Exploiting radiative cooling for uninterrupted 24-hour water harvesting from the atmosphere

Abstract

Atmospheric water vapor is ubiquitous and represents a promising alternative to address global clean water scarcity. Sustainably harvesting this resource requires energy neutrality, continuous production, and facility of use. However, fully passive and uninterrupted 24-hour atmospheric water harvesting remains a challenge. Here, we demonstrate a rationally designed system that synergistically combines radiative shielding and cooling-dissipating the latent heat of condensation radiatively to outer space-with a fully passive superhydrophobic condensate harvester, working with a coalescence-induced water removal mechanism. A rationally designed shield, accounting for the atmospheric radiative heat, facilitates daytime atmospheric water harvesting under solar irradiation at realistic levels of relative humidity. The remarkable cooling power enhancement enables dew mass fluxes up to 50 g m^-2 hour^-1, close to the ultimate capabilities of such systems. Our results demonstrate that the yield of related technologies can be at least doubled, while cooling and collection remain passive, thereby substantially advancing the state of the art.

Fig. 1. Design of system.

(A) Working principle with separated radiation and condensation side. The radiation shield—optimized by accounting for the surrounding radiative environment—allows one to improve substantially the dew harvesting potential of the system and can be applied for any selective emitter. (B) Structure of selective emitter. It consists of PDMS and silver, coated on a transparent glass substrate (chromium is used for oxidation protection and adhesion). (C) Measured spectral absorptivity/emissivity of the selective emitter. The average emissivity in the atmospheric transparency window is very high (${\overline{\mathrm{\epsilon }}}_{8-13}\approx 0.93$), while the average absorptivity in the solar spectrum range is very low (${\overline{\mathrm{\alpha }}}_{0.25-2.5}\approx 0.04$).

Fig. 2. Subcooling and dew harvesting potential enhancement through the radiation shield.

(A) Thermodynamic analysis of the radiation shield and involved heat fluxes. Styrofoam blocks condensation side, consequently ${\stackrel{·}{Q}}_{\text{dew}}=0$. The angular dependence of atmospheric emissivity is represented in blue. (B) Subcooling performance of OPUR, selective emitter with (β = 30°) and without (β = 90°) radiation shield. (C) Theoretical dew harvesting potential. The selective emitter, working collaboratively with the radiation shield, extends the harvesting window by more than 2 hours (t₁ to t₃) and to conditions with RH < 50% and 808 W m⁻² solar irradiation (10-min intervals). (D) Improvement of the state of the art. The selective emitter, collaboratively working with the radiation shield, outperforms existing dew harvesting technologies by a factor 2 to 3 depending on RH.

Fig. 3. Dew harvesting experiments.

(A) Qualitative images of dew harvesting under direct solar radiation. (B) Dew formation under solar irradiation, 90 min after the emitter is exposed to various (95 to 65%) levels of constant RH. (C) Dew mass flux and solar irradiation over 24 hours at RH > 90%. (D) Mean dew mass flux and mean irradiation for five experimental runs, demonstrating broad operational capability. More information can be found in table S1. Photo credit: Tobias Gulich, ETH Zurich.

Fig. 4. Self-removal mechanism of CNF coating.

(A) Working principle of superhydrophobic coating, promoting self-removal of droplets. Top right: SEM image of the CNF coating, verifying the hierarchical micro-/nanostructure. Bottom right: Long exposure image of coalescence-induced jumping, showing traces of detached droplets. (B) Dew mass flux rate of condensation coating (mean: 28.6 g m⁻² hour⁻¹; N = 9). Gray area represents SD. (C) Twelve-hour durability test of CNF coating (mean: 28.1 g m⁻² hour⁻¹; N = 2).