Scalable Micro/Nanostructured Surfaces for Thin-Film Condensation Heat Transfer Enhancement in Steam Power Plants

Abstract

Steam power plants, which contribute to over 50% of energy production globally, rely on condensers to control system-level energy efficiency. Due to the high surface energy of common heat exchanger materials, the vapor condenses by forming a continuous liquid film with low thermal conductivity (filmwise condensation), hindering heat transfer from the vapor side to the condenser surface. Hydrophobic surfaces achieved by either chemical methods (e.g., coating treatment) or physical methods (e.g. structures design) have shown great promise in enhancing condensation heat transfer by promoting dropwise condensation. However, the short lifetime and high fabrication cost of most of these hydrophobic surfaces remain a challenge for long-term and large-scale industrial applications. A promising solution to enhancing condensation heat transfer in a robust and scalable manner is to control the thickness and thermal conductivity of the condensate film, which we term thin-film condensation. This can be achieved by sandwiching a thin layer of porous metal wick between a hydrophobic membrane and the condenser surface to confine the condensed liquid, forming a thin liquid-metal composite film that significantly improves the effective thermal conductivity of the condensate-filled porous media.

In this work, we designed, fabricated, tested, and demonstrated thin-film condensation heat transfer using commercially available materials and scalable approaches. First, we proved the concept using biphilic, microchannel-assisted hierarchical copper surfaces made of commercially available copper foams and copper meshes. Condensation heat transfer on the hierarchical copper surfaces was characterized to be up to 2x as compared to the conventional filmwise condensation, even with flooding on the surface due to the defects on the mesh and the coating. Then, we investigated electrospinning as a potential approach to customize hydrophobic membranes for the thin-film condenser surfaces. The key benefit of the hydrophobic membrane in the surface design is to generate capillary pressure through micro/nanoscale pores, which acts as the driving force for the condensate flow in the metal wick. We conducted a parametric study on the effects of several key fabrication parameters on the pore size of the electrospun membrane, with the help of the fractional factorial design. Solution feeding rate was found to be the most impactful parameter on the membrane pore size and should be considered the most during membrane optimization. A heat and mass transfer model was developed to predict the heat transfer performance of the thin-film condenser surfaces made of electrospun membranes and porous copper wicks. Upon careful design of the surface structures, an over 5x heat transfer enhancement is expected on these thin-film condensers, which is comparable to the state-of-the-art dropwise condensation. Finally, a techno-economic analysis was conducted on the thin-film condensers. The result shows that the additional material for the condenser tube modification costs less than 10% of the condenser cost. However, with the expected 5x steam-side condensation heat transfer performance, thin-film condensers will be able to increase power plants' output by 2-6%, which is equivalent to over $10B of the value proposition for steam power plants across the globe.