That article's subtitle: "In principal,
but probably not in practice, fresh water can be extracted from our oceans for no expenditure of energy" [Italics mine]
Also - 231 meters deep is where, in theory, you'll just start getting fresh water coming through your membrane. Which is 231m deep. Fresh water that you'll spend plenty of $energy to pump up to the ocean surface.
(For reference - Hoover Dam, when brim-full, has a slightly lesser drop from the water surface in the reservoir to the output of the hydroelectric turbines. You'll be pumping $energy in, to lift each ounce of your "free" fresh water up to where it is useful.)
[Edit - from some quick calculations, and the density of sea water relative to fresh water...it looks like the "don't need to pay to pump the fresh water back up" version of this scheme will need to run its osmotic membranes at the bottom of the Challenger Deep (~11,000m), or something pretty close to that.]
Also - 231 meters deep is where, in theory, you'll just start getting fresh water coming through your membrane. Which is 231m deep. Fresh water that you'll spend plenty of $energy to pump up to the ocean surface.
(For reference - Hoover Dam, when brim-full, has a slightly lesser drop from the water surface in the reservoir to the output of the hydroelectric turbines. You'll be pumping $energy in, to lift each ounce of your "free" fresh water up to where it is useful.)
[Edit - from some quick calculations, and the density of sea water relative to fresh water...it looks like the "don't need to pay to pump the fresh water back up" version of this scheme will need to run its osmotic membranes at the bottom of the Challenger Deep (~11,000m), or something pretty close to that.]