Yes you do have to go down 900 feet = 300 meters. The osmotic pressure is 30 atmospheres, and the hydrostatic pressure of water is 10 meters per atmosphere. In fact, you need to go down in excess of 300 meters to provide a driving force. -- Gene From: Henry Baker <hbaker1@pipeline.com> To: math-fun <math-fun@mailman.xmission.com> Cc: rcs@xmission.com Sent: Thursday, April 23, 2015 7:20 PM Subject: Re: [math-fun] California water crisis 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.