We noticed you're browsing in private or incognito mode.

To continue reading this article, please exit incognito mode or log in.

Not an Insider? Subscribe now for unlimited access to online articles.

Sustainable Energy

Salty Solution for Energy Generation

Battery draws power from salinity difference between freshwater and saltwater.

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.  

Saline solution: This device generates electricity using differences in salinity between fresh and salt water. The two foil-like structures serve as positive and negative electrodes; the glass bulb is a reference electrode.

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.

Be the leader your company needs. Implement ethical AI.
Join us at EmTech Digital 2019.

Register now
More from Sustainable Energy

Can we sustainably provide food, water, and energy to a growing population during a climate crisis?

Want more award-winning journalism? Subscribe to Insider Plus.
  • Insider Plus {! insider.prices.plus !}*

    {! insider.display.menuOptionsLabel !}

    Everything included in Insider Basic, plus the digital magazine, extensive archive, ad-free web experience, and discounts to partner offerings and MIT Technology Review events.

    See details+

    Print + Digital Magazine (6 bi-monthly issues)

    Unlimited online access including all articles, multimedia, and more

    The Download newsletter with top tech stories delivered daily to your inbox

    Technology Review PDF magazine archive, including articles, images, and covers dating back to 1899

    10% Discount to MIT Technology Review events and MIT Press

    Ad-free website experience

You've read of three free articles this month. for unlimited online access. You've read of three free articles this month. for unlimited online access. This is your last free article this month. for unlimited online access. You've read all your free articles this month. for unlimited online access. You've read of three free articles this month. for more, or for unlimited online access. for two more free articles, or for unlimited online access.