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Osmotic Steam Condenser

Condense steam from a steam engine, using brine
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A steam engine's efficiency depends primarily on the pressure differential between it's intake and it's exhaust.

The typical way of providing a low pressure exhaust for a steam engine is to route the used steam into a condenser, which removes heat to turn the steam into water, and then use a pump to remove the condensed water, and a compressor to remove any air that leaked into the condenser.

This idea is to provide a low pressure exhaust for a steam engine using a different method. Namely, route the used steam across an osmotic membrane, and pump salt water across the other surface of the membrane. There is no condensate, so no pump is needed to remove that; a compressor is, however needed, for the same reason as it is needed with a condenser.

The salt water's affinity for fresh water will pull the steam through the membrane, in a manner identical to that which occurs in an pressure retarded osmosis power plant.

This method should require less salt water to remove a given amount of steam than if we were merely using that water to condense the steam by removing heat.

Furthermore, it should be able to operate at lower steam pressure than a condenser, producing a more efficient steam engine.

A disadvantage is that it doesn't produce condensate, which means that water for the steam engine must be continually resupplied.

The ideal location for a power plant using this method would therefor be the same as the ideal location of an osmotic power plant -- close to both a wastewater treatment plant and a desalination plant.

The wastewater plant produces water that's quite pure (but which (at least in America) can't legally be used as drinking water) and is thus ideal to be fed to the steam engine's boiler with minimal treatment, while the desalination plant produces concentrated salt brine, which has greater steam removal ability than mere seawater.

goldbb, Jul 03 2011

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       ... why ?
FlyingToaster, Jul 03 2011
  

       Ditch the membrane - a free brine/steam interface will do the same job. In a very real sense, the surface tension of the brine is equivalent to a nearly perfect membrane. (Why do people always think that osmosis occurs only at membranes?) [edit] The membrane means the brine be kept close to atmospheric pressure, which I assume is the point here, but it also reduces efficiency, as there is always a potential difference associated with flow across a membrane.   

       The brine and fresh water from the desalination plant have presumably been made using high-grade energy (such as electricity). I think that your scheme is simply stealing form Peter to feed Paul, and won't increase overall efficiency.
spidermother, Jul 03 2011
  

       ahh *negative* pressure exhaust.
FlyingToaster, Jul 03 2011
  

       //The brine and fresh water from the desalination plant have presumably been made using high- grade energy// But you only need the brine, and, for that, evaporation ponds will do.   

       In practice, you'll still site this near a desalination plant, but the point is that the extra "high-grade" energy isn't part of the price you pay for the osmotic steam condenser -- Peter is being robbed to provide Paul with potable water, not to provide the OSC with anything. The OSC uses non-potable water from a sewage treatment plant.   

       Of course, it's really the same water, after being passed through people's innards, and otherwise dirtied. Elegant: [+]
mouseposture, Jul 03 2011
  

       Aha! Simply position the steam/brine interface at the top of a 10m-ish column of brine, supported by atmospheric pressure. That way, you don't need a pressure exchanger or a membrane.
spidermother, Jul 03 2011
  

       [mp] Quite right; I hadn't read the last couple of paragraphs closely. The OSC is being subsidised by the waste brine from an existing RO plant and the freshwater from a water treatment plant. (+).
spidermother, Jul 03 2011
  

       // it doesn't produce condensate, which means that water for the steam engine must be continually resupplied //   

       That's the biggest hitch.   

       Most commercial "steam" wngines, be they turbines or reciprocating, run closed-cycle for a very good reason; the water in the system is very pure indeed.   

       If you keep injecting more water into the feed side, the problems are (a) heating this water to boiling point at the operating pressure of the system, and (b) Total Dissolved Solids - inorganic compounds that precipitates out when the water is, effectively, distilled off.   

       Thermodynamically, maybe, but practically, no.   

       Bun for ingenuity. [+].
8th of 7, Jul 03 2011
  
      
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