Abstract
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 CO2 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% 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.
Highlights
Introduction searchTo achieve efficient radiative cooling, the optical propertiesThe Earth keeps its thermal equilibrium by dissipating the excess heat from absorbed solar energy to the cold outer space (≈3 K) via thermal radiation
When the emitted power exceeds the power absorbed by the material over all other wavelengths, the material is able to cool below ambient temperatures
The performance of passive-daytime radiative cooling (PDRC) materials with high emittance in the IRAW can be enhanced by designing their morphology to minimize absorption of solar irradiance over a wide spectral range (λ ≈ 0.3–2.5μm)
Summary
Thin films of cellulose (acetate) with a disordered network morphology were produced by a phase separation method. The achieved porosity enables strong light scattering down to UV wavelengths and across the visible solar spectrum (Figure 2b), while showing thermal emission in the mid-IR and the IR atmospheric transparency window. We characterize the network morphology, optical transmittance and reflectance spectra as well as their thermal properties to show their applicability and performance for radiative daytime cooling and discuss their potential for sustainable large scale coating applications
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