A hybrid technology created by researchers at Penn State University could be the breakthrough needed to advance efforts to produce power based on the salt concentrations between two water sources.
Researchers in May unveiled the technology, which essentially seeks to generate power from regions where freshwater in rivers meets the seawater in oceans. Methods to capture energy from salinity gradients already exist and have long been studied. The two most successful methods currently in use are pressure-retarded osmosis (PRO) and reverse electrodialysis (RED).
In PRO, which has evolved since the 1970s as the most common system, water from a low-salinity feed solution permeates through a semi-permeable membrane into a pressurized, high-salinity draw solution, and power is captured by depressuring the permeate through a hydroturbine. According to Christopher Gorski, assistant professor in environmental engineering at Penn State, PRO is so far the best technology in terms of how much energy can be captured. “But the main problem with PRO is that the membranes that transport the water through foul, meaning that bacteria grows on them or particles get stuck on their surfaces, and they no longer transport water through them,” he explained. Holes in the membranes are incredibly small, and they become blocked easily. Also, PRO doesn’t have the ability to withstand the necessary pressures of super-salty waters, he said.
RED, on the other hand, uses an electrochemical gradient to develop voltages across ion-exchange membranes. It creates energy when chloride or sodium ions are kept from crossing the ion-exchange membranes. “Ion-exchange membranes only allow either positively charged ions to move through them or negatively charged ions,” Gorski said. “So only the dissolved salt is going through, and not the water itself.” But RED’s downfall is that it doesn’t have the ability to produce large amounts of power, he explained.
A relatively new method called capacitive mixing (CapMix) uses an electrode-based technology that captures energy from the voltage that develops when two identical electrodes are sequentially exposed to two different kinds of water with varying salt concentrations, such as freshwater and seawater. But, it, too, isn’t yet able to generate much power.
Funded by the National Science Foundation, the Penn State researchers’ solution is to combine CapMix and RED in an electrochemical flow cell. The custom-built flow cell (Figure 1) features two channels separated by an anion-exchange membrane. “A copper hexacyanoferrate electrode was then placed in each channel, and graphite foil was used as a current collector. The cell was then sealed using two end plates with bolts and nuts,” the university said. “Once built, one channel was fed with synthetic seawater, while the other channel was fed with synthetic freshwater. Periodically switching the water’s flow paths allowed the cell to recharge and further produce power.”
|1. Worth its salt. This image shows the concentration flow cell. Two plates clamp the cell together, which contains two narrow channels fed with either synthetic freshwater or seawater through the plastic lines. Courtesy: Penn State/Jennifer Matthews|
“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.”
The team said it recorded open-circuit cell voltages while feeding two solutions at 15 milliliters per minute. “At 12.6 watts per square meter, this technology leads to peak power densities that are unprecedentedly high compared to previously reported RED (2.9 watts per square meter), and on par with the maximum calculated values for PRO (9.2 watts per square meter), but without the fouling problems,” it said.
The researchers will now explore how stable the electrodes remain over time, as well as analyzing how other elements in seawater—like magnesium and sulfate—might affect the performance of the cell.
—Sonal Patel is a POWER associate editor