Highly effective organic draw solutions for renewable power generation by closed-loop pressure retarded osmosis

Abstract

An appropriate draw solution selection is a key to successful implementation of closed-loop pressure retarded osmosis (PRO) process for sustainable energy generation. In this study, the organic compounds potassium citrate, calcium acetate, potassium oxalate, potassium acetate, ammonium acetate, ammonium carbamate, ammonium formate, potassium formate, sodium glycolate, sodium propionate and calcium propionate were identified for the first time as highly effective draw solutions (except for NaP) using an easy desk-top screening method. This method identified these organic compounds 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 5.32–6.73 W/m2 at 2.8 MPa osmotic pressure. These peak power densities increased from 109% to 118% (11.1–14.64 W/m2) when increasing the osmotic pressure of the draw solutions by 50% (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 (0.029–0.0699 mol m−2 h−1 and 0.0325–0.0854 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. Membrane distillation could be used as a downstream separator technique for the distillable organic draw solutions; however, only the ammonium carbamate among the thermolytic compounds could be separated downstream by using a low-temperature thermal distillation process.