A Generator Draws Energy From Saltwater

The generator siphons energy from the process of osmosis—when the salts  spread out evenly into freshwater through a membrane. 

Where freshwater meets saltwater, Aleksandra Radenovic sees a world of untapped renewable energy. Radenovic is a nanoscientist at the Swiss Federal Institute of Technology, and today she published the schematics of a brand new type of flat, membrane-like power generator. The generator siphons energy from the process of osmosis—when the salts in salty water spread out evenly into freshwater through a membrane. Only three atoms wide at its thinnest point, the generator could be used in places like river mouths and estuaries, or other locations where waters of different salinities constantly mix.

Radenovic says the electric promise of her generator could be enormous. She estimates that just a three foot square made of her flat device could theoretically produce an entire megawatt of power. That's enough juice to run 50,000 energy-saving light-bulbs. The generator is described today in the journal  Nature .

Osmosis Power

At its core, Radenovic's membrane is a thin sheet studded with an array of incredibly tiny holes. The sheet is made of a relatively cheap compound called molybdenum disulfide. These holes are just large enough that only certain sized salts can shuffle through. Thanks to the molybdenum disulfide material, the holes are also naturally electrically charged to repel certain types of salts away.

The researchers created an experiment with a tiny scrap of this material that had just one of these holes, which they call a nano-pore. Radenovic then set up a box with two compartments—each filled with water with a different level of salinity. She connected the two compartments with this scrap of her material. As the salts started their shuffle toward equilibrium, pushing through the single nano-pore, they generated a small amount of electricity. That's because the salts have a small amount of electrical charge, and so can create current through their movement. 

Expanding on how much energy one single nano-pore could generate ("between 10 and 20 nanowatts," she says). Radenovic's team concluded that a three by three-foot square of the stuff, dotted with these tiny pores on only 30 percent of the material, could produce a megawatt of power through the osmosis of fresh and very salty water.

Approaching the Megawatt

Here's the holdup. Radenovic and her colleagues made a single scrap of her material with one lonely hole. Nobody is quite sure how you'd manage to evenly manufacture or puncture the millions of nano-sized holes you'd need for a larger sheaf of the material. "So we are still far away from this [megawatt] number," writes Radenovic in an email.

As even Radenovic will acknowledge, the team wouldn't be the first to see huge promise in osmosis power generation and then downsize their expectations under the cold, crushing fist of reality. Radenovic references a much hyped but never-completed prototype plant in Hurum, Norway, which was developed in 2009 but shuttered in 2013. This time, maybe this kind of energy can really punch through. 

Source: Popular Mechanics

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Taxonomy

• Membranes
• Forward Osmosis
• Nanotechnology