Tapping Salt Water's Energy Potential

It's rare to run across a novel form of renewable energy that hasn't already been touted as the Next Big Thing and the potential savior of humanity. Work on harnessing salinity gradient power, one aspect of which is also known as "osmotic power", has proceeded in relative obscurity, with only one demonstration-scale installation that I'm aware of, in Norway. However, recent developments at Stanford University, as reported in Technology Review, hold the promise of extracting electricity directly from the difference in salt concentration between fresh water and seawater. If it can be scaled up as indicated, it could offer yet another renewable energy option for coastal communities, though as with most others, it is unlikely to be free of undesired consequences. So far as I know, the law of no free lunches has not yet been repealed.

Most forms of energy production, renewable or otherwise, depend on taking advantage of some kind of gradient--differences in temperature, pressure, or another characteristic between two locations. In some cases, these gradients are inherent in the primary energy source involved, such as the pressure gradients that produce the wind harnessed by wind turbines, or the temperature gradients that drive geothermal power plants. In other cases, the gradient is created by the process of energy conversion in some device, such as an internal combustion engine, gas turbine or nuclear reactor. For most of us, it's probably easier to relate to the pressure and temperature gradients that drive such machines than to visualize the concentration gradients that Dr. Cui's team at Stanford was investigating. Reading about their work dusted off the cobwebs from my chemical engineering mass transfer studies, long ago.

The "mixing entropy battery" the researchers created uses electrodes chosen for their affinity for sodium and chloride ions, with surfaces optimized through the application of nanotechnology. By charging it from an external power source while immersed in fresh water and then discharging it while filled with salt water, they are able to extract more energy than they put into it. This is not perpetual motion, because the extra energy comes from exploiting the concentration difference between the two fluids. Impressively, they extracted 74% of that energy potential in their tests; few energy cycles recover more than 50% of the energy of their sources. The catch is that like any battery, the process stops when the electrodes are saturated. At that point, the battery must be recharged in freshwater. This cycle can be repeated many times.

Although this technique might be used to produce smaller-scale rechargeable batteries for consumer devices or perhaps even cars, the researchers apparently had in mind larger-scale applications that would exploit the differences in salinity where a river meets the sea. Not only are such locations quite common, but they also tend to be near centers of population, because of the historical relationship between waterborne commerce and settlements. They have apparently calculated that the global potential of the concentration gradients in estuaries could meet 13% of the world's energy needs, or around 2 terawatts (2 million Megawatts.)

What's not clear from what I've read of their work is how much of the salinity gradient in an estuary could be tapped without significantly affecting the local ecosystem. Even though the amount of salt returned to the estuary would match what was taken out, performing the relevant mixing somewhere else--inside a power plant--would alter the preexisting conditions. That might not be catastrophic, particularly if the scale of the plant were limited to the natural variation in water flow caused by seasonal rainfall patterns, but it would still count as an environmental impact. I don't regard that as a reason not to pursue this technology, because everything we do at the scale of our global civilization has an environmental impact somewhere, but it's at least cause to proceed cautiously. In the world in which we now live, anyone investing in this technology won't have a choice about that, anyway.

So it seems we should add salinity power to the existing long list of renewable energy options, including wind, solar, geothermal, ocean thermal, wave and tidal power. If historical precedents concerning the interval from laboratory to commercial application are any guide, we might expect the first large-scale salinity power plant to appear sometime in the mid-to-late 2020s, assuming that it doesn't encounter unexpected hurdles and can be scaled up economically. Even if it can't supply all our needs, salinity power could make an important contribution to the low-emission energy mix we anticipate by mid-century. I'll be watching developments with great interest.