Passive cooling concepts. Building form and orientation

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1 Passive cooling concepts Some of the well-known concepts of passive cooling include manipulating building form and orientation shading vegetation insulation ventilation evaporative and nocturnal cooling Building form and orientation The optimum shape of a building is widely considered to be the one that loses the least amount of heat in winter and accepts the least amount of heat in summer. Olgyay (1963) dispelled the myth that the square form was the optimum for most climates. An elongated form oriented along the east west axis can perform better. The maximum amount of solar radiation is interrupted by the horizontal surface of the roof followed by east and west walls and then by the south and north wall during the summer for latitudes below the Tropic of Cancer in the Northern Hemisphere. It is, therefore, desirable that the building is oriented with the longest wall facing north and south, and short walls facing east and west. The intention is to minimize the wall area exposed to the intense morning and evening sun. It is observed that significant heat gains occur between 06:30 a.m. and a.m. on the east walls and between 2:00 p.m. and 5:30 p.m. on the west walls, when the sun is low in the sky. Consequently, windows on these walls are sources of high heat gain and should either be eliminated or reduced in size. The volume of a building is approximately related to its thermal capacity, while the exposed surface area is related to the rate at which it gains or loses heat. A summary of requirements for conditions of optimum heat gain and heat loss for a simple rectangular building form in different climates is presented in Table 7.7 (Santamouris and Asimakopoulos 1996). It is also advisable to place service spaces that are unconditioned on the east and west sides. Garages, staircases, toilets, and service shafts can act as buffer spaces to minimize the ingress of heat into the habitable areas of the building. 425

2 426 Renewable energy engineering and technology Table 7.7 Requirements of building form for different climate types Climate Elements and requirements Purpose Warm humid Minimize building depth For ventilation Minimize west-facing wall To reduce heat gain Maximize south and north walls To reduce heat gain Maximize surface area For night cooling Maximize window wall For ventilation Composite Controlled building depth For thermal capacity Minimize west wall To reduce heat gain Limited south wall For ventilation and some winter heating Medium area of window wall For controlled ventilation Hot dry Minimize south and west walls To reduce heat gain Minimize surface area To reduce heat gain and loss Maximize building depth To increase thermal capacity Minimize window wall To control ventilation heat gain and light Mediterranean Minimize west wall To reduce heat gain (summer) Moderate area of south wall To allow (winter) heat gain Moderate surface area To control heat gain Small to moderate window To reduce heat gain but allow winter light Cool temperate Minimize surface area To reduce heat loss Moderate area of north and west walls To receive heat gain Minimize roof area To reduce heat loss Large window wall For heat gain and light Equatorial upland Maximize north and south walls Maximize west-facing walls Medium building depth Minimize surface area To reduce heat gain To reduce heat gain To increase thermal capacity To reduce heat loss and gain Orientation should also take into consideration prevailing local wind patterns; hot winds should be blocked in hot and dry climates and captured and channelled to improve ventilation in warm and humid climates. Shading Application of shading must take into consideration the interaction of several factors. Obstructing the solar heat gains from reaching the envelope and the interiors of the building Non-interference with winter solar gains Control of intense daylight by diffusing it in a uniform manner into the space 426

3 Elements of passive solar architecture 427 Unobstructed view from the windows Admission and regulation of the ventilation of adjacent spaces These design objectives and the corresponding shading techniques differ according to the latitude, location, type of building, its schedule of operation, occupant activities, internal heat gains, and expected comfort conditions. Good shading strategies can save 10% 20% of the energy required for cooling. A group of buildings in a cluster can be so spaced as to mutually shade each other. However, effectiveness of the shading depends on the configuration and layout of the cluster, which the architect may not have much control over. Internal and external shading devices can be used extremely effectively for solar control. Internal and external shading devices The main aim of shading devices is to protect openings from direct solar radiation, while their secondary intent is to protect openings from diffused and reflected radiation. There are two primary classifications of shading devices. Fixed elements Fixed elements are mainly external, and include horizontal overhangs, vertical fins, combination of horizontal and vertical elements, and balconies. Internal elements include light shelves and louvres. Adjustable elements Adjustable elements can be external shading elements in the form of tents, awnings, pergolas, or internal elements such as curtains, venetian blinds, rollers, and window shutters (Figure 7.8). Adjustable devices can be lifted, rolled, and drawn back from the window either manually or automatically in response to the changing radiation and daylighting levels. Figure 7.8 Pergolas for shading of south facades Source Santamouris and Asimakopoulos (1996) 427

4 428 Renewable energy engineering and technology External devices are more effective as they obstruct solar radiation even before it reaches the interior of the building. Internal devices stop from the radiation that has already penetrated inside only the portion that can be reflected by their surfaces, while the remainder is absorbed, convected, and radiated to the room. Thus, effectiveness of shading devices is mainly determined by their reflectivity. Additionally, these may conflict with daylighting and ventilation requirements as they block the openings in most cases. External shading devices are generally considered about 35% more efficient than the internal ones. Olgyay and Olgyay (1957) have suggested a method by which the designing of shading devices is carried out in four steps. 1 The times when shading is needed (overheated period) is determined. According to this methodology, provisions for shading are required at any time when the outdoor air temperature exceeds 70 ºF (21 ºC) at a latitude of approximately 40º. For every 5º latitude change towards the equator, the limiting temperature is elevated by 0.75 ºF (0.42 ºC). 2 The position of the sun when the shading is needed is determined by using a sun-path diagram. The overheated period is marked on the sun-path. This is done by constructing a table of average temperature for every hour in every month, thus obtaining the times of the overheated period. The boundary lines of the overheated period are then transferred to the sunpath diagram. 3 The type and position of the shading device are determined. The shading mask of a given shading device is plotted on a protractor having the same scale as the sun-path diagram. 4 The dimensions of the shading device are determined so as to interrupt the sunlight during the overheated period and to let it in during the underheated period. Maximum solar radiation is incident on the roof. Hence, it is helpful if the roof surface is protected from the sun as far as possible during the day. This can be designed as an integral part of the building or it can be a separate cover. Shading provided by external means should not interfere with night-time cooling. A cover of concrete or galvanized iron sheets over the roof not only provides protection from direct radiation but also prevents radiation from escaping into the cool night sky. An alternative method is to provide a cover of deciduous plants or creepers. Evaporation from the leaf surfaces lowers the daytime temperature of the cover and at night time, it may even be lower than the sky temperature. 428

5 Elements of passive solar architecture 429 Another shading device in some traditional buildings is developed by covering the entire roof surface by small, closely packed inverted earthen pots. In addition to shading, this system provides a layer of still air over the roof, which acts as insulation, impeding heat flow into the building during the day. This arrangement also permits an upward heat flow and loss to the night sky. Although this technique is thermally efficient, there are practical difficulties of maintenance and lack of usability of the rooftop. An effective roof shading system is a removable canvas roof, which can be mounted close to the roof during the day and can be rolled away during night. The upper side of the cover should be white or light to minimize the amount of radiation absorbed and the consequent heat gain through it. The various methods of roof shading are shown in Figure 7.9. Reflective surfaces Certain colours reflect solar radiation more (and absorb less) than others. If external surfaces are painted in such a manner that their emission in the long-wave region is high, then heat flux transmitted into the building is reduced considerably (Table 7.8). It is seen that whitewash has lower reflectivity than aluminium but stays cooler when exposed to solar radiation because of its high emissivity at low temperatures. External colour and finish influence light-weight structures more due to their low resistance to heat flow and low thermal capacity. Table 7.8 Reflectivity of the surfaces for solar radiation and their emission in the long-wave region Reflectivity Emissivity Material (solar radiation) (low temperature) Aluminium foil (bright) Aluminium foil (oxidized) Polished aluminium Aluminium paint Galvanized steel bright Whitewash new White oil paint Grey colour (light) Grey colour (dark) Green colour (light) Green colour (dark) Red brick Glass

6 430 Renewable energy engineering and technology Figure 7.9 Some methods of shading the roof Reproduced with permission from Elsevier Source Bansal, Minke, and Hauser (1994) 430

7 Elements of passive solar architecture 431 Vegetation Rational use of vegetation around the building can offer significant shading. Vegetation can reduce the impinging solar radiation and modify air temperatures, outdoors and indoors, to effectively reduce the cooling loads of buildings. The choice of trees should be very carefully based on the shape and character of the plant, both during summer and winter, and on the shadow shape they provide. Their position should be chosen strategically to provide shade at the most critical hours of the day. Trees on the east south-east and west south-west give the best results as the sun is at a low altitude in the morning and evening, and casts long shadows (Figure 7.10). Bushes can act as vertical wingwalls to shade windows on the east and west (Figure 7.11). Vertical trellis covered by vines or creepers are very useful to shade facades. Figure 7.10 Trees at east and west sides for shading in summer Source Santamouris and Asimakopoulos (1996) Figure 7.11 Bushes for shading of east and west windows Source Santamouris and Asimakopoulos (1996) 431

8 432 Renewable energy engineering and technology A study quoted by Bansal, Minke, and Hauser (1994) has shown that the ambient air temperature under a tree adjacent to a wall is about 2 to 2.5 ºC cooler than the unshaded areas. This cooler microclimate immediately adjacent to a building results in less heat gain through conduction by walls and windows. Insulation The external surface of the building gets heated due to solar radiation that is absorbed and due to convective heat gain from hotter ambient air. This heat is then transmitted inwards through the building envelope. One important factor that determines the quantity of heat that reaches the interiors of the building is insulation in the walls and the roof. The use of insulation is a very important aspect of energy efficiency in buildings. In hot climatic conditions, thermal mass of the building envelope should be strongly coupled with the interiors in order to absorb internal heat gains easily. This means that the layer of insulation in the building element should be placed on the external side. It is crucial that the performance of insulation is analysed in conjunction with the thermal mass as their thermal relationship is closely intertwined. There are two basic techniques of the application of insulation to building components: pre-formed material in the form of slabs or sheets and cast in situ. These are usually made of expanded polystyrene, extruded polystyrene, polyurethane, polyisocyanurate or perlite boards. Ventilation Ventilating the building by cooler ambient air is one of the most effective means of cooling the indoors. Convective cooling can take place by natural means in which internal air is replaced by the ambient air. Air flow in a building is either due to pressure differences caused by temperature differences (stack effect) or due to the pressure difference of the wind. Stack ventilation depends upon the buoyancy of warmer air to rise and leave the building through an outlet located at a higher level while heavier cooler air is pulled in through a lower inlet (Figure 7.12). The rate of flow induced by thermal force is given by the formula M v = A [H(T i T a )] 1/2...(7.28) 432

9 Elements of passive solar architecture 433 Figure 7.12 Stack effect Source Bansal, Minke, and Hauser (1994) where M v is the ventilation rate in m 3 /s, A is the unobstructed area of inlet in m 2, H is the vertical distance between inlet and outlet in m, T i is the average temperature of indoor air in ºC, T a is the average temperature of ambient air in ºC. However, by conventional design, it is usually not possible to achieve the required ventilation rates for occupant comfort. In order to maximize the advantage of ventilation, designers manipulate building components. Some elements typically used to facilitate the ventilation of indoor spaces are discussed below. Various parameters that affect wind forces entering through windows are: Climate Wind speed and direction Size and location of inlet and outlet openings Volume of the space Shading devices Internal partitions Wind movement inside a building is governed by aeromotive forces and the temperature differential between the inside and the outside. Air moves from a high-pressure zone to a low-pressure zone through openings in the building envelope. To assist sensible air movement, it is essential to provide cross ventilation. In the absence of an outlet opening, there is no effective air movement through a building even in case of strong winds. An opening only on the windward side leads to a pressure build-up indoors and increases discomfort. 433

10 434 Renewable energy engineering and technology Similarly, an opening only on the leeward side can lead to oscillating pressures which are uncomfortable for occupants. For effective convective cooling, air movement should be directed at the surface of the human body. Therefore, the inlets and outlets must be positioned such that movement is created up to 2 m from the floor level (Figure 7.13). The sizes of the inlet and outlet openings need to be carefully calculated to make the most of the available wind outside. For a given area and a total wind force (area pressure), the largest air velocity is attained through a small inlet opening with a large outlet. This is due to the air being forced through a small aperture and also due to the Venturi effect, that is, in the imaginary funnel connecting the inlet and the outlet, a sideways expansion of the air jet further accelerates the particles. Such an arrangement is useful when the direction of wind is fairly constant. However, when that is not the case, a large inlet is preferable, which enables the total volume of air that passes through to be larger. The best arrangement is large, full wall openings on both sides with adjustable shutters that can channel the air flow in the required direction, in keeping with the varying direction of the wind. Window shutters, sashes, canopies, shading devices, louvres, and other elements that control the openings also affect air flow pattern indoors. Courtyards and open spaces enclosed by built mass have been used traditionally to cool buildings. They operate on the principle of convective air movement (Figures 7.14[a] and [b]). During the day, solar radiation incident to the courtyard heats up the air that rises to escape the enclosure. To replace it, cool air from the ground level flows through the openings of the room, thus setting up the air movement. During the night, the process is reversed. As radiation and convection cool the warm roof surface, a stage is reached when its surface temperature equals the dry bulb temperature of ambient air. On further cooling by radiation, condensation may also occur. If the roof surface is sloping inwards, this Figure 7.13 Effect of opening positions Reproduced with permission from Orient Longman Pvt. Ltd Source Koenigsberger, Ingersoll, Mayhew, et al. (1973) 434

11 Elements of passive solar architecture 435 Figure 7.14(a) Courtyard effect (day) Reproduced with permission from Elsevier Source Bansal, Minke, and Hauser (1994) Figure 7.14(b) Courtyard effect (night) Reproduced with permission from Elsevier Source Bansal, Minke, and Hauser (1994) cool air sinks into the court and enters habitable spaces through low level openings. To make this system work efficiently, a parapet wall should be raised around the perimeter of the roof to prevent mixing of air. This technique works well in warm and humid climate. A temperature drop of 4 7 ºC below ambient is possible. Wind tower The principle of a wind tower is explained by Bahadori (1978). The hot ambient air enters through the top opening and gets cooled when it comes in contact with the cool tower, becomes heavier, and sinks down. An inlet is provided to the rooms from the tower and, along with an opening on the outer side, a cool draught is created. After a whole day of heat exchange, the wind tower becomes warm by the evening. At night, the cooler ambient air comes in contact with the bottom of the tower after passing through the rooms. It gets heated up by the warm surface of the wind tower and begins to rise due to buoyancy, and a reverse convective loop is created. 435

12 436 Renewable energy engineering and technology Figure 7.15 Daytime and night-time operations of a wind tower Reproduced with permission from Elsevier Source Bansal, Minke, and Hauser (1994) This system works well in hot and dry regions, where the diurnal range of temperatures is very high. For a wind tower approximately 4-m high, a temperature drop of about 4 ºC is observed with an indoor air velocity of 1 m/s (Figure 7.15). Solar chimney A solar chimney works on the principle of the stack effect but here the air is deliberately heated by solar radiation in order to extract indoor air. As the chimney is designed to maximize solar gain, the temperatures reached within are very high, and it is isolated from habitable spaces. The rate of ventilation is affected by the following factors. Vertical distance between the inlet and outlet Cross-sectional area of the inlet and outlet Geometrical construction of the solar absorbing plate Inclination angle Solar chimneys can be used effectively for ventilation in regions of low wind speed. Evaporative and nocturnal cooling The principles of evaporation are discussed in the section on mass transfer in Chapter 3. Briefly, evaporation of water from a film or a pond or a drop occurs if the difference between the vapour pressure at the given water temperature and the partial pressure of water in the atmosphere is positive. A water body exposed to the atmosphere loses/gains heat through conduction, convection, radiation, and evaporation. The temperature attained by such water bodies 436

13 Elements of passive solar architecture 437 can be calculated by a heat balance method (Kishore, Ramana, and Rao 1979). Experiments have shown that water ponds exposed to the night sky can attain temperatures below the wet bulb temperature in favourable conditions. A roof sack-cloth cooling technique has been developed in India (Jain and Rao 1974; Kumar and Jain 1981) and is found to be effective in hot arid conditions. The clear night sky can act as a sink with low temperatures. The sky temperatures can be obtained by the equation T sky = T a ( p a ) 1/4...(7.29) where T sky and T a are measured in Kelvin, and p a is the partial pressure of water vapour in the atmosphere in mm Hg. An ingenious method of cooling a building by nocturnal and evaporative cooling in summer, and heating by solar radiation in winter was developed by Hay and Yellot (1969), as described in the following section. Passive concepts for composite climates As mentioned earlier in this chapter, buildings in composite climate require both heating and cooling, which makes passive design more challenging and complex. A few systems based on passive concepts are as follows. Earth air tunnel Sky-therm system Solar chimney based hybrid systems Earth air tunnel It is well known that ground temperatures at a depth of about 4-5 m below the surface remain constant throughout the year at a value close to the annual average. Hence, a tunnel of suitable length in which ambient air enters at one end would provide heated/cooled air at the other end. Earth air tunnels are studied quite well (Sodha, Bansal, Kumar, et al. 1986). A schematic diagram of the earth air tunnel installed at TERI is shown in Figure A parametric prediction of the cooling potential has also been carried out (Mihalakakou, Santamouris, Asimakopoulos, et al. 1995). The sky-therm system The sky-therm system, originally conceived by H Hay in the 1960s and developed further in collaboration with J I Yellot (Hay and Yellot 1969; 437

14 438 Renewable energy engineering and technology Roof shading Figure 7.16 Schematic diagram of earth air tunnel system Yellot and Hay 1969), makes use of roof ponds with movable insulation for collection, storage, and dissipation of heat. In this simple but very effective system, black plastic bags filled with water are placed on a metallic roof. An insulation panel can be slid in or out so as to either expose the roof to the sky or to shield it (Figure 7.17). In winter, the black plastic bags are exposed to sunlight and they act as collection-cum-storage devices of solar energy. The bags are insulated in the Figure 7.17 Operating principles of sky-therm system 438

15 Elements of passive solar architecture 439 night by sliding the insulation panel. The metallic roof transfers the stored heat into the room by conduction and radiation. In summer, and under dry conditions, the black plastic bags are exposed to night sky and they lose heat by nocturnal radiation to sky. The roof is insulated during the daytime, so that room heat is transferred to the colder bags. During hot and humid conditions, the roof deck is flooded with water, so that heat loss at night is enhanced due to evaporation. The occupants reported that heating and cooling are even and comfortable. Sky-therm systems are useful in hot/arid zones and for single dwellings. Making a sturdy and leak-proof metallic roof and providing for sliding insulation pose some problems, which have been adequately addressed in practical buildings. A system similar in concept to the roof pond was experimented with at IIT Kanpur in the early 1980s under the sponsorship of TERI. Solar chimney-based hybrid system The solar chimney-based hybrid system, designed by T E R I, was constructed and studied at the Gual Pahari campus of T E R I under a project funded by the Ministry of Non-conventional Energy Sources. The design consists of the following elements (Raman, Mande, and Kishore 2001). Reduction of heat loads is achieved by insulating the outside surfaces, using double glazed windows and providing an ante-room at the entrance. The entire south wall is converted into a solar collector by painting it black and fixing a single glazing. Bottom and top vents into the room and a closable exhaust at the top edge of the wall are provided. The collector wall acts like a solar chimney in summer and like a Trombe wall in winter. Evaporative cooling of the roof is done by the sack-cloth system with a provision for air passage space below the cooling surface. Ambient air enters this space, gets cooled, and enters the room from a roof vent. The solar chimney would aid in creating a natural draught. A schematic diagram of the system for summer and winter operations is shown in Figure 7.18 (a) and (b) and a photograph of the system is shown in Figure Typical day performance of the system for summer and winter is shown in Figure It can be seen that when the ambient temperature varied from 26 ºC to 42 ºC in summer, the room could be maintained around 32 ºC. For a winter day when the temperature varied from 2 ºC to 17 ºC, the room was kept at about 16 ºC. The design of the system is such that existing structures can be modified to incorporate passive features. 439

16 440 Renewable energy engineering and technology Figure 7.18(a) Schematic diagram of the hybrid passive system during summer operation Figure 7.18(b) Schematic diagram of the passive system during summer operation Figure 7.19 Overall view of the heating-cum-cooling design developed at TERI 440

17 Elements of passive solar architecture 441 Figure 7.20(a) Temperature profile of the passive building for a typical winter day operation Figure 7.20(b) Temperature profile of the passive building for a typical summer day operation 441

18 442 Renewable energy engineering and technology Nomenclature A s Surface area of the body, Du Bois area (m 2 ) a Absorptivity of a surface A Unobstructed inlet area of a building opening (m 2 ) d Thickness of a conducting element (m) f cl Clothing surface ratio h c, h i, h o Convecting heat transfer coefficient (W/m 2 k) I Solar radiation intensity (W/m 2 ) I cl Clothing parameter (clo) k Thermal conductivity (W/mK) M v Ventilation rate (m 3 /s) p a Partial pressure of water vapour in ambient air (mm Hg) p s Vapour pressure of water at a given temperature (mm Hg) Q M, Q H, Q sw Heat exchange rates (kcal/h or kw ) q c, q Heat fluxes (kcal/h m 2 ) r Reflectivity RH Relative humidity R cl, R Heat transfer resistance (m 2 K/W) T a, T s, T cl Temperature (ºC or K) V Air speed (m/s) W Work done per unit time (kcal/h or kw) η Efficiency of conversion References ASHRAE (American Society of Heating, Refrigerating, and Air-conditioning Engineers) ASHRAE Handbook of Fundamentals Atlanta: ASHRAE ASHRAE (American Society of Heating, Refrigerating, and Air-conditioning Engineers) ASHRAE Handbook fundamentals: SI units Atlanta: ASHRAE Bahadori M N.1978 Passive cooling systems in Iranian architecture Scientific American 238(2): Bansal N K and Minke G (eds) Climatic Zones and Rural Housing in India Julich: German Indian Cooperation in Scientific Research and Technological Development/ KFA 442

19 Elements of passive solar architecture 443 Bansal N K, Minke G, and Hauser G Passive Building Design Elsevier Science B V. Cook J (ed.) Passive Cooling The MIT Press Fanger P O Thermal Comfort: analysis and applications in environmental engineering New York: McGraw-Hill Givoni B Passive and Low Energy Cooling of Buildings van Nostrand Reinhold Goetzberger A (ed.) Special issue on transparent insulation Solar Energy 49(5) Hay H R and Yellot J I Natural air conditioning with roof pond and movable insulation ASHRAE Trans. 75(1): Hoffman M E and Feldman M Calculation of the thermal response of buildings by the total thermal time constant method Building and Environment 16(2): Jain S P and Rao K R Experimental study on the effect of roof-spraying cooling on unconditioned and conditioned building Building Science 9: 9 16 Kishore V V N Assessment of natural cooling potential for buildings in different climatic conditions Building and Environment 23(3): Kishore V V N, Ramana M V, and Rao D P An experimental and theoretical study of a natural water cooler, pp [Proceedings of National Solar Energy Convention, IIT Bombay, Organized by Solar Energy Society of India] Koenigsberger O H, Ingersoll T G, Mayhew A, Szokolay S V. (1973) Manual of Tropical Housing and Building, Part 1 Orient Longman Pvt. Ltd Krishnan A, Baker N, Yannas S, Szokolay S V (eds) Climate Responsive Architecture New Delhi: Tata McGraw-Hill 443

20 444 Renewable energy engineering and technology Kumar V and Jain S P Automatic water actuated switch for cooling building by roof surface evaporation Energy Management 5: Mihalakakou G, Santamouris M, Asimakopoulos D, Tselepidaki I Parametric prediction of the buried pipes cooling potential for passive cooling applications Solar Energy 55(3): Olgyay V Design with Climate Princeton: Princeton University Press Olgyay V and Olgyay A Solar Control and Shading Devices Princeton: Princeton University Press Raman P, Mande S, and Kishore V V N A passive solar system for thermal comfort conditioning of buildings in composite climates Solar Energy 70(4): Santamouris M and Asimakopoulos D (eds) Passive Cooling of Buildings James & James (Science Publishers) Ltd Sodha M S, Bansal N K, Kumar A, Bansal P K, Malik M A S Solar Passive Building: science and design New York: Pergamon Press Szokolay S V Thermal comfort and passive design In Advances in Solar Energy, vol. 2, edited by K Boer and J Duffie New York: Plenum Press Yellot J I and Hay H R Thermal analysis of a building with natural air conditioning ASHRAE Trans. 75(1),