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).