Power by Osmosis

November 27, 2009

Back in July, I posted a note about some new research in power generation, which attempts to extract energy from the entropy increase that occurs when salt and fresh water are mixed.   Although that process is still at the research stage, it demonstrates that there are actually an awful lot of energy sources around us — we just need to figure out how to use them.

The New Scientist now has a report on another attempt to extract energy from combining salt and fresh water.  A pilot power plant has just been opened on the Oslo fjord in Norway that will use pressure differences resulting from osmosis to drive a turbine and thereby generate electricity:

Osmosis occurs wherever two solutions of different concentrations meet at a semipermeable membrane. The spontaneous passage of water from dilute to concentrated solutions through the membrane generates a pressure difference that can be harnessed to generate power.

Fresh and salt water naturally mix near the mouth of the fjord.  They are pumped into an osmotic cell, which generates water pressure equivalent to a column of water 120 meters high.

Tapping the energy potential inherent in mixing salt and fresh water, by whatever process, has some interesting advantages.  It is available on a nearly continuous basis, unlike wind or solar power, although seasonal variations in river flows would cause some fluctuations.  Also, many large cities are located on or near river estuaries, meaning that power could potentially be produced close to the ultimate customers.

The Norwegian plant is only a pilot; net of the power used to pump water into the plant, it can only produce a few kilowatts of power.  So it might be able to run the engineers’ coffee machines, but perhaps not much more.   However, building a pilot plant is normal procedure in engineering; it allows practical problems that may not have been apparent from the design work to be detected and, hopefully, solved.  (In the osmosis cell, for example, the possible effects of bacterial contamination or silt are not well understood.)  Still, it is a good sign that the work is being done.

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