h a l f b a k e r y
This ain't rocket surgery.
add, search, annotate, link, view, overview, recent, by name, random
news, help, about, links, report a problem
or get an account
A steam engine's efficiency depends primarily on the
pressure differential between it's intake and it's
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
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
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
||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?)  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.
||ahh *negative* pressure exhaust.
||//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
||Of course, it's really the same
water, after being passed through people's
innards, and otherwise dirtied. Elegant: [+]
||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.
||[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. (+).
||// 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.