Introducing Osmotic Power

Osmotic power is a concept first introduced almost 40 years ago that continues to evolve as a potentially significant source of energy. It uses the power of salt to create electricity as a renewable resource with zero carbon emissions. With its PX Pressure Exchanger (PX™) technology, Energy Recovery, Inc. (ERI) has helped advance osmotic power toward being a viable energy source.

Harnessing Osmosis

Osmotic power uses nature’s energy to make electricity. The driving force for the process is the natural phenomenon of osmosis. When salty water is exposed to water with less salt, osmosis causes water to flow, from low salinity to high salinity. If two water bodies are separated by a membrane that selectively blocks and filters out salt, only water flows across the membrane, drawn toward the saltier side. The force exerted by the water, called osmotic pressure, is highest when the salinity difference across the membrane is the greatest. The salt water serves as the “fuel” for the process. Dissolved sugar and other substances have osmotic potential as well.

Osmosis occurs in all living organisms and in the world around us. Osmosis is sustained with three things: a fresh water supply, a salt or sugar supply and a membrane that blocks the salt or sugar while allowing water to flow through. For example, plants use osmosis to draw water from their roots. In plants, irrigation supplies fresh water, photosynthesis supplies sugar and the surface of the roots and the walls of the plant’s cells serve as the membranes.

To sustain osmosis in a man-made process, the same three elements that are necessary for natural osmosis are required. Fresh water can be supplied by a river and salt water by the ocean, for example. Membranes capable of separating salt from water have been developed for modern desalination applications and may potentially be adapted for osmotic power. Therefore, wherever a river flows into the ocean, osmotic energy is potentially available if an efficient osmotic power process is used.

Such a process is illustrated in Figure 1. Fresh water and salt water are supplied to the opposite sides of a membrane. Fresh water is drawn across the membrane by the salt water, building pressure on the salt water side and diluting it. The pressure of the dilute brackish water is released through a turbine which turns a generator to make electricity. The dilute water flows to the sea just as it would have without the process. The result is clean, carbon-free, renewable power.

Figure 1 – Illustration of High-efficiency Osmotic Power Process
Statkraft, Norway

Copyright 2009 Osmotic Power Inc.