[Keynote] Zero Carbon Buildings Under Urban Heat Stress How Urban Microclimate Redefines Decarbonization Pathways

Zero Carbon Buildings (ZCBs) are currently evaluated using annual carbon balance methodologies that neglect urban microclimate dynamics, peak-hour grid carbon intensity, and thermodynamic feedbacks between buildings and cities. This work demonstrates that a building certified as net zero under standard static weather assumptions can become a significant carbon emitter under urban heat island (UHI) conditions due to coupled interactions between ambient temperature, HVAC system efficiency, urban morphology, and synchronized electrified cooling demand. The study introduces a multi-scale analytical framework linking building energy modelling, urban microclimate processes, and district-scale thermodynamic constraints. The framework identifies three coupled feedback mechanisms governing ZCB underperformance: (i) UHI-induced degradation of heat pump coefficient of performance (COP), (ii) morphology-driven thermal rejection and density constraints, and (iii) peak-hour carbon amplification caused by synchronized electrification loads. Results indicate that dense urban morphologies with Floor Area Ratios (FAR) above 3 fundamentally alter the feasibility of low-temperature fifth-generation district heating and cooling (5GDHC) systems and increase cooling-related peak emissions despite annual net-zero balances. The work further highlights a major modelling blind spot: the absence of coupled simulation between HVAC exhaust thermodynamics, pedestrian thermal comfort (PET/UTCI), and urban canopy microclimate. Comparative analysis between European and Chinese urban forms demonstrates that ZCB pathways are not transferable across climatic and morphological contexts because urban geometry determines system feasibility before technology selection occurs. The paper concludes that zero-carbon assessment must move from annual building-scale balances toward temporally resolved, urban-scale thermodynamic evaluation integrating microclimate, infrastructure density, and peak carbon dynamics.