Can batch or semi-batch processes save energy in reverse-osmosis desalination?

Energy savings in reverse osmosis (RO) are highly constrained by the design of conventional processes, for which the minimum practical energy of desalination substantially exceeds the thermodynamic minimum. Batch processes can theoretically approach the thermodynamic minimum, suggesting the possibility for further energy savings. In this study, we aim to quantify what energy reductions may be possible for batch-like processes when process inefficiencies such as frictional losses and concentration polarization are included. We first introduce a practical batch process that utilizes energy recovery devices and an unpressurized feed tank. We also consider a less practical pressurized-tank scenario, as well as semi-batch (closed-circuit) RO. We then derive analytical approximations and conduct numerical modeling to compare the energy requirements of batch, semi-batch, and staged RO processes under realistic conditions. Through this analysis, we find that practical batch-like processes and processes with increased staging offer comparable and significant energy savings. For example, semi-batch RO and two-stage RO would save 13% and 15% energy, respectively, over one-stage seawater RO at 50% recovery. We conclude with a discussion of other important factors, such as capital costs and process robustness and flexibility, that will affect the implementation of batch, semi-batch, and staged processes.