Osmotic Power

Did you ever wonder why the skin on your hands sometimes shrivels when you have them in water for too long? The underlying reason is called osmosis (a simple explanation in more detail below), and the same driving force is now being utilized as a power source.

Occasionally I encounter an energy story that catches me by surprise because it is so far under the radar. This morning I got one of those from a friend who e-mailed and referred me to this story:

The world’s first osmotic power plant opened!

My immediate reaction was skepticism that you could really make osmotic power work as a viable energy source. But first a bit of background before readers’ eyes glaze over at the usage of unfamiliar terminology. Students of chemistry or biology will have encountered the concept of osmosis, and most people have heard of reverse osmosis for the production of fresh water from saline or otherwise contaminated water.

In a simplified nutshell, water that is separated from a salt solution by a semi-permeable membrane (like a cell wall) will have a potential to migrate across into the salt solution – creating a pressure difference on the two sides of the membrane. (Lots of systems can create an osmotic pressure, but for illustration let’s focus on salt water and fresh water).

Osmosis is a very important concept in biology, as it is the mechanism by which water moves in and out of cells. A blood cell, for instance, will lose water and shrink if it encounters an outside environment that is more saline (saltier) than the internal environment. Water moves through plants by this process as well.

But to illustrate what is going on in the press release above, let’s talk about reverse osmosis. In reverse osmosis, a pressure is applied to the high salt concentration side to force fresh water back across the membrane – leaving the salt behind. The applied pressure must be greater than the osmotic pressure, or the fresh water will migrate to the saline side.

Now imagine that system in reverse. There is a saline solution on one side of the membrane, and it is allowed to build pressure from the migration of the fresh water across the membrane into the salt water. The build up of pressure – in this case osmotic pressure – could in theory be utilized for energy.

Imagine the way a dam works. Water pressure forces the water through a turbine, which generates electricity if it is coupled to a generator. If the osmotic pressure is likewise allowed to relieve through a turbine, then yes, in fact it could be used to produce electricity. Such a system would indeed produce osmotic power.

However, until this morning’s e-mail I had never heard of anyone actually building a system to do this. And I am skeptical that anyone can actually produce cost-effective electricity this way, because to generate a substantial pressure is going to require a lot of membrane surface area. A little bit of digging shows that the system above has a power output of only 4 kilowatts.

To put that into perspective, there are numerous power plants with outputs greater than 1,000 megawatts – which is 250,000 times the size of this osmotic power demonstration unit. So while this is perhaps newsworthy due to the novelty, they must prove that they can economically scale-up, and that is always a big hurdle.

One thing I wondered about as I read this article is whether it might not be more cost-effective to put in pipelines of fresh water to regions that are doing reverse osmosis of salt water. The idea being instead of using the fresh water in one location for osmosis and the salt water in the other for reverse osmosis, bypass the osmosis all together (reverse osmosis is very energy intensive).

Update: A reader just sent a link that says that IBM is looking into this as well: Energy From Sea Water? Consider IBM Intrigued

Footnote: I Googled the term “osmotic power” to see if that term had ever been used in this blog. My expectation was that it hadn’t, but I see that a reader linked to a story on this a couple of weeks ago in the comments following the story on OTEC (which I should be updating soon).

16 thoughts on “Osmotic Power”

  1. RR,
    Takes a lot of energy to move water around, especially if it involves lifting it to higher elevations.

    A good chunk of otherwise energy-efficient California's electrical power is used to move water to the dry southern part of the state.

    And the debate on the full environmental impacts (delta smelt, salmon, farmers ,etc.) is just beginning…

  2. “And here comes another PHEV…”

    “A little bit of digging shows that the system above has a power output of only 4 kilowatts.”

    If that is correct, projects pictures show lots of equipment for a small amount of electricity. Many roof top PV systems are about 4 kilowatts. This look to be about two orders of magnitude less practical than solar PV.

  3. Hey, look on the bright side. At least this is being subsidized by the Norwegian taxpayer, and not by the usual suspects.

    The things that people would rather do than build a nice renewable nuclear power plant!

  4. "Did you ever wonder why the skin on your hands sometimes shrivels when you have them in water for too long?"

    Don't try this at home folks (like this woman did), but did you know you can do the same to your brain by drinking enough water. It's called hyper-hydration or water intoxication and can be fatal. This is the opposite of the effect caused by diuretics like alcohol which increase blood concentration and reduce brain cell turgidity, causing headache and hangover. Moral: drink water with your moonshine.

  5. It's called hyper-hydration or water intoxication and can be fatal.

    Very interesting timing on that comment. I wrote an essay today that will go up tomorrow, and I used the example that water is critical to health, but too much can kill you. The essay is in response to someone who essentially argued that all biomass burning is bad. My point was that lots of things can be bad in specific situations, but you shouldn't paint with a broad brush.


  6. For Wendell:

    Built in New Holland’s Basildon, U.K.facility, the five models from the T7000 Auto Command Series are powered by a new NEF engine, with between 167 and 225 hp that can run on 100 percent biodiesel, and completely developed by Fiat Powertrain Technologies of Turin, Italy.

    Tractor of the Year

  7. What is clean energy?

    In the US, the generating electricity has insignificant environmental impact and has a perfect safety record with respect to the public.

    For the most part those who claim sources of 'clean' can not show it is any better than coal or natural gas.

  8. On the subject of reverse osomosis and osmotic energy I just wanted to bring up, as an engineer, one of the coolest energy saving devices I've seen.


    In reverse osomosis you can recover part of the energy used to pressurize the feed water from high pressure reject stream (the high salt solution that doesn't pass through the membrane). Typically a turbine is used to recover the energy at about at most 90% efficieny. The mechnical energy recovered is then used to power one of the feed pumps to RO system with the pump again working at at most 90% efficiency. So the total overall efficiency for the energy recovery scheme was about 80%.

    What the energyrecovery guys came up with is isobaric energy transfer where the high pressure reject stream is used to directly pump some of the the flow into thr RO membrane. with this system over 95% energy recovery is obtained.

  9. PeteS-
    Yeah, and they say they can bring the shale gas up for less than current methods.
    I suspect we see CNG available for $1 a gallon equivalent widely in 10years.
    And our leader RR, rather than being stranded on a coast-to-coast USA car trip (as he once opined), will cruise cross-country on CNG.
    Of course, we have gone from refinery shortages of a couple years ago to what looks like a refinery capacity perma-glut.
    Times change.

  10. I wouldn't expect too many cars to be converted to CNG. Electricity from natural gas is supposed to rise substantially in the coming decades. That is going to leave a whole lot less left for cars. Just something to think about..


  11. The thing about this osmosis process is that there are plenty of locations where vast amounts of fresh water meets seawater.

    River mouths.

    A low head pumping station could easily put huge volumes of both seawater and freshwater onto a shoreside location and then dump the saline water back out to sea.

    As RR pointed out, the biggest issues are likely to be capital cost issues.

    What is the likely capital cost per MW?


  12. "River mouths."

    Andy, river water has a lot of sediment in it. It would probably need to be cleaned up a lot before it could be allowed near an easily-clogged semi-permeable membrane. That would take energy.

    And if the river is really big, like the Amazon, it reduces the salinity of the sea water for miles around.

    It would be so much more efficient just to build a nuclear power plant.

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