Research Project
Efficient Removal of Carbon Dioxide from Air Using Low-grade Heat
The direct air capture (DAC) represents the direct carbon dioxide removal from atmospheric air. It is a potentially transformative technology for addressing climate change due to its inherent negative carbon footprint. In this project, I leverage the high CO2 uptake and selectivity of cooperative adsorbent and accessible low-grade heat to efficiently remove CO2. I designed a DAC system with integrated solar-heat injection and heat recovery components to achieve levelized cost of < 100 USD/tonCO2 and energy demand < 2 GJ/tonCO2, a tipping point of large-scale application of DAC. As an emerging technology and potentially game-changer of climate change, there remain many opportunities for material selection, structural design and operation control for novel DAC.
High-temperature Thermal Storage for Concentrating Solar Power (CSP)
Concentrating solar power coupled with thermal energy storage is a promising clean energy source due to its capability of low-cost thermal storage and on-demand electricity generation. One of the key objectives in the current R&D efforts in CSP is to improve the thermal efficiency by elevating the operation temperature to above 700℃ and achieving at least 12 hours of thermal energy storage using particle bed or molten salt as heat transfer media. In this project, I developed a non-contact modulated photothermal radiometry (MPR) system for in-situ high temperature thermal conductivity measurement of high-temperature alloys, refractory coating, moving particle bed and flowing molten salt in heat exchangers, which utilizes an intensity-modulated laser as heat source and the intrinsic thermal emission from the specimens as thermometry. For the first time, the thermal conductivity of moving particle bed under a wide range of operating conditions was probed and the bulk property and near-wall thermal resistance was differentiated. I also demonstrated the MPR thermal measurement of molten salt with faster sample screening and higher measurement accuracy than most contact measurement techniques that suffered from the corrosive and conductive nature of molten salts, which have promising applications spreading over thermal storage, nuclear reactors, and thermochemical reactors. I leveraged the fundamental understanding of heat transfer mechanism of high temperature media into the optimum design of the critical components in CSP.
https://doi.org/10.1016/j.ijheatmasstransfer.2021.120989
Continuous and High Efficiency Solar-driven Desalination
Solar steam generation has become an attractive research area due to the promising application for water purification and desalination.Efficient mass transport and selective salt rejection are highly desirabl for solar or thermally driven seawater desalination, but its realization is challenging. Here a new liquid supply mechanism is proposed, i.e., ionic pumping effect, using a polyelectrolyte hydrogel foam (PHF), demonstrated with poly(sodium acrylate) [P(SA)] embedded in a microporous carbon foam (CF). The PHF simultaneously possesses high osmotic pressure for liquid transport and a strong salt-rejection effect. The PHF is able to sustain high flux of ≈ 24 LMH flux and a salt rejection ratio over 80%. the PHF shows a continuous and stable solar-driven desalination flux of ≈1.3 LMH under one-sun over 72 h, which has not been achieved before. The successful demonstration of both efficient ionic pumping and strong salt rejection effects makes the PHF an attractive platform for sustainable solar-driven desalination.
https://doi.org/10.1002/aenm.201900552
Thermal Battery Using Hygroscopic Hydrogel for Electronic Cooling
Effective thermal management of electronic devices attracts widespread attention due to the increasing power and miniaturizing design of high-power electronics such as 5G devices, database server and optoelectronics. Sorption-based evaporative cooling using hydrogel materials provides high cooling power due to the latent heat of water, where water is absorbed during the off-peak hour to expand the cooling envelope during the peak usage time. In this project, thin hydrogel was coated on metallic heat sink with high thermal conductivity and ~ 10 to 100-fold extended surface area. Due to the low thermal resistance across the thin gel coating, high evaporative thermal conductance and high surface area provided by the heat sink, we achieved high overall thermal conductance of > 1000 W m-2 K-1 (comparable to single-phase forced convection cooling by liquid), high cooling power of > 27 kW m‑2 and high thermal capacity > 200 kWh m-2, which has not been achieved before for other passive cooling method. Due to the high-water adsorption capacity of the hydrogel (> 0.9 gwater ggel-1 from moisture and > 100 gwater ggel-1 from liquid water), and the short time constant for vapor diffusion across the thin hydrogel coating ( < 60 mins), the hygroscopic hydrogel coating adsorbed moisture from air during the off-peak hour within 3 h, enabling the fast autonomous cyclic operation of the heat sink in the long term without active liquid supply.
Thermoresponsive Hydrogel for Energy Efficient Forward Osmosis
The development of forward-osmosis (FO) desalination is significantly restricted by the lack of suitable draw agents for both high permeable flux and fast release of water. Inorganic electrolytes with high ionic strength, however, the regeneration of the diluted draw agent into the concentrated state usually requires an energy intensive process.One of the recently studied draw agents is based on thermo-responsive polymers, such as poly(N-Isopropylacrylamide) (P-NIPAAm) due to its potential to reduce the energy consumption.As a result of the high swelling ratio of P-NIPAAm (>10 times), the specific energy consumption for water production can be as low as 1.12 kWh/m3. Besides, due to the low LCST (~ 32 °C ) of P-NIPAAm, it is possible to further decrease the specific cost of energy by utilizing low-grade heat, such as solar-thermal energy ( e.g., mixing the P-NIPAAm, with light-absorbing particles) or waste heat. We optimize the structure of the P-NIPAAm based hydrogel to achieve both high water yield and low energy consumption for forward desalination.
Phase Change Heat Transfer on Functional Microstructures
Cooling occupies ~ 16% of the residential electricity consumption in U.S. and ~ 60% of the total energy consumption in the IT industry, and thus there is an urgent demand of high-efficiency thermal management techniques. Phase change devices, such as heat pipes, phase-change materials (PCM) and evaporation/boiling heat sinks, show great promise for passive cooling under high heat flux. As the heat flux keeps increasing to an unprecedented level, collective efforts from the heat transfer community have been paid to push the CHF towards its theoretical limit. However, the-state-of-the-art CHF is still limited by ~ 1 kW cm-2, which is an order of magnitude lower than the sound-limit (~ 10 kW cm-2), leaving vast space in designing novel micro/nanostructure for phase separation and probing unexplored phase-change regime. I designed functional nano/microstructures for extended surface, rupturing of boundary layer, phase separation, and instability mitigation, to significantly enhance both the CHF and HTC of phase change heat transfer. I leverage the understanding of phase change heat transfer at the nano/micro scale into design of electronics cooling for extreme heat flux condition, such as heat pipe and vapor chamber.
https://doi.org/10.1016/j.applthermaleng.2017.11.031
https://doi.org/10.1016/j.ijheatmasstransfer.2017.06.128
Nanoscale Heat and Mass Transfer
The next generation of nano-electronics and nanofluidis devices require more fundamental understanding in nano heat and mass transfer and accurate manipulation of heat flux and mass flow. In this project, to resolve the lack of efficient methods for the 3-D actuation of nano-droplet with high tunability, I proposed the electro-wetting-on-dielectric (EWOD) driving scheme on a graphene surface. The droplet could be actuated when the EWOD saturated contact angle was reached, which determined the critical magnitude of E-field. The droplet velocity could also be tuned by changing the initial wettability of graphene surface. Detailed examination of the liquid-solid interface revealed the significant penetration of water molecules into the inner Helmholtz plane (IHP) before the induction of droplet detachment, when the electric energy was converted into surface energy. For all cases studied, the saturated contact angle served as a sufficient condition for the actuation of droplet.
DOI: 10.1039/C8NR03330G