Thermal Transport in Granular and Hydrogel Materials for Solar-thermal Energy Harnessing and Management
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Thermal Transport in Granular and Hydrogel Materials for Solar-thermal Energy Harnessing and Management


Nowadays, 80% of the developed world’s energy comes from fossil fuels. With the aim to peak the carbon emission by 2030 and neutralize it by 2050, sustainable energy sources are being actively explored. It is estimated that 90% of the current energy budget is in the form of thermal energy, implying the significance of efficient harnessing and management of thermal energy towards an energy-efficient world. Concentrating solar power (CSP) coupled to thermal energy storage have gained extensive attention in recent years due to their capability to store high-grade thermal energy at low cost for on-demand electricity generation. However, current R&D effort in CSP has been hindered by the limited knowledge in heat transfer of high-temperature components in CSP, such as solar-absorbing coatings, alloy containments, granular flow and molten salt, etc. One of the challenges stems from the lack of suitable in-situ measurement tool for these materials due to their high temperature and corrosive natures. To tackle this difficulty, a non-contact laser-based modulated photothermal radiometry (MPR) thermal measurement system was developed. The MPR uses the thermal emission from the specimen surface as the thermometry probe, which is suitable for high-temperature measurement under harsh environment. The system was first characterized by bulk materials and solar-absorbing coating. The MPR system was modified for the measurement of the intrinsic thermal conductivity and convection heat transfer coefficient of flowing fluid in a channel. Afterwards, the MPR system was applied for the measurement of moving particle bed for CSP. The effective thermal conductivity of moving particle with variation in the flowing velocity, temperature, particle size, and particle morphology was obtained. We disseminated the near-wall thermal resistance from the bulk thermal resistance. The discrete element modelling (DEM) indicated that the near-wall thermal resistance was induced by the diffusive motion of particles away from the wall in a moving particle bed. Low-grade thermal energy is a universal form of energy with easy access and low cost. The low-grade heat was utilized for desalination and water harvesting using hydrogel materials. We developed a new class of thermo-responsive hydrogels for forward-osmosis desalination where the hydrogel draws water molecules across the membrane from seawater due to its high ionic strength. This new class of draw agent processed the unique LCST phase transition to release water in liquid phase at 35 ℃ to avoid the latent heat penalty in conventional FO desalination. Subsequently, we applied polyelectrolyte hydrogels for continuous solar-driven evaporative desalination, where solar energy was directly converted to heat to evaporate the seawater. Due to the unique osmotic pumping mechanism within the hydrogel, the salt crystallization problem was avoided as opposed to conventional capillary wicks. Lastly, we demonstrated that the hydrogel materials could also be applied for water harvesting from moisture in the air by the adsorption-desorption process. The generic idea was to coat a thin hydrogel layer on a metallic scaffold with 100-1000-fold enhanced effective surface for fast heat and mass transfer kinetics. We achieved a daily water yield an order-of-magnitude higher than the state-of-the-art techniques. Due to the evaporative latent heat in the desorption process, we found that the water harvesting device also manifested itself a superior heat sink for power electronic thermal management. Based on the similar geometric design, we developed a moisture thermal battery (MTB) and demonstrated its capability of passive thermal management of 5G device and CPU.

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