Reading Maria Surma Manka’s Maria Energia site tipped me to an interesting source of chemical energy that is abundant and close to many major metropolitan areas. Currently called osmotic energy it’s really a use of the salinity gradient energy that becomes available when fresh water contacts with seawater. The osmosis name comes from the membrane that separates the two kinds of water.
What happens is the fresh water seeks to reduce the salinity of the seawater so it pushes through the membrane to dilute the seawater thus increasing the seawater container pressure. The pressure can be harnessed to drive mechanical devices to do work such as generate electricity.
Osmosis then isn’t the energy; it’s the means to move the water from one circumstance to another by holding the salty seawater back as fresh water moves over for dilution. It is an essentially free source of energy, but so far the capital costs are thought to be out of sight so making the prospects for investment distant.
Which is not to say development isn’t worthwhile. Rivers will always run to the sea so the source of the energy will always be available. It’s the engineering and construction costs that will need attention. There is even more than one way to use the salinity gradients to find energy, with osmosis being a part of the process each time.
Dubious? Well, the osmotic effect is used by animal bodies to recover water from the kidneys. Plants use it to maintain pressure at the needed fixed levels. The semipermeable membranes are already developed, and the salinity gradient is well known. If you have some desalination experience then you’ll know that reverse osmosis is the principle in reverse requiring pressure manipulations to function rather than producing pressure to be used for work.
There are problems strikingly similar to reverse osmosis. One being that as the fresh water moves to the salt side the salty side will dilute so reducing the pressure potential. Then there is the plugging of the membrane which would need flushing to stay functional.
A couple of old attempts at design are on record. The Submarine Hydro Electric Osmosis Power Plant or SHEOPP is anchored to the seafloor with fresh water piped in to drive it. The design relies on pressures by being set deep in the sea so that after generating power the fresh water is discharged and depressurized into a submarine tank. If one could deliver clean freshwater and perfect the membranes a flushing pump system wouldn’t be needed and the power output could be maximized. The practicality of this at industrial scale is dubious at best.
Another design is built underground. Similar to the submarine unit this also relies on “pressure retarded osmosis” a way to describe the containment of pressure until its used. This design keeps the facility on shore still below the sea surface where pressure from the sea can be used. As the osmosis takes place the pressure in the contained seawater rises. Its that flow of water that drives the generator, not the osmosis effect itself.
What Maria was seeing is the current state of development from Statkraft of Norway which is a large conglomerate involved in a wide array of businesses including hydroelectric power. Statkraft knows that Norway has a technical potential in salinity gradient of up to 12 terrawatt hours per year – that would be enough to power over a half million Norwegian homes.
Osmotic or salinity gradient energy, if it can be made economically competitive, would be base load power; the steady reliable never stops coming power that is always there. If the engineering and design problems of getting economical facilities built are solved, there are a lot of places that could have installations built.
Now, who would have thought that something as simple as the differential between the salinity of a river and the sea could be processed to yield a head pressure of 120 meters, a very high waterfall, and plenty to push a turbine?
Statkraft says it’s in, building a prototype at Tofte southwest of Oslo. Try the Statkraft site, its link rich with several information links with many interesting details in English.