In 1974 a researcher at the University of Connecticut submitted a paper to the journal Science. He claimed that large amounts of energy could be extracted from the natural mixing of fresh water and salty sea water that occurs at river mouths around the world.
The author, Richard Norman, was surprised when the manuscript was positively reviewed. He had submitted it partly tongue-in-cheek in response to the journal’s apparent affinity for publishing what he described as “outlandish claims”.
However, on looking closely at the numbers, he realized that this really could be a viable source of sustainable energy.
As a biologist with an interest in biophysics, he was aware of the “enormous amount of energy” involved in the mixing of salt water with fresh water, but also of a way in which this energy could be extracted via the process of osmosis. During osmosis, water is transported through a wall while salt and other molecules are not.
Things have come a long way since then. On Nov. 24 2009, the world’s first power plant aimed at harnessing this energy came online.
Statkraft, Norway’s national energy producer, is behind the project. After 10 years of research and $20 million USD invested, they will be looking to this experimental plant to show that this technology is commercially viable.
In the 10 minutes it takes you to read this story, 82 million mega joules of energy will have been dissipated by the natural mixing of fresh water and salty water at river mouths and estuaries all over the world. That’s enough energy to power more than 2,000 BC homes for a year.
To see where the energy comes from, imagine a tank with a partition down the middle: sea water on one side, salt water on the other. Remove this partition and the two liquids will spontaneously mix. At this point a physicist might tell you that there has been a dissipation of free energy as the liquids mixed.
Free energy is a lot like the proverbial free lunch in the sense that it’s not really free. Rather, it is the energy that can be extracted from the system during a reversible process.
The mixing as described above is not reversible, but if a reversible method of allowing salt water and fresh water to mix could be found, the ‘free energy’ could be extracted.
In theory, 2.2 kilo-joules of energy could be extracted for every liter of fresh water dispersed into the sea, equivalent to what you would get from burning one gram of coal.
Extracting the energy
The first ideas on how to go about extracting this energy came soon after the publication of Norman’s paper, from a researcher who was working on desalination – the process of making fresh, potable water from sea water.
Sidney Loeb was developing semi-permeable membranes that could act as salt filters. By pressurizing sea water, which requires energy, water would pass through the membrane, leaving the salt behind. They realized that this process should run equally well in reverse, with fresh water passing through the membrane producing pressurized salt water, which produces energy. This is the process at the heart of the new Statkraft power plant.
To get a better idea of just how this power plant will work, imagine again that same tank of water, now split into two by a semipermeable membrane.
Fresh water from the river fills one side, while sea water fills the other. Fresh river water passes through the membrane on its own, thanks to osmosis, which is driven by the difference in salinity.
The fresh water dilutes the salt water, but it also increases the pressure in the salt water chamber. The pressurized water can then be released through a turbine, producing energy in the same way as a hydropower station.
The pressure buildup is the manifestation of the free energy in the system. By introducing a semipermeable membrane, this energy can be extracted from the mixing process.
The technology at the center of osmotic power is the membrane. The speed with which it can pass water through will determine how much energy is produced, and whether the process is commercially viable.
Karen Gerstandt is one of the researchers who worked in collaboration with Statkraft to develop new membranes. She explains that the membranes used for desalination that had been in development since the 70s were poorly suited to the application of power generation.
New membrane technology needed to be developed from scratch, and it was not until the late 90s that this research began in earnest.
The membranes produced and tested in the lab by Gerstandt and the group at the Institute for Polymer Research in Hamburg, Germany, are right on the borderline of what is required to run a commercially viable power plant. The the energy produced by the membranes depends on how quickly they allow water to pass through, and is measured in terms of the power produced by a square meter of membrane.
Stein Erik Skilhagen, head of osmotic power at Statkraft said that five watts per square meter is the magic power density that is needed. This has been proven possible in lab trials, and one of the objectives of the Statkraft plant is to see if it’s also possible in a full commercial power station.
Both Statkraft and the scientists working on the membranes admit that the large scale, real world efficiency of the membranes could well be less than what was observed in the lab. This does not worry Skilhagen, however.
He believes better membranes are not far off, saying “it’s just a matter of finding the right material. I just hope we can do it faster than has been the case with other renewables.”
Researchers too are optimistic about improvements in membrane technology.
Gerstandt is currently looking to nature for inspiration.
“Osmosis is a natural process, and there are membranes in nature 50,000 times better than what we can make in the lab,” said Gerstandt. She points to the kidneys and cell walls as examples where hugely efficient membranes play an important role.
This optimism suggests that even if the Statkraft plant fails to meet the five watt per square meter efficiency in this first installation, the problem could be easily resolved with the next generation of membrane technology.
Potential of osmotic power
The energy produced by mixing of salt and fresh water is huge. It is the ability of power producers to harness it that leaves some uncertainty as to how significant this resource really is.
According to Statkraft, in Norway alone they could generate 12 terra-watt hours annually, equivalent to 10% of national power consumption. Europe could harness 200 tWhs, while globally there is potential to produce 1200 tWhs, enough to satisfy the entire energy consumption of China.
Government and industry will need to get involved, Skilhagen says, for this renewable energy source to really take off.
“Although there has been a lot of interest from other power generators, the interest needs to be visible. Governments need to send clear signals to power producers that they support sustainable energy,” said Skilhagen.