Osmotic power

When fresh water and more saline water are separated by a membrane that allows water molecules to cross, water tends to move toward the more concentrated solution. This osmosis can be used to harness part of the free energy associated with mixing waters of different salinities. River mouths are one possible resource, while concentrated brines from desalination or industrial processes can provide other pairings. The available energy is a theoretical resource; the amount that a real plant can recover depends on membrane behavior, water treatment, pumping and local conditions.

In pressure-retarded osmosis (PRO), a semipermeable membrane separates a less concentrated feed, often fresh water, from a pressurized, saltier draw solution. Water crosses into the draw side, increasing its flow while it remains pressurized. Part of that pressurized stream can drive a turbine, with the remaining flow used in a pressure-recovery arrangement. PRO therefore converts osmotic transport into hydraulic power and then electricity. Its output must be assessed after accounting for pumping and pretreatment energy.

Reverse electrodialysis (RED) uses alternating cation- and anion-exchange membranes arranged in a stack. Saline and dilute streams flow through alternating channels. Ions move selectively down their concentration gradients across the membranes, producing an electrical potential across the stack; electrode reactions provide the circuit through which current is collected. Unlike PRO, RED relies on ion transport rather than water permeation to generate an electrical output. Both approaches depend on membrane selectivity, resistance, durability and stack or module design.

Research has explored both stand-alone systems near estuaries and hybrid installations that recover energy from concentrated waste streams. A 2014 International Renewable Energy Agency technology brief documented early demonstration projects, including a PRO prototype in Norway and RED pilots in the Netherlands. These were demonstration-scale projects, not evidence of broad commercial electricity supply. In 2013, Statkraft announced that it would discontinue its osmotic-power development, stating that it could not make the technology sufficiently cost-competitive in the foreseeable future.

Reviews identify membrane cost and performance, biofouling, water pretreatment, pumping demand and plant design as important engineering challenges. Environmental assessment is also necessary: water withdrawals, intake design and discharge of mixed brackish water can affect estuary and river ecosystems. Salinity-gradient energy remains a research and development field, with possible value in settings where concentrated brines and existing water infrastructure are available, but theoretical potential should not be confused with practical, economically recoverable power.