It wasn’t possible to exploit Pattle’s idea for power generation until the 1970s. That’s when artificial materials for making semi-permeable membranes became commercially available. An Israeli scientist named Sidney Loeb suggested that they could be used in what he called “osmotic power plants”; Loeb hoped they might harness the energy released as the Jordan River mixed with the salty Dead Sea.

Such power plants actually work best not when the flow rate across the membrane is as large as possible, but when it is slowed down a little. This can be done by squeezing the salt water so that the pressure hinders the influx of fresh water from the other side of the membrane. Consequently, this technology is known as “pressure-retarded osmosis”.