Urban heat mitigation with building-scale green technologies: vertical greenery systems, cool materials and building-integrated photovoltaics

Urban microclimates have been experiencing increasing stress from intense heat events due to global warming and urbanization. A key strategy to mitigate heat in cities is improving the thermal performance of building envelopes with green technologies. Three envelope-based technologies, vertical greenery systems (VGS), cool materials (CM) and building-integrated photovoltaics (BIPV), have emerged with distinct advantages, but each has limitations. While many studies have focused on individual technologies, comprehensive comparisons of the thermal impact throughout outdoors and indoors remain limited. Therefore, this study evaluated the cooling performance of these three technologies using multiple thermal indicators and a coupling approach combining energy simulation and computational fluid dynamics modelling. Comparative analyses were conducted for a city block in Ningbo under local extreme hot conditions, considering different plant species, materials and systems’ ages for VGS, CM and BIPV, respectively. The results indicate that VGS and CM provided overall cooling benefits; however, aged CM could trap radiative heat within street canyons, which intensified local extremes. Despite BIPV’s energy advantages, its potential to induce warming warrants climate-conscious assessments before deployment. The three technologies exhibited divergent carbon outcomes under extreme heat conditions, where VGS acted as a temporary source (increasing emissions by approximately 6.6%), CM reduced emissions and BIPV functioned as a sink (with on-site electricity generation fully offsetting its cooling energy demand). The findings can support integrated designs strategies for building-scale green technologies and highlight the need for multi-criteria evaluation frameworks to guide climate-responsive urban renewal.