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. Rich ------- Quoting Eugene Salamin via math-fun <math-fun@mailman.xmission.com>:
There is a minimum energy requirement for desalination, independent of the technology, based on thermodynamics. The osmotic pressure of seawater is 30 atm = 3 MPa. This is the minimum pressure required to force water molecules from seawater through a membrane to pure water on the other side. Thus the minimum process energy is 3 mJ/m^3. Any other process, such as evaporation or freezing, has the same minimum energy cost, since this depends only on the initial and final thermodynamic states.
This value assumes the source to be an infinite reservoir with constant salinity as water is extracted. Reverse osmosis requires prefiltering to remove crud that would clog the critical filter. Thus there is an investment in the source brine. If a fraction x of the water has been extracted, the brine salinity is greater by a factor 1/(1-x), and the osmotic pressure is greater by the same factor, assuming the brine sufficiently dilute for van't Hoff's law to hold. To extract a further dx of water requires effort dx/(1-x), and so the effort to extract a total fraction f of the water is int(dx/(1-x),x=0 to f) = -log(1-f). This gets f units of water, so the minimum energy requirement is greater by a factor of (-log(1-f))/f. For example, if half the water is extracted, discharging brine at twice the salinity, the energy required is greater by 2 log 2 = 1.38, or 4.14 MJ/m^3 = 1.15 kWh/m^3. According to [ http://en.wikipedia.org/wiki/Desalination ], reverse osmosis requires 3-5.5 kW h/m^3, just for the reverse osmoses, and excluding the energy needed to operate the facility. -- Gene
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