Osmosis leads to energy harvesting

Osmosis was first discovered in 1748 by physicist Jean-Antoine Nollet when he immersed a pig’s bladder filled with wine in a trough full of fresh water.



Osmosis was first discovered in 1748 by physicist Jean-Antoine Nollet when he immersed a pig’s bladder filled with wine in a trough full of fresh water.

(He is also remembered for an experiment involving 200 monks forming a circle almost a mile in circumference while holding an iron wire. He proved the speed of electricity when he connected the ends of a wire to a Leyden jar battery and instantaneously shocked all of them.)

To finish with the pig’s bladder story, after being submerged for a short period, it burst as the diffusion of water into the bladder caused the pressure inside to build to the point of rupture.

Now, some 261 years later, this osmotic process works in desalination plants to purify drinking water from sea water, using a technique known as reverse osmosis.

In Norway, using a procedure known as pressure-retarded osmosis, energy is harvested from the difference in salinity between fresh and sea water.

In November 2009, the Norwegian utility Stratkraft built the world’s first osmotic power plant prototype.

This plant produces a whopping 4,000 watts of power, enough for about two small homes.

The Norwegian company now has plans to build a one-to-two-megawatt plant in 2015.

Pressure retarded-osmosis (PRO) works when the volume and pressure of the fluid on the sea water side of permeable membrane increases to a level where it can be used to power a turbine at a constant flow.

Like gravity in a hydroelectric dam, the fresh water moves through the membrane and into the salt water at a calculated rate.

However, PRO is not the only method of producing power by using salt and fresh water.

Reversed electro-dialysis (RED) is in essence a salt water battery. This device uses anion and cation exchange membranes to generate electric power.

This process was discovered in the 1950s and has faced problems with efficiencies and contamination of the membranes from impurities, found in fresh and salt water, like algae.

Institut Lumière Matière in Lyon France, collaborating with the Institut Néel, are working on an experimental membrane that is both impermeable and electrically insulating into which they inserted a boron nitride nanotube.

This approach is claiming an electrical output 1,000 times that of current semipermeable membrane technology and promises viable options to offshore wind farms.

In Monza, Italy, in a lab at the University of Milan, another method of producing power from the salinity difference between sea and fresh water is being developed.

Based on electric double layer (EDL) capacitor technology, two carbon electrodes are immersed in sea water and given polarity by introducing a small electrical current.

When fresh water is injected into the chamber, the decrease in salinity creates an electrostatic energy that develops an increase in voltage.

Blue power, as these technologies are becoming known, uses the power in the salinity of sea water and eventually may provide countries with large coastal areas and river deltas with an alternate energy source that has no environmental impact.

Lorne Oja is an energy consultant, power engineer and a partner in a company that installs solar panels, wind turbines and energy control products in Central Alberta. He built his first off-grid home in 2003. His column appears every second Friday in the Advocate. Contact him at: lorne@solartechnical.ca.

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