S4E9

Abstract

The sky is a natural heat sink that has been extensively used for passive radiative cooling of households. A lot of focus has been on maximizing the radiative cooling power of roof coatings in the hot daytime using static, cooling-optimized material properties. However, the resultant overcooling in cold night or winter times exacerbates the heating cost, especially in climates where heating dominates energy consumption. To address this problem, we recently approached thermal regulation from an all-season perspective by developing a mechanically flexible coating that adapts its thermal emittance to the environment, driven by a photonically amplified metal-insulator transition. In this talk, I will first briefly introduce the development of radiative cooling technologies and then present our temperature-adaptive radiative coating (TARC) that optimally absorbs the solar energy and automatically switches thermal emittance according to environmental temperatures. Our simulations show that TARC outperforms existing roof coatings for annual energy saving in most climates, especially those with substantial seasonal variations.

Introduction of speaker

Dr. Kaichen Dong is a postdoctoral researcher with Prof. Junqiao Wu and Prof. Jie Yao in the Department of Materials Science and Engineering at the University of California, Berkeley and a research affiliate at the Electronic Materials (E-Mat) Program at the Materials Sciences Division of Lawrence Berkeley National Laboratory. His research is focused on phase-change materials, nano-photonics, and micro-machines. Dr. Dong earned his B.Eng. in 2012 and Ph.D. in 2017, both from the Department of Precision Instrument at Tsinghua University. He was a visiting Ph.D. student at U.C. Berkeley (2014-2016) and a research associate at Tsinghua University (2017-2019). He received the National Excellent Doctoral Dissertation Award in the field of Measurement, Control and Instrument from the China Instrument and Control Society (2018).

Abstract

Liquid-gas interfacial transport plays a critical role in 30% of global desalination, 70% of the world’s electricity generation, and almost all heating and cooling systems. Fundamental understanding of interfacial transport, which can broadly impact energy and water applications, has been limited by the difficulty of experimentally isolating and characterizing the interfacial thermal resistance. In the first part of my talk, I will discuss how we overcame this challenge and elucidated fundamental phase change kinetics with a specially designed ultrathin nanoporous membrane. This configuration allowed us to show a unified relationship for evaporation under different working conditions and subsequently provide a general figure of merit for phase change systems. In the second part of my talk, I will discuss how we translate the obtained understanding into performance enhancement in cooling systems leveraging novel engineered materials. For electronics cooling, we created a membrane-based hierarchical evaporator and demonstrated a record pure evaporation heat flux for dielectric fluids. More importantly, our design enabled a new paradigm for phase change heat transfer, favoring low surface tension fluids rather than water. For thermal management of buildings and perishable goods, we invented hydrogel-aerogel bilayer structures which achieves cooling with evaporation while resisting environmental heating. Consequently, we extended the cooling time by 400% compared to the conventional single layer design. Overall, we show that combining fundamental interfacial transport physics with novel materials and interface engineering presents unique opportunities for innovations toward more sustainable energy and water technologies.

Introduction of speaker

Dr. Zhengmao Lu is a postdoctoral scholar advised by Prof. Jeffrey Grossman in the Department of Materials Science and Engineering at MIT. Prior to this appointment, he obtained his Ph.D. in Mechanical Engineering at MIT, advised by Prof. Evelyn Wang. His Ph.D. thesis focused on establishing modeling frameworks and experimental platforms to elucidate evaporation kinetics and create high flux phase change devices for thermal management of electronics. Currently, Zhengmao is developing novel passive cooling solutions for buildings, food, and pharmaceutical products and engineering natural carbonaceous materials for industrial separation processes. Zhengmao is a recipient of the Keck Travel Award in Thermal Sciences and the Wunsch Foundation Silent Hoist and Crane Award for Outstanding Graduate Research from MIT Mechanical Engineering.