I seem to recall someone suggesting doing this using the ocean's own pressure: stick a 900' pipe with an RO membrane on the end vertically into the ocean & suck all the seawater out of it. You now have to pump a much smaller amount of fresh water up from the bottom of the pipe & keep the pipe empty (i.e., full of just air at atmospheric pressure). Perhaps you don't have to go down as far as 900' for this version to work; I haven't done the math. At 05:49 PM 4/23/2015, rcs@xmission.com wrote:
During the 197x Energy Crisis, there was an article in Science pointing out the osmotic pressure difference between fresh water & sea water was equivalent to a 900 foot dam. The authors had a proposal to extract some of that as usable energy, building some dam-like gadget at the mouth of a river. Points in favor: sizable new energy source, tolerable cost, known physical chemistry. Against: serious eco-damage, possible membrane issues.