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Seawater moves to fill brine chamber via osmosis.
Osmosis causes water level in brine chamber to reach
above sea level.
Brine water passes via gravity to freshwater chamber,
which is below top of brine chamber (and can even be
below sea level using weights).
Seawater Chamber chSea inserted in seawater.
Brine Chamber chBrine inserted to right of chSea, in
Between chSea and chBrine: Membrane mBrine - which
allows only water to pass from chSea to chBrine
Freshwater Chamber chFresh currently empty of liquid
with air only) inserted in seawater to right of chBrine.
Top of Freshwater chamber is a membrane mFresh -
allows only water to pass from chBrine to chFresh (this is
not exactly reverse osmosis, but rather filtering with
When water reaches sea level (and mFresh) the fresh
chamber is sealed and replaced or fresh water is pumped
drawing of idea
[pashute, Dec 10 2010]
Museum of Unworkable Devices: Reverse Osmosis
"Not only will this device give an endless stream of fresh water but can be used to run a small generator." [jutta, Dec 10 2010]
Museum of Unworkable Devices: Answer to the above
[jutta, Dec 10 2010]
||I've linked to a related discussion in the Museum of Unworkable Devices. There, the pressure needed for the filter comes from lowering a long tube into the ocean, not from generating a column of water - which, in view of it taking 20 atmospheres pressure differential for reverse osmosis to start, seems more realistic.
||The principle involved here is more general than "flow of water across a semipermeable membrane"; in a sense, considering osmosis / reverse osmosis at the membrane in isolation is a bit of a red herring.
||There are three sources of water potential gradient involved here: 1) osmotic, 2) gravitational potential (height), and 3) pressure. Summing these gives the total potential, and this sum is what determines how, or if, water will flow.
||That is what is missing in the discussion linked by [jutta], and what needs to be considered to show why both that idea, and this, will not work (beyond the uninteresting "in this house we obey the laws of thermodynamics" explanation).
||(Hint: In a column of water of uniform salinity, the potential of the water at the bottom is greater than the potential of the water at the top. In other words, such a column is not at equilibrium.)
||Reverse osmosis is where there is water on both
sides of a membrane, with one side being saltier
than the other. But in this case there is no water
on the bottom side of the mFresh membrane.
There is only air. This is acting as a filter similar to
mesh sifting of wheat flower.
||Do I need such a high pressure in this case as in
full reverse osmosis? If yes, please explain why.
||My assumption is (or was) that the pressure
needed for reverse osmosis is to overcome the
"osmosis pressure" when one side is salty and the
other side has water entering the membrane in
order to "counter" the higher concentration of
salt. With gravity pulling the water down, and on
the other side of the membrane only air, I would
expect a much lower pressure to be needed.
||But then my skin does not burst in air, so maybe
||the pressure differential required is identical in both cases.
||WcW: Could you explain why this is so?
||When there is water on both sides: the less
concentrated water tends to push its way to the
more concentrated side. This pressure needs to
be overcome. But when there is air on one side -
there is no back
pressure to be overcome. Why then is the required
pressure differential still
||Where's the hat now that we need to pass it?