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Thermal Comfort Conditions in Outdoor Urban Spaces: Hot Dry Climate -Ghardaia- Algeria

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Thermal Comfort Conditions in Outdoor Urban Spaces: Hot Dry Climate -Ghardaia- Algeria Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 169 ( 2016 ) 207 – 215 4th International Conference on Countermeasures to Urban Heat Island (UHI) 2016 Thermal Comfort Conditions in Outdoor Urban Spaces: Hot Dry Climate -Ghardaia- Algeria Islam Boukhelkhala , Pr. Fatiha Bourbiab a University of Larbi Ben M’hidi, Oum El Bouaghi, 04000, Algeria b Laboratoire Architecture Bioclimatique Et Environnnement, Constantine, 25000, Algeria Abstract One of the great challenges facing our generation of scientists is how to mitigate the effects of urban heat island. This problem become more and more important for the wellbeing of man. The aim of this study is to enable designers to produce comfortable outdoor spaces. Many researchers believe that the causes of the microclimatic variation in cities are linked to urban geometry, and the effect of shade trees. The main aim of this paper is to discuss and assess the impact of the geometry and shade trees on the open spaces climate, in Ghardaia (hot dry climate) –Algeria. ©© 20162016 The The Authors. Authors. Published Published by Elsevier by Elsevier Ltd. This Ltd. is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of the 4th IC2UHI2016. Peer-review under responsibility of the organizing committee of the 4th IC2UHI2016 Keywords: Urban heat island; open space geometry; outdoor thermal comfort; vegetation. 1. Introduction The habitat, much more than a basic unit in the town, it is a multifunctional space. It ensures safety and shelter against the elements, but it is especially the place of human relationships, the place where the feeling is the desire for better living together where share. Housing is in a more limited sense of where we distinguish one from another and more [1]. Corresponding author. Tel.: +213 661 86 42 80; fax: +213 31 97 56 16. E-mail address: [email protected] 1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 4th IC2UHI2016 doi: 10.1016/j.proeng.2016.10.025 208 Islam Boukhelkhal and Fatiha Bourbia / Procedia Engineering 169 ( 2016 ) 207 – 215 The Urban Heat Island (UHI) effect occurs when city temperatures run higher than those in suburban and rural areas do, primarily because growing numbers of buildings have supplanted vegetation and trees [2]. The main causes of the different microclimatic conditions in cities are linked to urban geometry, which influences incoming and outgoing radiations as well as surface material properties. Hence appropriate urban planning and design, could play important roles in preventing, or mitigating the negative effects of urbanization [3, 4]. In hot climate, increased temperatures lead to decreased thermal comfort in urban areas. This has a negative effect on the well-being of people and could have negative social and economic consequences, e.g. less use of urban space. Deteriorated outdoor climate conditions also lead to worse indoor comfort and use of active acclimatization mainly air conditioning [5]. The reduction of vegetation, the increase of thermally massive and non-porous surfaces, the presence of drainage systems, the emission of heat and pollution contribute to altering the urban microclimate [6,4]. This makes environmental designers responsible not only for the internal conditions of the buildings but also for the external climatic environment created by those buildings. Most of the literature reviewed shows that the first control mechanism against the climate is the city layout. The update researches suggest that careful planning; design and attention to the climatic forces, can significantly improve the urban environment from both the human comfort and the energy use perspective. Open spaces in cities have a large variety of forms and surface characteristics. The microclimate of these spaces are influenced by several parameters such as the urban geometry, the vegetation, the water levels and the properties of surfaces [7,8]. The main objective of this research is to evaluate the thermal comfort level of the urban open spaces located in the city of Tafilelt (Ghardaia, Algeria). The tools used in the simulation are the numerical microclimate model ENVI-met 3.0 [9], and COMFA [10]. Several techniques based on bioclimatic architectural criteria have been analyzed, in order to improve the microclimate in an outdoor space and propose a set of climatic guidelines for architects and planners. Nomenclature UHI Urban Heat Island COMFA Comfort Formula SVF Sky View Factor PMV Predicted Mean Vote TB Thermal Balance M Metabolic Heat Rabs Heat due to Terrestrial and Solar Radiation. Remit Heat due to Radiation emitted by the person. Evap Heat dissipated by evaporation. Conv Heat gained or lost by convection The COMFA method mainly consists of the following basic formula expressing the energy budget of a person in the outdoor environment [10]. Budget = M + Rabs - Remit - Evap – Conv (1) Table 1. The human comfort feelings related to the budget values Budget W/m2 Description BUDJET -201 Very cold -200 BUDJET -120 cold -120 BUDJET -50 cool -50 BUDJET +50 Comfort Islam Boukhelkhal and Fatiha Bourbia / Procedia Engineering 169 ( 2016 ) 207 – 215 209 +50 BUDJET +120 warm +120 BUDJET +200 hot +201 BUDJET Very hot 2. Presentation of the Site (Case Study) The investigation was conducted in city of Tafilelt (Ghardaia, Algeria) located at (32.48° North and 03.67° East). This city characterized by Islamic architecture, reported to hidden spiritual elements. Its Islamic architecture expresses a common vision of the sacred and of the divine; it connects the community of believers to god [11]. The climate of the Ghardaia city is hot and dry in the summer with an average temperature of 38°C occurring in July. The average air temperatures recorded reach a maximum of 40°C and 42°C, with occasional peaks of 46°C that occurs at about 15:00. The humidity is relatively low, it varies between 23% and 27% during some periods of drought and it can reach the minimum rates between 2 and 6%. The intensities of solar radiation on the region are very high; the horizontal solar radiation is important, it can reach 1040 Wh/m² between June and July. The wind is dry and hot steering north - east with an average speed between one and 2.5 m/s. The study area is located in the central parts of the city, and characterize by Islamic art in an arid area that has mission to remember the god as well as keep the intimacy indoor units [12]. The selected site corresponds to different geometric configurations of the urban streets. These are characterized by the buildings height to street width ratios (H/W). A specific SVF value will determine the particular solar radiation received, and consequently the different temperature values at each site. In fact, the degree of sunlight penetration depends on the direction of the streets; the width, length, and height of the surrounding buildings. The SVF of the different stations are presented in Table 2. Fig. 1. Plan of the site (case study) 3. Geometry Analysis The street geometry has an important effect on the urban climate modifying the air temperature near the ground as well as the surface temperature [5]. The relationship between building height and street width is considered as the main contributor of the heat island effect in the cities [3]. The selected site represents different geometry of the urban canyons, the station selection is based on a course that passes through several outdoor spaces (plot and urban canyon) of different shape and orientation, this choice is based on several criteria (SVF, RATIO, Orientation, Vegetation, Albedo ....). This diversity of sky view factor determines the different solar radiation received. Actually, the sun penetration depends on the direction of the streets, the width the length and the height of the surrounding buildings. 210 Islam Boukhelkhal and Fatiha Bourbia / Procedia Engineering 169 ( 2016 ) 207 – 215 When the long axis of the street is pointing east-west, the solar access to the façades of the buildings is identical to the ground of the streets, meaning that direct solar radiation does not touch the ground. On the contrary, when the axis of the street is pointing north-south, the sun does penetrate the first floors in the winter period for just a couple of hours at noon on account of the obstructions by buildings on the east and west [5]. Table 2. Schematic determining the geometry of the stations 1, 2, 3, 4, 5 and 6 N STATION CHARACTERISTIC SVF RATIO H/W Small square designed 1 for the people - SVF= 0.88% It serves as a passage 2 for the vehicle; the floor is completely in - SVF= 74.8% concrete and bitumen. Oriented East-West, H/L = 3.75 characterized by ratio 3 H / L = 3.75, and SVF = 24.1, the floor SVF= 24.1% covering is by stone and concrete. It has the same H/L = 3.40 characteristics as the 4 station 03, except the orientation (north - SVF= 26.4% south). Asphalt street, it has H/L = 0.78 the same orientation as 5 the station 4 but H / L different (0.78). SVF= 58.7% H/L = 2.22 Oriented East-West, 6 characterized by its ratio H / L = 2.22, and SVF= 17.6% FCS = 17.6, the soil is sandy. Islam Boukhelkhal and Fatiha Bourbia / Procedia Engineering 169 ( 2016 ) 207 – 215 211 4. Discussion and Results 4.1. Scenario A The measurements recorded with DELTA OHN -HD32.3 in a several stations well defined on June 2013 as shown in Fig 2.
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