street design and thermal comfort in hot and dry climate [PDF]

Key words: hot and dry climate, street design, outdoor thermal comfort, PET. 1. INTRODUCTION. The ratio height to width

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STREET DESIGN AND THERMAL COMFORT IN HOT AND DRY CLIMATE *

Fazia Ali Toudert , Helmut Mayer Meteorological Institute, University of Freiburg, Germany

Abstract The presentation discusses the contribution of the street design in the definition of a comfortable microclimate for pedestrians at the street level. The analysis is carried out by using ENVImet, a three-dimensional numerical model which estimates the microclimatic changes within urban environments. Simulations are run for a typical summer day in Ghardaia, Algeria, located in the Sahara and characterized by a hot and dry climate. Complex street geometries are compared to simple urban canyons. The role of architectural details such as galleries and horizontal overhangs is investigated as a way to improve the thermal comfort. A special emphasis is given to a human-biometeorological assessment of these microclimates by use of the thermal index PET. Key words: hot and dry climate, street design, outdoor thermal comfort, PET

1. INTRODUCTION The ratio height to width and the street orientation in relation to the sun are among the parameters already known to be of great influence for the characterization of the street microclimate. Also the shadowing seems to be an efficient strategy to mitigate heat stress in summer. However, quantitative information about these effects are still lacking which investigate the whole space of the street and consider the design details. The main difficulty faced by the planner is the conflicting seasonal objectives, e.g. the necessary protection from the sun in the summer and the need for solar access in winter. Thus, thumb rules about the optimal street design regarding the climate comfort are needed. It is also worthwhile to notice that the climate of the Sahara is so extreme that passive design strategies are very likely insufficient to ensure the required thermal comfort. However, several dispositions may increase the time of frequentation of outdoor spaces and at least avoid more critical thermal situations. 2. OBJECTIVES AND METHODOLOGY For this purpose we used ENVImet, a three-dimensional model which simulates the microclimatic modifications due to buildings and vegetation (Bruse, 1997). Its advantage is the high spatial resolution which allows a fine analysis of the microclimate at street level and the possibility to represent complex geometries including galleries and horizontal overhangs. ENVImet also needs relatively few input parameters and calculates all meteorological parameters of which air temperature, wind speed and direction, air humidity, short-wave and long-wave radiation fluxes as well as the mean radiant temperature are important for this investigation. If air temperature, wind speed, air humidity and the mean radiant temperature are known, then it is possible to calculate the PET index by using a VDI program (VDI, 1998). The Physiologically Equivalent Temperature PET is one of thermal indices well suited to describe the human comfort outdoors. PET is based on the human energy balance model MEMI (Höppe, 1999) including the physiological processes of human beings to adjust to a climatic situation. PET transfers the heat stress experienced outdoors to an internal standard situation. PET has the unit deg C and, therefore, can be easily applied for planning purposes. Thermal comfort outdoors is strongly dependent on the radiation fluxes which are well represented by the mean radiant temperature and consequently by PET. Previous investigations have shown that the significance of the air temperature in describing the comfort conditions in summer is weak due to the well mixed air within the street space in the daytime (e.g. Matzarakis et al., 1999). Consequently, this analysis is based principally on PET. 3. RESULTS A profile of an aspect ratio of H/W = 2 oriented in an east-west direction has been chosen as a street reference in the following discussion. The orientation E-W is privileged because of its high potential of solar access indoors and outdoors needed in winter. The wind above the roof level is kept perpendicular to the street axis. The domain simulated is composed of 2 buildings separated by a street of 8 m width (Table 1). The simulations are run for a typical summer day for the location of Ghardaia (32.40 N; 3.80 E, 469 m a.s.l.) in the south of Algeria.

*

Corresponding author address: Fazia Ali Toudert, Meteorological Institute, University of Freiburg, Werderring 10, D-79085 Freiburg, Germany; e-mail: [email protected]

Table 1: Schemes of the simulated streets canyons street width: 8 m bldg height H: variable building length: 6 H wind speed: 5 m/s at 10m Asphalt road: albedo: 0.1 Brick walls: albedo: 0.3 roof: albedo: 0.15

(a) reference street: E-W oriented street with an aspect ratio of H/W = 2

(b) (c) E-West oriented E-W oriented street street with an aspect with an asymmetrical ratio of H/W = 4 profile H/W = 2 and 1, including galleries

(d) NE-SW oriented street with horizontal overhangs and galleries

Plan

H = 16 m

H =32 m

H1 = 16 m and

wind

H = 16 m

H2 = 12 m

3.1. The street reference: H/W = 2 Fig. 1(a) represents the spatial and temporal distribution of the PET index for the reference street. Calculations were performed for a 1 m grid net at the human representative height of 1.2 m above ground level. In spite of the high walls, the street remains exposed at the major part of day and the PET values are extremely high even for the shaded part. This is due to the subtropical latitude of Ghardaia where the sun elevation is about 80° with extreme high air temperatures and intense solar radiation. PET minimum is about 40 °C and develops in the sidewalk in shade zones during 6 hours from10:00 to 16:00 LST. This extends to 40% of the street at midday hours from12:00 and 14:00 LST. 60% of the street starts to cool from 18:00 and cools completely from 19:00 LST, reaching a PET value of 34 °C at 20:00 LST. The largest and exposed part of the street experiences PET values higher than 60 °C with a peak of 68 °C round 16:00 LST. It appears clearly that during almost all the day the street is highly uncomfortable which would exclude any leisure activity unless an improvement of the thermal quality based on further strategies is planned. Potentially this is possible through dispositions which enhance shade. These are among others: ! a higher aspect ratio of the street, ! a different solar orientation of the street, ! use of galleries and horizontal overhangs, ! planting of suited trees. The combinations of these alternatives lead to a great number of alternatives. We are proposing in the following a discussion of three from these alternatives (Table 1). 3.2. The deep profile: HW = 4 Compared to H/W = 2, this street as illustrated by Figure 1(b) shows spatially a larger area of shade and consequently lower PET values which, however, still lie around 40 °C. This area develops for 6 hours in the shaded part of the street and for 1 to 4 hours in the opposite part. Temporally the morning hours from 8:00 to 10:00 LST remain highly uncomfortable due to the exposition to the sun rays coming from the east direction and which make the high walls inefficient in keeping the street in shade. Symmetrically the street experiences the highest thermal discomfort in the late afternoon between 16:00 and 17:00 LST, also due to the sun exposition but this time from the west direction. In this case PET exceeds 60 °C. The street cools rapidly from 17:00 to 18:00 LST and reaches 36 °C at 19:00 LST and 34 °C at 20:00 LST. This scheme is to some extend comparable to the street with H/W = 2 with a rate appreciably similar, but the deeper street cools somewhat faster due to the shorter time of exposition of its surfaces and thus of heat storage. This example shows that despite the depth of the street, PET values are still above the comfort level. An aspect ratio of 4 does not allow to reach yet the so-called ”cool-island”. This can be probably reached only for even deeper profiles. This helps to explain partly the typical design of very deep streets in traditional desert cities. Provided that sunlight and daylight inside the buildings are ensured by internal courts or patios, the need of solar access in winter is resolved, but in the case of conventional day lighting through facades this proportion could compromise this goal. Effectively one of the disadvantages of such profiles is the impossibility to use the space of the street as a source of sunshine and light for indoor spaces and to keep a minimum of exposition outdoors in winter. As well the dilution of pollutants can be strongly reduced compromised if any source of pollution at street level exists such as motor traffic. Thus this kind of street or even deeper is more appropriate as pedestrian paths in housing areas of a dense urban plan.

3.3. Asymmetrical profile with galleries The following example introduces an alternative which is opposite to the previous one. Here the street is asymmetric with a greater openness to the sky in the south direction in order to keep a higher potential of solar access in winter. Of course, it is expected that this geometry leads to more exposition of the street in summer. Therefore, the use of galleries as a way to protect pedestrian spaces is added. Effectively, the need for thermal comfort within a street, as already mentioned, depends on its use. A pedestrian street requires a thermal comfort in the whole space, while a street designed also for motor traffic limits these requirements to the sidewalks at the edges of the street. Compared to the reference street in Fig. 1, the spatial and temporal evolution of PET in the street area is noticeably similar (Fig. 1c). The warming up of the street is only about 3 °C in comparison to a regular profile. On the contrary, in almost the whole space of the galleries PET varies between 32 °C and 38 °C for both sides. This is due to the E-W orientation for which the horizontal overhangs are an efficient solution for the obstruction of sun rays. This geometry is also interesting regarding the nocturnal cooling of the street. Although it has not been studied in the present paper, however, this street could normally cool faster than a regular profile with an aspect ratio of 2 due to its larger sky view factor (about 0.47 to 0.66 instead of 0.47 to 0.53). This aspect is especially important if materials with a high thermal inertia are used which store an important amount of heat during the day and release it after sunset. Such materials are justly recommended for these regions to regulate the internal temperatures. 3.4. NE-SW street with horizontal overhangs and galleries In this example, two further design possibilities are discussed, namely the use of NE-SW orientation instead of an E-W orientation and the use of horizontal overhangs represented schematically here by the whole facades. The following hypotheses are verified: The use of a NE-SW orientation is intended as an alternative to mitigate heat stress due to the orientation that would make the vertical surfaces of the walls more efficient in increasing shade in the morning and in the afternoon where the sun position is relatively low. At the same time this orientation still allows a solar exposition even not as optimal as an E-W orientation. The asymmetry increases the potential of sun rays capture in winter on the south exposed wall but the overhangs keep the street in shade during a large part of the day. Compared to the cases presented previously, Fig. 1d shows a noticeable small area of overheat with extreme PET values being lower. This area extends from the NW gallery exposed in the morning between 9:00 and 11:00 LST up to the opposite wall about 15:00 LST with only 2 hours of maximal PET values for each point of the street. Only a small part of the street is highly uncomfortable even at the hottest hours of the day which means that an alternative is always available to walk in a comfortable part of the street during the whole day. The gallery SE remains in shade all time and PET varies between 32 °C and 42 °C. After 16:00 LST, PET lies below 42 °C in the whole street. 4. DISCUSSION AND CONCLUSION These simulations show the dependence of the thermal comfort on the design of the street including geometry, orientation and other design strategies such as galleries, and horizontal overhangs. One conclusion is that thermal comfort is very hard to reach passively for such climates but an improvement is possible. The shading effects of the walls for a E-W orientation are only effective for high proportions of 4 and more. For shallower streets, shading strategies have to be implemented at the lower part of the street. The rotation of the street to a NE-SW orientation leads to better comfort conditions because in this case the effects of the walls are more affirmed. For this orientation as well as the E-W one, the most uncomfortable time is the afternoon but a NESW orientation offers a longer time of comfort in the morning since the street is comfortable up to 10:00 – 11:00 LST while it is already overheated at 9:00 LST for an E-W. orientation. Further and according to these examples, several aspects could be discussed : 1. The need of comfort is strongly dependent of the use of the street. It is difficult to keep a whole street in optimal comfort situations because of other design imperatives. Fortunately, this is also not indispensable, and important is to keep the space of the street at least partly comfortable. Studies based on social surveys have shown that the frequentation of urban spaces are favored if a choice is given to people to adjust to a climatic situation by moving to shaded sub-spaces. The last example shows that a judicious combination of geometric aspects (aspect ratio, gallery, overhangs) and the orientation can lead to a substantial amelioration of the microclimate at street level even with keeping a minimum of solar access in winter for indoors and outdoors. 2. The discussion of the comfort situation for different streets is possible by the use of PET values. However, it is still difficult to quantify precisely the meaning of one PET value for the human comfort. The scale of comfort, if available, is mostly based on indoor situations, which may differ outdoors (Spagnolo and de Dear 2003) and thus further sociological surveys are needed for the assessment of the impact of subjective aspects of comfort such as acclimatization or expectation.

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The use of vegetation is an additional solution which can cool further the space of the street. This has not been verified in this paper but an observation of the precedent PET patterns gives some information about the most suitable locations of green within the street to improve its comfort level. Because the vegetation is essentially used for its shade to mitigate the heat stress, it can be seen for example that an E-W oriented street is likely the one where implementation of vegetation is the most desirable. For the cases (a) and (c) a row of trees would ameliorate the microclimate if planted on the north half of the space which is permanently exposed to sun. A further strategy for hot and dry climate is to use materials with a high thermal inertia, recommended as well for indoors as for outdoors. A precaution, however, would be to keep high view factors for streets which are largely exposed to the sun in the daytime such as E-W oriented streets in order to facilitate the nocturnal cooling and thus to avoid an overheated situation in the nighttime. The dimensions of the street discussed here are likely appropriate as pedestrian streets, but in fact general recommendations could be applied for larger streets with dimensions including for instance motor traffic. In fact, the relative dimensions (expressed as proportions) of the street are more relevant than the absolute dimensions. The positive effects of the wind are absent in this discussion because of the perpendicular incidence which reduced noticeably the wind speed at a pedestrian level, but a wind blowing at an oblique or parallel direction would theoretically lead to higher velocities and consequently reduce further the PET values of several deg C.

References

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(d) NE-SW street with overhangs and galleries

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Bruse, M., 1997, www.envi-met.com.de. Höppe, P., 1999, The physiological equivalent temperature - a universal index for the biometeorological assessment of the thermal environment, Int. J. Biometeorol., 43, 71-75. Matzarakis, A., Mayer, H., Iziomon, M.G., 1999, Applications of a universal thermal index: physiological equivalent temperature, Int. J. Biometeorol., 43, 76-84. VDI, 1998, Methods for the human-biometeorological assessment of climate and air quality for urban and regional planning, part 1: climate, VDI 3787 Part 2, Beuth, Berlin. Spagnolo, J., De Dear,R.,2003, A field study of thermal comfort in outdoor and semi-outdoor environments in subtropical Sydney Australia, Building and Environment, 38, 721-738.

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Figure 1: Spatial and temporal distribution of the Physiologically Equivalent Temperature PET at the height of 1.2 m above the ground (dotted line) for different streets. The schemes left show the XY resolution at street level for street with and without galleries (Grid equal to 1m horizontally and 0.4 m vertically).

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