|Fig. 1: Osmotic Power Plant Diagram. (Source: Brandon Wulff. After Aaberg. )|
When freshwater and seawater are mixed together, like when a freshwater river flows into the ocean, enormous amounts of energy are released. If the freshwater and seawater are then separated via a semi-permeable membrane, then the freshwater will pass through the membrane and dilute the saltwater due to the chemical potential difference. This process is called osmosis. If the salt ions are captured completely by the membrane, the passing of water through the membrane will create a pressure known as osmotic pressure. This pressure can be converted into energy.  This technology has existed since the 1970s, but the membranes that have been developed are not effective enough for the process to be cost effective. Companies like Statkraft, though, are making breakthroughs in technology and hope to bring osmotic power for consumer use in the near future.
The technology that captures this energy is called pressure retarded osmosis (PRO). (Fig. 1) Fresh water is pumped into a module containing these special membranes mentioned above. The freshwater mixes with the seawater in the membrane module and this osmotic process increases the volumetric flow of high pressure water and is the key energy transfer in the power production process. The diluted, brackish water from the module is split into two flows: freshwater and brackish water.  The brackish water is flowed through a turbine to generate power.  The PRO process can generate up to 1 MW of power per cubic meter of freshwater that passes through the system per second. 
In the past, osmotic power has had an environmental impact due to the size of the footprint that it takes. In the past, a waterfall was necessary in order to generate power. Now, the location doesn't matter so long as freshwater and seawater is available. The need for dams and water management systems and its effects on the fish and other wildlife being minimal will reduce the impact that osmotic power has on the environment.  The cleaning of the membranes involve cleaning agents that are similar to those that are use in drinking water plants and are environmentally friendly.
In 2001, Statkraft joined with a few other companies to help develop the PRO technology further. The major goal of the company is to develop a membrane that has a power production of 4 Watts per square meter or more. Also they are looking to lower the production costs and lifetime of the membranes. In 2003, Statkfrat opened its first osmotic power testing facility in Norway. However, while Statkraft is working on this, it is not really feasible in most parts of the world. While Norway has an abundant amount of fresh water, the freshwater is much more valuable in a place like California than Norway. Also, the amount of energy put in to get energy out of fresh water is the same amount of energy needed to get freshwater from seawater.
© Brandon Wulff. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 R. J. Aaberg, "Osmotic Power: A New and Powerful Renewable Energy Source?" Refocus 4, 48 (2003).
 K. Gerstandt et al., "Membrane Processes in Energy Supply For an Osmotic Power Plant," Desalination 224, 64 (2008).
 S. E. Skilhagen, J.E. Dugstad, and R. J. Aaberg, "Osmotic Power - Power Production Based on the Osmotic Pressure Difference Between Waters With Varying Salt Gradients," Desalination 220, 476 (2008).