Highly-Scattering Cellulose-Based Films for Radiative Cooling.

Passive radiative cooling (RC) enables the cooling of objects below ambient temperature during daytime without consuming energy, promising to be a game changer in terms of energy savings and CO 2 reduction. However, so far most RC surfaces are obtained by energy-intensive nanofabrication processes or make use of unsustainable materials. These limitations are overcome by developing cellulose films with unprecedentedly low absorption of solar irradiance and strong mid-infrared (mid-IR) emittance. In particular, a cellulose-derivative (cellulose acetate) is exploited to produce porous scattering films of two different thicknesses, L ≈ 30 µm (thin) and L ≈ 300 µm (thick), making them adaptable to above and below-ambient cooling applications. The thin and thick films absorb only ≈ 5 % ${\approx}5\%$ of the solar irradiance, which represents a net cooling power gain of at least 17 W m -2 , compared to state-of-the-art cellulose-based radiative-cooling materials. Field tests show that the films can reach up to ≈5 °C below ambient temperature, when solar absorption and conductive/convective losses are minimized. Under dryer conditions (water column = 1 mm), it is estimated that the films can reach average minimum temperatures of ≈7-8 °C below the ambient. The work presents an alternative cellulose-based material for efficient radiative cooling that is simple to fabricate, cost-efficient and avoids the use of polluting materials.