Multiple-scale urban form renewal strategies for improving diffusion of building heat emission—A case in Xi'an, China

Air conditioning systems transfer heat from indoors to the outdoors during hot summers, exaggerating the urban heat island (UHI) phenomenon and the risks of relevant climatic disasters. Urban form renewal strategies are urgently required to improve the diffusion of building heat emissions (BHEs). In this study, the impacts of BHEs on the outdoor airflow and air temperature fields of 42 typical urban blocks in Xi'an were simulated using scSTREAM program. BHEs increased the near-surface UHI intensities by 1.0 °C and 1.4 °C in residential and mixed-use blocks, respectively. The diffusion conditions of the BHEs were better for blocks with lower building densities. However, the UHI increase caused by the BHEs was positively correlated with the floor area ratio only in the mixed-use blocks. Regression formulas describing the relationships between the UHI increase caused by the BHEs and block-form indicators were then proposed. In a district renewal project for urban climate optimization, these formulas were used as the fitness functions of genetic algorithm to determine the form indicators for all blocks therein. The district UHI intensity caused by BHEs could be reduced by 0.3 °C with the maintaining of the district development intensity.