Effects of tree shading and transpiration on building cooling energy use

Building cooling energy use is regarded as the hot topic of building research due to its increasing trend at a rate proportional to the total direct energy use in urban areas nowadays. Urban greening as an effective countermeasure in reducing building cooling load has been attracting close attention although there is a shortage of quantitative measures for applying the general idea to a specific site or building. This study addresses this problem. The cooling effect of trees around buildings and the influence of tree characteristics on building cooling load reduction was quantified using microclimate field measurements and sap flow rate of trees. By integrating approaches of field measurements of microclimate and tree characteristics into EnergyPlus simulation of building energy use, the cooling effects of trees due to both shading and transpiration on buildings were quantified and discussed. Four scenarios were developed based on different tree characteristics for quantifying the influence of trees on saving of building energy use: Scenario 1 with treeless condition, Scenario 2 with existing trees condition in a case of Nanjing City, Scenario 3 with more tree shading condition, and Scenario 4 with more tree transpiration condition. The simulation results showed that compared to Scenario 1, Scenario 2 can reduce building energy use by 10.3% due to the benefits from existing trees shading and transpiration. More shading in Scenario 3 and a higher transpiration rate in Scenario 4 can reduce building energy use by 15.2% and 12.4%, respectively in comparison to that in Scenario 1.