Carbon – based nanofluids and hybrid natural polymers for enhanced solar-driven evaporation of water: synthesis and characterization

The scarcity of freshwater is becoming a global challenge worldwide due to limited resources availability and increasing demand both for manufacturing and household use. For this reason, there is an important need to develop efficient, economic and sustainable desalination technologies able to take advantage of unconventional sources of water (seawater, brackish groundwater and wastewater) in order to produce freshwater. Sun is considered as the most promising abundant renewable (and free) energy source that can be employed in steam and vapor generation processes, which has a great importance in many applications such as: water desalination, domestic water heating, and power generation. This doctoral dissertation presents a study on the efficiency of different carbon based systems - nanofluids and hybrid natural composites - for the improvement of direct-solar evaporation systems, for the production of freshwater. The two main goals of this work consist of: (i) the synthesis and characterization of stable carbon-based nanofluids in water and of re-usable, economical and ecological hybrid composite materials, and (ii) the comparison of such carbon-based systems applied to water evaporation, understanding mechanisms, advantages and limitations. Carbon based materials (carbon black, graphene and multi-walled carbon nanotubes) were chosen because of their high sunlight absorption ability, unique thermal properties, as well as low cost and abundant availability. However, the hydrophobic character of such materials makes necessary to find efficient strategies to overcome this problem when dealing with water. In this work, the suspension stability of graphene-based nanofluids in water - a key parameter for the application of nanofluids in any field - was effectively improved by combining physical (by RF Sputtering coating) or chemical (by NaClO-NaBr solution) graphene surface modification treatments, and the use of common additives (Triton X-114, SDBS and gum arabic) showing different stabilization mechanisms. The best strategy to obtain long-time graphene suspension stability in water (both deionized water and saline solution with 3.5 wt% NaCl) turned out to be the combination of the easy chemical treatment with the electro-steric stabilization effect of gum arabic. In addition to nanofluids, a re-usable devices based on gum arabic cross-linked gelatin hydrogel were synthesized and characterized. Hydrophobic carbon-based materials were easily and uniformly embedded into the porous hydrogel matrix, thanks to the amphiphilic character of both gelatin and gum arabic. The effect of carbon-nanoparticles nature, morphology and concentration on the measured effective thermal conductivity of the composite material was studied and the thermal conductivity of the nanoparticles was evaluated applying several models based on the effective medium approach. The values obtained for the nanoparticles were far from the tabulated thermal conductivity values because of the combination of the composite features (such as nanoparticles concentration, Kapitza resistance) and the particles characteristics (such as aspect ratio, crystalline structure). The performance of carbon-based nanofluids and hybrid hydrogels on direct-solar evaporation of water was tested and compared to that of carbon-wood bilayer composite (which presents both hydrophilic character and natural channels for water transportation) under solar simulator. The effect of surface temperature, light-to-heat conversion efficiency of carbon-based materials, heat losses, water transport through a porous medium and suspension stability (in the case of nanofluids) were investigated in order to understand the advantages and limitations of such systems. All the tested systems were able to improve water evaporation rate and evaporation efficiency up to 70% and 82% under 1 sun and 2 suns respectively using a small amount of nanoparticles: the same amount of particles dispersed in nanofluid (0.01 wt%) was embedded into hydrogels or deposited onto wood. The high sunlight absorption ability of carbon-based nanoparticles appeared as a dominant parameter for the improvement of water evaporation rate. In fact, enhanced light absorption was directly related to a high photothermal conversion efficiency, which caused an improvement in the surface temperature, leading to a consequent enhancement in evaporation rate. It has been found that an adequate supply of water to the evaporation surface represents a fundamental parameter as well considering floating systems.