Solar energy is the most abundant and easily accessible source of renewable energy, however, its efficient use is not an easy task. Absorption of solar energy directly by the working fluid is an emerging trend in solar collection, known as direct solar absorption. On the other hand, almost one-third of the world population is living in water stressed conditions and this figure is expected to continuously increase in next a few decades. Desalination of sea and brackish water and reclamation of wastewater is being progressively practiced worldwide through different techniques. Forward osmosis (FO) is an emerging desalination technology, which operates under an internal osmotic gradient across the FO membrane. The lack of proper draw solutions with high osmotic pressure, minimum reverse solute flux and easy regeneration properties, however, limits the FO’s development.
This work develops a novel concept of combining solar energy, nanoparticles and FO to produce potable water suitable in arid areas, far away from the grids. Two independent functions i.e. osmotic pressure and direct solar absorption, are integrated for the first time into purposely formulated nanofluids for FO solar desalination. The direct solar absorptive nanofluid based novel draw solutions (NDS) are aimed at developing high osmotic pressure for enhanced water flux across FO membrane and, at the same time, to absorb solar energy efficiently for their regeneration.
A number of nanofluids were formulated and characterized in terms of their morphologies, structures and elemental compositions. The characterized nanofluids were investigated experimentally for their direct solar absorption behaviour and FO performance. A unique hybrid of direct absorptive nanofluid and osmotically active matters was developed that sufficiently performed the proposed two functions in FO solar desalination.
The photothermal conversion performance of engineered NDS was examined under a solar simulator and the results revealed the influence of nanoparticle type and concentration. The inclusion of low concentrations of nanoparticles could improve solar capture significantly. In direct solar absorption and steam generation experiments, nine nanofluids were examined and an enhancement in bulk photothermal efficiency (PTE) of about 95%, 100% and 105% over the base fluid was observed with gold, silver and carbon nanofiber (CNF) nanofluids respectively. The most absorptive carbon nanofiber based nanofluid was surface functionalized for osmotic pressure enhancement and experimented as NDS for FO performance in terms of osmotic pressure, water flux, reverse solute flux and water recovery. The NDS developed sufficient osmotic pressure and an enhancement of 80% in water flux was observed over 1M salt solution used as reference. The reverse solute flux of the NDS was negligible and the quality of product water was within the potable water standards.
The experimental results showed that the proposed novel draw solutions can sufficiently develop osmotic pressure to permeate water across the FO membrane and in the same time, significantly enhance the potable water generation using solar energy. Moreover, the quality of produced water suggested that these novel draw solutions could be potential candidates for future FO desalination in arid areas by using the energy from the Sun
