Recent progress in batch and semi-batch reverse osmosis processes such as CCRO have shown the promise to be the most efficient desalination systems. Despite their progress, it is critical to further increase their efficiencies, and reduce the downtime between cycles that worsens their cost performance. In this study, we model in new detail a further improved batch desalination system that uses a high pressure feed tank with a reciprocating piston. A high-pressure pump fills the inactive side with the following cycle’s feedwater, providing two main benefits. First, no tank emptying step is needed because feed is already present, thus reducing downtime. Second, the tank fully empties each cycle, thus avoiding the small energy losses from brine mixing with the new feed that past best designs had. The modeling methodology is the most thorough yet for batch processes, as it uses a discretized module that includes transient mass transport equations for salt boundary layers, membrane permeability effects, and minute salt permeation through the membrane. Comparing the new configuration to standard reverse osmosis with and without energy recovery, the new process vastly outperforms, with the potential to be below 2 kWh/m3 for seawater. The new process has less downtime too, around 2% of cycle time, compared with 10% for CCRO or 16% from past batch studies.

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