• Energy Research

Better prediction of turbulent airflow around urban trees is necessary because it impacts urban pollutant dispersion, outdoor thermal comfort, energy efficiency, pedestrian-level comfort and urban heat island mitigation. A major challenge is how to obtain an accurate numerical model of the turbulent flow field around trees and an improved parameterization of the turbulent momentum deficit using the drag coefficient (Cd). To address this challenge, we use wind tunnel tests and real trees to measure Cd for four common subtropical tree species: Ficus microcarpa, Mangifera indica, Michelia alba and Bauhinia blakeana. The results show that the Cd of the four trees ranges from 0.523 to 0.932, and that the negative relationship between Cd and wind speed (U) can be expressed by the exponential formula Cd=a*U−b. A three-dimensional model of wind modification by trees is proposed, and its accuracy is verified with wind tunnel measurements. With precise Cd and model, the predictability of the influence of trees on urban wind in subtropical areas is enhanced (root mean square error RMSE ranging from 2.89 m/s to 0.45 m/s). Future development and applications of the numerical model will help provide useful guidelines for urban landscape management and sustainable urban planning in terms of urban ventilation.

Better prediction of turbulent airflow around urban trees is necessary because it impacts urban pollutant dispersion, outdoor thermal comfort, energy efficiency, pedestrian-level comfort and urban heat island mitigation. A major challenge is how to obtain an accurate numerical model of the turbulent flow field around trees and an improved parameterization of the turbulent momentum deficit using the drag coefficient (Cd). To address this challenge, we use wind tunnel tests and real trees to measure Cd for four common subtropical tree species: Ficus microcarpa, Mangifera indica, Michelia alba and Bauhinia blakeana. The results show that the Cd of the four trees ranges from 0.523 to 0.932, and that the negative relationship between Cd and wind speed (U) can be expressed by the exponential formula Cd=a*U−b. A three-dimensional model of wind modification by trees is proposed, and its accuracy is verified with wind tunnel measurements. With precise Cd and model, the predictability of the influence of trees on urban wind in subtropical areas is enhanced (root mean square error RMSE ranging from 2.89 m/s to 0.45 m/s). Future development and applications of the numerical model will help provide useful guidelines for urban landscape management and sustainable urban planning in terms of urban ventilation.

Abstract Vertical green facades (VGFs) are one of the most promising technologies for reducing building energy consumption and mitigating the urban heat island phenomenon. Many studies have investigated the cooling effects of VGFs; however, research on the thermal behavior of VGFs and the impacts on indoor and outdoor thermal environments are scarce, which limits the understanding and application of VGFs. Therefore, field measurements were conducted in the subtropical city of Guangzhou, China, during hot days. First, the thermal balance of the vegetation canopy was investigated. In particular, the net photosynthesis and transpiration of foliage were measured to estimate the thermal effect of plant physiological activities. Moreover, the operative temperature (OT) and wet bulb globe temperature (WBGT) were measured to assess the comprehensive effects of the VGF on the indoor and outdoor thermal environments, respectively. The results indicated that transpiration could consume approximately 50% of solar radiation absorbed by the vegetation canopy. Furthermore, the thermal effect ratio of net photosynthesis to transpiration was less than 5.5%, suggesting that omission of the thermal effect of the net photosynthesis of climbing plants in thermal balance calculations could result in an error lower than 2.9%; such a low error may be acceptable for most engineering applications of VGFs. The VGF caused a decline in room air temperature and mean radiation temperature, resulting in a peak OT reduction of 3.6 °C. Moreover, the peak WBGT in the outdoor environment could be reduced by up to 2.7 °C due to the shading effect and transpiration cooling of the VGF. These findings help advance our understanding of the thermal transfer process of VGFs and extend the application of VGFs from a single cooling purpose to comprehensive improvement of the thermal environment.

Abstract Vertical green facades (VGFs) are one of the most promising technologies for reducing building energy consumption and mitigating the urban heat island phenomenon. Many studies have investigated the cooling effects of VGFs; however, research on the thermal behavior of VGFs and the impacts on indoor and outdoor thermal environments are scarce, which limits the understanding and application of VGFs. Therefore, field measurements were conducted in the subtropical city of Guangzhou, China, during hot days. First, the thermal balance of the vegetation canopy was investigated. In particular, the net photosynthesis and transpiration of foliage were measured to estimate the thermal effect of plant physiological activities. Moreover, the operative temperature (OT) and wet bulb globe temperature (WBGT) were measured to assess the comprehensive effects of the VGF on the indoor and outdoor thermal environments, respectively. The results indicated that transpiration could consume approximately 50% of solar radiation absorbed by the vegetation canopy. Furthermore, the thermal effect ratio of net photosynthesis to transpiration was less than 5.5%, suggesting that omission of the thermal effect of the net photosynthesis of climbing plants in thermal balance calculations could result in an error lower than 2.9%; such a low error may be acceptable for most engineering applications of VGFs. The VGF caused a decline in room air temperature and mean radiation temperature, resulting in a peak OT reduction of 3.6 °C. Moreover, the peak WBGT in the outdoor environment could be reduced by up to 2.7 °C due to the shading effect and transpiration cooling of the VGF. These findings help advance our understanding of the thermal transfer process of VGFs and extend the application of VGFs from a single cooling purpose to comprehensive improvement of the thermal environment.

Abstract Pervious concrete is regarded as a contributing material to reduce surface runoff, purify water pollution, and mitigate the urban heat island. However, the poor water absorption and retention capabilities of it could not meet the need of evaporation. To prolong its evaporative cooling effect, this study introduces a modified method of incorporating biochar (BC) particles into pervious concrete as hygroscopic filler. The albedo, emissivity, porosity, microstructure morphology, water absorption, and evaporation of pervious concrete that was prepared by replacing the cement in weight, by 5.0% of BC particles were investigated. It was found that the BC addition had little impact on the emissivity and porosity of pervious concrete, while significantly reduced the albedo. Contributing to the abundant micro-pores and higher specific surface area of carbonaceous particles, they would significantly enhance the water absorption and retention capability of pervious concrete. Using 5.0% pulverized BC as the admixture maximally increased the total water absorption of pervious concrete from 100 ± 2 kg/m3 to 117 ± 8 kg/m3. During the evaporation process, the water absorption increment could effectively decrease the surface temperature by up to 3–6 °C, with an extra cooling period of 6 h, compared to conventional BC-free pervious concrete. Based on the findings, we conclude that replacing a small quantity of cement by pulverized BC in conventional pervious concretes could effectively improve the evaporative cooling performance.

Abstract Pervious concrete is regarded as a contributing material to reduce surface runoff, purify water pollution, and mitigate the urban heat island. However, the poor water absorption and retention capabilities of it could not meet the need of evaporation. To prolong its evaporative cooling effect, this study introduces a modified method of incorporating biochar (BC) particles into pervious concrete as hygroscopic filler. The albedo, emissivity, porosity, microstructure morphology, water absorption, and evaporation of pervious concrete that was prepared by replacing the cement in weight, by 5.0% of BC particles were investigated. It was found that the BC addition had little impact on the emissivity and porosity of pervious concrete, while significantly reduced the albedo. Contributing to the abundant micro-pores and higher specific surface area of carbonaceous particles, they would significantly enhance the water absorption and retention capability of pervious concrete. Using 5.0% pulverized BC as the admixture maximally increased the total water absorption of pervious concrete from 100 ± 2 kg/m3 to 117 ± 8 kg/m3. During the evaporation process, the water absorption increment could effectively decrease the surface temperature by up to 3–6 °C, with an extra cooling period of 6 h, compared to conventional BC-free pervious concrete. Based on the findings, we conclude that replacing a small quantity of cement by pulverized BC in conventional pervious concretes could effectively improve the evaporative cooling performance.

AbstractThe between blinds glass is a new product which installs aluminium blinds between the hollow glasses. It is mainly used in the developed country nowadays. The purpose of this article is to discuss the adaption of between blinds glass in different climate regions of China. The heat load in winter and cooling load in summer of the residential buildings using three kinds of window glass such as ordinary hollow glass, low emissivity glass and between blinds glass in different climate regions of China are calculated. The thermal performances of three kinds of glass are compared. The results show that between blinds glass is fit for the residential buildings in different climate regions of China.

AbstractThe between blinds glass is a new product which installs aluminium blinds between the hollow glasses. It is mainly used in the developed country nowadays. The purpose of this article is to discuss the adaption of between blinds glass in different climate regions of China. The heat load in winter and cooling load in summer of the residential buildings using three kinds of window glass such as ordinary hollow glass, low emissivity glass and between blinds glass in different climate regions of China are calculated. The thermal performances of three kinds of glass are compared. The results show that between blinds glass is fit for the residential buildings in different climate regions of China.