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•An integrated model of EnergyPlus and ENVI-met was employed for this study.•We studied energy saving and cooling potential by different green-roof types.•Both models’ results show a ...good agreement for hourly roof surface temperature.•Lower electricity peak and temperature was found with green-roof types.•The observed lowered energy and temperature varies per urban density and climate.
This paper presents a parametric study on the effect of four green-roof types on outdoor/indoor temperature and cooling demand under four different climates and three urban densities using co-simulation approach with ENVI-met and EnergyPlus. Results reveal an outdoor nighttime warming effect of not more than 0.2°C which is most obvious with the semi-extensive while the outdoor and indoor cooling effect ranges between 0.05–0.6°C and 0.4–1.4°C, respectively depending on the green-roof type, urban density and time of the day. These daytime temperature reductions also vary per prevailing climates and follow this order: hot-dry, hot (or warm)-humid, and temperate which can be explained by the interplay between solar intensity/air temperature and relative humidity between the regions. In a hot-humid region, the evaporative cooling potential of greenery is dampened when compared to hot-dry region. This is also true for region with low solar intensity and humidity like the temperate region. In terms of cooling demand reduction, 5.2% was observed in hot-dry climate on the hottest day of the year with full-intensive green-roof while the least saving of 0.1% was found with semi-extensive green-roof in temperate climate. In general, for both outdoor temperature and cooling demand reduction, semi-intensive green-roof was found more effective than its full-extensive counterpart while the higher spatial green-roof is most important for indoor temperature reduction irrespective of the leaf density of the greenery. Therefore, the intent of green-roof installation should be a determining factor for the type and spatial extent to be implemented.
Camping has become a popular outdoor activity in China. However, the long and scorching summers in China's hot and humid regions pose challenges for campsites in maintaining thermal comfort. ...Therefore, we explored the impact of tree species and planting methods on the thermal comfort of urban campsites in hot and humid areas using the ENVI-met model to simulate the conditions of the study area. The reliability of the model was validated by comparing the simulated values of air temperature (T
) and relative humidity (RH) with field measurements. We conducted an in-depth analysis of common trees in hot and humid areas and analyzed the effects of five tree species and four tree planting forms on the microclimate of campsites in such areas, using the physiological equivalent temperature (PET) as the evaluation index of thermal comfort. The results indicated that: (1) trees with larger crown widths were most effective in improving outdoor thermal comfort. The ability of trees to regulate microclimate was more influenced by crown width than by leaf area index (LAI), and (2) trees planted in patches provided the highest level of thermal comfort, whereas single trees provided the lowest. However, relying solely on tree planting made it difficult to significantly reduce outdoor heat stress. Therefore, other methods such as increasing ventilation or mist spray should be adopted to modify camping area. This study provides a reference for the planting design of outdoor campsites in hot and humid regions of China.
•An optimal spatial scale for examining greenspace cooling effects was identified.•The effect of greenspace configuration on the land surface temperature was quantified.•Spatial configuration of a ...mainland-island greenspace enhances the cooling effect.•Fragmented greenspace is also effective for cooling given a fixed amount of forest cover.•Greenspace cooling intensity can indicate cool island characteristics well.
Urban areas will experience the greatest increases in temperature resulting from climate change due to the urban heat island (UHI) effect. Urban greenspace mitigates the UHI and provides cooler microclimates. Field research has established that temperatures within parks or beneath trees can be cooler than in non-greenspaces, but little is known about the effects of the spatial pattern of greenspace on urban temperatures or the optimal spatial patterns needed to cool an urban environment. Here, urban cool islands (UCIs) and greenspace in Nanjing, China were identified from satellite data and the relationship between them analyzed using correlation analyses. The results indicate the following: (1) Areas with a higher percentage of forest-vegetation experience a greater cooling effect and a 10% increase in forest-vegetation area resulted in a decrease of about 0.83°C in surface temperature; (2) A correlation analysis between mean patch size, patch density, and an aggregation index of forest vegetation with temperature reduction showed that for a fixed amount of forest vegetation, fragmented greenspaces also provide effective cooling; (3) The spatial pattern of UCIs was strongly correlated with greenspace patterns; a mainland-island greenspace spatial configuration provided an efficient means of enhancing the cooling effects; and (4) the intensity of the cooling effect was reflected in cool island characteristics. These findings will support better prediction of the effects of specific amounts and spatial arrangements of greenspace, helping city managers and planners mitigate increasing temperatures associated with climate change.
The urban heat island (UHI) effect has been a great threat to human habitation, and how to mitigate this problem has been a global concern over decades. This paper addresses the cooling effect of a ...novel permeable pavement called evaporation-enhancing permeable pavement, which has capillary columns in aggregate and a liner at the bottom. To explore the efficiency of mitigating the UHI, bench-scale permeable pavement units with capillary columns were developed and compared with conventional permeable pavement. Criteria of capillary capacities of the column, evaporation rates, and surface temperature of the pavements were monitored under simulated rainfall and Shanghai local weather conditions. Results show the capillary column was important in increasing evaporation by lifting water from the bottom to the surface, and the evaporation-enhancing permeable pavement was cooler than a conventional permeable pavement by as much as 9.4°C during the experimental period. Moreover, the cooling effect of the former pavement could persist more than seven days under the condition of no further rainfall. Statistical analysis result reveals that evaporation-enhancing permeable pavement can mitigate the UHI effect significantly more than a conventional permeable pavement.
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•The capillary column enhances evaporation of the new pavement.•The new pavement is cooler 9.4°C in maximum than conventional pavement.•The new pavement provides a longer cooling duration.•Figuline pavers present a better cooling effect than fine concrete pavers.
Increasing vegetation is considered one of the most efficient ways to mitigate the urban heat island (UHI) effect due to the cooling effect of evapotranspiration. Considering the adverse consequences ...of the UHI effect on sustainable development and health problems among city dwellers, the effectiveness of urban vegetation on regulation of the UHI effect should be quantitatively evaluated and analyzed to satisfy the needs of urban planning and policy making. Therefore, in this study we analyzed an one-year field experiment data along an 8-km-long belt transect with various land use types and measured the air temperature at 2-h intervals in Shenzhen, a subtropical megacity in China. The results showed that (1) a higher UHI intensity (UHII) was found during the nighttime throughout the year, with a maximum UHII of 4.02 °C during a heat wave night. (2) Both UHI intensity and the Discomfort Index showed a significant negative linear relationship to vegetated area ratio, especially during nighttime with a strong UHI effect. (3) The UHII decreases by 0.16–0.55 °C during nighttime and 0.05–0.15 °C during daytime, when the vegetated area ratio increases by 10%. (4) Increasing vegetation coverage could significantly reduce air temperature fluctuations and areas with vegetation coverage greater than 55% could maintain a relative stable thermal environment for the dwellers. These results could be useful for urban thermal environmental management and urban planning.
•Nighttime UHII is higher than daytime UHII throughout the year.•A 10% increase in vegetation coverage cools UHII by 0.16–0.55 °C during nighttime.•A 10% increase in vegetation coverage cools UHII by 0.05–0.15 °C during daytime.•Increasing vegetation coverage could significantly reduce air temperature fluctuation.
Trees could provide notable cooling by intercepting solar radiation and evapotranspiration. Human-made shelters in urban areas also serve as shading devices. However, few studies have compared the ...cooling efficacy of trees and artificial shelters. This study systematically quantified and compared the daytime and nighttime cooling effects of a large Chinese Banyan tree (Ficus microcarpa) with dense foliage and an extensive concrete shelter, in an urban park in Hong Kong's subtropical summer. Microclimatic parameters at the two sites were monitored to compare air temperature, and the computed values of PET (Physiological Equivalent Temperature) and UTCI (Universal Thermal Climate Index). The mean daytime cooling effects generated by the tree were 0.6 °C (air temperature), 3.9 °C (PET) and 2.5 °C (UTCI), which were higher than the shelter at 0.2 °C, 3.8 °C and 2.0 °C respectively. The differences were significant for air temperature and UTCI (p < .001 and p < .05 respectively, t-test) but not for PET (p = .261). The tree's mean daytime maximum cooling effects were 2.1 °C (air temperature), 18.8 °C (PET) and 10.3 °C (UTCI). The tree's mean nighttime cooling was significantly higher than the shelter for all three indices (p < .001, t-test). The thermal stress classifications by PET and UTCI were significantly different on the neutral or warmer side (p < .001, Chi-squared test), suggesting that they cannot be used interchangeably. The findings could inform decisions on natural versus artificial shelters in urban thermal design, and trigger comparative investigations in using PET and UTCI for outdoor thermal comfort assessment.
•Daytime cooling effects of a large tree were 3.9 °C (PET) and 2.5 °C (UTCI).•Cooling effect of a concrete shelter was significantly weaker in UTCI (2.0 °C).•Nighttime cooling effects of the tree were significantly higher than the shelter.•PET was more sensitive to ‘extreme heat stress’ compared to UTCI.•Classifications on PET and UTCI scales were different on the neutral or warmer side.
Urban trees are one of the most effective strategies to mitigate excessive heat stress in cities. To understand the underlying mechanisms of their cooling effect and to assess their use in urban ...planning, the accurate simulation of how trees interact with the ambient built environment is critical and imperative. However, the representation of urban trees in existing urban canopy models (in particular the single-layer ones) remains oversimplified. Here we develop a new Monte Carlo ray tracing method to explicitly resolve the canopy transmittance and evaluate its impact on radiative view factors between trees and regular building facets. The new method is highly accurate in reproducing analytical solutions. Sensitivity tests of radiative view factors suggest the importance of canopy transmittance in changing the radiation exchange. We then incorporate the ray tracing algorithm into the new version of the Arizona State University (ASU) Single-Layer Urban Canopy Model (ASLUM v3.1). In addition to radiation transmittance, ASLUM v3.1 explicitly resolves the radiative shading, evapotranspiration, and root water uptake of urban trees in street canyons, with significantly improved performance in predictions (especially latent heat flux) when compared to previous versions. We further apply ASLUM v3.1 to evaluate the impacts of trees with varying characteristics on urban radiation exchange and turbulent heat fluxes. Results show that urban trees reduce the net radiation of ground and wall as well as the daytime temperature via shading and transpiration, but may slightly warm the nighttime street canyons through radiative trapping effect.
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•We develop a new stochastic method to include canopy transmittance in view factors.•Sensitivity of view factors to urban geometries and transmittance is evaluated.•The new urban canopy model with tree modeling outperforms its previous versions.•Radiation exchange and cooling effect of trees depend on foliage characteristics.
Urban heat island (UHI) effect has serious negative impacts on urban ecosystems and human well-being. Mitigation of UHI using nature-based solutions is highly desirable. It was well known that urban ...green infrastructure (UGI), i.e., urban vegetation, can effectively mitigate UHI effect. However, the potential of urban blue infrastructure (UBI), i.e., urban surface water, on UHI mitigation is not well understood, although its potential to lower UHI effect via evaporation is similar to the biophysical mechanism of evapotranspiration through vegetation. In this paper, we study the relationship between UBI and land surface temperature (LST) in Wuhan city in central China, using a normalized difference water index (NDWI), maximum local cool island intensity and the maximum cooling distance as indicators for the cooling effects of UBI, respectively. We found a significant negative linear relationship between mean LST and NDWI after NDWI passes a critical threshold value. NDWI is an effective biophysical parameter to delineate the spatial distribution of UBI. The cooling effects of UBI are influenced both by its size and shape. Water surface temperature decreased logarithmically with increasing UBI size, critical threshold values of UBI size corresponding to maximum cooling efficiency do exists. Maximum cooling distance and maximum local cool island intensity are also affected by the shape and size of UBI, and exhibit seasonal and spatial variations. These results provide insights for urban landscape planning regarding how to use UBI as a nature-based solution to improve urban thermal environment.
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•NDWI can effectively delineate UBI and relate it to LST.•Linear relationship exists between mean LST and NDWI large than a threshold value.•Negative logarithmic relationship exhibits between UBI size and its mean LST.•Quantitative relationships exist between UBI and its cooling indicators.•The cooling effect of UBI varies with seasons and UBI's geographical location.
•Floating PV panels experience cooling by deployment on water-bodies.•Open system designs leads to an increase in the heat loss coefficient of floating PV panels.•Similar trends of the cooling effect ...in two climates have been observed.•Simulations show an increase in annual energy yield of FPV systems, up to 6%, due to the cooling effect.
An enormous area could potentially be unlocked, when more photovoltaic (PV) systems would be deployed on water bodies. Especially in densely populated areas this opens a pathway for PV to contribute to the energy transition in a large scale. Another potential benefit of floating PV (FPV) systems is that they can outperform conventional PV systems in terms of energy yield due to the cooling effect from the water. However, there is very little field data available to quantify the cooling effect and compare it across different climatic conditions. The research presented here has thoroughly studied this effect and translated it into an estimated specific yield comparison between conventional and floating PV systems. The study is based on field tests that are located in two different climate zones: a temperate maritime climate (the Netherlands) and a tropical climate (Singapore). Irradiance weighted average temperatures of FPV systems have been compared with a land-based system in Netherlands and a rooftop system in Singapore as references. The best performing FPV systems showed 3.2 °C (Netherlands) and 14.5 °C (Singapore) lower weighted temperatures compared to their benchmarks. Open system designs, where the PV panels of the floating system are widely exposed to the water surface, lead to an increase in the heat loss coefficient of floating PV panels (a measure for the cooling effect) of up to 22 W/m2K compared to reference PV systems. Annual specific yields of the PV systems were estimated by the measured irradiance-weighted temperature difference and by a PVsyst model with inputs of the heat loss coefficients. Based on these calculations, we observe that the gain in energy yield from the cooling effect of FPV systems compared to the reference PV systems is up to 3% in the Netherlands and up to 6% in Singapore.
Green areas in the city can greatly improve the outdoor thermal environment, as well as mitigate the urban heat island effect by reducing summer air temperature. In a context of climate change, with ...the expected increase in temperature and intensity of heat waves, cooling by green areas is set to become increasingly important. In this study, field observations were carried out to investigate the thermal performance of a large urban vegetated park and its influence on thermal environment of the surrounding urban areas in Beijing, China. Measurements were conducted along a selected path during three summer days with clear skies and light winds by mobile traverses. The results showed that the park was cooler than the surrounding urban areas both during the daytime and at night. The mean air temperature differences between the park and the surrounding areas were in the range of 0.6–2.8°C at different times, with a maximum of 4.8°C observed at midnight. The results also found that as the distance from the park border increased, the ambient air temperature exhibited a gradually increasing trend. This indicated the park has a cooling effect on the urban environments adjacent to the park, and this cooling effect extended approximately 1.4km from the park boundary. The air temperature variations also depended strongly on the land cover characterizing the immediate environment of the measurement sites. Increasing the percent vegetation cover could significantly decrease air temperature, while the increase of percent impervious surface area would significantly increase it.
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•We study the cooling effects of a large park and observed significant cooling effects.•The park was 0.6 to 2.8°C cooler than the surrounding urban environment.•The park cooling was variable but could extend almost 1.4km beyond the park's border.•Large differences in temperature exist both within the park and surrounding urban areas.•The air temperature differences depended strongly on land cover features of each site.
