Cooling performance of near-infrared and traditional high-reflective coatings under various coating modes and building area densities in 3D urban models: Scaled outdoor experiments

• Impacts of 3D layouts and cool coatings on urban thermal environment are studied. • Coating types (roofs, walls or both) and densities ( λ p =0.44/0.25) are considered. • Near-infrared reflective materials induce much cooler walls/indoor air for RW-type. • Among most cases, λ p =0.44 attain greater overall urban albedo than at λ p =0.25. • Cool walls induce cooler walls (2.3–4.4 °C)/indoor air (2.3–3.8 °C) at λ p =0.25. The urban heat island can be mitigated by using surface cool coatings. However, it is challenged to conduct field measurements in real cities for quantifying the integrated effects of urban geometries and cool coatings on urban thermal environments. We conducted scaled outdoor experiments of 3D urban models(building height H = 1.2 m, street width W = 0.5 m/0.25 m) during May-July 2023 in a subtropical city(Guangzhou). Significant urban parameters including two plan area index( λ p =0.25/0.44), three cool materials of traditional white/yellow coatings(TR-W, TR-Y with reflectance: 0.73, 0.76) and near-infrared reflective yellow coating(NIR-Y, 0.76, 0.92 in the near-infrared spectrum), and three coating modes(only roof, R-type; only vertical walls, W-type; both roof and walls, RW-type) were considered. In uncoated cases, overall average urban albedo α a = 0.13/0.16 when λ p =0.25/0.44. Under TR-W coatings, λ p =0.44 experienced greater α a than λ p =0.25, and all three coating modes significantly enhanced α a , with more effective by R-type and RW-type. Moreover, at λ p =0.44, average wall/indoor cooling effects of W-type and RW-type were 0.6–1.6 °C/0.7–1.6 °C, less than those at λ p =0.25(2.3–4.4 °C/2.3–3.8 °C) with a larger sky view factor. As λ p changing from 0.25 to 0.44, the overall urban heat storage flux( ΔQ S ) significantly rose, whereas cool coatings had little influence on ΔQ S . Besides, the NIR coating increased α a to 0.28/0.47 under W-type/RW-type, which was an increase of 33.3 %/30.6 % compared to the TR-W coating, and reduced < ΔT̅ wall >(0.9–1.3 °C/2.2–2.5 °C for W-/RW-type) by 125–225 %, and < ΔT̅ indoor_air >(1.4 °C/2.9 °C) by 55.6 %-61.1 % more than TR-Y (0.9 °C/1.8 °C). Street air temperature showed minimal change with cool coatings. These experiments can validate and improve numerical simulations, providing a scientific basis for urban planning.