Energy can be extracted from all sorts of physical processes. That wind can provide energy is very intuitive. So is solar heating. Solar PV takes a bit more to understand, but we’ve all seen it in action. Tidal energy extraction? Also fine. Biomass energy? Understandable.
What baffles the mind a bit more though is extracting energy from salinity gradients. It has been studied for some time. For example, Dr. Wick, from the Scripps Institution of Oceanography in La Jolla, wrote a paper titled “Power from Salinity Gradients” in the 70s. In these earlier concepts, osmotic pressure was utilized, but costs and lack of robustness of the devices kept them from being commercialized.
More than 30 years later, the idea is revisited. The Norwegian company Statkraft built a first demonstration plant to generate energy from osmosis in 2009. It’s been a little quiet around them, but they are still hoping to open a large scale plant in the next decade. Osmosis is the transport of water through a semi-permeable membrane. Plants use osmosis, for example, to absorb moisture through their leaves. Our body cells use osmosis all the time for transfer of fluids. In an osmotic power plant, fresh water and salt water are channelled into separate chambers. The chambers are separated by an artificial membrane that is good at passing through water, but not at passing through solutes like salt. Now comes the interesting part: the fresh water will be pushing through the membrane to the saltier side. This leads to an increase in pressure on the salt water side of the membrane. How much depends on the quality of the membrane. If the membrane is really good at drawing through the fresh water, the pressure difference thus obtained may be large enough to drive an actual turbine. Statkraft and other companies have invested a lot of money in developing ever better membranes and seem to be getting somewhere.
But, there are other ways to exploit salinity gradients. My colleague Yi Cui from Materials Science and Engineering at Stanford recently demonstrated a new technique. It’s nice and uses batteries rather than membranes. The following explanation of the mechanism was recently described in the Stanford Report.
The battery itself is simple, consisting of two electrodes – one positive, one negative – immersed in a liquid containing electrically charged particles, or ions. In water, the ions are sodium and chlorine, the components of ordinary table salt. Initially, the battery is filled with freshwater and a small electric current is applied to charge it up. The freshwater is then drained and replaced with seawater. Because seawater is salty, containing 60 to 100 times more ions than freshwater, it increases the electrical potential, or voltage, between the two electrodes. That makes it possible to reap far more electricity than the amount used to charge the battery. “The voltage really depends on the concentration of the sodium and chlorine ions you have,” Cui said. “If you charge at low voltage in freshwater, then discharge at high voltage in sea water, that means you gain energy. You get more energy than you put in.” Nice. Once the discharge is complete, the seawater is drained and replaced with freshwater and the cycle can begin again.
To enhance efficiency, the positive electrode of the battery is made from nanorods of manganese dioxide. That increases the surface area available for interaction with the sodium ions by roughly 100 times compared with other materials. The nanorods make it possible for the sodium ions to move in and out of the electrode with ease, speeding up the process.
Why are they even interested? Cui’s team calculated that if all the world’s rivers were put to use, their batteries could supply about 2 terawatts of electricity annually – that’s roughly 13 percent of the world’s current energy consumption. But, Cui realizes that river mouths and estuaries, while logical sites for their power plants, are environmentally sensitive areas. The good thing about his device is that it just needs to route some of the river water through the system. A power plant operating with 50 cubic meters of freshwater per second could produce up to 100 megawatts of power, according to the team’s calculations. That would be enough to provide electricity for about 100,000 households. After use, the water is simply returned. The process itself should have little environmental impact.
The water used does not have to be clean fresh water, and this is one thing I really like about Cui’s device. Storm runoff would be useable and maybe even sewage water might work.