Study of Hybrid-Nanofluids in Superhydrophilic Wick Structure for Heat Transfer Enhancement in an Adsorption Cooling System

Cooling buildings consumes over 25% of Hong Kong’s total energy. Adsorption cooling systems (ACS) are an excellent replacement for vapor-compression refrigeration systems due to their environmental friendliness and energy-saving potential. Powered by renewable energy, they significantly reduce the dependence on fossil fuels. However, due to low cooling efficiency and bulky size, ACSs are not widely commercialized. Previous research focused on new adsorbents to enhance performance. Indeed, an energyefficient evaporator together with an enhanced adsorbate is another research direction to improve efficiency. Here, a superhydrophilic wick-structured evaporator using hybrid-nanofluids as the adsorbate in ACSs is proposed. Because the capillary effect of wick-structures creates the orientation-independent property, increasing the heat transfer surface area in the evaporator, with the same cooling capacity, ACSs can be more compact. However, the evaporation rate of a wick-structured plate is lower than that of normal plates due to a larger contact angle on the wick-structure. Coating superhydrophilic patterns on the plate is one solution to address this issue since the superhydrophilic-coating increases the solid-liquid contact area. Our preliminary results show the evaporation rate of a superhydrophilic wick-structured plate using water improves by 430% and 450% compared to that of an untreated-plate and a wickstructured plate, respectively. Prior studies show that hybrid-nanofluids, dispersion of two types of nanoparticles in a base-fluid, exhibit better thermo-physical properties, i.e. higher effective thermal conductivity, than single-particle nanofluids, and much better than water. Therefore, the evaporation performance of the superhydrophilic wickstructured plate using hybrid-nanofluids can further be enhanced, substantially improving ACSs’ cooling performance and facilitating a more compact design. However, a number of academic challenges need addressing since hybrid-nanofluids heat transfer on superhydrophilic wick-structures has never been studied. This project will investigate and characterize thermo-physical properties of hybrid-nanofluids with different nanoparticle types and concentrations so that suitable adsorbates could be developed. Two mathematical models will be developed, with results being verified by experiments. One model will determine the effective thermal conductivity of hybrid-nanofluidssaturated superhydrophilic wick-structured plate, while the other model will predict the evaporation rate of hybrid-nanofluids on a superhydrophilic wick-structured plate. Based on the models, a hybrid-nanofluids based superhydrophilic wick-structured evaporator will be designed, built and tested. Finally, the evaporator will be integrated into an ACS for performance evaluation. This research will reveal the impact of hybridnanoparticles deposition in porous media and the related heat transfer characteristics. The findings will enhance the thermo-physical knowledge of hybrid-nanofluids, and steer new development of ACSs.