Marine energy technologies usually rely on the movement of the water source to generate electricity whether it's from the bobbing of waves, the pull of tides or the running of a river, but there are other sources of energy beneath the surface. Researchers at Pennsylvania State University have developed a technology that harnesses the energy in the mixing of freshwater and saltwater where rivers empty into the sea.
"The goal of this technology is to generate electricity from where the rivers meet the ocean," said Christopher Gorski, assistant professor in environmental engineering at Penn State. "It's based on the difference in the salt concentrations between the two water sources."
This new technology could harness a so far barely-tapped source of energy. If the technology was installed at coastal convergences around the world, it could meet 40 percent of global energy demand.
The technology to harvest energy from the mixing of fresh and saltwater has existed for a long time, but the three main methods for extracting that energy -- pressure retarded osmosis (PRO), reverse electrodialysis (RED) and capacitive mixing (CapMix) -- all have drawbacks from easily clogged membranes to low efficiencies. The Penn State engineers were able to come up with a new approach that combined the RED and CapMix methods to produce a robust technology that avoids the shortcomings of its predecessors.
The result is an electrochemical flow cell where two channels are separated by a membrane. An electrode is placed in each channel and the cell contains a graphite foil as a current collector. Freshwater is fed into one channel and saltwater into the other and the waters' flow paths are switched back and forth to generate electricity.
"There are two things going on here that make it work," said Gorski. "The first is you have the salt going to the electrodes. The second is you have the chloride transferring across the membrane. Since both of these processes generate a voltage, you end up developing a combined voltage at the electrodes and across the membrane."
In testing, the device produced higher power densities than all of the previous methods.
The device still needs further testing to determine how long the electrodes remain stable and whether other elements in ocean water like magnesium or sulfate hamper the cell's performance, but the potential for the technology is big. With some work and some tweaking, this innovation could become a major source of clean electricity around the world.