Comparison of Energy Efficiency between Atmospheric Batch Pressure-Retarded Osmosis and Single-Stage Pressure-Retarded Osmosis.

Batch pressure-retarded osmosis (PRO) with varied-pressure and multiple-cycle operation using a pressurized variable-volume tank has been proposed as a high-efficiency osmotic energy harvesting technology, but it suffers scalability constraints. In this study, a more scalable batch PRO, namely, atmospheric batch PRO (AB-PRO), was proposed, utilizing an atmospheric tank to receive and store the intermediate diluted draw solution (DS) and a pressure exchanger to recover the pressure energy from the diluted DS before being recycled into the tank. Its performance was further compared with single-stage PRO (SS-PRO) at different flow schemes via analytic models. The results show that the AB-PRO with an infinitesimal per-cycle water recovery () approaches the thermodynamic maximum energy production under ideal conditions, outperforming the SS-PRO with lower efficiencies caused by under-pressurization (UP). However, when considering inefficiencies, a ~40% efficiency reduction was observed in AB-PRO owing to UP and entropy generation as the optimal is no-longer infinitesimal. Nonetheless, AB-PRO is still significantly superior to SS-PRO at low water recoveries () and maintains a stable energy efficiency at various , which is conducive to meeting the fluctuating demand in practice by flexibly adjusting . Further mitigating pressure losses and deficiencies of energy recovery devices can significantly improve AB-PRO performance.