Investigation of air-conditioning anthropogenic heat dissipation and thermal discomfort in street canyons toward sustainable urban cooling

Global climate change and rapid urbanization exacerbate urban heat islands, increasing cooling demands and making air conditioner running a major source of anthropogenic heat emissions. This creates a vicious cycle where high temperature induces high energy consumption, resulting in substantial heat dissipation within dense street canyons, further threatening public health by significantly reducing pedestrian thermal comfort. However, systematic understanding of the heat accumulation mechanism and pedestrian discomfort impact is limited under concurrent heat dissipation of air conditioner outdoor units, combined with the influence of real heat sources (e.g., solar radiation, traffic), in dense street canyons. This study used a full-scale simulation method to evaluate air conditioner outdoor unit heat dissipation and potentially induced thermal discomfort across different scenarios (varying street orientation/width/length; outdoor unit running status and period). Based on the evidence from Nanjing, air conditioner outdoor units largely elevated pedestrian-level temperature by 1.1–1.9 °C and 1.9–3.3 °C in daytime and nighttime, respectively. Narrow street (15 m width) was 0.6–1.0 °C hotter than wide street (40 m width) due to restricted airflow. In north-south streets, east-side pedestrian space could be 3.5 °C hotter than west-side space due to shading. The Discomfort Index reached the medical emergency state in over 60 % of pedestrian spaces within narrow-long streets (15 m width and 60 m length) at night. Vertical greenery can effectively reduce temperatures by 2.3 °C and alleviated thermal discomfort. This work offers significant insights pertaining to anthropogenic heat mitigation of air conditioners in dense cities, which is instructive for sustainable urban cooling. • Air conditioner outdoor units elevate temperatures by 1.9 °C (day) and 3.3 °C (night). • Narrow streets (15 m width) trap heat with 1 °C hotter than wide (40 m width) streets. • 60 % of pedestrian areas in narrow-long streets reach medical emergency thermal state. • Vertical greenery reduces temperature by up to 2.3 °C to mitigate thermal discomfort.