The impact of neighborhood layout heterogeneity on carbon emissions in high-density urban areas: A case study of new development areas in Hong Kong

Under the climate change context, there is a universal appeal for a paradigm shift towards environmentally sensitive urban design. Previous studies focused on density parameters, while ignoring the rational layout of buildings in the neighborhood, especially the layout heterogeneity, which would cause different environmental performance outcomes under the intricate and dynamic interdependence among buildings. This study systematically explores the impact of five neighborhood layout heterogeneity indicators, including the coefficient of variation of building height, footprint area, volume, footprint aspect ratio, and surface-to-volume ratio, on carbon emissions, with consideration of several scenarios regarding different renewable energy application ratios. Totally 384 neighborhood cases are modeled under the same planning conditions in a new development area in Hong Kong and examined with the same simulation assumptions so as to exclude the impact of non-design related factors. The joint influences of different neighborhood layout indicators are examined by multiple linear regression analysis. The statistical results confirm the significant impacts of heterogeneity of building density and shape on neighborhood carbon emissions. It is found that the heterogeneity of building height would impede carbon reduction efforts due to higher mutual shading on rooftops and thus less solar harvesting, while the carbon reduction by building façade solar energy collection could be improved by the heterogeneity of building shape, including the variation of aspect ratio and surface-to-volume ratio. When considering both energy consumption and renewable energy collection, the heterogeneity of building height can lead to less net carbon emissions while that of building shape results in more net carbon emissions. Notably, the heterogeneity of neighborhood layout design would lead to the opposite result in carbon reduction efforts regarding solar energy collection and building energy consumption, which leaves a trade-off in decision-making when designing low-carbon neighborhoods.