Energy benefits of green-wall shading based on novel-accurate apportionment of short-wave radiation components

Climber green wall can alleviate urban heat island and conserve energy, mainly attributed to shading of solar irradiance by vegetation, yet the mechanism has remained poorly understood. The radiation properties of transmissivity, reflectivity and absorptivity, and thermal properties of the building envelope equipped with green wall, jointly govern radiation transfer processes. This field-experimental study monitored in-situ the radiation regime of a climber green wall on a windowed building envelope using high-precision radiometers. The northeast-oriented green wall in humid-subtropical Hong Kong has Lonicera japonica climbers, with 0.24 leaf area index. Radiation properties and shading-induced energy savings were determined in summer and validated. An innovative radiation apportionment model (RAM) was developed to determine the short-wave transmissivity, reflectivity and absorptivity of the climber canopy which were respectively 0.382, 0.074 and 0.543 in sunny weather and 0.449, 0.098 and 0.454 in cloudy weather. For further model development, the extinction coefficient (κ) obtained from Beer’s Law was estimated at 4.00 and 3.34 in respective weather conditions. According to RAM outputs, shading alone could shield against insolation up to 497 W/m2 behind the canopy and 356 W/m2 in the indoor space. Taking the electricity tariff and the carbon intensity of electricity generation of a local power company, the average daily energy savings at 0.226 kWh/m2 were translated into monetary and carbon units, registering approximately USD0.03 and 0.062 kg CO2 respectively. The extrapolated seasonal savings from a total of six green walls installed at the experimental site could reach USD75.8 and 157.9 kg CO2.