Passive cooling design options to ameliorate thermal comfort in urban streets of a Mediterranean climate (Athens) under hot summer conditions

Abstract

This work investigates how to reduce, by appropriate urban design, air temperature at the street level and to improve pedestrian thermal conditions in summer. The case study is a street in Athens with low aspect ratio H/W (H: building height; W: street width), low trees canopy coverage level, and high traffic. Urban variables for design are trees canopy coverage area ratio, traffic load, walls surfaces albedo, and H/W. Their thermal effect is separately and altogether estimated by applying the microclimatic Green-CTTC model whereas thermal stress is assessed using the Physiological Equivalent Temperature (PET) index. The studied day is relatively hot (36.5 °C at midday) with eight hours of heat stress (two and six hours of extreme and strong heat stress, respectively) at street level. Acceptable near-to-discomfort limits for local pedestrians are considered whereas PET classification heat stress levels are adjusted to local conditions by applying a correction equation to obtained PET values. Results show that the examined scenarios are associated with air temperature decrease and improvement of thermal comfort in the shade, especially during the day's hottest hours. The trees thermal effect is the dominant factor followed by the increase of H/W and of the walls surfaces albedo. All the studied design scenarios are found able to reduce the number of heat stress hours but the trees canopy coverage area ratio increase scenario was found by itself able to be associated only with moderate heat stress for local pedestrians. Alternative local smart controls for thermal comfort (SCAT) options are also discussed.