warm. As a result cooling is not achieved.

DESICCANT COOLING Desiccant cooling is effective in warm and humid climates. Natural cooling of human body through sweating does not occur in highly humid conditions. Therefore, a person’s tolerance to high temperature is reduced and it becomes desirable to decrease the humidity level. In the desiccant cooling method, desiccant salts or mechanical dehumidifiers are used to reduce humidity in the atmosphere. Materials having high affinity for water are used for dehumidification. Air from the outside enters the unit containing desiccants and is dried



adiabatically before entering the living space. The desiccants are regenerated by solar energy. Sometimes, desiccant cooling is employed in conjunction with evaporative cooling, which adjusts the temperature of air to the required comfort level. Desiccant materials have a low vapor pressure at their surface, which help attracting moisture from the air because the pressure exerted by the water in the air is higher. The pressure gradient causes water molecules to move from the air to the surface of the desiccant at a reduced vapor pressure, resulting in dehumidification of the air. Note that desiccant materials do not cool the air to condense its moisture, but rather shifts the latent cooling load to a sensible cooling load. Desiccants can be either solids or liquids. Their surface vapor pressure is a function of their temperature and moisture content. They are further classified as two types; 1) Adsorbent desiccant and 2) Absorbent desiccant. Adsorbent desiccants collect moisture like a sponge and are usually solid materials. Water is collected on the surface and in the crevices of the material. Absorbent desiccants undergo a chemical or physical change as they collect moisture and are usually liquids or solids that change phase to liquids as they absorb moisture. Desiccant Cooling Equipment The most common desiccant cooling system is a combination of rotary air-to-air heat exchanger (desiccant wheel) with inner surfaces covered with a solid desiccant, e.g., silica gel, a regeneration heater (solar, gas-fired, electrical or water-heated) for removing adsorbed water from the desiccant and evaporative cooling system. Components needed to build a solid desiccant cooling system are: 1. Desiccant rotor 2. Heating coil 3. Energy recovery system 4. Two evaporative humidifiers These components are installed into any traditional air handling unit in a certain relation to each other. By controlling the components in sequences the system can provide cooling of supply air in summer operation. Concept Description Refer figure below to understand the process in steps:



Node 1 to 2: The desiccant cooling process begins when the air (1) is dehumidified by a rotating desiccant wheel. During dehumidification, the dry bulb temperature of the air rises because the removal of moisture results in a release of heat. The desiccant vapor pressure approaches that of the surrounding air and is no longer capable of acquiring moisture. The desiccant is then regenerated in the process of desorption by removing it from the moist air stream and heating it. Node 2 to 3: The dry-bulb temperature is then reduced by a heat recovery system without altering moisture content Node 3 to 4: An evaporative humidifier increases the moisture content of the air in proportion to reduction in dry bulb temperature (state condition 4). Node 4 to 5: The coil on the supply stream is in operation only for heating conditions. Not applicable for cooling conditions. Node 5 to 6: The fan delivers air to the space. In process the supply air temperature is increased a little owing to fan heat. Node 6 to 7: Air is supplied to the space at condition 6 and is returned back at thermal condition 7 for heat recovery. Node 7 to 8: Return air is generally passed through another evaporative humidifier where the return air temperature is reduced and moisture content increased close to saturation (8). This guarantees the maximum potential for indirect cooling of the supply air stream through the heat exchanger for heat recovery. Node 8 to 9: This colder air is transferred via the sensible heat recovery wheel from (8) to (9), subsequently leads to an increase in the temperature of the air, which is the use as regeneration air. Node 9 to 10: This return air is subsequently reheated in the coil until it reaches state (10). Node 10 to 11: The temperature of the latter is elevated to a level sufficient to reactivate the desiccants in the desiccant wheel (10 to 11). The use of heat for the regeneration process is the highest energy requirement of the system. The elevated temperature of the return air duct removes moisture from the desiccants and the wheel will then absorb moisture again. Typical desiccant reactivation temperatures required in the return air duct are 70-80°C and air will be exhausted from the system at typically 40-50°C. A proportion of return air may bypass the heater and desiccant wheel in order to minimise energy consumption.



Liquid Desiccants The desiccant system described previously uses solid desiccants i.e. silica gel or lithium chloride; liquid desiccant systems use a liquid spray of desiccant solution such as lithium bromide. The development of liquid desiccant systems compared to solid desiccant systems is still in its infancy although it is claimed that liquid desiccant systems have the potential advantage of better efficiency as dehumidification and heat transfer take place simultaneously rather than sequentially. Types of Desiccant Materials Types of materials used as a basis for desiccant systems include the following materials: • Silica Gel • Lithium Chloride (Liquid or Dry) • Lithium Bromide • Activated Alumina • Titanium Silicate Commercially available desiccant systems are based on five configurations or technologies. • Liquid Spray Towers • Solid Packed Tower • Rotating Horizontal Bed • Multiple Vertical Bed • Rotating Desiccant Wheel INDUCED VENTILATION Passive cooling by induced ventilation can be very effective in hot and humid climates as well as hot and dry climates. This method involves the heating of air in a restricted area through solar radiation, thus creating a temperature difference and causing air movements. The draft causes hot air to rise and escape to the ambient, drawing in cooler air and thereby causing cooling. In effect, a solar chimney is created to cause continuous air circulation. Figure illustrates the principle of induced ventilation and some of its variations. Variations and controls: Arrangements may be made to draw air from the coolest part of the structure as replacement, to set up a continuous circulation and cool the living spaces. Curved roofs and vents are used in combination for passive cooling of air in hot and dry climates, where dusty winds make wind towers impracticable. The system works on the principle of cooling by induced ventilation, caused by pressure differences. This principle is illustrated in Fig. Wind flowing over a curved surface creates a pressure difference across it. If vents are provided on the surface, air is sucked out of the structure through the openings. Therefore, the hot internal air forces its way out through the vents inducing air-circulation. Air vents are usually placed above living rooms.



Induced ventilation: principle and variations

Air vents Curved roofs and air vents are used in combination for passive cooling of air in hot and dry climates, where dusty winds make wind towers impracticable. Suited for single units, they work well in hot and dry and warm and humid climates. A hole in the apex of the domed or cylindrical roof with the protective cap over the vent directs the wind across it. The opening at the top provides ventilation and provides an escape path for hot air collected at top. Arrangements may be made to draw air from the coolest part of the structure as replacement, to set up a continuous circulation and cool the living spaces. The system works on the principle of cooling by induced ventilation, caused by pressure differences.



Section showing induced ventilation through curved roof and air vents

The cooling effect can be enhanced by providing evaporative cooling. A pool of water is usually kept on the floor directly below the vents so that the air flowing into the room gets cooled, in turn cooling the living space. The air vents are usually provided with protective caps which help to direct the winds across them.

EARTH-AIR PIPE SYSTEM The earth-air pipe system consists of a pipe of appropriate dimensions buried at a depth of about 4 to 5m in the ground. Ambient air is blown through it by a blower at one end of the pipe. The other end is connected to the building to which it supplies conditioned air. Figure shows the schematic of such a system. As explained earlier, the temperature at a depth of about 4 to 5m is very stable and is equal to the annual average ambient temperature. It remains unaffected even if heat is withdrawn from or supplied to the ground, due to its large thermal capacity. The earth-air pipe system takes advantages of this fact. Ambient air flowing through the pipe gets cooled (in summer) or heated up (in winter) before entering the living space of a building. If the pipe is of adequate length (for a given air flow rate), the desired heating or cooling effect can be realised. To meet the thermal load requirement of the building, one may use more than one pipe buried at the same depth a few metres apart. However, it is possible for the relative humidity of the air from the earth-air pipe system to be higher than the ambient humidity, depending on the soil conditions. If the air fed to living spaces is not reused, the system is called single pass system. The earth-air pipe system can also be used in the re-circulation mode. In that case air from the living space is re-circulated through earth-air pipe and is supplied back to the living space. By using an earth-air pipe system, energy and peak load requirements for space conditioning of a building can be significantly reduced. This would lead to energy conservation. The use of such systems has gained increasing acceptance during the last few years, and a number of them are being installed in India, China, USA and Europe.



Earth-air pipe system: working principle Performance Analysis The performance of the earth-air pipe system depends on the rate of heat transfer between the air and surrounding earth, which in turn is governed by the resistances offered by: (i) the convection between air and inner surface of the pipe, (ii) conduction through the thickness of the pipe wall, and (iii) conduction through the surrounding earth. Thus, the performance of the earth-air pipe system depends on : system parameters (depth of the pipe from the earth's surface, its length and radius, thermal

conductivity of the pipe material, and air speed through the pipe) soil parameters surrounding the pipe (thermal conductivity, specific heat, density and moisture content) weather conditions (solar radiation, ambient temperature) earth’s surface conditions (shaded, blackened, white-painted or wetted with water) The longest resistance to the heat flow is due to the soil surrounding the pipe, and it is the main factor in controlling the rate of cooling/heating of the air in the pipe. Soil having higher thermal conductivity is desirable, and so wet soil is more effective in heat transfer. EARTH COUPLING This technique is used for both, passive cooling as well as heating of buildings, a feat which is made possible by the earth acting as a massive heat sink. The temperature of the earth's surface is controlled by the ambient conditions. However, the daily as well as seasonal variations of the temperature reduce rapidly with increasing depth from the earth's surface. At depths beyond 4 to 5m, both daily and seasonal fluctuations die out and the soil temperature remains almost stable throughout the year. It is equal to the annual average ambient air temperature at that place. The temperature of the soil at depths beyond 4 to 5m can however be modified by suitable treatment of the earth's surface. For increasing the temperature, the earth's surface can be blackened/ glazed, and for



decreasing it's value the surface can be shaded, painted white, wetted with water spray or can have thick vegetation. Thus, the underground or partially sunk buildings would provide both cooling (in summer) and heating (in winter) to the living space. Besides, load fluctuations are reduced by the addition of earth mass to the thermal mass of the building. The infiltration of air from outside is reduced, and there is a decrease in noise and storm effects. An earth-sheltered structure has to be heavier and stronger to be able to withstand the load of the earth and the vegetation above. Besides, it should be suitably waterproofed and insulated to avoid ground moisture. For this, a high level of design and supervision in construction is required. A building may be coupled with the earth by burying it underground or berming. Figure shows an example of earth berming. Another possibility of utilising the ground effect is through earth-air pipe systems, and is discussed in the following section.

Working principle of earth-berming

WIND TOWER Wind tower is generally used in hot and dry climates for cooling purposes. The tower is meant to “catch” the wind at higher elevations and direct it into the living space. The air flow passages in the tower may have equal or different areas. The tower may have only one opening facing the wind, if wind is predominantly in one direction, or may have openings in all directions in locations with variable wind directions. Such systems have been used for centuries in West Asian countries for natural ventilation and passive cooling. A prerequisite for using a wind tower is that the site should experience winds with a fairly good and consistent speed. A wind tower operates in various ways according to the time of day and the presence or absence of



wind. The cardinal principle of its operation lies in changing the temperature and thereby the density of the air in and around the tower. The difference in density creates a draft, pulling air either upwards or downwards through the tower, shown schematically in Fig. Working: Night The tower area is so designed that the top part provides large heat storage capacity, and also has a large surface area for heat transfer. The tower walls and the internal walls of the air-flow passages absorb heat during the day and release it at night, warming the cool night air in the tower. Warm air moves up creating an upward draft and is exhausted through the openings. The pressure difference thus created pulls the cool night air through the doors and windows into the building. In the absence of wind, the tower acts as a chimney. The nocturnal radiation through the roof and the external walls brings about further cooling.

Working principle of a wind tower In the presence of wind, the cool night air enters the tower and forces itself down into the structure. Though it is warmed slightly during the process, sufficient cooling can be achieved due to forced circulation. Again, cooling due to nocturnal radiation adds to this process.



Detail of a wind tower Working : Day The hot ambient air coming in contact with the cool upper part of the tower gets cooled. It becomes cold and dense, and sinks through the tower and into the living spaces, replacing the hot air. In the presence of



wind, the air is cooled more effectively and flows faster down the tower and into the living area. It must be noted that the temperature of the tower soon reaches that of the ambient air and hence, in the absence of wind, the downward flow ceases, the tower then begins to act like a chimney. The operation of the tower depends greatly on the ambient fluctuations like the wind velocity, air temperature changes, etc. Variations and controls: Variations in wind tower design can be achieved by altering tower heights, cross section of the air passages, locations and number of openings, and the location of the wind tower with respect to the living space to be cooled. The variations are aimed at providing the desired air-flow rates, heat transfer area and storage capacity. Air flow through different parts of the buildings can be controlled by the doors and windows. Due to small storage capacity, the sensible cooling may stop after several hours of operation on hot summer days. In order to improve the efficiency of its operation, evaporative cooling may be introduced. The air flowing down the tower is first sensibly cooled, and then further cooled evaporatively. This can be achieved by providing a shower/spray or dripping of water at top of tower, or a fountain at the bottom. The reduction in the temperature of air can be as much as 10 – 15o C in arid climates. Wind towers can easily be incorporated in low-rise buildings. It may be noted that wind towers may need to be shut off when cooling is not required, and hence, such provisions may be included in the design. Due consideration must also be given to prevent the entry of dust, birds and insects.

Insulation The effect of insulation is to reduce heat gain and heat loss. The more insulation in a building exterior envelope, the less heat transferred into or out of the building due to temperature difference between the interior and exterior. Insulation also controls the interior mean radiant temperature (MRT) by isolating the interior surfaces from the influence of the exterior conditions, and also reduces draughts produced by temperature differences between walls and air. Insulation is of great value when a building requires mechanical heating or cooling and helps reduce the space-conditioning loads. Location of insulation and its optimum thickness are very important. In hot climates, insulation is placed on the outer face (facing exterior) of the wall or roof so that thermal mass of the wall is weakly coupled with the external source and strongly coupled with the interior. Use of 40 mm thick expanded polystyrene insulation on walls and vermiculite concrete insulation on the roof has brought down space-conditioning loads of the RETREAT building in Gurgaon by about 15% [8]. Air cavities within walls or an attic space in the roof ceiling combination reduce the solar heat gain factor, thereby reducing space-conditioning loads. The performance improves if the void is ventilated. Heat is transmitted through the air cavity by convection and radiation.



Advanced techniques in different climates

If cooling is the major requirement, the techniques listed under hot and dry climate may be adopted. In case of heating requirement, the techniques for cold climates may be used. Techniques such as roof pond, roof radiation trap, solar chimney, earth berming, etc. which find dual usage can also be incorporated. 1. Direct gain 7. Roof radiation trap 13. Earth berm 2. Trombe wall 8. Solarium 14. Wind tower 3. Water wall 9. Evaporative cooling 15. Earth-air tunnel 4. Solar chimney 10. Nocturnal radiation cooling 16. Curved roof / air vents 5. Transwall 11. Desiccant cooling 17. Cavity wall / insulation 6. Roof pond 12. Induced ventilation 18. Varytherm wall

19. Daylighting











MIXED MODE VENTILATION Some buildings may not be suitable for pure natural ventilation due to their depth or other constraints, but may make use of mixed mode with mechanical ventilation to supplement the natural ventilation. The main benefit of some augmentation by mechanical systems is that there is less unpredictability with indoor environment conditions, though it will result in greater energy use. "Mechanical" or "forced" ventilation use fans or blowers to provide ventilation and air movement in the absence of breezes. Mechanical ventilation systems provide closer control of the area than natural ventilation systems and are less susceptible to sudden changes in ambient weather conditions. It is predictable in its operation and the airflows are controlled. The fans can be arranged in extract only, supply only and supply and extract modes. Evaporative cooling, Nocturnal radiation cooling, Passive Dessicant cooling, induced ventilation, earth sheltering, Berming, Wind Towers, earth – Air tunnels, Curved Roofs & Air Vents, Insulation

Mechanical Air Supply with Natural Air Exhaust Here the air supply is controlled mechanically and the air exhaust takes place on a natural way by ventilation openings, windows or shafts. The arrangement shall result in positive pressurization of building with ventilator controls adjusting the air supply. An air filter can be used to ensure clean incoming air. Natural air supply with mechanical air exhaust This is a popular form of ventilation in residential buildings and offices. The mechanical air exhaust system creates negative pressure in the building, making sure the air is sucked in naturally through the inlet openings. Mechanical Supply and Exhaust In this system, the supply air and the exhaust air are transported mechanically. This arrangement offers many advantages over other form of mechanical ventilation:



Good control of pressurization of building. Supplying more air than exhaust will result in positive pressurization of the building.

Good control of the ventilation capacity; no dependence of the outdoor weather conditions and despite possible noisy environment

The possibility of extracting heat from the exhaust air and use it to preheat the fresh air supply (heat recovery)

The possibility of preheating and pre-cooling of the air supply The possibility of humidify and dehumidify of the air supply The possibility of cleaning the air by an air filter or supplying the air from a relative clean site of the

building For proper functioning of the system the building has to be sufficiently airtight. EVAPORATIVE COOLERS Evaporative coolers utilize the natural process of water evaporation along with an air-moving system to create effective cooling. Fresh outside air is pulled through wetted filters that cool the air through water evaporation. A blower wheel then circulates the cool air throughout a room, home, or business. DESIGN APPROACHES There are two general methods of evaporative cooling: direct and indirect. Direct Evaporative Cooling: Direct evaporative cooling involves the movement of air past a water spray (air washer/ water spraying chamber) or other wetted medium (evaporative pads, rigid media or evaporating wheel). The air in process gets cooled due to evaporation of water and is allowed to move directly to where it is needed.

Direct evaporative cooling is represented on the psychrometric chart by a displacement along a constant wet-bulb temperature line AB:

Psychrometric chart of Direct Evaporative Cooling



In process, the dry-bulb temperature of air is reduced, both relative humidity and absolute humidity is increases while the wet-bulb temperature and the enthalpy are not changed. Since the cooled and humid air is directed into the building, the indoor air is generally not re-circulated through the evaporative cooler. Therefore, a very high rate of air flow is necessary, about 15 to 30 air changes per hour. Indirect Evaporative Cooling: The high level of humidity that is produced by direct evaporative cooling may be undesirable for some applications. Indirect evaporative cooling systems attempt to solve this problem with the help of a secondary heat exchanger. The cooled air leaving the direct evaporative cooler is made to exchange heat with the outside supply air in the secondary heat exchanger. In process, the supply air temperature decreases but its water content remains constant.

Since the humidity content of the cooled air does not rise, indirect evaporation cooling is represented on the psychrometric chart by a displacement along a constant humidity ratio-line CD (as cooling at any other cool surface).

Psychrometric chart of Indirect Evaporative Cooling

Indirect evaporative cooling reduces both the dry-bulb and the wet-bulb temperature of the primary air while the humidity ratio i.e. the water content of the cooled inlet air remains unchanged (unlike in the direct evaporative cooling which adds humidity to the airstream). It must be noted that, since the air temperature drops, its relative humidity will still increase. The cooling process is represented by the line C to D.

Combined direct/indirect evaporating cooling process

The two-stage evaporative cooler combines an indirect cooler in the first stage and a direct cooler in the second stage. The psychrometric chart below shows the principle function of this system: Air to be cooled,



initially at point A, is sensibly cooled by an indirect evaporative process until it reaches point B. Since the water content of the air

has not changed, line AB is parallel to the dry-bulb temperature axis. Then the air enters the second stage, where it becomes cooled down by a direct evaporative process to point C. Since this is a constant wet-bulb temperature process, line BC is parallel to the wet-bulb temperature lines.

Line AB represents the indirect cooling in the first stage, whereas line BC represents the direct cooling in the second stage. Two stage indirect/direct evaporative cooling results in reducing both the dry bulb and wet bulb temperatures



ROOF SURFACE EVAPORATIVE COOLING (RSEC) - SOLAR ENERGY CENTRE, GURGAON Administration block of Solar Energy Centre, Gurgaon Location : Gurgaon, Haryana. Climate : Composite (predominantly hot) Brief description of building : It is a single storeyed research centre. The buildings include a guest house, a workshop, offices and laboratories. Being situated on a large open plot of land, the buildings are spread out and possess courtyards around which the various activities are clustered. A plan and section of the administrative block of the same is given in Fig. Energy conscious features: Roof surface evaporative cooling system Appropriate planning in which laboratories requiring air conditioning are put together in a well-insulated building Hollow concrete block walls to resist heat gain by conduction Reflective finish on roof surface Windows designed for cross ventilation and daylighting. The east and west facing windows incorporate

openable louvered shutters



Performance of the building: The Solar Energy Centre conducted a post-occupancy evaluation of this building. It was observed that the roof surface evaporative cooling (RSEC) system caused a lowering of temperature by 2-3ºC in comparison with rooms without RSEC system. Figure shows the comparison of measured temperatures of the reception room (with RSEC system) with those of A.O. room and verandah, both being without RSEC system. One advantage of the RSEC system is that it cools in a healthy manner as it does not humidify the ambient air of the room. On the other hand if a desert cooler were to be used, it would pump moist air inside the room and increase the humidity, which would cause discomfort and affect the health of occupants.

EVAPORATIVE POND Outdoor breezes create air movement through the house interior by the ‘push-pull’ effect of positive air pressure on the windward side and negative pressure (suction) on the leeward side. Good natural ventilation requires locating openings in opposite pressure zones. Also, designers often choose to enhance natural ventilation using tall spaces called stacks in buildings. With openings near the top of stacks, warm air can escape whereas cooler air enters the building from openings near the ground. Ventilation by creating stacks has been effectively used in the WBREDA office building in Calcutta. Located in a warm humid climate, induced ventilation was a primary design strategy for this building.



PASSIVE DOWNDRAFT EVAPORATIVE COOLING (PDEC) - TORRENT RESEARCH CENTRE (TRC) Project Period: 1994-99 Size: Built-up area is approximately 19700 m² Vernacular architecture in India is rich with examples of passive cooling systems. The TRC at Ahmadabad, Gujarat, India was completed and occupied in 1997, and has been reported as a unique example of climate responsive design integrating a Passive Downdraft Evaporative Cooling (PDEC) system, a modified adaptive vernacular technology. The complex comprises of pharmaceutical research facilities and related support services. Two of the five laboratory buildings are air conditioned, while the other three equipped with the PDEC system. The entire complex covers 22,600 square metres of floor space of which around 3,200 square metres is air-conditioned. The PDEC system is designed to operate under critical climatic conditions during hot-dry season when mid afternoon outside temperatures regularly reach 40°C or more. The design of the building facilitates generating an air draft, assuming still air conditions. The air heats up in the peripheral shafts, rises and escapes through the openings at the top. The air in this volume gets replaced from the usable spaces, which in turn receives its own replacement through the concourse area, on top of which the air inlets are located. The PDEC system pipes water through nozzles at a pressure of 50 Pa to produce a fine mist at the top of the three large air intake towers located above the central corridors of each laboratory building. Evaporation of the fine mist serves to cool the air which then descends slowly through the central corridor space via the openings on each side of the walkway.



At each level, sets of hopper windows designed to catch the descending flow can be used to divert some of this cooled air into the adjacent spaces. Having passed through the space, the air may then exit via high level glass louvered openings which connect directly to the perimeter exhaust air towers. During the warm humid monsoon season when the use of micro ioniser would be inappropriate, the ceiling fans can be brought into operation to provide additional air movement in the offices and laboratories. In the cooler season, the operating strategy is designed to control the ventilation, particularly at night, to minimize heat losses by the users adjusting the hopper windows and louvered openings in their individual spaces to suit their requirements. Advantages associated with Passive Design Some of the benefits of this approach are, 72% of the central building has achieved human comfort by using PDEC The building has been able to establish extremely low levels of energy consumption per square metre. Summer temperatures are maintained at 28°-32°C; 6 to 9 air changes/hour on different floors in summer The temperature fluctuations inside do not exceed 3°-4 °C, over 24 hour period, when outside

fluctuations are 14°-17° C. Air-conditioning plant capacity saved, is about 200 M. Tons. The annual savings in the electrical

consumption including the savings on account of less use of artificial lighting during the day is in excess of 100,000 dollars per annum.

Fig 2.30: Typical Cross section of the Torrent Research Centre with inlet shafts.



TERI –RETREAT BUILDING





The earth-air pipe system is used to cool about 120 m2 of floor area in the Dilwara Bagh house. Two rectangular pipes of cross sectional area of 0.6m x 0.8m, and a length of 60m are employed at a depth of 4m. A blower of 3 hp is used to force air into the system, and is housed in a blower room about 67.5m away from the house. It maintains an air velocity of about 6m/s in the pipe, which is made of brick and sand stone. A cross-section of the pipe and a sketch plan of the system are shown in Figs. 3.35 and 3.36. The outlets of the system to the rooms are protected by earth-berms. A cross-section of the same is shown



Extensive post occupancy evaluation studies have been carried out by Thanu et al. Figure 3.38 shows a typical performance of the system during summer and winter conditions. It is seen that in summer, the exit or delivery temperature is about 290C when outside can be as high as 380C. Further, the fluctuation in room temperature is only 2.20C as compared to 11.80C for outside air. In winters, the delivery temperature is maintained at about 200C, when outside air is about 80C − an increase in temperature by about 120C. Thus, the earth-air pipe system performs well both in summers as well as in winters. The system provides an average daily cooling potential of 242 kWh (thermal) in a summer month and about 365 kWh in a winter month. As the blower’s power is 3 hp (2.2 kW), the coefficient of performance (COP) of the system is 4.5 in summer and 6.8 in winter.



WIND TOWER - JODHPUR UNIVERSITY HOSTEL, JODHPUR

Jodhpur University Hostel is designed for a hot-dry climate. The main design constraints were a great water scarcity and strong desert winds.This building was put up as part of the research project undertaken by the Centre for Energy Studies, IIT, Delhi. Although energy conservation was stated as the objective, the design attempted to test and demonstrate suitable methods of providing thermal comfort in the hot, dry climate of Rajasthan. The air being very dry, evaporative cooling in summer can prove to be extremely effective in Jodhpur. The design, therefore, uses a favourable orientation, massive structure and air gap in the roof for insulation, reflective external finishes, deep sunshades and a wind tower for making use of the cool winds. An experimental evaporative cooling system using wires for water distribution, has also been installed on the windtower. The prevailing direction for cool winds in Jodhpur is the south-west. Windows apertures are difficult to provide in this orientation as it is least favourable as regards solar radiation. To overcome this, a wind tower concept was used. The tower faces the wind direction and is located over the staircase, thus minimizing costs. Cool air is provided to each room from this tower and normal windows or smaller shafts (towers) facing the lee of the wind have been provided to distribute the cool air throughout the building. The tower only catches the cool wind from the south-west avoiding warmer air from other directions. Walls are built of local light coloured stone. Large slabs of the same stone have been used for roofing, staircase, partitions and for lintels over windows. Roof insulation is provided by using small inverted terra-cotta pots over the stone slabs and filling up the intervening spaces with lime concrete. Since very few manufactured materials have been used, this is a low-embodied energy building.

Ground floor plan, first floor plan and section of the Hostel Block of Unviersity of Jodhpur



View of Hostel Block, University of Jodhpur

Green Buildings-passive cooling techniques

Environment