Micro/Nanostructures for Enhanced Heat Transfer and Energy Storage(應用微奈米結構增強熱傳與熱能儲存)
Approximately 90% of the world’s total power is generated by converting thermal energy into electricity. Thus, to address energy security, we must look at the science and engineering of thermal energy. This talk focuses on using micro/nanostructures to enhance heat transfer and energy storage in various thermal energy systems. Condensation plays an essential role in many thermal systems. Condensation which involves heterogeneous nucleation, growth, and departure of liquid droplets is intrinsically a random process. We report the ability to spatially control heterogeneous nucleation on a superhydrophobic (SHB) silicon nanowire array-coated surface. Moreover, superior heat and mass transfer performances were found on an SHB silicon nanowire (SiNW) array surface. Nevertheless, the condensation heat transfer on SHB surfaces is greatly deteriorated by the flooding phenomenon at high subcooling. Thus, a novel threedimensional (3D) hybrid surface was proposed to enhance the condensation at high subcooling temperatures. The 3D hybrid surface consisted of SHB SiNW arrays and hydrophilic microchannels. The heat transfer coefficient on the 3D hybrid surface could be enhanced over a large subcooling range. More remarkably, a high heat flux of 655 ± 10 kW∙m-2 was obtained on the 3D hybrid surface. Ice formation may cause many adverse effects in a number of ways on many natural and industrial systems. SHB surfaces lost their ice-phobic property during the frosting. We demonstrated the abilities of spatial control of ice formation and confinement of icestacking direction by manipulating the free energy barrier to nucleation. The controlled ice formation enhances anti-icing and deicing performances. The concept demonstrated in the work could lead to the development of new engineered ice-phobic surfaces Among all the renewable energies, solar energy is the most promising energy-harvesting resource because of its abundance. Solar-thermal power generation, which stores sunlight as heat and converts it into electricity when power is needed, could overcome the diurnal limitation of solar power. We demonstrated the enhancement of energy storage for solarthermal power plants by using latent heats of Sn/SiOx core-shell nanoparticles embedded in a salt. We also proposed a concept to create enhanced latent heat absorption in a temperature range, rather than at a specific temperature, by using metal-based phasechange materials. A wide endothermic plateau from 370 to 407 °C for the Hitec salt was obtained by releasing the latent heat of alloy particles embedded in the salt. Amorphous materials are generally regarded as thermal insulators. Here, we report that amorphous polymer nanofibers can exhibit a very large thermal conductivity, e.g., 56 W/mK. This value is one of the highest reported values for polymers. Besides, it is observed that heat transfer in the nanofibers is time- and annealing temperature-dependent. The thermal conductivity of the nanofibers can be modulated to span three orders of magnitudes from being nearly insulated (e.g., 0.35 W/m-K) to being highly thermally conductive (e.g., 56 W/m-K). The non-equilibrium feature of the polymer chains in the nanofibers is responsible for the high and tunable heat transfer. The finding renovates our knowledge of poor heat transfer within amorphous polymers.
Prof. Ming-Chang Lu received his Ph.D. degree in Mechanical Engineering from the University of California at Berkeley in 2010. He worked at National Chiao Tung University from 2010 to 2019. He joined the Mechanical Engineering Department of National Taiwan University in August 2019. He currently serves as the chairman of the Heat and Mass Transfer Society of Taiwan. He was awarded as Distinguished Young Scholar by the Society of Theoretical and Applied Mechanics of Republic of China in 2015. He also received the 2015 Ta-You Wu Memorial Award of Ministry of Science and Technology of Taiwan. His research focuses on enhancing thermal energy transportation, storage, and conversion efficiency using micro/nanostructures.Ming-Chang Lu