Do membrane transport properties vary under dynamic and cyclic RO? Evidence for constant permeability and hysteresis from channel history

Dynamic and cyclic operation of reverse osmosis (RO) systems is increasingly investigated for integration with renewable energy sources. Pilot-scale data from the National Renewable Energy Laboratory (NREL) are re-examined using a steady-state correlation framework derived from the solution–diffusion model under steady, ramped, and sinusoidal operating conditions. Results show that a constant permeability coefficient accurately describes all operating regimes when feed salinity is properly accounted for. Ramped experiments align with steady-state correlations, with the only deviation explained by a lower feed conductivity (approximately 10% lower EC). Under sinusoidal operation, the cycle-averaged water flux matches steady-state predictions, while the observed hysteresis originates not from pressure-dependent transport but from finite-rate salt redistribution within the feed channel, reflecting the transient coupling between pressure variation and concentration dynamics. These findings simplify the modeling of wave- and renewable-driven desalination systems, confirming that intrinsic membrane permeability can be treated as constant and that transient deviations result primarily from feed-salinity variation and feed-channel history, supporting constant-parameter frameworks for system control and design optimization. • Constant membrane permeability captures both steady and dynamic RO behavior. • Flux–pressure hysteresis arises from finite-rate salt redistribution in the feed channel. • Spatiotemporal modeling confirms hydrodynamic, not intrinsic, permeability effects. • Results validate constant-parameter models for renewable-driven desalination systems.