«CSAAR (7: 2010: Amman) Sustainable Architecture and Urban Development \ Edited by Steffen Lehmann, Husam Al Waer, Jamal AI-Qawasmi. Amman: The Center ...»
In recent years, several investigations done by Verma, Bansal, and Garg (1986) were performed and showed that there can be multiple solutions to the excessive heat problem through the roof. The use of low emissivity material in the attic of a building reduced the underside ceiling surface temperature, which lowered the room air temperature, Nahar, Sharma, Purohit (2003). The evaporative cooling approach for passive cooling of buildings in hot arid climates has also become an attractive subject of investigation for many researchers. The relative advantages of evaporative cooling in relation to many other approaches (cavity wall, insulation, whitewash and large exposure orientations, vegetable pergola shading, roof with removable canvas, water film, soH humid grass and roof with white pots as cover) were demonstrated in R.
Lambert (1988). The reduction of heat gain through the roofs using evaporative cooIing systems was extensively investigated with open roof ponds by Nayak, Srivastava, Singh, Sodha (1982), Sodha, Singh, Tiwari (1980), on water spraying over the roof, moving water layer over the roof, thin water film and roofs with wetted gunny bags Sodha, Srivastava, Kumar, Tiwari (1980). Chandra and Chandra (1983) have developed a periodic heat transfer model to study the effects of evaporative cooling using water spray and variable ventilation on the temperature control of a non-air-conditioned building.
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Figure. I. Roofs exposed to solar radiations
The present study suggests an improved roof design by combining the advantages ofthe previously described cooling techniques (water ponds, low emissivity surfaces) and inserted rocks of high thermal capacity. The resulting design can be more advantageous and effective than other systems for reducing heat during daytime and storing coolness at night. High thermal capacity materials (rock bed) will delay the entry of daytime heat into the building by such aperiod that it reaches the interior during the evening, when it is least bothersome and often welcome. The roof is composed of steel plate ceiling and a tlat aluminium plate separated by an air space partially filled with high thermal capacity rocks placed in a small quantity of water.
The system is properly closed to prevent water vapour escaping outside. A schematic diagram ofthe model design is shown in Fig. 2.
The choice of the roof for our investigation comes from the fact that 50% heat load passes through it. The reduction of heat transmission via the roof was investigated for a typical summer day of June for Laghouat in Algeria (Latitude
33.43°N, Longitude 2.56 E). Theoretical results presented Ben Cheikh H. Bouchair A (2003) shows that the most significant factors affecting the cooling power of the passive cooling roofwere the rocks, water volume, aluminium roofthickness and roof air space width. The rocks water volume was 6.5 litres. The weight of the rocks was 65kg. The width of the air gap was variable from 17cm to 27cm.
206 Hamida Ben Cheikh
Abbreviations Tae = Outside air temperature (0C), Tai Inside air temperature (0C) Qrav Reat change by radiations between the roof and the sky (w) Qcae Real change by convections between the roof and the outside air temperature Tae (w) Qrs = Reat gain from solar radiations (w) surface (w) Qcti Reat change by convection between the inside air and the roof inside surface (w) Gvdt = Real change between Tai and Tae, through exterior walls and by natural ventilation(w) Sustainable Architecture and Urban Development 207 2 Experimental Measurements Field measurements were conducted at Laghouat University. The experimental set-up consisted of two identical test cells (A) and (8) fabricated of steel structure, each having dimensions (0.70 XO.7 X 0.90 m). Figure 3 shows the configuration of tested cells. The experimental cell (A), is made of metal frame of (0.70 XO.7 X 0.90
m) interior edge, all sides were strongly insulated by 4 cm thick polystyrene except the roof. The cell was elevated by 50 cm above the ground using four metal supports as shown in Fig. 3. In the North wall a steel door of 30cm x 60 cm dimensions upon which a 4 cm thick extruded polystyrene foam panel was fixed. In the south wall a window of 35 cm x 37 cm dimensions, plastic netting, of fine meshes was fixed on the window exterior face, to limits the transmission ofthe solar radiation Fig. 3. The door and the window are used two allow night natural ventilation. The experimental cell (8) was the basic reference unit. The roof was constructed of simple aluminium sheet painted white (Fig. 4).
3 Temperature Measurements Air temperatures outside the room were measured using a me theologieal station installed near the laboratory, far from the test eell by 150m. The temperature at different positions under the roof level has been measured by eopper constant thermoeouples eonneeted to digital thermometer. Thermoeouples fixed under the roof surfaee the end of the thermoeouples were enveloped in thin aluminium paper to refleet the radiation from the surrounding interior surfaees. The readings of all thermoeouples have been averaged to give the average temperature.
4 Results and Discussion Hourly variations of the inside air temperature for typical summer day by using evaporative refleetive roof measured and presented in Fig. 5. for different values of air gap width (17,22 and 27 em). The ambient air temperature is also given in these figures to observe the effeetive eooling. Roof without any treatment gives the maximum inside air temperature (48°C ) when the ambient air temperature was
38.5°C during day hours. However during night ho urs the inside air temperature fall down to the ambient air temperature. Roof with evaporative refleetive roof, when the air gap width fixed at l7em gives higher inside air temperature (42,5° C) than 22 em air gap by two degree (40,5° e ), 27 cm air gap in the roof gives the same inside air temperature as the 22 cm air gap, that means the optimum air gap is 22 em. Fig.6.
shows a comparison of room air temperatures with eooling roof system and with bare roof without room night natural ventilation. It can be seen from this figure that the evaporative reflective roof ean reduee the internal room air temperatures during the day up to 10 oe in eomparison to the air temperatures for a bare roof over the room.
Fig.7. is the eomparison of room air temperatures with cooling roof system and with bare roof when room night natural ventilation is alJowed. The ventilation was allowed from 8 pm till 9 am, aperiod when the outside air temperature is relatively low. This can significantly improve cooling of room air temperatures, as shown in feg 5 Sustainable Architecture and Urban Development 209
Figure. 5. Comparison of measured room air temperature when roof system is functioning and when the roof is bare (for ventilated and non ventilated cases for variable air gap dimensions
Figure.7. Comparison ofroom air temperature when roof system is functioning and with bare roof (with ventilate).
5 ConcJusion Under hot arid eonditions a prototype model for an evaporative refleetive roof used to improve spaee eooling in buildings has been tested. The experimental results examined the effeetiveness of such a roof eooling system in eomparison to a bare roof. The results showed that eooling inside buildings ean be improved by the applieation of such a eooling design. lt was also seen that eombining evaporative refleetive roof with night ventilation inereases such eooling more signifieantly. From the previous graphs the eooling system had a great effeet on the time lag and deerement faetor, it inereases the time lag so the maximum outside temperature aeeurse at 15pm, where the inside maximum at 18 30pm at these time the outside temperature was aeeeptable, which improve the inside eomfort in buildings during day times and reduees the eooling load whieh the energy eonsumption.
Ben Cheikh, H. Bouehair A. (2004). Passive eooling by evapo-refleetive rooffor hot dry elimates. Renewable Energy 29: 1877-1886.
Bouehair, A. (1989). Solar indueed ventilation in the Algerian and similar climates. PhD thesis. UK:University ofLeeds.
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R. Lambert. (1989). Heat transfer through roofs of low eost Brazilian houses.
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Sustainable Architecture and Urban Dcvelopmcnt Design and Validation Premises and Concept for tbe Design of an Affordable Low Energy Arcbitecture Ulrike Heine Clemson University. Clemson, USA Abstract In the upstate of North Carolina, South Carolina and Georgia there i8 an intriguing climatic eondition that results from the region's position between two eomplementarily opposing e1imate zones. Speeifically, the eonditions of these two zones offer the eombined ehallenges of warm, humid, cool and temperate climates, plaeing eonflicting demands on the design of energy-efficient buildings. Warm, humid summers require high volumes of air movement and air exchange to aehieve eomfortable conditions. However, cool, wet winters require striet ventilation control to prevent heat loss and regulate relative humidity levels. By examining the speeific nature of these zones it becomes apparent that some sustainable design strategies are more appropriate than others for use in this region. Effeetive energy-efficient design begins with an ace urate understanding of loeal c1imate conditions combined with the knowledge of whieh design principles, techniques and materials are appropriate for those eonditions. The e10se relationship between comfort, c1imate and control when using passive design strategies demands a e10se examination of a1l three players and how they interact in a given design. This paper examines and evaluates these relationships in the unique Southem U.S. context in order to determine and prove wh ich passive teehniques are most appropriate for the suceessful implementation of sustainable design in the region and how to combine them most efficiently with afTordable aetive strategies. The goal of the current research is to lay academieally proven foundations for the later physical realization of the Zero Energy House as a showcase for Clemson University, for the Southem States and for American Architeeture.