Scalable Drop-to-Film Condensation on a Nanostructured Hierarchical Surface for Enhanced Humidity Harvesting

Abstract

Active cooling-based atmospheric water generators, despite their growing demand, continue to be energy intensive and offer poor collection efficiencies (energy consumption per liter of water production). Despite progress in micro-/ nanofabrication techniques and functional coatings, advanced surfaces have not been successfully scaled onto such harvesters to accelerate condensation and improve their efficiencies. Here, we present a scalable dualnanostructured hierarchical surface that comprises sporadically distributed bundles of randomly oriented faceted microcones having facets composed of nanostructures, which are either bumps or ridges. Condensate removal on this surface occurs via drop-to-film coalescence, followed by film shedding in the form of macrodrops. Compared to a conventional plain metal surface used for condensation, the improvement in latent heat transfer coefficient using a hierarchically textured surface ranged from 19.9% at a subcooling of ∼8 °C to 1048.4% at a subcooling of ∼1 °C in laboratory scale experiments, subcooling being defined with respect to the dew point. To demonstrate utility at industrial scale and to ensure scalability of the modified surfaces, we create a prototype assembly comprising a tube-fin heat exchanger with hierarchically textured fins, cooled using a standard refrigeration cycle, producing ∼25 L of water per day. The prototype containing hierarchically textured fins provides ∼10.8% enhanced water collection at ∼10.4% improved average collection efficiency compared to the traditional water generator when tested in outdoor conditions.

Inducing hydrophobicity in Stainless Steel 304 by mechanical texturing and chemical functionalization

“…Assuming spherical droplet shapes for SS-PA-P, SS-PY-PA-P, and SS-CA-PA-P, the calculated condensate water masses for each droplet are 0.38 mg, 2.14 mg, and 1.15 mg, respectively. Higher water mass indicates higher water harvest rate and higher latent heat transfer coefficient (HTC) in accordance to equation 8 [3].…”

Preferred Mode of Atmospheric Water Vapor Condensation on Nanoengineered Surfaces: Dropwise or Filmwise?

“…This may have caused, for the range of degree of subcooling and humidity ratio difference investigated here, the peak of the rough structures on the SHL copper and aluminum substrates to remain dry and protruded above the thin film. 32 , 35 , 52 These regions act as the sites for a high rate of heterogeneous nucleation despite having the substrate exhibiting an overall thin-filmwise condensation. As a result, at steady state, the rate of condensate formation in the thin-film regime is higher than that in the rivulet regime.…”

“…Table summarizes the research articles that reported the overall condensation rate on the nonwetting superhydrophobic and wetting superhydrophilic surfaces in the atmospheric humid air conditions. Clearly, the majority of the works cited in Table report that filmwise mode is more effective than dropwise mode for humidity harvesting. − However, these observations did not receive comprehensive attention and, as a consequence, have remained a mystery. This result is contradictory to the general perception about the heat transfer efficiency of dropwise and filmwise modes of condensation under pure vapor conditions, but a plausible reason has not yet been addressed.…”

Fundamentals and Applications of Surface Wetting

“…Copyright 2013, American Chemical Society, Washington, DC. Panel (b)­(i) was reproduced with permission from ref . Copyright 2021, American Chemical Society, Washington, DC.…”