The difference in salinity between freshwater and saltwater holds promise as a large source of renewable energy. Energy is required to desalinate water, and running the process in reverse can generate energy. Now a novel approach based on a conventional battery design that uses nanomaterials could provide a way to harvest that energy economically.
The new device, developed by researchers at Stanford University, consists of an electrode that attracts positive sodium ions and one that attracts negative chlorine ions. When the electrodes are immersed in saltwater, they draw sodium and chlorine ions from the water, and the movement of the ions creates an electrical current. The electrodes are recharged by draining the saltwater, replacing it with freshwater, and applying a relatively low-voltage electrical current, which draws the ions back out of the electrodes. When the freshwater is drained, the electrodes are ready to attract more ions from the next batch of saltwater.
"It is the opposite process of water desalination, where you put in energy and try to generate freshwater and more concentrated saltwater," says Yi Cui, a materials science and engineering professor at Stanford University and the study's lead author. "Here you start with freshwater and concentrated saltwater, and then you generate energy."
Cui's group converted to electricity 74 percent of the potential energy that exists between saltwater and freshwater, with no decline in performance over 100 cycles. Placing the electrodes closer together, Cui says, could allow the battery to achieve 85 percent efficiency.
A power plant using this technology would be based near a river delta where freshwater meets the sea. Drawing 50 cubic meters of river water per second, Cui says, a power plant could produce up to 100 megawatts of power. He calculates that if all of the freshwater from all of the world's coastal rivers were harnessed, his salinity-gradient process could generate 2 terawatts, or approximately 13 percent of the energy currently used around the world.
Such wide-scale use, however, would seriously disturb sensitive aquatic environments. "I think you would only be able to utilize a very small fraction of this or it would be an ecological disaster," says Menachem Elimelech, director of the Environmental Engineering Program at Yale University. Elimelech says it would be necessary to pretreat the water to remove suspended material including living organisms. Such processing would require energy, add costs, and itself seriously disturb the ecosystem if done on a large scale.
Prior efforts to harvest energy from the salinity differential between saltwater and freshwater have focused primarily on a process known as pressure-retarded osmosis. In this approach, freshwater and saltwater are housed in separate chambers, which are divided by an artificial membrane. The higher salinity of the saltwater draws freshwater through the membrane, increasing the pressure on the saltwater side. The pressurized water is then used to drive a turbine and generate electricity.
Norwegian electric company Statkraft is currently testing pressure-retarded osmosis at a pilot plant outside Oslo and also working to develop more efficient and durable membranes. Statkraft officials say their goal is to convert 80 percent of the available chemical energy to electricity. Cui says he doubts that the approach will be able to exceed an efficiency of 40 percent. "Efficiency-wise we are certainly much better," he says.
To achieve high efficiency, Cui's group used manganese-dioxide nanorods for its battery's positive electrode. The material gives the sodium ions roughly 100 times more surface area to interact with than conventional electrode materials do. And the nanostructure allows the ions to quickly attach and detach from the electrode, making the entire battery more efficient.
Cui's team used a silver electrode to bond with the negatively charged chlorine ions. Silver, however, is prohibitively expensive for large-scale deployments, and it's also toxic, capable of causing environmental harm if it dissolves into the water being cycled through the battery. Cui says his group is looking for a substitute, but an alternative may be hard to find.
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