Osmotic Heat Engine

Abstract

A method of converting thermal energy into mechanical work that uses a semi-permeable membrane to convert osmotic pressure into electrical power. A closed cycle pressure-retarded osmosis (PRO) process known as an osmotic heat engine (OHE) uses a concentrated ammonia-carbon dioxide draw solution to create high osmotic pressures which generate water flux through a semi-permeable membrane against a hydraulic pressure gradient. The depressurization of the increased draw solution volume in a turbine produces electrical power. The process is maintained in steady state operation through the separation of the diluted draw solution into a re-concentrated draw solution and deionized water working fluid, both for reuse in the osmotic heat engine.

1 . A method of generating power comprising:
 pressurizing a concentrated draw solution comprising ammonia and carbon dioxide in a ratio of greater than 1 to 1 on a first side of a semi-permeable membrane;   introducing a dilute working fluid on an opposite side of the semi-permeable membrane;   promoting flow of the dilute working fluid across the semi-permeable membrane into the pressurized draw solution to generate a dilute draw solution; and   inducing flow of the dilute draw solution through a turbine.   
     
     
         2 . The method according to  claim 1 , wherein the concentrated draw solution has an ammonia to carbon dioxide ratio of between about 1 to 1 and to 2.5 to 1. 
     
     
         3 . The method according to  claim 1 , wherein the draw solution has a concentration of between 0.1 and 12 molar. 
     
     
         4 . The method according to  claim 3 , wherein the draw solution has a concentration of between about 3 to about 6 molar. 
     
     
         5 - 16 . (canceled) 
     
     
         17 . The method of  claim 1 , further comprising regenerating the concentrated draw solution and the dilute working fluid from the dilute draw solution downstream of the turbine. 
     
     
         18 . The method of  claim 17 , further comprising recycling the regenerated concentrated draw solution and the regenerated dilute working fluid. 
     
     
         19 . The method of  claim 17 , wherein regenerating the concentrated draw solution and the dilute working fluid is performed at a temperature of between about 35° C. and 250° C. and a pressure of between about 0.05 atm and about 10 atm. 
     
     
         20 . The method of  claim 1 , wherein promoting flow of the dilute working fluid across the semi-permeable membrane comprises promoting a water flux of at least about 25 m 3 /m 2 -s. 
     
     
         21 . An osmotic heat engine, comprising:
 a first chamber;   a second chamber;   a semi-permeable membrane fluidly coupling the first chamber and the second chamber;   a source of a dilute working fluid fluidly connected to the first chamber;   a source of a concentrated draw solution comprising ammonia and carbon dioxide in a ratio of greater than 1 to 1 fluidly connected to the second chamber;   a pressure exchanger to pressurize the concentrated draw solution in the second chamber; and   a turbine fluidly connected downstream of the second chamber.   
     
     
         21 . The method according to  claim 21 , wherein the concentrated draw solution has an ammonia to carbon dioxide ratio of between about 1 to 1 and 2.5 to 1. 
     
     
         22 . The method according to  claim 22 , wherein the concentrated draw solution has a concentration of between 0.1 and 12 molar. 
     
     
         23 . The osmotic heat engine of  claim 21 , further comprising a separator fluidly connected downstream of the turbine. 
     
     
         24 . The osmotic heat engine of  claim 23 , further comprising a source of low grade heat thermally connected to the separator. 
     
     
         25 . The osmotic heat engine of  claim 23 , further comprising a recycle system fluidly connecting the separator to the first and second chambers. 
     
     
         26 . The osmotic heat engine of  claim 21 , wherein power output per membrane area of the osmotic heat engine is at least about 150 W/m 2 .