Aqueous double-layer paint of low thickness for sub-ambient radiative cooling

Abstract

Radiative cooling may serve as a promising option to reduce energy consumption for space cooling. Radiative cooling paints provide a cost-effective and scalable solution for diverse applications and attract great attention, but the state-of-art cooling paints generally use non-eco-friendly organic solvents and need large thicknesses (>400 μm) to realize high performance, which leads to high cost and environmental issues in implementation. This work aims to address these challenges by developing eco-friendly aqueous paints with low thickness (below 150 μm) by adopting a double-layer design based on a complementary spectrum strategy. The structure consists of a wide bandgap top layer to scatter short-wavelength light and a bottom layer with high reflectance to visible and near-infrared (NIR) irradiation. Effects of different design factors are studied using numerical simulation and experiments to attain the optimal design. The resulting Y₂O₃-ZnO paints show a strong reflectance of 95.4 % and a high atmospheric window emissivity of 0.93 at a low thickness of 150 μm. Field tests in the subtropic humid climate of Hong Kong demonstrated sub-ambient cooling of 2 °C at noon and 4 °C at night without shielding convection. The paints also show high robustness and excellent resistance to water and UV light attacks, rendering them promising for large-scale applications.

Keywords: Monte Carlo simulation; multilayer; radiative cooling; water-based acrylic paint.

Figure 1:

Schematic of double-layer paint structure and its working principle.

Figure 2:

Simulation and calculations. (a) Schematic of simulation process. (b) Reflectances of 50-μm-thick paints with 800-nm fillers of different refractive indexes. (c) Required film thicknesses for fillers with different refractive indexes to achieve a reflectance of 92 %. (d) Scattering efficiency of ZnO and Y₂O₃ particles of different sizes. (e) Variation of the spectral reflectance of the paint with respect to the top layer thickness while maintaining the bottom layer thickness of 100 μm. (f) Variation of the spectral reflectance of the paint with respect to the bottom layer thickness while maintaining the top layer thickness of 50 μm.

Figure 3:

Fabrication and characterization. (a) Fabrication process of double layer paint. (b) Double-layer paint sample on a 10 × 10 cm aluminium substrate. (c) Contact angle of commercial paint (top) and radiative cooling paint (bottom). (d) SEM of Y₂O₃ paint. (e) SEM of ZnO paint. (f) Y₂O₃ particle size distribution. (g) ZnO particle size distribution. (h) SEM of cross-section double layer paint with EDS of cross-section showing ZnO and Y₂O₃. (i) Comparison of different paints. (j) Full spectrum of multilayer paint, Y₂O₃ paint, and ZnO paint.

Figure 4:

Field testing setup and results. (a) Field test setups for cooling temperature (top) and cooling power (bottom). (b) Sky condition in HKUST. (c) Relative humidity and wind speed during the field test. (d) Temperature variation during the field test. (e) Cooling power versus ambient temperature throughout the field test. (f) Calculated daytime cooling power at various surface temperature in different climate regions.

Figure 5:

Weather tests. (a) Comparison of reflectance of different durations of UV light. (b) Comparison of reflectance of before and after water submersion. (c) Comparison of reflectance of double layer paint and commercial paint before and after soiling test. (d) Comparison of soiling test of commercial paint and radiative cooling paint. (e) Adhesion test.