Abstract

The urge to use sustainable green energy to meet the ever-increasing energy demand is inevitable due to the depletion of existing fossil fuels sources. Closed-loop pressure retarded osmosis (PRO) process is one of the renewable technologies which can produce green energy without having any deleterious effects on nature. It is a process which combines both PRO (source of generating power) and a downstream separation process, such as membrane distillation (MD), for the regeneration of draw solution. An appropriate draw solution selection is a key to successful implementation of closed-loop PRO process for sustainable energy generation. In this study, few organic compounds such as potassium citrate (KCit), calcium acetate (CaAc), potassium oxalate (KOxa), potassium acetate (KAc), ammonium acetate (NH4Ac), ammonium carbamate (NH4C), ammonium formate (NH4F), potassium formate (KF), sodium glycolate (NaGly), sodium propionate (NaP) and calcium propionate (CaP) were identified for the first time as highly effective draw solutions (except for NaP). Using a desktop screening method, the organic compounds were identified by considering physical state at ambient condition, water solubility, and osmotic pressure. The draw solutions were comprehensively evaluated for water flux, power density and reverse salt flux through a laboratory-based investigation of the PRO process. The peak power densities achieved for the identified draw solutions were 5.32 W/m2 to 6.73 W/m2 at a 2.8 MPa osmotic pressure. These peak power densities increased from 109% to 118% (11.1 W/m2 to 14.64 W/m2) when increasing the osmotic pressure of the draw solutions by 50% (i.e. 4.2 MPa). A significant increase in the peak power density was obtained due to the very low reverse salt flux for the organic draw solutions (0.029 to 0.0699 mol m-2 h-1 and 0.0325 to 0.0854 mol m-2 h-1 at osmotic pressures of 2.8 MPa and 4.2 MPa, respectively). The identified organic draw solutions were also analyzed as distillable and thermolytic through gravimetric method for the identification of potential downstream recovery methods to recycle the draw solutions in the closed-loop PRO process. This research concludes that, except for ammonium carbamate, all other aforementioned draw solutions could be potentially recovered using the membrane distillation process.
As the temperature is directly associated with solution physicochemical properties, this research has been further extended to observe the effect of temperature on the performance enhancement of the closed-loop PRO process. The effect of temperature has been studied on two organic draw solutions, potassium acetate (KAc) and sodium propionate (NaP), due to a similar osmotic pressure with NaCl. It has been found that KAc and NaP show ~31% (8.5 Wm-2 to 11.1 Wm-2) and ~27% (8.1 Wm-2 to 10.3 Wm-2) increase respectively in power density while increasing the operating temperature from 20 degree C to 40 degree C. It has been further investigated that reversal salt flux is ~5-8 times lower for organic salt than NaCl. A comparison shows that at 40 degree C, potassium acetate and sodium propionate produce 23% and 14.5% higher power density over sodium chloride draw solution. Based on the result of this study, increased power production coupled with a lower reversal salt flux has made organic salt as a potential draw solution for the future research in the PRO process.