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An investigation of outdoor thermal environments with different ground surfaces in the hot summer-cold winter climate region
Journal of Building Engineering 27 (2020) 100994
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An investigation of outdoor thermal environments with different ground surfaces in the hot summer-cold winter climate region Zefeng Huang a, Zhonghua Gou b, *, Bin Cheng a a b
School of Civil Engineering and Architecture, Southwest University of Science and Technology, Mianyang, 621010, China School of Engineering and Built Environment, Griffith University, Gold Coast, QLD 4215, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Outdoor thermal environment Ground surface Reflectivity Materials
This study aims to find the impact of different ground surfaces on thermal environments in outdoor activity space. Field measurements were conducted in a university campus located in the hot summer-cold winter climate zone. Five typical outdoor ground surfaces were selected: light-colored marble brick, plastic track, bluestone slate under the shade, lawn under the shade, and unobstructed lawn. Thermal environment parameters, including air temperature, relative humidity, solar radiation, surface temperature, mean radiant temperature (Tmrt) and the physiological equivalent temperature (PET) were measured and calculated for summer and winter, respectively. It is confirmed that the property of the ground surface had a significant effect on surface temper ature and Tmrt in winter and had a significant effect on relative humidity and PET in summer. The use of ground surfaces with low reflectivity which could increase the surface temperature by 4.5 � C in winter while reduce the heat stress (PET) by 3.7 � C in summer, should be encouraged in outdoor activity space.
1. Introduction Thermal environment is closely associated with the comfort and safety. At present, the studies of indoor thermal environment are more than that of outdoors [1–3]. One of important reasons is that the outdoor thermal environment is more complex than indoors. Especially, outdoor thermal environment is affected by solar radiation, which is not considered in terms of indoor thermal environment; thus, it is more complicated and there is no unified evaluation index at present. According to the results of relevant studies [4,5], the change of outdoor thermal environment is closely related to the properties of ground surfaces, which is the interface of energy budget of land surface regulating and controlling the hydrologic balance and energy balance of an ecosystem, and influencing thermal environment ultimately [6]. The effects from ground surfaces on the outdoor thermal environment mainly include the emitted radiation from the surface itself and the re flected short-wave solar radiation. The emitted radiation is mainly concentrated in the far-infrared band of more than 6 μm, and the re flected radiation to the solar radiation is mainly concentrated in the visible light and near-infrared bands of 0.3–2.5 μm. In the visible and near-infrared band (0.3–2.5 μm), the emitted radiation of the ground surface itself is almost zero, so the effect of the ground surface on the
outdoor thermal environment is mainly reflected by the reflected solar radiation. Table 1 shows the reflectivity of common ground surface materials. Ran [7] made human body equivalent to a cylinder "human body cylinder" (human body cylinder and human body have the same height and circumference), built physical and mathematical models of the ra diation emitted and reflected from the ground surface to the human body. There have been several empirical studies that investigated the thermal environment or comfort in relation to different ground surfaces properties. For example, Taleghani [8] conducted the research over the influence of increased reflectivity of the ground surface on outdoor thermal comfort in a university of the Netherlands; the result showed that the material with high reflectivity can significantly affect the thermal comfort of pedestrians in open space, and the mean radiant temperature would rise 1.2 � C and PET would rise 0.8 � C as every 0.1 increase of reflectance ratio. Campra et al. [9] found that white surfaces could make a city’s temperature lower than its outskirts of 0.3 � C in Almeria, Spain. However, in Athens, Santamouris et al. [10] studied the effect of highly reflective pavement of 4,500 m2 on the microclimate of local parks; they observed that the air temperature decreased by 1.9 � C, and the surface temperature decreased by 12 � C in typical summer time. In the meantime, Kyriakodis et al. [11] observed in Athens that asphalt
* Corresponding author. E-mail address: [email protected] (Z. Gou). https://doi.org/10.1016/j.jobe.2019.100994 Received 22 May 2019; Received in revised form 8 August 2019; Accepted 12 October 2019 Available online 14 October 2019 2352-7102/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Building Engineering 27 (2020) 100994
subtropical mountainous humid monsoon climate zone. The annual average temperature is from 14.7 � C to 17.3 � C and annual average precipitation is from 825.8 mm to 1,417 mm. Measuring point selection and ambient conditions are shown in Fig. 1. In this survey, five different ground surfaces were selected as data measurement areas, including light-colored marble brick, plastic track, unobstructed lawn, lawn under the shade and bluestone slate under the shade. These ground surfaces are common materials for outdoor activity space paving, and the in clusion of measuring points under the shade can further study the effects of solar radiation on the thermal environment above different ground surfaces. All points were measured simultaneously. The winter measurement took place on January 15th, 17th and 20th, 2018, a total of three days; the measuring time is from 9:00 to 17:00. The summer measurement took place on 22nd and 23rd, 2018, a total of two days, with measuring time from 9:00 to 18:00. There were no raining days during the measurement period (including the days before the measurement), under which circumstance the water absorption by the permeable ground surfaces and its related water evaporation are minimal. Thermal environment parameters include air temperature, relative humidity, wind speed, solar radiation, globe temperature and surface temperature have been recorded in this study. The instruments used to measure the parameters are shown in Table 2. All instruments are in compliance with ISO7726 standard [14]. Except for the ground surface temperature that was measured and recorded manually by the infrared thermometer at a 30 min interval, other thermal environment parame ters were recorded automatically with a 10 min interval and at a height of 1.1 m. Five sets of instruments were used for the simultaneous mea surement of the five ground surfaces. However, due to the failure of some instruments, some data for the bluestone slate under the shade in summer was missing; therefore, we excluded the point in the summer analysis. The average, instead of each of the measurement days was used for the analysis, to reduce the bias due to some data missing and the climate variation.
Table 1 Reflectivity of common ground surface materials. Category
Reflectivity
Category
Reflectivity
Dry bare land Wet bare land Dry lawn Wet lawn Plastic track
0.1–0.2 0.08–0.09 0.15–0.25 0.14–0.26 0.10
Light-colored marble brick Painted floor Steel plate floor Concrete floor Asphalt ground
0.19 0.10 0.15 0.20 0.10
pavement and concrete pavement can cool the air temperature and ground surface temperature by 1.5 � C and 11.5 � C respectively. The abovementioned is the positive influences of the ground surface of highly reflective material on the outdoor thermal environment. How ever, some studies have shown that reflectivity does not affect the thermal conditions near the ground. Erell et al. [12] indicated that the use of highly reflective materials can cool the temperature but also in crease the thermal stress for pedestrians. In this regard, further research is required to investigate the influence of ground surface properties on the thermal environment especially for pedestrian comfort. In this paper, we analyze the influence of outdoor ground surfaces of different reflectance properties on the outdoor thermal environment in a university campus with the hot summer-cold winter climate; at the same time, this paper uses the physiological equivalent temperature (PET) which considers human adaptation as the outdoor thermal comfort index [13], and further analyze the correlation between main outdoor thermal environment parameters and PET. This study also explores which thermal environment parameters in winter and summer are the main factors affecting PET, aiming to provide evidence for the optimi zation and improvement of the outdoor thermal environment. 2. Methodology The measurement took place in the campus of Southwest University of Science and Technology which located in the northeast of Mianyang City, Sichuan Province (104.73� E, 31.48� N), China. It belongs to the
Fig. 1. Layout of measuring points. Table 2 The measurement instrument set and accuracy. Instrument
Measured parameter
Model
Range
Accuracy
Manufacturer
Hand-held weather station
Air temperature Relative humidity Wind speed Solar radiation Globe temperature Underlying surface temperature
PH-II-C
50-80 � C 0–100% 0–45 m/s 0–2000 W/m2 0–120 � C 30-400 � C
�0.3 � C �5% �0.3 m/s �5% �0.1 � C �1.5 � C
XINPUHUI
Solar radiation meter Globe temperature probe Infrared thermometer
JTR05 Testo 480 Testo 830-T2
2
JANTYTECH Testo Testo
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Journal of Building Engineering 27 (2020) 100994
Fig. 2. Comparison of air temperature above the ground surface of different properties.
3. Analysis and results
the direct radiation from sunlight may be an important factor affecting the air temperature above the ground surface. In summer, the change of air temperature was different from that in winter. The air temperature increased continuously, and the trend of temperature decrease was not significant even during the period from 17:00 to 18:00. The lowest mean air temperature appeared at the lawn under the shade (31.8 � C), fol lowed by the unobstructed lawn (36.5 � C), indicating that the plant can effectively reduce the air temperature in summer. Fig. 3 shows the data of relative humidity and their descriptive sta tistics are shown in Table 4. The relative humidity above all the measuring points shows a gradual decreasing trend regardless of the seasonal difference. In winter, the relative humidity above the measuring points of the lawn under the shade and bluestone slate under the shade was 62.6% and 60.0% respectively higher than the other measuring points. The difference reached the peak (20%) at around 10:30 a.m. In summer, the variation of relative humidity was almost the same as that in winter. The measuring point at the lawn under the shade had the highest mean value (52.8%), followed by the unobstructed lawn (44.0%). Fig. 4 shows the data of solar radiation and Table 5 shows the descriptive statistics. The solar radiation of non-shaded points showed a single-peak trend with the maximum value appearing during the time from 13:00 to 15:00. However, the solar radiation of shaded points was stable at a low level and was basically the same in winter and summer without significant changes. Regardless of the seasonal difference, the solar radiation at the shaded points were significantly lower than that
First, all measured data were illustrated in graphs to show their variations during a day, and their descriptive statistics was summarized in tables. After that, ANOVA (Analysis of Variance) and LSD (Least Significant Difference) methods were used for comparisons of thermal environment parameters between different ground surfaces. In partic ular, ANOVA was used to explore whether the ground surface of different properties had effect on the measured thermal environment parameters; LSD method was used to further compare which properties of the ground surface had a most significant impact. Finally, correlation analysis was used to determine the relationships between different thermal environment parameters. 3.1. Data description Fig. 2 shows the air temperature change above the ground surface of different properties in winter and summer and Table 3 summarizes their basic statistics. In winter, the air temperature shows a single-peak trend of increasing first and then decreasing during the measured time of a day. Except that the temperature of two measuring points above the lawns reached the highest around 16:00, the temperature of other measuring points reached the highest around 15:00. The plastic track measuring point had the highest mean air temperature at 14.4 � C, while the bluestone slate under the shade and lawn under the shade measuring point had the lowest at 11.3 � C and 11.9 � C respectively, indicating that Table 3 The descriptive statistics for air temperature (� C).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
13.7 42.0
10.1 33.9
16.9 48.1
2.1 3.7
14.4 38.6
10.4 33.0
17.8 45.9
2.5 3.5
11.9 31.8
8.8 27.8
15.0 34.4
2.1 1.7
12.9 36.5
8.9 29.0
18.0 44.4
3.0 3.1
11.3 –
8.2 –
13.0 –
1.6 –
3
Unobstructed lawn
Bluestone slate under the shade
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Journal of Building Engineering 27 (2020) 100994
Fig. 3. Comparison of relative humidity above the ground surface of different properties.
without being shaded. The maximum difference in winter could reach 313 W/m2 at around 14:30, while the maximum difference in summer occurred at 13:00, for 877W/m2. The air flow causes the short-wave radiation heat absorbed by the surface to be transmitted to the air in the form of convection and is transmitted upward through the turbulent heat exchange. Therefore, the air temperature at the pedestrian height is affected to some extent by the surface temperature. In winter and summer, the higher the surface temperature is, the stronger the exchange of heat with the human body is. Fig. 5 shows the ground surface temperature and Table 6 shows their descriptive statistics. Regardless of the seasonal difference, the surface temperature at all measuring points showed generally a single-peak trend of increasing first and then decreasing during the measured time period of one day. The mean surface temperature of the plastic track (16.2 � C) was the highest in winter, followed by the unobstructed lawn (13.3 � C) and the light-colored marble brick (11.6 � C). The mean surface temperature of the lawn under the shade (9.7 � C) and the bluestone slate under the shade (8.9 � C) was much lower than others. The summer sit uation was similar to that in winter. The mean surface temperature of the plastic track (52.2 � C) was the highest, and the lawn under the shade (25.7 � C) was the lowest, but the mean surface temperature of the lightcolored marble brick (43.7 � C) was higher than that of the unobstructed lawn (35.3 � C). This shows that the ground surfaces of different prop erties had different temperature changes under the same solar radiation due to their different heat capacity and reflectivity. Among them, vegetation could properly reduce the surface temperature, and shading is an important factor to reduce the surface temperature.
In the outdoor thermal environment, mean radiant temperature (Tmrt) is a popular parameter which is often used to evaluate thermal comfort or to calculate the radiative heat loss of the human body. Tmrt is defined as the surface temperature of a hypothetical isothermal enclosed surface. The radiation heat transfer between it and the human body is equivalent to that between the actual non-isothermal enclosed surface and the human body [15]. The calculation formula of Tmrt is in (1) [16]: � ��1=4 � �4 1:10 � 108 v0:6 Tg Ta Tmrt ¼ Tg þ 273 þ 273 (1) 0:4 εD Wherein, Tmrt is mean radiant temperature, � C; Tg is globe temperature, C; Ta is air temperature, � C; v is wind speed, m/s; D is the globe diameter, m; (The standard globe with D ¼ 0.15 m is used in this paper). ε is the absorption rate of the globe (0.95 in this paper). Fig. 6 shows the variation of Tmrt and Table 7 shows their descriptive statistics. From the winter results, the difference of Tmrt at each measuring point is not significant in the morning; however, in the af ternoon when the solar radiation was gradually increasing, the differ ence became more significant, reaching a maximum at around 15:00, and the difference found between the unobstructed lawn and the blue stone slate under the shade was 21 � C. In the measurement period, the two measuring points under the shade received limited direct sunlight, which caused that Tmrt was significantly lower than the other three measurement points. In the afternoon of winter, the unobstructed lawn and plastic track could directly receive the solar radiation and had higher Tmrt. No doubt, solar radiation was an important factor affecting �
Table 4 The descriptive statistics for relative humidity (%).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
53.5 36.6
46.5 29.7
62.0 55.5
4.5 6.1
53.0 39.1
45.0 30.0
62.7 47.3
5.6 5.5
62.6 52.8
51.6 47.3
76.5 68.8
8.4 5.3
52.8 44.0
45.5 33.4
58.3 61.0
3.6 5.9
60.0 –
48.2 –
72.7 –
8.1 –
4
Unobstructed lawn
Bluestone slate under the shade
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Journal of Building Engineering 27 (2020) 100994
Fig. 4. Comparison of solar radiation at the ground surface of different properties.
Tmrt. In summer, Tmrt of all measuring points maintained at a high level during the measurement time, and the maximum value of Tmrt was measured in unobstructed lawn at 11:00, which is 71.9 � C; the minimum value of Tmrt was measured in the lawn under the shade at 9:00, which is 28.1 � C. At the same time, compared with the two measuring points of light-colored marble brick and the plastic track, Tmrt at the unobstructed lawn was also higher in most of the time. This shows that the ground surface properties had little effect on Tmrt, and even the lawn with a certain cooling effect on the air temperature may not improve the sur rounding Tmrt. In addition, Tmrt at the measuring point of the lawn under the shade was much lower than the rest measuring points in most of the time, which further proved that solar radiation is an important factor affecting Tmrt. PET is an index for comprehensive evaluation of thermal environ ment and is a physiological equilibrium temperature under a given environment. Its value is equal to the temperature corresponding to a typical indoor environment that reaches the same outdoor heat state [17]. The influence of main thermal environment parameters, activity and clothing insulation on human comfort is considered comprehen sively, and the outdoor thermal environment condition can be evaluated synthetically. In this paper, RayMan 1.2 software was used to input the relevant parameters such as air temperature, relative humidity, wind speed, water vapor pressure, Tmrt, clothing insulation and activity, and the PET value was calculated. Among them, the clothing insulation value was set at 1.5 clo and 0.5 clo respectively for winter and summer according to the typical daily dress in the two seasons, and the activity
was 80W for a slowly walking adult. Literature [18] studied the rela tionship between PET and thermal sensation in the outdoor environment in different regions, and divided the thermal sensation into nine different scales, which are very cold, cold, cool, slightly cool, neutral, slightly warm, warm, hot, and very hot, and correspond to different ranges of PET values. Fig. 7 shows the variation of PET and Table 8 shows the descriptive statistics. The PET values at the two measuring points (the bluestone slate under the shade and the lawn under the shade) in winter were at a low level throughout the day, and the values fluctuated mostly within the range of 7–13 � C which are corresponding to the thermal sensation: cold and cool, and categorized as less comfortable. The PET values of the other three measuring points in the morning time period were only slightly higher than those measured under the shade. The difference was not large; however, starting from around 12:00, with the increase of solar radiation, the PET value at these three measuring points began to rise, and was significantly higher than the PET value at the two measuring points under the shade, reaching a maximum of 22.9 � C at around 15:00. In the time range of 14:00 to 16:00, PET values at these three measuring points were basically within the range corresponding to the thermal sensation of neutral. It shows that solar radiation had a significant effect on improving the thermal sensation of people in winter, especially in the place where the ground surface can receive direct sunlight in the afternoon. In summer, except for the measuring point of lawn under the shade, the PET at the other three measuring points was at a high level during the measurement period of the day, mainly fluctuating within the range
Table 5 The descriptive statistics for solar radiation (W/m2).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
210 693
88 210
367 936
85 213
214 634
90 28
360 943
78 320
58 49
22 30
139 78
23 12
191 738
80 257
341 971
78 217
36 –
8 –
64 –
18 –
5
Unobstructed lawn
Bluestone slate under the shade
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Journal of Building Engineering 27 (2020) 100994
Fig. 5. Comparison of surface temperature of ground surface of different properties.
of 40–50 � C, which are corresponding to the thermal sensation: hot and very hot. The PET at the measuring point of the lawn under the shade was lower, mainly fluctuating within the range of 27–34 � C, corre sponding to the thermal sensation neutral and slightly warm. This showed that effective shading to reduce direct exposure to solar radia tion is an important method for people to improve their thermal sensation in summer.
(16.2 � C) and Tmrt (21.3 � C) appeared at the plastic track while the lowest mean surface temperature (11.6 � C) and Tmrt (17.9 � C) of appeared at the light-colored marble brick. In summer, the highest (44.0%) and the lowest (36.6%) mean relative humidity appeared at the unobstructed lawn and the light-colored marble brick respectively; the highest (45.4 � C) and the lowest (41.7 � C) mean PET appeared at the light-colored marble bricks and the plastic runways, respectively. Results of further comparisons using LSD are shown in Table 9. The surface temperature and Tmrt of the plastic track was respectively 4.5 � C and 3.4 � C higher than that of the light-colored marble brick in winter. In addition, by comparing the relative humidity above the ground surface under different properties in summer, the following significant differ ences can be found: light-colored marble brick < plastic track < unob structed lawn, in which the unobstructed lawn can conduct transpiration in summer and have the strongest humidification effect. The multiple comparison tests show that the PET above the unobstructed lawn and the light-colored marble brick was respectively 3.5 � C and 3.7 � C higher than that above the plastic track.
3.2. Anova and Lsd To ensure the effectiveness of ANOVA, the test of variance homo geneity must be conduct first to determine whether the variance of the measured thermal environmental parameters (air temperature, relative humidity and surface temperature) and the calculated thermal comfort index (Tmrt and PET) above each ground surface in winter and summer are equal. The results show that the variance of surface temperature (F ¼ 1.732, Sig. ¼ 0.188) and Tmrt (F ¼ 1.767, Sig. ¼ 0.175) at the ground surface under different properties in winter, the variance of relative humidity (F ¼ 0.724, Sig. ¼ 0.487) and PET (F ¼ 2.104, Sig. ¼ 0.125) at the ground surface under different properties in summer are equal, which meets the premise requirements of ANOVA. The results of ANOVA show that the effects of ground surface properties on surface temperature (F ¼ 5.064, Sig. ¼ 0.010), Tmrt (F ¼ 3.521, Sig. ¼ 0.032) in winter, and on relative humidity (F ¼ 23.259, Sig. ¼ 0.000), PET (F ¼ 13.098, Sig. ¼ 0.000) in summer were significant. In winter, the highest mean surface temperature
3.3. Correlation During the measurement time, the variation of PET value above the ground surface of different properties with time in winter and summer is basically consistent with the variation of air temperature and Tmrt with time. To further understand which thermal environment parameters in winter and summer had a significant effect on PET, a correlation analysis
Table 6 The descriptive statistics for surface temperature (� C).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
11.6 43.7
6.8 30.2
16.3 52.9
3.3 6.4
16.2 52.2
9.8 36.4
22.7 65.8
4.7 9.5
9.7 25.7
7.0 23.8
11.5 27.5
1.3 1.1
13.3 35.3
7.2 31.1
19.8 40.4
4.5 2.6
8.9 –
6.4 –
10.7 –
1.6 –
6
Unobstructed lawn
Bluestone slate under the shade
Journal of Building Engineering 27 (2020) 100994
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Fig. 6. Comparison of Tmrt above the ground surface of different properties.
is needed. Fig. 8 and Fig. 9 respectively shows the scatter plots of air temperature, Tmrt and PET at the two measuring points of the lawn under the shade and unobstructed lawn in winter and summer. The curve estimation was carried out, and the corresponding regression equation was obtained. To ensure that the regression equation can better reflect the statis tical relationship between air temperature, Tmrt and PET, the signifi cance test of the regression equation is needed. Table 10 and Table 11 show the results of the significance test of the regression equation of air temperature, Tmrt and PET in the lawn under the shade measuring point in winter. The two-tailed probability P value corresponding to the F statistic observation value is approximately zero. When the significance level α is 0.05, the null hypothesis of significance test of the regression equation should be rejected; it is considered that there is a significant secondary relationship between air temperature, Tmrt and PET in the lawn under the shade measuring point in winter. The regression equa tions of air temperature, Tmrt and PET at the other four measuring points in winter and all measuring points in summer can also pass the signifi cance test, indicating that the regression equations can better reflect the statistical relationship between the explained variable PET and the explanatory variable air temperature and Tmrt. From the comparison of the regression equations in Fig. 8, the air temperature, Tmrt and PET above the lawn under the shade and the unobstructed lawn in winter show a significant secondary relationship, and the effect of air temperature at the two measuring points on PET is significantly stronger than that of Tmrt on PET. By comparing Fig. 9, it
can be found that the relationship between the air temperature, Tmrt and PET above the lawn under the shade and the unobstructed lawn in summer is slightly different from that in winter. In summer, at the measuring point at the lawn under the shade, the effect of air temper ature on PET was significantly stronger than that of Tmrt on PET. But at the measuring point in the unobstructed lawn, it is opposite. The effect of Tmrt on PET is significantly stronger than that of air temperature. This may be due to the increase in air temperature, which causes Tmrt to rise at the same time; while the rate at which Tmrt rises is much higher than the rate at which air temperature rises. Therefore, at the unobstructed lawn where the air temperature is high in summer, Tmrt is the main factor to affect PET. Table 12 shows the correlation between air temperature, Tmrt and PET above all the ground surface of different properties measured in winter and summer. The results show that the correlation between air temperature, Tmrt and PET at two measuring points under the shade in winter were weak compared with those measuring points where they could receive direct solar radiation, indicating that in winter outdoor environment, solar radiation can enhance the influence of air tempera ture and Tmrt on PET. By comparing the correlation between the measuring point at the lawn under the shade in summer and the other three measuring points, it can be shown that solar radiation can also enhance the influence of Tmrt on PET in summer, but it reduces the in fluence of air temperature on PET. At the same time, the result is different: the correlation between air temperature and PET at the lawn under the shade in summer was stronger than that of Tmrt and PET. The
Table 7 The descriptive statistics for Tmrt (� C).
Winter Summer
Light-colored marble brick
Plastic track
Lawn under the shade
Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
17.9 47.5
11.5 38.2
33.4 56.8
5.1 4.2
21.3 43.3
11.8 33.9
34.9 55.1
6.7 6.0
14.5 33.6
9.1 28.1
20.7 38.6
3.5 2.1
19.7 50.7
9.3 34.0
35.7 71.9
7.0 8.8
13.0 –
9.3 –
17.8 –
2.5 –
7
Unobstructed lawn
Bluestone slate under the shade
Journal of Building Engineering 27 (2020) 100994
Z. Huang et al.
Fig. 7. Comparison of PET above the ground surface of different properties.
correlation between Tmrt and PET at the other three unobstructed measuring points where the air temperature was relatively higher in summer was stronger than the correlation between air temperature and PET, which further confirms the conclusion that Tmrt is the main factor affecting PET at the unobstructed lawn with high air temperature in summer.
track was respectively 4.5 � C and 3.4 � C higher than that of the lightcolored marble brick. A lower reflectivity to solar radiation can help to absorb more solar energy, causing the surface temperature rising and further increasing Tmrt. The study has two limitations: (1) The effects of ground surfaces on the thermal environment were only investigated in winter and summer; (2) Subject to the restrictions of the field measurement area, the ground surfaces that have been investigated were some common ground surface type for outdoor activity spaces, and there is a lack of investigation into the ground surface with some new materials (such as water permeable materials). In the future study, considering the complexity of the out door environment, more internal factors, such as material thermal properties, and external factors such as heat reflectance from the sur roundings, should be investigated. Especially, given that materials with high water permeability can take away heat through evaporation and reduce ambient temperature, the study of permeable ground surfaces needs to address water absorption, evaporation and related ambient temperature reduction. At the same time, multiple sets of control sim ulations can be performed using thermal environment simulation soft ware such as ENVI-met to further determine the influence of the area of the ground surface on the thermal environment.
4. Discussion In summer, the mean air temperature at the unobstructed lawn measuring point was 2.1–5.5 � C lower than that at those hard pave ments. The air temperature above the lawn under the shade was 4.7 � C lower than the air temperature above the unobstructed lawn. The findings prove the fact that vegetations (including lawns and shades) have cooling effects in summer. Besides vegetation, ground surface reflectivity also plays an important role in influencing the thermal environment. The PET above the light-colored marble brick was 3.7 � C higher than that above the plastic track. This indicates that in summer, the high reflectivity ground surface may increase people’s thermal stress, which echoes the studies from Erell, E. et al. [12] who suggested using low reflectivity materials as a floor covering can improve people’s thermal comfort in summer. In winter, the mean air temperature above the plastic track was higher than that above the light-colored marble brick, which is probably due to the color and related reflectivity difference. This was confirmed by the fact that the mean surface temperature and Tmrt of the plastic
5. Conclusion Based on the analysis and discussion of the above results, the following conclusions can be drawn:
Table 8 The descriptive statistics for PET (� C).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
15.3 45.4
10.9 38.0
20.1 50.9
2.5 3.5
15.9 41.7
10.7 33.7
21.8 50.8
3.5 4.7
12.4 32.9
7.1 27.2
16.7 35.4
2.5 2.1
15.8 45.1
8.1 35.7
22.9 54.8
3.9 4.5
11.4 –
7.1 –
14.3 –
1.7 –
8
Unobstructed lawn
Bluestone slate under the shade
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Journal of Building Engineering 27 (2020) 100994
Table 9 Multiple comparison tests of different ground surface properties on surface temperature, Tmrt, relative humidity and PET.
Surface temperature
Ground surface property (I)
Ground surface property (J)
Mean difference (I-J)
Standard error
Sig.
light-colored marble brick
plastic track unobstructed lawn light-colored marble unobstructed lawn light-colored marble plastic track plastic track unobstructed lawn light-colored marble unobstructed lawn light-colored marble plastic track plastic track unobstructed lawn light-colored marble unobstructed lawn light-colored marble plastic track plastic track unobstructed lawn light-colored marble unobstructed lawn light-colored marble plastic track
4.5412a 1.6647 4.5412a 2.8765 1.6647 2.8765 3.3763a 1.8112 3.3763a 1.5651 1.8112 1.5651 2.4782a 7.4218a 2.4782a 4.9436a 7.4218a 4.9436a 3.7364a 0.2636 3.7364a 3.4727a 0.2636 3.4727a
1.4438 1.4438 1.4438 1.4438 1.4438 1.4438 1.2734 1.2734 1.2734 1.2734 1.2734 1.2734 1.1080 1.1080 1.1080 1.1080 1.1080 1.1080 0.8148 0.8148 0.8148 0.8148 0.8148 0.8148
0.003 0.255 0.003 0.052 0.255 0.052 0.009 0.157 0.009 0.221 0.157 0.221 0.027 0.000 0.027 0.000 0.000 0.000 0.000 0.747 0.000 0.000 0.747 0.000
plastic track unobstructed lawn Tmrt
light-colored marble brick plastic track unobstructed lawn
Relative humidity
light-colored marble brick plastic track unobstructed lawn
PET
light-colored marble brick plastic track unobstructed lawn
a
brick brick
brick brick
brick brick
brick brick
95% confidence interval Lower limit
Upper limit
7.444 4.568 1.638 0.026 1.238 5.779 5.893 4.328 0.859 0.952 0.706 4.082 4.666 9.610 0.290 7.132 5.234 2.756 2.127 1.345 5.345 5.082 1.873 1.864
1.638 1.238 7.444 5.779 4.568 0.026 0.859 0.706 5.893 4.082 4.328 0.952 0.290 5.234 4.666 2.756 9.610 7.132 5.345 1.873 2.127 1.864 1.345 5.082
Significance level<0.05.
(1) Solar radiation is an important factor affecting air temperature, surface temperature and Tmrt above the ground surface of different properties in winter and summer. The stronger the solar radiation is, the more significant the difference is. At the same
time, solar radiation also has a significant effect on improving people’s thermal sensation in winter and summer. (2) The difference in the property of the ground surface affects the air temperature above it. The plastic track has a darker color and a
Fig. 8. Relationship between air temperature, Tmrt and PET above the lawn under the shade and unobstructed lawn in winter. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 9
Z. Huang et al.
Journal of Building Engineering 27 (2020) 100994
Fig. 9. Relationship between air temperature, Tmrt and PET above the lawn under the shade and unobstructed lawn in summer. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
lower reflectance to solar radiation, which results in a higher surface temperature, and in turn increases the air temperature above it. (3) Vegetation can humidify the surrounding environment to a certain extent both in summer and winter, and the humidification will be far more significant as the vegetation area expanding. (4) The property of the ground surface has a significant effect on surface temperature and Tmrt in winter, and has a significant ef fect on relative humidity and PET in summer. The ground surface with high reflectivity may increase the immediate temperature above the ground and consequently increase people’s thermal stress in summer. (5) When directly exposed to solar radiation, air temperature is the main factor that influences PET in winter while Tmrt is the main factor that influences PET in summer. Solar radiation can enhance their influences.
Table 10 Significance test results of the regression equation of air temperature and PET above the lawn under the shade in winter.
Regression Residual Sum
Sum of square
Degree of freedom
Mean square
F
Sig.
171.149 138.384 309.533
2 46 48
85.574 3.008
28.446
.000
Table 11 Significance test results of the regression equation of Tmrt and PET above the lawn under the shade in winter.
Regression Residual Sum
Sum of square
Degree of freedom
Mean square
F
Sig.
128.715 180.818 309.533
2 46 48
64.358 3.931
16.373
.000
Based on the above conclusions, the use of ground surfaces with low reflectivity should be encouraged in outdoor planning and design, because it can significantly increase the surface temperature in winter and reduce the people’s thermal stress in summer and can play a positive role to improve the thermal environment in winter and summer.
Table 12 Correlation between air temperature, Tmrt and PET above the ground surface of different properties. R2 Winter Summer
Air temperature Tmrt Air temperature Tmrt
Bluestone slate under the shade
Lawn under the shade
Light-colored marble brick
Unobstructed lawn
Plastic track
0.4832 0.32
0.5529 0.4172 0.949 0.798
0.7243 0.5198 0.747 0.923
0.8651 0.628 0.01 0.912
0.9064 0.5932 0.061 0.953
10
Journal of Building Engineering 27 (2020) 100994
Z. Huang et al.
References
[10] M. Santamouris, N. Gaitani, A. Spanou, M. Saliari, K. Giannopoulou, K. Vasilakopoulou, et al., Using cool paving materials to improve microclimate of urban areas – design realization and results of the flisvos project, Build. Environ. 53 (2012) 128–136. [11] G.E. Kyriakodis, M. Santamouris, Using reflective pavements to mitigate urban heat island in warm climates - results from a large scale urban mitigation project, Urban Clim. 24 (2018) 326–339. [12] E. Erell, D. Pearlmutter, D. Boneh, P.B. Kutiel, Effect of high-albedo materials on pedestrian heat stress in urban street canyons, Urban Clim. 10 (2014) 367–386. [13] E. Ng, V. Cheng, Urban human thermal comfort in hot and humid Hong Kong, Energy Build. 55 (2012) 51–65. [14] Z. Gou, D. Prasad, S.S.Y. Lau, Impacts of green certifications, ventilation and office types on occupant satisfaction with indoor environmental quality, Architect. Sci. Rev. 57 (2014) 196–206. [15] Z. Gou, S.S.Y. Lau, Contextualizing green building rating systems: case study of Hong Kong, Habitat Int. 44 (2014) 282–289. [16] C. ASHRAE, 8—Physiological Principles and Thermal Comfort. Handbook of Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, 2001, pp. 8.1–8.20. [17] Z. Gou, S.S.Y. Lau, H. Ye, Visual alliesthesia: the gap between comfortable and stimulating illuminance settings, Build. Environ. 82 (2014) 42–49. [18] T.-P. Lin, A. Matzarakis, R.-L. Hwang, Shading effect on long-term outdoor thermal comfort, Build. Environ. 45 (2010) 213–221.
[1] G. Fan, J. Xie, H. Yoshino, U. Yanagi, K. Hasegawa, C. Wang, et al., Investigation of indoor thermal environment in the homes with elderly people during heating season in Beijing, China, Build. Environ. 126 (2017) 288–303. [2] Y. Wang, L. Wang, E. Long, S. Deng, An experimental study on the indoor thermal environment in prefabricated houses in the subtropics, Energy Build. 127 (2016) 529–539. [3] N. Zhang, B. Cao, Z. Wang, Y. Zhu, B. Lin, A comparison of winter indoor thermal environment and thermal comfort between regions in Europe, North America, and Asia, Build. Environ. 117 (2017) 208–217. [4] T.-P. Lin, Y.-F. Ho, Y.-S. Huang, Seasonal effect of pavement on outdoor thermal environments in subtropical Taiwan, Build. Environ. 42 (2007) 4124–4131. [5] J. Lu, M. Zhang, W. Pei, Hydro-thermal behaviors of the ground under different surfaces in the Qinghai-Tibet Plateau, Cold Reg. Sci. Technol. 161 (2019) 99–106. [6] Z. Gou, S.Y.S. Lau, P. Lin, Understanding domestic air-conditioning use behaviours: disciplined body and frugal life, Habitat Int. 60 (2017) 50–57. [7] Khoshbakht M, Gou Z, Dupre K. Cost-benefit Prediction of Green Buildings: SWOT Analysis of Research Methods and Recent Applications. Procedia Engineering2017. p. 167-178. [8] M. Taleghani, The impact of increasing urban surface albedo on outdoor summer thermal comfort within a university campus, Urban Clim. 24 (2018) 175–184. [9] P. Campra, M. Garcia, Y. Canton, A. Palacios-Orueta, Surface temperature cooling trends and negative radiative forcing due to land use change toward greenhouse farming in southeastern Spain, J. Geophys. Res. 113 (2008).
11
Contents lists available at ScienceDirect
Journal of Building Engineering journal homepage: http://www.elsevier.com/locate/jobe
An investigation of outdoor thermal environments with different ground surfaces in the hot summer-cold winter climate region Zefeng Huang a, Zhonghua Gou b, *, Bin Cheng a a b
School of Civil Engineering and Architecture, Southwest University of Science and Technology, Mianyang, 621010, China School of Engineering and Built Environment, Griffith University, Gold Coast, QLD 4215, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Outdoor thermal environment Ground surface Reflectivity Materials
This study aims to find the impact of different ground surfaces on thermal environments in outdoor activity space. Field measurements were conducted in a university campus located in the hot summer-cold winter climate zone. Five typical outdoor ground surfaces were selected: light-colored marble brick, plastic track, bluestone slate under the shade, lawn under the shade, and unobstructed lawn. Thermal environment parameters, including air temperature, relative humidity, solar radiation, surface temperature, mean radiant temperature (Tmrt) and the physiological equivalent temperature (PET) were measured and calculated for summer and winter, respectively. It is confirmed that the property of the ground surface had a significant effect on surface temper ature and Tmrt in winter and had a significant effect on relative humidity and PET in summer. The use of ground surfaces with low reflectivity which could increase the surface temperature by 4.5 � C in winter while reduce the heat stress (PET) by 3.7 � C in summer, should be encouraged in outdoor activity space.
1. Introduction Thermal environment is closely associated with the comfort and safety. At present, the studies of indoor thermal environment are more than that of outdoors [1–3]. One of important reasons is that the outdoor thermal environment is more complex than indoors. Especially, outdoor thermal environment is affected by solar radiation, which is not considered in terms of indoor thermal environment; thus, it is more complicated and there is no unified evaluation index at present. According to the results of relevant studies [4,5], the change of outdoor thermal environment is closely related to the properties of ground surfaces, which is the interface of energy budget of land surface regulating and controlling the hydrologic balance and energy balance of an ecosystem, and influencing thermal environment ultimately [6]. The effects from ground surfaces on the outdoor thermal environment mainly include the emitted radiation from the surface itself and the re flected short-wave solar radiation. The emitted radiation is mainly concentrated in the far-infrared band of more than 6 μm, and the re flected radiation to the solar radiation is mainly concentrated in the visible light and near-infrared bands of 0.3–2.5 μm. In the visible and near-infrared band (0.3–2.5 μm), the emitted radiation of the ground surface itself is almost zero, so the effect of the ground surface on the
outdoor thermal environment is mainly reflected by the reflected solar radiation. Table 1 shows the reflectivity of common ground surface materials. Ran [7] made human body equivalent to a cylinder "human body cylinder" (human body cylinder and human body have the same height and circumference), built physical and mathematical models of the ra diation emitted and reflected from the ground surface to the human body. There have been several empirical studies that investigated the thermal environment or comfort in relation to different ground surfaces properties. For example, Taleghani [8] conducted the research over the influence of increased reflectivity of the ground surface on outdoor thermal comfort in a university of the Netherlands; the result showed that the material with high reflectivity can significantly affect the thermal comfort of pedestrians in open space, and the mean radiant temperature would rise 1.2 � C and PET would rise 0.8 � C as every 0.1 increase of reflectance ratio. Campra et al. [9] found that white surfaces could make a city’s temperature lower than its outskirts of 0.3 � C in Almeria, Spain. However, in Athens, Santamouris et al. [10] studied the effect of highly reflective pavement of 4,500 m2 on the microclimate of local parks; they observed that the air temperature decreased by 1.9 � C, and the surface temperature decreased by 12 � C in typical summer time. In the meantime, Kyriakodis et al. [11] observed in Athens that asphalt
* Corresponding author. E-mail address: [email protected] (Z. Gou). https://doi.org/10.1016/j.jobe.2019.100994 Received 22 May 2019; Received in revised form 8 August 2019; Accepted 12 October 2019 Available online 14 October 2019 2352-7102/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Building Engineering 27 (2020) 100994
subtropical mountainous humid monsoon climate zone. The annual average temperature is from 14.7 � C to 17.3 � C and annual average precipitation is from 825.8 mm to 1,417 mm. Measuring point selection and ambient conditions are shown in Fig. 1. In this survey, five different ground surfaces were selected as data measurement areas, including light-colored marble brick, plastic track, unobstructed lawn, lawn under the shade and bluestone slate under the shade. These ground surfaces are common materials for outdoor activity space paving, and the in clusion of measuring points under the shade can further study the effects of solar radiation on the thermal environment above different ground surfaces. All points were measured simultaneously. The winter measurement took place on January 15th, 17th and 20th, 2018, a total of three days; the measuring time is from 9:00 to 17:00. The summer measurement took place on 22nd and 23rd, 2018, a total of two days, with measuring time from 9:00 to 18:00. There were no raining days during the measurement period (including the days before the measurement), under which circumstance the water absorption by the permeable ground surfaces and its related water evaporation are minimal. Thermal environment parameters include air temperature, relative humidity, wind speed, solar radiation, globe temperature and surface temperature have been recorded in this study. The instruments used to measure the parameters are shown in Table 2. All instruments are in compliance with ISO7726 standard [14]. Except for the ground surface temperature that was measured and recorded manually by the infrared thermometer at a 30 min interval, other thermal environment parame ters were recorded automatically with a 10 min interval and at a height of 1.1 m. Five sets of instruments were used for the simultaneous mea surement of the five ground surfaces. However, due to the failure of some instruments, some data for the bluestone slate under the shade in summer was missing; therefore, we excluded the point in the summer analysis. The average, instead of each of the measurement days was used for the analysis, to reduce the bias due to some data missing and the climate variation.
Table 1 Reflectivity of common ground surface materials. Category
Reflectivity
Category
Reflectivity
Dry bare land Wet bare land Dry lawn Wet lawn Plastic track
0.1–0.2 0.08–0.09 0.15–0.25 0.14–0.26 0.10
Light-colored marble brick Painted floor Steel plate floor Concrete floor Asphalt ground
0.19 0.10 0.15 0.20 0.10
pavement and concrete pavement can cool the air temperature and ground surface temperature by 1.5 � C and 11.5 � C respectively. The abovementioned is the positive influences of the ground surface of highly reflective material on the outdoor thermal environment. How ever, some studies have shown that reflectivity does not affect the thermal conditions near the ground. Erell et al. [12] indicated that the use of highly reflective materials can cool the temperature but also in crease the thermal stress for pedestrians. In this regard, further research is required to investigate the influence of ground surface properties on the thermal environment especially for pedestrian comfort. In this paper, we analyze the influence of outdoor ground surfaces of different reflectance properties on the outdoor thermal environment in a university campus with the hot summer-cold winter climate; at the same time, this paper uses the physiological equivalent temperature (PET) which considers human adaptation as the outdoor thermal comfort index [13], and further analyze the correlation between main outdoor thermal environment parameters and PET. This study also explores which thermal environment parameters in winter and summer are the main factors affecting PET, aiming to provide evidence for the optimi zation and improvement of the outdoor thermal environment. 2. Methodology The measurement took place in the campus of Southwest University of Science and Technology which located in the northeast of Mianyang City, Sichuan Province (104.73� E, 31.48� N), China. It belongs to the
Fig. 1. Layout of measuring points. Table 2 The measurement instrument set and accuracy. Instrument
Measured parameter
Model
Range
Accuracy
Manufacturer
Hand-held weather station
Air temperature Relative humidity Wind speed Solar radiation Globe temperature Underlying surface temperature
PH-II-C
50-80 � C 0–100% 0–45 m/s 0–2000 W/m2 0–120 � C 30-400 � C
�0.3 � C �5% �0.3 m/s �5% �0.1 � C �1.5 � C
XINPUHUI
Solar radiation meter Globe temperature probe Infrared thermometer
JTR05 Testo 480 Testo 830-T2
2
JANTYTECH Testo Testo
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Journal of Building Engineering 27 (2020) 100994
Fig. 2. Comparison of air temperature above the ground surface of different properties.
3. Analysis and results
the direct radiation from sunlight may be an important factor affecting the air temperature above the ground surface. In summer, the change of air temperature was different from that in winter. The air temperature increased continuously, and the trend of temperature decrease was not significant even during the period from 17:00 to 18:00. The lowest mean air temperature appeared at the lawn under the shade (31.8 � C), fol lowed by the unobstructed lawn (36.5 � C), indicating that the plant can effectively reduce the air temperature in summer. Fig. 3 shows the data of relative humidity and their descriptive sta tistics are shown in Table 4. The relative humidity above all the measuring points shows a gradual decreasing trend regardless of the seasonal difference. In winter, the relative humidity above the measuring points of the lawn under the shade and bluestone slate under the shade was 62.6% and 60.0% respectively higher than the other measuring points. The difference reached the peak (20%) at around 10:30 a.m. In summer, the variation of relative humidity was almost the same as that in winter. The measuring point at the lawn under the shade had the highest mean value (52.8%), followed by the unobstructed lawn (44.0%). Fig. 4 shows the data of solar radiation and Table 5 shows the descriptive statistics. The solar radiation of non-shaded points showed a single-peak trend with the maximum value appearing during the time from 13:00 to 15:00. However, the solar radiation of shaded points was stable at a low level and was basically the same in winter and summer without significant changes. Regardless of the seasonal difference, the solar radiation at the shaded points were significantly lower than that
First, all measured data were illustrated in graphs to show their variations during a day, and their descriptive statistics was summarized in tables. After that, ANOVA (Analysis of Variance) and LSD (Least Significant Difference) methods were used for comparisons of thermal environment parameters between different ground surfaces. In partic ular, ANOVA was used to explore whether the ground surface of different properties had effect on the measured thermal environment parameters; LSD method was used to further compare which properties of the ground surface had a most significant impact. Finally, correlation analysis was used to determine the relationships between different thermal environment parameters. 3.1. Data description Fig. 2 shows the air temperature change above the ground surface of different properties in winter and summer and Table 3 summarizes their basic statistics. In winter, the air temperature shows a single-peak trend of increasing first and then decreasing during the measured time of a day. Except that the temperature of two measuring points above the lawns reached the highest around 16:00, the temperature of other measuring points reached the highest around 15:00. The plastic track measuring point had the highest mean air temperature at 14.4 � C, while the bluestone slate under the shade and lawn under the shade measuring point had the lowest at 11.3 � C and 11.9 � C respectively, indicating that Table 3 The descriptive statistics for air temperature (� C).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
13.7 42.0
10.1 33.9
16.9 48.1
2.1 3.7
14.4 38.6
10.4 33.0
17.8 45.9
2.5 3.5
11.9 31.8
8.8 27.8
15.0 34.4
2.1 1.7
12.9 36.5
8.9 29.0
18.0 44.4
3.0 3.1
11.3 –
8.2 –
13.0 –
1.6 –
3
Unobstructed lawn
Bluestone slate under the shade
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Journal of Building Engineering 27 (2020) 100994
Fig. 3. Comparison of relative humidity above the ground surface of different properties.
without being shaded. The maximum difference in winter could reach 313 W/m2 at around 14:30, while the maximum difference in summer occurred at 13:00, for 877W/m2. The air flow causes the short-wave radiation heat absorbed by the surface to be transmitted to the air in the form of convection and is transmitted upward through the turbulent heat exchange. Therefore, the air temperature at the pedestrian height is affected to some extent by the surface temperature. In winter and summer, the higher the surface temperature is, the stronger the exchange of heat with the human body is. Fig. 5 shows the ground surface temperature and Table 6 shows their descriptive statistics. Regardless of the seasonal difference, the surface temperature at all measuring points showed generally a single-peak trend of increasing first and then decreasing during the measured time period of one day. The mean surface temperature of the plastic track (16.2 � C) was the highest in winter, followed by the unobstructed lawn (13.3 � C) and the light-colored marble brick (11.6 � C). The mean surface temperature of the lawn under the shade (9.7 � C) and the bluestone slate under the shade (8.9 � C) was much lower than others. The summer sit uation was similar to that in winter. The mean surface temperature of the plastic track (52.2 � C) was the highest, and the lawn under the shade (25.7 � C) was the lowest, but the mean surface temperature of the lightcolored marble brick (43.7 � C) was higher than that of the unobstructed lawn (35.3 � C). This shows that the ground surfaces of different prop erties had different temperature changes under the same solar radiation due to their different heat capacity and reflectivity. Among them, vegetation could properly reduce the surface temperature, and shading is an important factor to reduce the surface temperature.
In the outdoor thermal environment, mean radiant temperature (Tmrt) is a popular parameter which is often used to evaluate thermal comfort or to calculate the radiative heat loss of the human body. Tmrt is defined as the surface temperature of a hypothetical isothermal enclosed surface. The radiation heat transfer between it and the human body is equivalent to that between the actual non-isothermal enclosed surface and the human body [15]. The calculation formula of Tmrt is in (1) [16]: � ��1=4 � �4 1:10 � 108 v0:6 Tg Ta Tmrt ¼ Tg þ 273 þ 273 (1) 0:4 εD Wherein, Tmrt is mean radiant temperature, � C; Tg is globe temperature, C; Ta is air temperature, � C; v is wind speed, m/s; D is the globe diameter, m; (The standard globe with D ¼ 0.15 m is used in this paper). ε is the absorption rate of the globe (0.95 in this paper). Fig. 6 shows the variation of Tmrt and Table 7 shows their descriptive statistics. From the winter results, the difference of Tmrt at each measuring point is not significant in the morning; however, in the af ternoon when the solar radiation was gradually increasing, the differ ence became more significant, reaching a maximum at around 15:00, and the difference found between the unobstructed lawn and the blue stone slate under the shade was 21 � C. In the measurement period, the two measuring points under the shade received limited direct sunlight, which caused that Tmrt was significantly lower than the other three measurement points. In the afternoon of winter, the unobstructed lawn and plastic track could directly receive the solar radiation and had higher Tmrt. No doubt, solar radiation was an important factor affecting �
Table 4 The descriptive statistics for relative humidity (%).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
53.5 36.6
46.5 29.7
62.0 55.5
4.5 6.1
53.0 39.1
45.0 30.0
62.7 47.3
5.6 5.5
62.6 52.8
51.6 47.3
76.5 68.8
8.4 5.3
52.8 44.0
45.5 33.4
58.3 61.0
3.6 5.9
60.0 –
48.2 –
72.7 –
8.1 –
4
Unobstructed lawn
Bluestone slate under the shade
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Journal of Building Engineering 27 (2020) 100994
Fig. 4. Comparison of solar radiation at the ground surface of different properties.
Tmrt. In summer, Tmrt of all measuring points maintained at a high level during the measurement time, and the maximum value of Tmrt was measured in unobstructed lawn at 11:00, which is 71.9 � C; the minimum value of Tmrt was measured in the lawn under the shade at 9:00, which is 28.1 � C. At the same time, compared with the two measuring points of light-colored marble brick and the plastic track, Tmrt at the unobstructed lawn was also higher in most of the time. This shows that the ground surface properties had little effect on Tmrt, and even the lawn with a certain cooling effect on the air temperature may not improve the sur rounding Tmrt. In addition, Tmrt at the measuring point of the lawn under the shade was much lower than the rest measuring points in most of the time, which further proved that solar radiation is an important factor affecting Tmrt. PET is an index for comprehensive evaluation of thermal environ ment and is a physiological equilibrium temperature under a given environment. Its value is equal to the temperature corresponding to a typical indoor environment that reaches the same outdoor heat state [17]. The influence of main thermal environment parameters, activity and clothing insulation on human comfort is considered comprehen sively, and the outdoor thermal environment condition can be evaluated synthetically. In this paper, RayMan 1.2 software was used to input the relevant parameters such as air temperature, relative humidity, wind speed, water vapor pressure, Tmrt, clothing insulation and activity, and the PET value was calculated. Among them, the clothing insulation value was set at 1.5 clo and 0.5 clo respectively for winter and summer according to the typical daily dress in the two seasons, and the activity
was 80W for a slowly walking adult. Literature [18] studied the rela tionship between PET and thermal sensation in the outdoor environment in different regions, and divided the thermal sensation into nine different scales, which are very cold, cold, cool, slightly cool, neutral, slightly warm, warm, hot, and very hot, and correspond to different ranges of PET values. Fig. 7 shows the variation of PET and Table 8 shows the descriptive statistics. The PET values at the two measuring points (the bluestone slate under the shade and the lawn under the shade) in winter were at a low level throughout the day, and the values fluctuated mostly within the range of 7–13 � C which are corresponding to the thermal sensation: cold and cool, and categorized as less comfortable. The PET values of the other three measuring points in the morning time period were only slightly higher than those measured under the shade. The difference was not large; however, starting from around 12:00, with the increase of solar radiation, the PET value at these three measuring points began to rise, and was significantly higher than the PET value at the two measuring points under the shade, reaching a maximum of 22.9 � C at around 15:00. In the time range of 14:00 to 16:00, PET values at these three measuring points were basically within the range corresponding to the thermal sensation of neutral. It shows that solar radiation had a significant effect on improving the thermal sensation of people in winter, especially in the place where the ground surface can receive direct sunlight in the afternoon. In summer, except for the measuring point of lawn under the shade, the PET at the other three measuring points was at a high level during the measurement period of the day, mainly fluctuating within the range
Table 5 The descriptive statistics for solar radiation (W/m2).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
210 693
88 210
367 936
85 213
214 634
90 28
360 943
78 320
58 49
22 30
139 78
23 12
191 738
80 257
341 971
78 217
36 –
8 –
64 –
18 –
5
Unobstructed lawn
Bluestone slate under the shade
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Journal of Building Engineering 27 (2020) 100994
Fig. 5. Comparison of surface temperature of ground surface of different properties.
of 40–50 � C, which are corresponding to the thermal sensation: hot and very hot. The PET at the measuring point of the lawn under the shade was lower, mainly fluctuating within the range of 27–34 � C, corre sponding to the thermal sensation neutral and slightly warm. This showed that effective shading to reduce direct exposure to solar radia tion is an important method for people to improve their thermal sensation in summer.
(16.2 � C) and Tmrt (21.3 � C) appeared at the plastic track while the lowest mean surface temperature (11.6 � C) and Tmrt (17.9 � C) of appeared at the light-colored marble brick. In summer, the highest (44.0%) and the lowest (36.6%) mean relative humidity appeared at the unobstructed lawn and the light-colored marble brick respectively; the highest (45.4 � C) and the lowest (41.7 � C) mean PET appeared at the light-colored marble bricks and the plastic runways, respectively. Results of further comparisons using LSD are shown in Table 9. The surface temperature and Tmrt of the plastic track was respectively 4.5 � C and 3.4 � C higher than that of the light-colored marble brick in winter. In addition, by comparing the relative humidity above the ground surface under different properties in summer, the following significant differ ences can be found: light-colored marble brick < plastic track < unob structed lawn, in which the unobstructed lawn can conduct transpiration in summer and have the strongest humidification effect. The multiple comparison tests show that the PET above the unobstructed lawn and the light-colored marble brick was respectively 3.5 � C and 3.7 � C higher than that above the plastic track.
3.2. Anova and Lsd To ensure the effectiveness of ANOVA, the test of variance homo geneity must be conduct first to determine whether the variance of the measured thermal environmental parameters (air temperature, relative humidity and surface temperature) and the calculated thermal comfort index (Tmrt and PET) above each ground surface in winter and summer are equal. The results show that the variance of surface temperature (F ¼ 1.732, Sig. ¼ 0.188) and Tmrt (F ¼ 1.767, Sig. ¼ 0.175) at the ground surface under different properties in winter, the variance of relative humidity (F ¼ 0.724, Sig. ¼ 0.487) and PET (F ¼ 2.104, Sig. ¼ 0.125) at the ground surface under different properties in summer are equal, which meets the premise requirements of ANOVA. The results of ANOVA show that the effects of ground surface properties on surface temperature (F ¼ 5.064, Sig. ¼ 0.010), Tmrt (F ¼ 3.521, Sig. ¼ 0.032) in winter, and on relative humidity (F ¼ 23.259, Sig. ¼ 0.000), PET (F ¼ 13.098, Sig. ¼ 0.000) in summer were significant. In winter, the highest mean surface temperature
3.3. Correlation During the measurement time, the variation of PET value above the ground surface of different properties with time in winter and summer is basically consistent with the variation of air temperature and Tmrt with time. To further understand which thermal environment parameters in winter and summer had a significant effect on PET, a correlation analysis
Table 6 The descriptive statistics for surface temperature (� C).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
11.6 43.7
6.8 30.2
16.3 52.9
3.3 6.4
16.2 52.2
9.8 36.4
22.7 65.8
4.7 9.5
9.7 25.7
7.0 23.8
11.5 27.5
1.3 1.1
13.3 35.3
7.2 31.1
19.8 40.4
4.5 2.6
8.9 –
6.4 –
10.7 –
1.6 –
6
Unobstructed lawn
Bluestone slate under the shade
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Fig. 6. Comparison of Tmrt above the ground surface of different properties.
is needed. Fig. 8 and Fig. 9 respectively shows the scatter plots of air temperature, Tmrt and PET at the two measuring points of the lawn under the shade and unobstructed lawn in winter and summer. The curve estimation was carried out, and the corresponding regression equation was obtained. To ensure that the regression equation can better reflect the statis tical relationship between air temperature, Tmrt and PET, the signifi cance test of the regression equation is needed. Table 10 and Table 11 show the results of the significance test of the regression equation of air temperature, Tmrt and PET in the lawn under the shade measuring point in winter. The two-tailed probability P value corresponding to the F statistic observation value is approximately zero. When the significance level α is 0.05, the null hypothesis of significance test of the regression equation should be rejected; it is considered that there is a significant secondary relationship between air temperature, Tmrt and PET in the lawn under the shade measuring point in winter. The regression equa tions of air temperature, Tmrt and PET at the other four measuring points in winter and all measuring points in summer can also pass the signifi cance test, indicating that the regression equations can better reflect the statistical relationship between the explained variable PET and the explanatory variable air temperature and Tmrt. From the comparison of the regression equations in Fig. 8, the air temperature, Tmrt and PET above the lawn under the shade and the unobstructed lawn in winter show a significant secondary relationship, and the effect of air temperature at the two measuring points on PET is significantly stronger than that of Tmrt on PET. By comparing Fig. 9, it
can be found that the relationship between the air temperature, Tmrt and PET above the lawn under the shade and the unobstructed lawn in summer is slightly different from that in winter. In summer, at the measuring point at the lawn under the shade, the effect of air temper ature on PET was significantly stronger than that of Tmrt on PET. But at the measuring point in the unobstructed lawn, it is opposite. The effect of Tmrt on PET is significantly stronger than that of air temperature. This may be due to the increase in air temperature, which causes Tmrt to rise at the same time; while the rate at which Tmrt rises is much higher than the rate at which air temperature rises. Therefore, at the unobstructed lawn where the air temperature is high in summer, Tmrt is the main factor to affect PET. Table 12 shows the correlation between air temperature, Tmrt and PET above all the ground surface of different properties measured in winter and summer. The results show that the correlation between air temperature, Tmrt and PET at two measuring points under the shade in winter were weak compared with those measuring points where they could receive direct solar radiation, indicating that in winter outdoor environment, solar radiation can enhance the influence of air tempera ture and Tmrt on PET. By comparing the correlation between the measuring point at the lawn under the shade in summer and the other three measuring points, it can be shown that solar radiation can also enhance the influence of Tmrt on PET in summer, but it reduces the in fluence of air temperature on PET. At the same time, the result is different: the correlation between air temperature and PET at the lawn under the shade in summer was stronger than that of Tmrt and PET. The
Table 7 The descriptive statistics for Tmrt (� C).
Winter Summer
Light-colored marble brick
Plastic track
Lawn under the shade
Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
17.9 47.5
11.5 38.2
33.4 56.8
5.1 4.2
21.3 43.3
11.8 33.9
34.9 55.1
6.7 6.0
14.5 33.6
9.1 28.1
20.7 38.6
3.5 2.1
19.7 50.7
9.3 34.0
35.7 71.9
7.0 8.8
13.0 –
9.3 –
17.8 –
2.5 –
7
Unobstructed lawn
Bluestone slate under the shade
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Fig. 7. Comparison of PET above the ground surface of different properties.
correlation between Tmrt and PET at the other three unobstructed measuring points where the air temperature was relatively higher in summer was stronger than the correlation between air temperature and PET, which further confirms the conclusion that Tmrt is the main factor affecting PET at the unobstructed lawn with high air temperature in summer.
track was respectively 4.5 � C and 3.4 � C higher than that of the lightcolored marble brick. A lower reflectivity to solar radiation can help to absorb more solar energy, causing the surface temperature rising and further increasing Tmrt. The study has two limitations: (1) The effects of ground surfaces on the thermal environment were only investigated in winter and summer; (2) Subject to the restrictions of the field measurement area, the ground surfaces that have been investigated were some common ground surface type for outdoor activity spaces, and there is a lack of investigation into the ground surface with some new materials (such as water permeable materials). In the future study, considering the complexity of the out door environment, more internal factors, such as material thermal properties, and external factors such as heat reflectance from the sur roundings, should be investigated. Especially, given that materials with high water permeability can take away heat through evaporation and reduce ambient temperature, the study of permeable ground surfaces needs to address water absorption, evaporation and related ambient temperature reduction. At the same time, multiple sets of control sim ulations can be performed using thermal environment simulation soft ware such as ENVI-met to further determine the influence of the area of the ground surface on the thermal environment.
4. Discussion In summer, the mean air temperature at the unobstructed lawn measuring point was 2.1–5.5 � C lower than that at those hard pave ments. The air temperature above the lawn under the shade was 4.7 � C lower than the air temperature above the unobstructed lawn. The findings prove the fact that vegetations (including lawns and shades) have cooling effects in summer. Besides vegetation, ground surface reflectivity also plays an important role in influencing the thermal environment. The PET above the light-colored marble brick was 3.7 � C higher than that above the plastic track. This indicates that in summer, the high reflectivity ground surface may increase people’s thermal stress, which echoes the studies from Erell, E. et al. [12] who suggested using low reflectivity materials as a floor covering can improve people’s thermal comfort in summer. In winter, the mean air temperature above the plastic track was higher than that above the light-colored marble brick, which is probably due to the color and related reflectivity difference. This was confirmed by the fact that the mean surface temperature and Tmrt of the plastic
5. Conclusion Based on the analysis and discussion of the above results, the following conclusions can be drawn:
Table 8 The descriptive statistics for PET (� C).
Winter Summer
Light-colored marble brick
Plastic track
Mean
Min
Max
SD
Mean
Min
Max
SD
Lawn under the shade Mean
Min
Max
SD
Mean
Min
Max
SD
Mean
Min
Max
SD
15.3 45.4
10.9 38.0
20.1 50.9
2.5 3.5
15.9 41.7
10.7 33.7
21.8 50.8
3.5 4.7
12.4 32.9
7.1 27.2
16.7 35.4
2.5 2.1
15.8 45.1
8.1 35.7
22.9 54.8
3.9 4.5
11.4 –
7.1 –
14.3 –
1.7 –
8
Unobstructed lawn
Bluestone slate under the shade
Z. Huang et al.
Journal of Building Engineering 27 (2020) 100994
Table 9 Multiple comparison tests of different ground surface properties on surface temperature, Tmrt, relative humidity and PET.
Surface temperature
Ground surface property (I)
Ground surface property (J)
Mean difference (I-J)
Standard error
Sig.
light-colored marble brick
plastic track unobstructed lawn light-colored marble unobstructed lawn light-colored marble plastic track plastic track unobstructed lawn light-colored marble unobstructed lawn light-colored marble plastic track plastic track unobstructed lawn light-colored marble unobstructed lawn light-colored marble plastic track plastic track unobstructed lawn light-colored marble unobstructed lawn light-colored marble plastic track
4.5412a 1.6647 4.5412a 2.8765 1.6647 2.8765 3.3763a 1.8112 3.3763a 1.5651 1.8112 1.5651 2.4782a 7.4218a 2.4782a 4.9436a 7.4218a 4.9436a 3.7364a 0.2636 3.7364a 3.4727a 0.2636 3.4727a
1.4438 1.4438 1.4438 1.4438 1.4438 1.4438 1.2734 1.2734 1.2734 1.2734 1.2734 1.2734 1.1080 1.1080 1.1080 1.1080 1.1080 1.1080 0.8148 0.8148 0.8148 0.8148 0.8148 0.8148
0.003 0.255 0.003 0.052 0.255 0.052 0.009 0.157 0.009 0.221 0.157 0.221 0.027 0.000 0.027 0.000 0.000 0.000 0.000 0.747 0.000 0.000 0.747 0.000
plastic track unobstructed lawn Tmrt
light-colored marble brick plastic track unobstructed lawn
Relative humidity
light-colored marble brick plastic track unobstructed lawn
PET
light-colored marble brick plastic track unobstructed lawn
a
brick brick
brick brick
brick brick
brick brick
95% confidence interval Lower limit
Upper limit
7.444 4.568 1.638 0.026 1.238 5.779 5.893 4.328 0.859 0.952 0.706 4.082 4.666 9.610 0.290 7.132 5.234 2.756 2.127 1.345 5.345 5.082 1.873 1.864
1.638 1.238 7.444 5.779 4.568 0.026 0.859 0.706 5.893 4.082 4.328 0.952 0.290 5.234 4.666 2.756 9.610 7.132 5.345 1.873 2.127 1.864 1.345 5.082
Significance level<0.05.
(1) Solar radiation is an important factor affecting air temperature, surface temperature and Tmrt above the ground surface of different properties in winter and summer. The stronger the solar radiation is, the more significant the difference is. At the same
time, solar radiation also has a significant effect on improving people’s thermal sensation in winter and summer. (2) The difference in the property of the ground surface affects the air temperature above it. The plastic track has a darker color and a
Fig. 8. Relationship between air temperature, Tmrt and PET above the lawn under the shade and unobstructed lawn in winter. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 9
Z. Huang et al.
Journal of Building Engineering 27 (2020) 100994
Fig. 9. Relationship between air temperature, Tmrt and PET above the lawn under the shade and unobstructed lawn in summer. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
lower reflectance to solar radiation, which results in a higher surface temperature, and in turn increases the air temperature above it. (3) Vegetation can humidify the surrounding environment to a certain extent both in summer and winter, and the humidification will be far more significant as the vegetation area expanding. (4) The property of the ground surface has a significant effect on surface temperature and Tmrt in winter, and has a significant ef fect on relative humidity and PET in summer. The ground surface with high reflectivity may increase the immediate temperature above the ground and consequently increase people’s thermal stress in summer. (5) When directly exposed to solar radiation, air temperature is the main factor that influences PET in winter while Tmrt is the main factor that influences PET in summer. Solar radiation can enhance their influences.
Table 10 Significance test results of the regression equation of air temperature and PET above the lawn under the shade in winter.
Regression Residual Sum
Sum of square
Degree of freedom
Mean square
F
Sig.
171.149 138.384 309.533
2 46 48
85.574 3.008
28.446
.000
Table 11 Significance test results of the regression equation of Tmrt and PET above the lawn under the shade in winter.
Regression Residual Sum
Sum of square
Degree of freedom
Mean square
F
Sig.
128.715 180.818 309.533
2 46 48
64.358 3.931
16.373
.000
Based on the above conclusions, the use of ground surfaces with low reflectivity should be encouraged in outdoor planning and design, because it can significantly increase the surface temperature in winter and reduce the people’s thermal stress in summer and can play a positive role to improve the thermal environment in winter and summer.
Table 12 Correlation between air temperature, Tmrt and PET above the ground surface of different properties. R2 Winter Summer
Air temperature Tmrt Air temperature Tmrt
Bluestone slate under the shade
Lawn under the shade
Light-colored marble brick
Unobstructed lawn
Plastic track
0.4832 0.32
0.5529 0.4172 0.949 0.798
0.7243 0.5198 0.747 0.923
0.8651 0.628 0.01 0.912
0.9064 0.5932 0.061 0.953
10
Journal of Building Engineering 27 (2020) 100994
Z. Huang et al.
References
[10] M. Santamouris, N. Gaitani, A. Spanou, M. Saliari, K. Giannopoulou, K. Vasilakopoulou, et al., Using cool paving materials to improve microclimate of urban areas – design realization and results of the flisvos project, Build. Environ. 53 (2012) 128–136. [11] G.E. Kyriakodis, M. Santamouris, Using reflective pavements to mitigate urban heat island in warm climates - results from a large scale urban mitigation project, Urban Clim. 24 (2018) 326–339. [12] E. Erell, D. Pearlmutter, D. Boneh, P.B. Kutiel, Effect of high-albedo materials on pedestrian heat stress in urban street canyons, Urban Clim. 10 (2014) 367–386. [13] E. Ng, V. Cheng, Urban human thermal comfort in hot and humid Hong Kong, Energy Build. 55 (2012) 51–65. [14] Z. Gou, D. Prasad, S.S.Y. Lau, Impacts of green certifications, ventilation and office types on occupant satisfaction with indoor environmental quality, Architect. Sci. Rev. 57 (2014) 196–206. [15] Z. Gou, S.S.Y. Lau, Contextualizing green building rating systems: case study of Hong Kong, Habitat Int. 44 (2014) 282–289. [16] C. ASHRAE, 8—Physiological Principles and Thermal Comfort. Handbook of Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, 2001, pp. 8.1–8.20. [17] Z. Gou, S.S.Y. Lau, H. Ye, Visual alliesthesia: the gap between comfortable and stimulating illuminance settings, Build. Environ. 82 (2014) 42–49. [18] T.-P. Lin, A. Matzarakis, R.-L. Hwang, Shading effect on long-term outdoor thermal comfort, Build. Environ. 45 (2010) 213–221.
[1] G. Fan, J. Xie, H. Yoshino, U. Yanagi, K. Hasegawa, C. Wang, et al., Investigation of indoor thermal environment in the homes with elderly people during heating season in Beijing, China, Build. Environ. 126 (2017) 288–303. [2] Y. Wang, L. Wang, E. Long, S. Deng, An experimental study on the indoor thermal environment in prefabricated houses in the subtropics, Energy Build. 127 (2016) 529–539. [3] N. Zhang, B. Cao, Z. Wang, Y. Zhu, B. Lin, A comparison of winter indoor thermal environment and thermal comfort between regions in Europe, North America, and Asia, Build. Environ. 117 (2017) 208–217. [4] T.-P. Lin, Y.-F. Ho, Y.-S. Huang, Seasonal effect of pavement on outdoor thermal environments in subtropical Taiwan, Build. Environ. 42 (2007) 4124–4131. [5] J. Lu, M. Zhang, W. Pei, Hydro-thermal behaviors of the ground under different surfaces in the Qinghai-Tibet Plateau, Cold Reg. Sci. Technol. 161 (2019) 99–106. [6] Z. Gou, S.Y.S. Lau, P. Lin, Understanding domestic air-conditioning use behaviours: disciplined body and frugal life, Habitat Int. 60 (2017) 50–57. [7] Khoshbakht M, Gou Z, Dupre K. Cost-benefit Prediction of Green Buildings: SWOT Analysis of Research Methods and Recent Applications. Procedia Engineering2017. p. 167-178. [8] M. Taleghani, The impact of increasing urban surface albedo on outdoor summer thermal comfort within a university campus, Urban Clim. 24 (2018) 175–184. [9] P. Campra, M. Garcia, Y. Canton, A. Palacios-Orueta, Surface temperature cooling trends and negative radiative forcing due to land use change toward greenhouse farming in southeastern Spain, J. Geophys. Res. 113 (2008).
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