Researchers at Penn State University have developed a new technology that they believe will be capable of producing large amounts of energy – possibly more than one-third the amount needed to meet global energy demands – in coastal areas where seawater and freshwater meet.
“The goal of this technology,” assistant environmental engineering professor Christopher Gorski explained earlier this month in a statement, “is to generate electricity from where the rivers meet the ocean. It’s based on the difference in the salt concentrations between the two water sources.”
According to Gorski and his colleagues, that difference in salinity levels could potentially create enough energy to meet nearly 40 percent of the world’s electricity needs. However, methods that experts currently use to harness that power have failed to fully capitalize on that potential.
The most commonly used technique, pressure retarded osmosis (PRO), uses a semi-permeable membrane to filter salt out of the water, generating osmotic pressure which is then turned into power using turbines. While Gorski explained that PRO is “the best technology” developed thus far “in terms of how much energy you can get out,” it is not without issues – namely, the technique uses tiny membranes to filter out the salt, and those membranes can easily become blocked.
A second technique, reverse electrodialysis (RED), uses an electrochemical gradient to develop voltages across ion-exchange membranes, and a third, capacitive mixing (or CapMix), harnesses electricity from two identical electrodes when they are sequentially exposed to a pair of different kinds of water. However, neither is able to mass-produce energy, the researchers explained.
Combined method produces more electricity than current techniques
Now, however, Gorski and his colleagues have developed an electrochemical flow cell that uses elements from both the RED and CapMix methods to harness electricity. “By combining the two methods,” the Penn State professor explained, “they end up giving you a lot more energy.”
First, they created a custom-built flow cell in which they used an anion-exchange membrane to separate two channels, then put a copper hexacyanoferrate electrode in each channel. Next, they added graphite foil (to collect the current) and sealed each of the cells with end plates, nuts and bolts. Synthetic seawater was fed into one chamber, and synthetic freshwater into the other.
By switching the flow paths of the water from time to time, the researchers were able to recharge the cell and produce additional power. They then studied the impact that different variables, such as changing flow paths, salinity levels and external resistance, had on energy production.
“There are two things going on here that make it work,” Gorski explained in a statement. “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.”
While the RED technique produced a maximum of only 2.9 watts of energy per square meter, and PRO managed 9.2 watts per square meter, the new method is able to produce a peak of 12.6 watts per square meter, the researchers said. While they called the results promising, they added that they hope to further improve the system, making the electrodes more stable and determining how its performance could be affected by some of the elements found in actual seawater.
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