Scientists have discovered a reliable way to extract renewable energy from ordinary seawater. The research team, from the Ecole Polytechnique Fédérale de Lausanne’s Laboratory of Nanoscale Biology in Switzerland, employed a natural process called osmosis, where a fluid permeates through a membrane from one side to another.
In this case, salt water is on one side of the membrane, and fresh water on the other. Simply put, as ions from the salt water cross over into the fresh water, they affect the total electrical charge found in the water on each side of the membrane, a charge that can be tapped for electrical use. In this case, the researchers reduced the thickness of a molybdenum disulfide membrane to a mere three atoms — the thinner the membrane, the more electrical current it can generate.
However, the key to making the device more effective at generating electricity was found in the size of the tiny pores in the membrane that allow the ions through: if the holes are too small, the device produces more voltage, but has a weak electrical current; conversely, if the holes are too big, the device’s efficiency drops, resulting in a higher current, but lower voltage. By tweaking the size of the nanopore used, the researchers found its optimal size, enabling them to squeeze a large amount of electricity out of the device: they estimate that a one-meter square membrane (10.8 square feet) membrane could generate 1 megawatt of electricity.
The advantage this form of power generation has over other renewable sources is that it can potentially maintain constant power generation, as opposed to solar or wind generation, methods that must rely on the availability of their respective energy sources. The osmosis method would be most effective in river estuaries, where large volumes of fresh water meet the sea, and manufacturing costs should be relatively cheap, as the molybdenum disulfide used to make the membranes is relatively abundant. The only hurdle seen by the research team is devising a method of reliably manufacturing the nanopores in the membrane on a large scale, to allow for large-scale manufacturing of the devices.