Cooling effect of direct green façades during hot summer days: An observational study in Nanjing, China using TIR and 3DPC data

Abstract

Thermal regulation is a key ecosystem service provided by direct green façades (DGFs), as vegetated walls absorb short wave radiation, reduce solar re-radiation from hard surfaces, and provide cooling due to shading and evapotranspiration. Few studies have investigated the correlation between the cooling effect of DGFs and vegetation characteristics at a fine spatial and temporal scale. This paper presents a new methodology to evaluate the cooling effect of DGFs related to fine-scale plant characteristics for hot summer days using thermal infrared (TIR) and three-dimensional point cloud (3DPC) data, through a case study conducted at the Executive Office Building on Nanjing University's Xianlin Campus, China. Results show that daily mean DGF surface temperature is significantly lower than the average bare wall surface temperature, with a maximum reduction of 4.67 °C. The cooling effect of the DGF is most obvious during midday (10:30 h to 16:00 h) and significantly decreases at night. At the pixel scale, the DGF exhibits a significant spatial variation of surface temperatures, which may be closely related to the DGF's canopy structure. Among the four vegetation indices acquired based on 3DPC data, the percentage of green coverage and the cooling effect of the DGF exhibited a linear relationship, while plant thicknesses, point density, and volume of the green façade were power function distributions. Incoming solar radiation and air temperature are the dominant independent variables in cooling effect and surface temperature fitting models. Our findings can guide DGF design to cool the thermal environment more effectively and to enhance building energy savings.