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

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.