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109-2-seminar 5/20Micro/Nanostructures for Enhanced Heat Transfer and Energy Storage
2021.05.19
Speaker: Prof. Ming-Chang Lu
Title:
Micro/Nanostructures for Enhanced Heat Transfer and Energy Storage
Abstract:
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 different thermal energy systems.
Condensation plays an essential role in various thermal systems. Condensation which involves heterogeneous nucleation, growth, and departure of liquid droplets is intrinsically a random process. Here, we report the ability to spatially control heterogeneous nucleation on a superhydrophobic (SHB) silicon nanowire array-coated surface. The control is achieved by manipulating the free energy barrier to nucleation through parameterizing regional roughness scale on the Si nanowire array-coated surface. Moreover, a closed system was built to investigate the heat transfer performance on the SHB silicon nanowire (SiNW) array-coated surface. It was found that the SHB surface could have superior heat and mass transfer performance than plain hydrophobic and hydrophilic surfaces. Although the SHB surface can enhance condensation heat transfer, the condensation heat transfer is greatly deteriorated by the flooding phenomenon that occurs at high subcooling temperatures. Thus, we proposed a novel three-dimensional (3D) hybrid surface to enhance the condensation at high subcooling temperatures. The 3D hybrid surface consisted of SHB SiNW arrays and hydrophilic microchannels. The microchannels could confine the liquid film thickness, and the liquid bridges formed on the 3D hybrid surfaces could be self-removed. Both of these characteristics prevent the surfaces from flooding. In addition, liquid droplets formed in the SiNW regions were dragged into the microchannels, which also improved the heat transfer. 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 have been demonstrated as a potential candidate for anti-icing because of the excellent water repellency associated with the surfaces. However, frosting on the SHB surfaces deprives the ice-phobic property on the SHB surfaces. A feasible way to alleviate the influence of frosting on a solid surface is to spatially control the ice formation at its early stage and to address the ice issue locally and immediately on the solid surface. We demonstrated the abilities of spatial control of ice formation and confinement of ice-stacking direction by manipulating the free energy barrier to nucleation. The v-shaped microgroove patterned surface, which possessed the abilities, exhibited the best anti-icing and deicing performances among the studied surfaces. The concept demonstrated in the work could lead to the development of new engineered ice-phobic surfaces.
Renewable energies, such as wind, hydroelectricity, and solar energy have attracted significant attention due to the limited availability of fossil fuels and global concerns about climate change. 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. In this work, we demonstrated the enhancement of energy storage for solar-thermal power plants by using latent heats of Sn/SiOx core-shell nanoparticles embedded in a salt. However, latent-heat-based improvements in energy storage occur at the melting points of the phase change materials. To overcome this shortcoming, we propose a concept to create enhanced latent heat absorption in a temperature range, rather than at a specific temperature, by using the metal-based phase-change 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. With the advantages of scalable synthesis and superior thermal properties, the alloy particles have potential applications in energy storage enhancement in various thermal energy systems.
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 Wm‑1K‑1. 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. The very large heat transfer associated with the amorphous polymer enables it being applied in enhancing heat transfer in many systems, such as electronic packaging, LED, solar panels, and heat exchangers.
Time:5/20
Venue:watch online video on moodle
Title:
Micro/Nanostructures for Enhanced Heat Transfer and Energy Storage
Abstract:
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 different thermal energy systems.
Condensation plays an essential role in various thermal systems. Condensation which involves heterogeneous nucleation, growth, and departure of liquid droplets is intrinsically a random process. Here, we report the ability to spatially control heterogeneous nucleation on a superhydrophobic (SHB) silicon nanowire array-coated surface. The control is achieved by manipulating the free energy barrier to nucleation through parameterizing regional roughness scale on the Si nanowire array-coated surface. Moreover, a closed system was built to investigate the heat transfer performance on the SHB silicon nanowire (SiNW) array-coated surface. It was found that the SHB surface could have superior heat and mass transfer performance than plain hydrophobic and hydrophilic surfaces. Although the SHB surface can enhance condensation heat transfer, the condensation heat transfer is greatly deteriorated by the flooding phenomenon that occurs at high subcooling temperatures. Thus, we proposed a novel three-dimensional (3D) hybrid surface to enhance the condensation at high subcooling temperatures. The 3D hybrid surface consisted of SHB SiNW arrays and hydrophilic microchannels. The microchannels could confine the liquid film thickness, and the liquid bridges formed on the 3D hybrid surfaces could be self-removed. Both of these characteristics prevent the surfaces from flooding. In addition, liquid droplets formed in the SiNW regions were dragged into the microchannels, which also improved the heat transfer. 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 have been demonstrated as a potential candidate for anti-icing because of the excellent water repellency associated with the surfaces. However, frosting on the SHB surfaces deprives the ice-phobic property on the SHB surfaces. A feasible way to alleviate the influence of frosting on a solid surface is to spatially control the ice formation at its early stage and to address the ice issue locally and immediately on the solid surface. We demonstrated the abilities of spatial control of ice formation and confinement of ice-stacking direction by manipulating the free energy barrier to nucleation. The v-shaped microgroove patterned surface, which possessed the abilities, exhibited the best anti-icing and deicing performances among the studied surfaces. The concept demonstrated in the work could lead to the development of new engineered ice-phobic surfaces.
Renewable energies, such as wind, hydroelectricity, and solar energy have attracted significant attention due to the limited availability of fossil fuels and global concerns about climate change. 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. In this work, we demonstrated the enhancement of energy storage for solar-thermal power plants by using latent heats of Sn/SiOx core-shell nanoparticles embedded in a salt. However, latent-heat-based improvements in energy storage occur at the melting points of the phase change materials. To overcome this shortcoming, we propose a concept to create enhanced latent heat absorption in a temperature range, rather than at a specific temperature, by using the metal-based phase-change 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. With the advantages of scalable synthesis and superior thermal properties, the alloy particles have potential applications in energy storage enhancement in various thermal energy systems.
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 Wm‑1K‑1. 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. The very large heat transfer associated with the amorphous polymer enables it being applied in enhancing heat transfer in many systems, such as electronic packaging, LED, solar panels, and heat exchangers.
Time:5/20
Venue:watch online video on moodle