Quantitative evaluation of passive cooling of the UCL microclimate in hot regions in summer, case study: urban streets and courtyards with trees

Building and Environment 39 (2004) 1087 – 1099

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Quantitative evaluation of passive cooling of the UCL microclimate in hot regions in summer, case study: urban streets and courtyards with trees Limor Shashua-Bar, Milo E. Ho/man∗ Faculty of Architecture and Town Planning, Technion-Israel Institute of Technology, Haifa 32000, Israel Received 8 October 2003; accepted 12 November 2003

Abstract This paper presents a quantitative analysis for predicting the air temperature variations within urban clusters with trees. The clusters considered are streets and attached courtyards which together constitute a major part of the residential areas. In this study, the cooling e/ect of trees is quanti8ed, using the analytical “Green CTTC model” developed recently by the authors. The results are validated by empirical estimates of measurements in situ. The empirical and analytical approaches provide corroborative estimates and conclusions. Sensitivity analysis on the thermal impact of certain major control factors for design purposes, such as cluster deepening, albedo modi8cation, and orientation in the presence of shade trees were obtained by simulations using the analytical model. The results indicate that the combined simulated cooling e/ect of the above three factors is about 4:5 K, at midday in summer (July–August) in the Mediterranean coastal region of Israel, a cooling which is about 50% of the air temperature rise from sunrise to noon hours. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Shade trees; Albedo; Cluster deepening; Orientation e/ect; UCL microclimate; Passive cooling design tools; Analytical Green CTTC model

1. Introduction Open spaces usually cover more than two-thirds of the urban area in modern architecture; thus their microclimate dominantly a/ects the urban canopy layer (UCL) climate. Recent studies [1–3] show the importance of passive cooling in modeling the relevant control elements. Modeling has also been used in suggesting ways for mitigating the urban heat-island problem [1]. This paper presents empirical and analytical procedures for predicting the air temperature variations within the urban cluster and determining the thermal e/ects of major design control elements, namely the cooling e/ects of shade trees and the e/ect of albedo modi8cation, of cluster deepening and of orientation. The ultimate objective of the sensitivity analysis is evaluation of the effects in cooling the UCL microclimate in summer in hot regions. ∗

Corresponding author. E-mail address: mho/[email protected] (M.E. Ho/man).

0360-1323/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2003.11.007

The urban typology studied in this paper is the canyon-type cluster, which serves as the basic structural unit in many urban climate studies [2,3]. Four green sites are analyzed: a street planted with trees along the sidewalks, a wide boulevard planted with trees along a median strip, and two canyon-type courtyards similar in geometry but slightly di/ering in planting density. Daily data on climatic variables (air temperature, humidity, solar radiation and wind velocity) were taken from a previous 8eld study carried out by the authors in the summer of 1996 [4] on 11 green sites in the Tel-Aviv metropolitan area near the Mediterranean coast (Tel-Aviv proper and the adjoining cities of Givatayim and Ramat-Gan). Two approaches were used in this study to quantify the cooling e/ect of the trees in the sites studied: one is empirical, using standard statistical tools (averages, regressions) applied to measurements in situ [4], and the other analytical, using the “Green CTTC model” [5]. The empirical analysis provides a useful comparison in validating the 8ndings of its analytical counterpart as regarding the role of the shade trees in the UCL passive cooling. The sensitivity analysis in

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Nomenclature Br

C CTTC 1−f FA H h hroof I (t) IV (t) Ipen (t) K m N PSA

Brunt number, e/ective √ atmosphere emissivity, Br = a + b · VP, where a and b are coeOcients, 0.51 and 0.076 respectively as calculated by Holden [10] convective heat exchange factor of the trees cluster thermal time constant (h) tree canopy transmissivity IV (t)=I (t) plan area of building roofs in cluster (m2 ) av. height of buildings in cluster (m) overall heat transfer coeOcient at surface (W=m2 K) overall heat transfer coeOcient at roof surface (W=m2 K) unobstructed solar radiation intensity (W=m2 K) solar radiation intensity under the tree canopy (W=m2 ) solar radiation intensity incident on the ground surface in a built-up environment (W=m2 ) temperature degree Kelvin surface solar radiation absorptivity cloud cover measured in tenths on a scale of zero to unity partial shaded area

modeling the UCL microclimate is obtained by simulations using the Green CTTC model. 2. Empirical approach The Tel-Aviv metropolitan area is characterized by a hot and humid climate during the summer season, with an average noontime air temperature of 29:7◦ C and a relative humidity of 65.8% as recorded in July–August 1996 at Sdeh-Dov, a nearby meteorological station, representative of the non-built-up area. Two streets and two canyon-type courtyards with trees were chosen from among the 11 green sites. Inside each site, several observation points spaced at about 20 m were chosen over its length. Dry and wet bulb temperatures were measured at 6:00, 9:00, 15:00, 18:00 and 24:00 h, at approximately 1:80 m above the ground. The measurements were taken on calm days, with wind velocity in the sites not exceeding 0:5 m=s. Besides the climatic variables, the partial shaded area (PSA) around each observation point was also determined at 15:00 h (14:10 solar time), the time at which air temperature reaches its maximum in summer in the region. The two streets studied di/er in their geometry and in the layout of the planted trees. Herzl Street in Ramat-Gan is a

S SVF SVF
t Ta Ts T (t) VP W

plot area (m2 ) sky view factor of the cluster sky view factor of the roof surfaces time (h) absolute air temperature (K) absolute surface temperature (K) air temperature (◦ C) at time t mean partial water vapor pressure in the air (mmHg) av. width of streets in cluster (m)

Greek letters QI () QTSOLAR (t) QTNLWR (t) QTAHR (t)   

step change in solar radiation intensity (W=m2 ) = I ( + 1) − I () contribution of solar radiation absorption to air temperature (K) contribution of net long-wave radiation exchange to air temp. (K) contribution of anthropogenic heat to air temperature (K) time (h) of indexing Stefan–Boltzmann constant (=5:67 × 10−8 W=m2 K 4 ) ground surface thermal emissivity (about 0.92)

typical 20 m wide two-lane thoroughfare with Ficus trees (about 60 years old) of moderate canopy height (from 5 m and more) along the sidewalks. Rothschild Blvd. in Tel-Aviv is 45 m wide with well-developed Ficus trees (about 70 years old) with relatively high canopies (from 8 m and more) along a 12-m median strip. Both street axes are oriented close to north–south, with heavy traOc during the day. The geometry of the street is represented by the aspect ratio H=W (bordering building height, H to width of the street, W ) which is 0.6 (12/20) in Herzl Street and 0.267 (12/45) in Rothschild Blvd. The two courtyards, A and B, are situated in the same neighborhood in Tel-Aviv. Both sites consist of a 20 –25 m open space enclosed between two stretches of four-story buildings about 12 m high. Both courtyards are planted with a variety of shade trees, Olive, Poinciana and well-developed Ficus predominating. The axes of both courtyards are also oriented close to north–south. The aspect ratio is 0.6 (12/20) in Courtyard A and 0.48 (12/25) in Courtyard B. The cooling e/ect of the trees at each observation point, as determined from measurements in situ, was de8ned as the di/erence between the measured air temperature at the site and that at a nearby reference point 50 –100 m from the site, with no trees, chosen to represent the background microclimate. The humidity e/ect of the trees is evaluated

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Fig. 1. Site maps, obs. plans and patterns of the maximum cooling (K) and humidity (%) e/ects in the streets.

in the same way, as the di/erence between the humidity (absolute or relative) at the site and that at the reference point [4]. Maps of the two streets and of the two courtyards are given in Figs. 1 and 2 respectively, together with the patterns of the maximum cooling and humidity e/ects at 15:00 h at the observation points along each site. In the vicinity of Herzl street and almost parallel to it is Hayeled Avenue, with similar street geometry (aspect ratio) and tree characteristics, but closed to traOc. The patterns in Figs. 1 and 2 indicate signi8cant variations in the cooling (K) and relative humidity (RH) e/ects along the sites. Statistical analysis of the cooling

e/ect observed inside the green sites [4], indicates the following: (a) An inverse linear relationship exists between the cooling and relative humidity e/ects. The relationship is especially strong in Rothschild Blvd. (r = 0:978) and also strong in the courtyards (r = 0:884), with a maximum rise of 12 percentage points in the relative humidity effect at 15:00 h. By contrast, the corresponding humidity rise in Herzl Street is only 1.2 percentage points. For all sites, for every 1 K cooling, a rise of 3.1 percentage points in the relative humidity is expected at 15:00 h (the regression slope is b = −3:1).

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Fig. 2. Site maps, obs. plans and patterns of the maximum cooling (K) and humidity (%) e/ects in the courtyards. Table 1 Diurnal average cooling e/ect (K) (averages for the days of measurement) Site

0600 h

Av. daily cooling e/ect (K) inside the sites Streets Hayeled Blvd. −0.05 Herzl Street 0.18 Rothschild Blvd. — Courtyards Courtyard A — Courtyard B — Av. daily air temperature (◦ C) at the reference sites Herzl Street 24.2 Rothschild Blvd. — Courtyards A and B —

0900 h

1500 h

1800 h

2400 h

−0.93 0.33 −1.38

−2.14 −1.00 −2.46

−1.05 −0.37 −1.64

−0.16 0.22 —

−2.29 −2.50

−2.47 −3.26

−1.31 −1.78

— —

27.9 28.6 29.5

31.8 32.3 33.7

29.7 30.6 31.7

26.3 — —

— Not measured.

(b) The variations in the cooling e/ect along the sites are explained mainly by the trees shade coverage. Using linear regressions, the total e/ect of trees shade coverage (partial shaded area, PSA = 1) in Rothschild Blvd. and in the two courtyards was found to be −3:3 K. The relationship is statistically highly signi8cant. In Herzl Street, the shade e/ect could not be estimated statistically because of invariance of the PSA. In Hayeled Avenue it is −2:92 K. The total e/ect (b = −3:3 K) is signi8cant, considering that the average rise of the air temperature at the reference sites from sunrise to its maximum at 15:00 h was about 8:2 K during the measurement period (July–August: 24:5◦ C at 6:00 h and 32:7◦ C at 15:00 h).

(c) Apart from di/erences in the shade coverage, the site’s cooling e/ect is a/ected by the di/erences in the built-up area geometry, albedo, tree characteristics and traOc load. The speci8c thermal e/ect of these factors was estimated by the authors [4] using an empirical model, and found to be −0:25 K at the Rothschild site and 0:75 K at the Herzl site. This di/erence of about 1 K may be due mainly to the traOc heat component. Absence of the traOc e/ect in Rothschild Blvd., as indicated by its small speci8c e/ect (−0:25 K), is probably due to the low aspect ratio and to the relatively high tree canopy: both factors make for improved ventilation and thus facilitate dissipation of the traOc heat. The speci8c e/ects in the courtyards were +0:25 K

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Table 2 Diurnal average relative humidity e/ect (percentage points) (averages for the days of measurement) Site

0600 h

0900 h

Av. daily relative humidity e/ect (percentage points) inside the sites Streets Hayeled Blvd. 1.0 5.8 Herzl Street 0.9 −0.4 Rothschild Blvd. — 3.8 Courtyards Courtyard A — 10.8 Courtyard B — 12.7 Av. daily relative humidity (%) at the reference sites Herzl Street 85.5 72.8 Rothschild Blvd. — 75.3 Courtyards A and B — 71.4

1500 h

1800 h

2400 h

6.0 1.2 10.4

2.8 0.2 6.4

1.1 −1.3 —

5.2 10.9

6.1 8.6

— —

64.4 68.1 69.1

71.5 77.2 70.3

82.7 — —

— Not measured.

and −0:7 K, respectively. The higher cooling e/ect of courtyard B is probably due to its lower tree canopy. The diurnal average cooling and relative humidity e/ects in the sites over the period of observation are summarized in Tables 1 and 2, respectively, together with the air-temperature and relative-humidity data at the corresponding reference sites.

QTSOLAR (t) is the contribution of solar radiation (direct, di/used and reTected) to air temperature variations, given by the following equation:   =t m
−(t − ) ; (2) QTSOLAR (t) = QIpen () 1 − exp h CTTC =0

where  denotes time ( = 0 being the moment preceding the start of the current prediction). Ipen (t) = I (t)(1 − PSAG (t)) + Idi/ (t) SVF

3. Analytical approach

− (I (t) + Idi/ (t) SVF)f (1 − C) PSATr ;

3.1. The Green CTTC model—outline The Green CTTC model, developed recently by the authors [5] is based on the same principles as the Cluster Thermal Time Constant (CTTC) model, developed earlier by Ho/man and colleagues [6–8], and incorporates additional thermal e/ects—those of the adjoining walls, of solar radiation reTected by the participating cluster’s surfaces, and of the shade trees. Like its predecessor, it comprises design parameters related directly to the physical structure and properties of the built-up complex (Toor and walls albedo, partial shaded areas, the cluster’s open space geometry, etc.) and to the density of shade trees. The predicted air temperature of the site is calculated through the contribution of the heat received from external sources, mainly the net solar radiation, anthropogenic heat release, and vegetation. A brief outline of the model’s basic theory is given below. The predicting equation for the cluster’s air temperature at time (t) is given by the following equation: T (t) = T 0 + QTSOLAR (t) − QTNLWR (t) + QTAHR (t);

(1)

where T0 is the regional base (or background) temperature, which was found to be equal to the mean daily air temperature measured at a rural meteorological station [7], representative of the non built-up area near the site.

I (t) is the unobstructed solar radiation intensity incident on a horizontal surface, and Idi/ (t) the sky di/used solar radiation intensity (W=m2 ), at time t. SVF is the cluster sky view factor. PSAG (t) is the partial shaded area from the walls on the ground at time t, PSATr is the partial shaded area from the trees on the ground at noon. The product f(1 − C)PSATr represents the thermal e/ect of shade trees [5], the coeOcient (1 − f) denoting the tree canopy solar radiation transmissivity, and C the convective heat exchange factor of the trees. The thermal e/ect of the shade trees is estimated by the e/ect of solar radiation penetration through the tree canopy and by the intensity of evapotranspiration. For well-developed Ficus trees, the decrease in insolation due to the trees was estimated in this study, for the summer period, to be about 40% (1 − C = 0:4) of the global solar radiation intensity incident on the tree canopy [5]. In other words, under complete shade coverage (PSATr = 1), the cluster would still receive an equivalent energy of 60% of the global solar radiation. The CTTC parameter, measured in hours, expresses the heat stored by the cluster’s surface areas per unit of heat transmitted through it. Its reciprocal (1/CTTC) in Eq. (2) acts as an attenuating factor in determining the cluster’s air temperature. The CTTC value for the ground was found to be about 8 hours for typical asphalt and concrete materials,

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and 6 hours for common massive external walls [7]. The relevant attenuating factors in the cluster’s Toor and walls are 18 and 16 per hour, respectively. The parameter m denotes the solar radiation absorptivity of the cluster’s surfaces. The quantitative thermal effect of changes in the wind velocity is expressed in the model through the overall surface heat transfer coeOcient, h (W=m2 K). The term QTSOLAR (t) is also calculated for the walls, and includes the e/ect of reTected radiation from the cluster’s surfaces. The overall value of QTSOLAR (t) in Eq. (1) is the weighted average of QTSOLAR (t) of the relevant surfaces weighted according to their surface areas. This weighting procedure was applied by Ho/man and Mosseri [8] for closed inner courtyards. QTNLWR (t) is the contribution of the net outgoing long-wave radiation exchange Tux to air cooling is given as ( Ts4 (t)−Br Ta4 (t)) SVF CC (t) QTNLWR (t) = h +



FA 1− S



(Ts4 (t) − BrTa4 (t))SVF
CC (t) FA ; hroof S (3)

where Ta (t) is the air absolute temperature (K) at the relevant meteorological station. Ts (t) is the absolute temperature (K) of the site’s relevant surface (Toor, wall, roof). In Eq. (3), the terms are the weighted average of QTNLWR (t) of the relevant surfaces, weighted according to the surface areas.  is the ground thermal emissivity of the cluster. Br is the Brunt atmosphere equivalent emissivity, which depends on the air partial water vapor pressure [9,10]. SVF and SVF
are the sky view factors of the cluster and roof surfaces, respectively. CC (t) = 1 − bN (t) is the cloudiness correction factor [8], where N (t) is the cloud cover at hour t measured in tenths on a scale of zero to unity, and b a coeOcient related to the cloud height, equals 0.9 for heights up to 1:5 km [11, Table A2.3]. The two terms on the right-hand side of Eq. (3) represent the cooling contributions of the cluster surfaces and building roof-tops, respectively. For the other parameters, see the list of symbols below. QTAHR (t) is the anthropogenic contribution to the air temperature, including man-made e/ects such as heat release owing to transportation, fuel consumption for domestic use (mainly air conditioning) and so forth. It is estimated in W=m2 and added to the absorbed solar radiation at time t (i.e. to mIpen (t)) [12]. 3.2. Application of the model to open space at a meteorological station Since the Green CTTC model is designed to predict the microclimate of di/erent urban typologies in a given

climatic region, the model is 8rst revalidated by applying it in predicting the diurnal air temperature variations of an urban open space without the e/ects of built-up forms and trees. This prediction is illustrated on data of a meteorological station, Beit-Dagan, representing the climatic coastal region in Israel. The diurnal estimates at Beit-Dagan calculated for July 1996 are shown in Fig. 3 versus the actual values of average monthly data, as well as the actual data of 21st July. The 8t is very close. Diurnal estimates were also obtained for January 1996. In contrast to the summer season—which in the Tel-Aviv metropolitan region is characterized by stable climatic conditions throughout—the winter season comprises clear, cloudy and rainy days. Due to this fact, the model was applied to the data of two consecutive partly cloudy days, 14th and 15th January rather than to the monthly average. The maximum predicted deviation from the actual data was 0.5 to 1:5 K. The close 8t of both the summer and winter data enhances the validity of the Green CTTC model. The parameters involved in the simulations in Fig. 3 were: mean solar radiation absorptivity of the ground, m=0:45, the overall heat transfer coeOcient of the surface, h, which is related to the wind velocity, was on the average 20 (W=m2 K) in July and 16 (W=m2 K) in January. The Brunt number (effective atmosphere emissivity), Br, which is related to the vapor pressure and cloudiness of the climatic region, was on the average 0.76 in July and 0.72 in January. Ground surface thermal emissivity,  = 0:92, ground CTTC = 7 h, cloud cover N for 14th and 15th January was 0.3 for the whole day with a maximum of 0.6 from 12:00 to 18:00 h. 3.3. Estimating the cooling e&ect of trees in urban canyon-type typologies In this section, the Green CTTC model is applied to simulate the diurnal air temperature at the four studied sites, with and without trees. The simulated values are compared with the measured values in situ at 9:00, 15:00 and 18:00 h, in summer 1996. The cluster’s parameter values are listed in Table 3. The simulated diurnal patterns of air temperature for the four sites as obtained using the Green CTTC model are shown in Fig. 4 (upper curve—without trees, lower curve— with trees). The di/erence between the levels of the two curves at each site gives the estimated cooling e/ect of the trees. In the case of Herzl Street, the lower curve shows that the measured values with trees are higher than the simulated ones. This discrepancy indicates a heating e/ect of other factors not included in the simulations, mainly the traOc heat release. By contrast, the measured values in Rothschild Blvd. and in the two courtyards are very close to the simulated ones, indicating no other signi8cant e/ects.

32

32

31

31

30

30

29

29

Air Temperature [C]

Air Temperature [C]

L. Shashua-Bar, M.E. Ho&man / Building and Environment 39 (2004) 1087 – 1099

28 27 26 25 24 23

28 27 26 25 24 23

22

22

21

21

20

20 5

7

9

11 13 15 17 19 21 23

1

3

5

5

7

9

11 13 15 17 19 21 23

Time [h] (a)

(b)

July - monthly average

20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 5

7

9

11 13 15 17 19 21 23

(c)

1

3

5

1

3

5

Time [h]

Air Temperature [C]

Air Temperature [C]

1093

1

Time [h] January 14th

3

July 21th

20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

5

5

7

9

11 13 15 17 19 21 23

Time [h] January 15th

(d)

= Measured data at meteorological station

= Simulated data

Fig. 3. Simulation values of diurnal air temperatures at Beit-Dagan meteorological station. July–January 1996.

Table 3 Parameter values for simulations using the Green CTTC model Site

Herzl street

Rothschild Blvd.

Courtyard A

Courtyard B

Length, L (m) Width, W (m) Height, H (m) Emissivity,  E/ective atmosphere emissivity, Br Sky view factor, SVF f parameter Tree density, PSATr Overall surface heat transfer coeOcient, h Surfaces solar radiation absorptivity, m for ground for walls Cluster thermal time constant (CTTC) (h) for ground for walls

220 20 12 0.92 0.74 0.625 0.933 0.63 14

245 45 12 0.92 0.74 0.789 0.948 0.74 16

75 20 12 0.92 0.74 0.612 0.956 0.66 12

67 25 12 0.92 0.74 0.645 0.956 0.75 12

0.52 0.50

0.55 0.50

0.70 0.50

0.70 0.50

8 6

8 6

8 6

8 6

L. Shashua-Bar, M.E. Ho&man / Building and Environment 39 (2004) 1087 – 1099 35

35

34

34

33

33

32

32

Air Temperature [C]

Air Temperature [C]

1094

31 30 29 28 27 26

31 30 29 28 27 26

25

25

24

24

23

23 5

7

9

11 13 15 17 19 21 23

1

3

5

5

7

9

11 13 15 17 19 21 23

Time [h]

5

3

5

Rothschild Blvd. (H/W = 0.26) Data : 25,28,29 Aug. 1996

(b)

35

35

34

34

33

33

32

32

Air Temperature [C]

Air Temperature [C]

3

Time [h]

Herzl St. (H/W = 0.60) Data : 2,5,10,12 July , 18 Aug. 1996

(a)

1

31 30 29 28 27 26

31 30 29 28 27 26

25

25

24

24

23

23 5

7

9

11 13 15 17 19 21 23

1

3

5

5

7

9

11 13 15 17 19 21 23

Time [h] Courtyard A (H/W = 0.60) Data : 11,13 Aug. 1996

(c)

Cluster without trees

1

Time [h] Courtyard B (H/W = 0.48) Data : 11,13 Aug. 1996

(d)

Cluster with trees

Measured values at the site

Fig. 4. Simulated patterns and measured in situ values of diurnal air temperature at the sites. Table 4 Comparison between measured and simulated estimates of cooling and speci8c e/ects (K) at 15:00 h Site

Herzl street

Rothschild Boulevard

Courtyard A

Courtyard B

(◦ C)

Air temperature From measurements 1. At the site 2. In the reference place

30.82 31.82

29.79 32.25

31.23 33.70

30.44 33.70

Estimated by simulations 3. At the site with trees 4. At the site without trees

29.30 31.30

30.30 33.10

30.80 33.30

31.00 34.10

−1.00 −2.00

−2.46 −2.80

−2.47 −2.50

−3.26 −3.10

0.75 1.00

−0.25 0.34

0.25 0.03

−0.70 −0.16

Cooling e/ect (K) 5. Measured: (1) – (2) 6. Simulated: (3) – (4) Speci8c e/ect (K) Estimated by empirical model Simulated: (5) – (6)

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Air Temperature [C]

The empirical estimates of cooling and speci8c e/ects by trees and the simulated ones at 15:00 h are compared in Table 4. The speci8c e/ect is de8ned as the di/erence between the measured value in situ and the estimated one [4], either by simulations or by the empirical model. At Herzl Street, the speci8c e/ect according to the empirical model (0:75 K) is close to its simulated counterpart (1 K). In the case of Rothschild Blvd. and courtyard A, the speci8c effects are also close. In courtyard B, the simulated e/ect is very low (−0:16 K) while the empirical estimate is relatively high (−0:7 K) but falls within the range of measurement accuracy. These facts, taken together with the close 8t with the Beit-Dagan simulation, add to the validity of the Green CTTC model for predicting the diurnal air temperature values in urban streets with shade trees.

1095

32 31 30 29 28 27 26 25 24 23 5

7

9

11

13

15

(a)

17

19

21

23

1

3

5

21

23

1

3

5

21

23

1

3

5

Time [h] 35 34

In the preceding sections, the validity of the Green CTTC model in predicting air temperature variations in the UCL was demonstrated 8rst on data of meteorological station and then on data of two urban streets and two canyon-type courtyards, with trees. The control variable studied in these sites was the density of shade trees. Modeling the UCL microclimate is performed through changing the levels of some major control variables besides the e/ect of shade trees. In this section, the thermal e/ects of building-scale control variables, such as the cluster geometry and the albedo of the walls, on the UCL air temperature are studied by simulations. Examination of the cluster’s orientation e/ect in the presence of shade trees and building-scale modi8cation is also considered. 4.1. The e&ect of cluster deepening Vernacular architecture made much use of deep urban spaces (high H=W ratio) and narrow open ones in enhancing cooling in hot and dry regions. Modern technology and traOc problems led to reversal of this trend. Lately, however, the increasing need for residential housing makes for high buildings on existing open ground, with the resulting deepening, but not necessarily in narrow spaces. Municipal ordinances now allow increase in existing building height from 3– 4 to 6 –8 stories in certain streets in Tel-Aviv. The deepening e/ect is evaluated in this study using the Green CTTC model by three separate simulations, conducted for a N–S axis canyon street for H=W of 0.25, 0.5 and 1.0, and 70% tree shading by developed Ficus trees in each case. Solar radiation and wind data used are for July 1996, from the meteorological station at Beit-Dagan. The simulated diurnal air temperature values for the three cases are shown in Fig. 5. All the curves systematically show lower diurnal air temperatures as the deepening process evolves, i.e. the higher the H=W , the lower the UCL air temperature. The simulated air temperature values at 15:00 h for the three cases and the cooling e/ects of deepening are summa-

32 31 30 29 28 27 26 25 24 23 5

7

9

11

13

15

(b)

17

19

Time [h] 35 34 33

Air Temperature [C]

4. Modeling the UCL microclimate

Air Temperature [C]

33

32 31 30 29 28 27 26 25 24 23 5

7

9

11

(c)

13

15

17

19

Time [h] Cluster without trees (m walls = 0.5)

Cluster with trees (m walls = 0.5)

Cluster with trees (m walls = 0.3)

Fig. 5. Simulated patterns of diurnal air temperature in an urban N–S canyon-street with 70% Ficus tree shading. Data: July 1996, Beit-Dagan, Israel.

rized in Table 5. It is seen that the drop in air temperature due to deepening by doubling the cluster height is stronger, the higher the H=W . Thus in a cluster of H=W = 0:5, the air temperature is expected to be 1:18 K lower than in its counterpart H=W = 0:25 while in that of H=W = 1, the expected drop is 1:56 K relative to the 0.5 cluster. At the same time, deepening reduces the cooling e/ect of trees, in view of a larger volume of air to be cooled in the deep courtyard than in a shallow one. The total cooling e/ect of the UCL air

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L. Shashua-Bar, M.E. Ho&man / Building and Environment 39 (2004) 1087 – 1099

Table 5 Simulated air temperature values (◦ C) and deepening e/ects (K) in an N–S canyon—street with and without trees at 15:00 h, July 1996 Site (◦ C)

Simulated air temperature 1. In cluster without trees 2. In cluster with trees 3. Simulated cooling e/ect of trees (K) (2) − (1) 4. Simulated e/ect of deepening H=W (K) (a) −1:18 = 32:97 − 31:79 (◦ C) (b) −1:56 = 31:79 − 30:23 (◦ C) 5. Total cooling e/ect of trees and deepening (K) (3)+(4)

H=W = 0:25

H=W = 0:5

H=W = 1:0

32.97 30.20 −2.77

31.79 29.41 −2.38

30.23 28.40 −1.83

—

−1:18a

−1:56b

−2.77

−3.56

−3.39

Parameters used: CTTC (h)=8 for ground, 6 for walls, m=0:55 for ground, 0.5 for walls, h (W=m2 K)=18, 16, 14 for H=W =0:25, 0.5, 1.0 respectively.

temperature due to 70% tree shading and to deepening is expected to reach up to 3:5 K at 15:00 h in summer in the Tel-Aviv metropolitan area. 4.2. The e&ect of albedo modi7cation Albedo of a surface (or reTectivity) is represented in the Green CTTC model by the parameter (1−m). Light-colored surfaces usually have high albedo values in the range of 0.70 – 0.80, in contrast to dark surfaces with low albedo values, less than 0.50. Increasing the albedo reduces the surface solar radiation absorption and thus the surface temperature which in turn a/ects that of the air in contact. Modi8cation of a surface albedo is easily obtained by means of suitable colors such as white paints, or of suitable materials such as light-colored marble, white cementitious coatings, etc. In the following, the e/ect of albedo modi8cation of the walls is assessed through simulations with the albedo changed from 0.5 to 0.7. These two levels are representative of the various colors over time: light colors are darkened by pollution, while dark colors are lightened on prolonged exposure to solar radiation. The albedo of the ground was 0.45 for all the modeling simulations. The cooling e/ect of albedo modi8cation is summarized in Table 6. The maximum e/ect is 1:18 K, obtained in the H=W = 1 case. As the studied change in albedo concerns the walls alone, the higher H=W the stronger the e/ect. In a shallow cluster, such as H=W = 0:25, the e/ect is relatively small: −0:56 K. The total modeling e/ect expected in canyon streets by means of 70% density of shade trees, deepening, and albedo change is given in the last row of Table 6 by adding the albedo e/ect to those of the trees and deepening from Table 5. The total expected sustainability e/ect is about 4:5 K, which amounts to about half of the daily temperature rise (max–min) in July–August in the coastal region of Israel.

4.3. Orientation e&ect The modeling e/ects discussed in the preceding sections refer to N–S oriented urban streets. Many studies have demonstrated empirically that orientation a/ects the distribution of insolation between the cluster Toor and the walls, and thereby the UCL air temperature (e.g. [2,7,13]). Shade trees, however, reduce the orientation e/ect considerably, as was found by the authors in a previous study [14]. The analysis was conducted experimentally on measurements taken in situ along two boulevards situated in the same part of Tel-Aviv, one of them is oriented approximately N–S and the other approximately E–W, similar in terms of street width and height of the Tanking buildings. Both are planted along a median strip with developed Ficus trees of about the same age. The 8ndings in that experiment indicate no signi8cant cooling e/ect due to orientation. In this context, simulations similar to those in Sections 4.1 and 4.2 were conducted on E–W longitudinal axis streets, planted with Ficus trees. The modeling e/ects resulting from the albedo modi8cation and deepening in a E–W canyon streets are summarized in Table 7, for H=W = 0:25, 0.5 and 1.0, and 70% shading by developed Ficus trees in each case. Comparison between the total e/ect of modeling in the N– S and that in the E–W streets (Tables 6 and 7, respectively) shows similar cooling e/ects, the N–S oriented being cooler by a maximum of 0:64 K at 15:00 h (air temperature of 27:22◦ C in N–S as against 27:86◦ C in E–W, for H=W =1:0). The albedo modi8cation of walls has slightly stronger e/ect in the N–S than in the E–W street (−1:18 K in the N–S for H=W = 1:0 as against −0:69 K in the E–W). The simulated diurnal air temperature values in the N–S and E–W streets with 70% Ficus trees shading, for the two extreme aspect ratios, H=W = 0:25 and 1.0, and albedo of 0.7 for walls are shown in Fig. 6. The simulated patterns in Fig. 6 indicates that in the presence of shade trees the E–W street is warmer than the

L. Shashua-Bar, M.E. Ho&man / Building and Environment 39 (2004) 1087 – 1099

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Table 6 Simulated air temperature (◦ C) and albedo e/ects of walls (K) in a N–S canyon street with trees, data: 15:00 h in July 1996 Site Simulated air temperature (◦ C) 1. In cluster with trees, 0.5 albedo of walls (m = 0:5) 2. In cluster with trees, 0.7 albedo of walls (m = 0:3) 3. Simulated cooling e/ect of albedo (K) (2) − (1) 4. Simulated cooling e/ect of trees and albedo (K) (3) + (3) from Table 5 5. Total cooling e/ect of trees, albedo and deepening (K) (4) + (4) from Table 5

H=W = 0:25

H=W = 0:5

H=W = 1:0

30.20 29.64 −0.56

29.41 28.52 −0.89

28.40 27.22 −1.18

−3.33

−3.27

−3.01

−3.33

−4.45

−4.57

Table 7 Simulated air temperature (◦ C) and albedo e/ects of walls (K) in a E–W canyon street with trees, data: 15:00 h in July 1996 Site Simulated air temperature (◦ C) 1. In cluster without trees, 0.5 albedo of walls (m = 0:5) 2. In cluster with trees, 0.5 albedo of walls (m = 0:5) 3. In cluster with trees, 0.7 albedo of walls (m = 0:3) 4. Simulated cooling e/ect of trees (m = 0:5) (K) (2) − (1) 5. Simulated e/ect of deepening H=W (m = 0:5) (K) (a) −1:14 = 33:07 − 31:93 (◦ C) (b) −1:55 = 31:93 − 30:38 (◦ C) 6. Simulated cooling e/ect of albedo (K) (3) − (2) 7. Simulated cooling e/ect of trees and albedo (K) (4)+(6) 8. Total cooling e/ect of trees, albedo and deepening (K) (5)+(7)

H=W = 0:25

H=W = 0:5

H=W = 1:0

33.07 30.30 29.98 −2.77

31.93 29.56 29.04 −2.37

30.38 28.55 27.86 −1.83

—

−1:14a

−1:55b

−0.32

−0.52

−0.69

−3.09

−2.89

−2.52

−3.09

−4.03

−4.07

32 31 H/W = 0.25 E-W st. N-S st.

Air Temperature [C]

30 29 28 27 26 H/W = 1.0 E-W st. N-S st.

25 24 23 22 5

7

9

11

13

15

17

19

21

23

1

3

5

Time [h] H/W = 0.25:

E-W st. with trees

N-S st. with trees, H/W = 1.0:

E-W st. with trees

N-S st. with trees,

Fig. 6. Simulated patterns of diurnal air temperature in urban N–S vs. E–W canyon streets with 70% Ficus tree shading and albedo 0.7 of walls. Data: July 1996, Beit-Dagan, Israel.

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N–S street by 0:34 K for H=W = 0:25 and by 0:64 K for H=W =1:0: the lower the H=W ratio, the weaker the e/ect of orientation. These results are subject to the assumption that a change in orientation in streets with trees does not considerably a/ect the wind regime within the cluster. The quantitative thermal e/ect of airTow is expressed in the Green CTTC model through the parameter h (Eqs. (2) and (3)). The same values of h for the relevant geometry (H=W ratio) were used in the orientation simulations. 5. Summary and conclusions The Green CTTC analytical model is applied in simulating diurnal air temperature patterns in the UCL of canyon streets and attached courtyards. Sensitivity analysis of modeling e/ects in the UCL are examined by simulations conducted on summertime data for four major control elements: trees, albedo modi8cation of the walls, thermal e/ect of deepening the cluster, and orientation. The main 8ndings are: (1) The Green CTTC model was found suitable for predicting the air temperature of an open space at a meteorological station in summer as well as in winter. This fact enhances its applicability to urban clusters. (2) The model is suitable for predicting the air temperature of the UCL. The cluster types examined are urban streets and attached outdoor canyon-type courtyards with and without trees. Comparison with daily measurements in situ shows deviations less than 0:5 K at 15:00 h in summer. (3) The cooling e/ect of shade trees in streets and courtyards in Tel-Aviv was found to depend on the tree density and on the cluster geometry. The average measured cooling e/ects in streets was 2:5 K in Rothschild Blvd. and 1:0 K in Herzl St., with partial shaded area (PSATr ) 0.74 and 0.63, respectively. Cooling e/ects of 2.5 –3:1 K were obtained in the courtyards, with PSATr 0.66 and 0.75, respectively. (4) Deepening of the street and the adjoining courtyard by increased building height reduces the UCL air temperature. The simulated e/ect of changing the height from 12 m (four stories) to 24 m (eight stories) produces a cooling e/ect of 1:5 K in a street or a courtyard 24 m wide. (5) The cooling e/ect of trees depends on the tree shade coverage (PSA level) as well as on the cluster geometry. This e/ect is reduced by deepening the cluster, as the tree cooling energy serves a larger volume of air in a deep cluster than in a shallow one with equal Toor area. These 8ndings are illustrated by simulations with clusters of di/erent H=W , and equal tree shade coverage.

(6) Abedo of the walls has a signi8cant e/ect on the UCL microclimate. Albedo modi8cation from 0.5 to 0.7 has a cooling e/ect of as much as 1:2 K. The deeper the cluster, the stronger the cooling e/ect of albedo change. In N–S clusters, albedo modi8cation has a slightly stronger cooling e/ect (about 0:5 K) than in the E–W clusters. (7) The orientation e/ect on air cooling in N–S wooded clusters was found to be only slightly stronger than their E–W counterparts. The orientation e/ect is stronger, the deeper the cluster and the higher the albedo of the walls. In the H=W = 1:0 cluster with high albedo, the N–S orientation is about 0:64 K cooler at 15:00 h than in the E–W oriented one. (8) The combined simulated cooling e/ect of modeling with trees, high albedo and deepening reaches about 4:5 K at midday in summer (July–August) in the coastal region of Israel. The indoor AC energy savings and the improvement in outdoor comfort conditions through a 4:5 K cooling in the UCL air temperature and with more shade coverage, illustrate the potential bene8ts inherent in sustainability modeling, especially in hot and temperate climates.

Acknowledgements The authors are indebted to E. Goldberg for editional assistance and comments and also to Dr. Alan Seter from the Meteorological Station Beit Dagan for providing climatic data.

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[13] Barring L, Mattsson JO, Lindqvist S. Canyon geometry, street temperatures and urban heat islands in Malmo, Sweden. Journal of Climatology 1986;5:433–44. [14] Shashua-Bar L, Ho/man ME. Geometry and orientation aspects in passive cooling of canyon streets with trees. Journal of Energy and Buildings 2002;35:59–66.