Exploitable limits of osmotic energy recovery from Japanese rivers and brine

This study estimates comparative upper bounds on pressure-retarded osmosis (PRO) power and specific energy for five Japanese river estuaries and desalination-brine scenarios. We compare co-current and counter-current operation. For the five river–seawater cases, feed flows are calculated using a common 10% gross-discharge screening rule. The separate brine–freshwater comparison uses a capacity-derived brine-flow benchmark paired with an equal stipulated freshwater feed flow. Under these distinct resource-flow conditions, the Shinano River has the highest estimated mean power among the rivers in counter-current operation (74.6 MW), whereas the most concentrated brine case yields 8.6 MW in the same operating mode despite its higher specific energy. The power contrast is driven mainly by the specified flows, while brine salinity increases specific energy. A hypothetical pairing of the low-flow Ashida River with brine increases both specific energy and power relative to its river–seawater benchmark across the concentrations examined. These screening bounds require site-specific assessment of water allocation and operating constraints before practical power potential can be estimated.