Session: 12-06-02: Condensation and Phase Change Materials

Paper Number: 173007

Condensation Dynamics of Ultrawhite Radiative Cooling Paints 

Water is fundamental to societal development, public health, and global sustainability. However, potable water scarcity represents a critical global challenge, particularly for marginalized and vulnerable communities. Atmospheric water harvesting by radiative cooling offers a promising passive strategy to mitigate water scarcity. This technology works by reflecting solar radiation and emitting thermal radiation through the atmospheric sky window (8–13 µm), enabling below-ambient surface temperatures that promote dewing. Recent advances in ultrawhite radiative cooling coatings, using highly reflective pigments such as calcium carbonate (CaCO₃), barium sulfate (BaSO₄), and hexagonal boron nitride (hBN), have shown below-ambient cooling capability, but their potential to enhance condensation has yet to be explored.

In this study, we systematically investigate the influence of ultrawhite paints’ surface wettability, subcooling (2–10 °C below the dew point), and inclination angle (30° and 60°) on condensation dynamics and water collection efficiency. These factors play critical roles: the surface wettability controls droplet nucleation, mobility, and shedding; the inclination angle impacts both the radiative view factor to deep space and the ability of gravity to harvest condensed droplets; and differing pigments exhibit distinct surface morphologies that influence condensation behavior.

The results of our controlled experiments demonstrated the coupled effects of surface wettability (owing to different paint formulations), subcooling, and inclination on condensation dynamics and dew water yield. The paints offer distinct condensation modes, with filmwise condensation observed for the CaCO₃ paint and dropwise condensation for BaSO₄ and hBN based paints. The hBN-based paint achieved the highest condensation mass flux (486 g/m²·h at 10 °C subcooling and a 60° inclination angle), this was attributed to the enhanced nucleation and low receding contact angle of this paint, resulting in a dropwise condensation mode that supports continuous re-nucleation of droplets. It was found that the condensation mass flux increased nearly linearly with subcooling, while the onset time for water collection followed  an asymptotic trend, with significant delays at low subcooling (up to 3 hr). The inclination angle had minimal effect on mass flux at both low and high subcooling but played a significant role in determining the onset time. The 60°-inclined surface enabled faster droplet removal, reducing water collection onset time by nearly half. Minimizing delays is especially critical in real-world scenarios, where ideal environmental conditions may be short-lived or variable, requiring the optimal selection of inclination angle in order to maximize water collection. These findings in a controlled environment provide insights and pathways into engineering efficient outdoor radiative cooling water harvesting systems.

Presenting Author: Orlando Rivera Gonzalez Purdue University

Presenting Author Biography: Orlando is a doctoral candidate at Purdue University, working on phase change and thermal transport and surface characterization of ultrawhite, nanoparticle-based, radiatively cooled surfaces for condensation applications such as water harvesting. He is currently conducting his research under the guidance of Professor Justin Weibel and Professor Xiulin Ruan. He is a recipient of the NSF Graduate Research Fellowship (2022–2025) and Purdue's Ross Fellowship (2021–2022).

Authors:

Orlando Rivera Gonzalez Purdue University
Didier Ojeda Sanchez National University of Colombia
Abdulrahman Aljwirah Purdue University
Dudong Feng Purdue University
Akshay Rao Stanford University
David Warsinger Purdue University
Xiulin Ruan Purdue University
Justin Weibel Purdue University