[extropy-chat] energy from osmosis

[spike]

The osmotic pressure of ocean water to fresh
water is about 2.4 MPa. If we put an osmotic 
membrane across the end of a pipe and shove 
the empty pipe down into the ocean when the 
pipe reaches a depth of about 240 meters 
fresh water will start colleting on the 
inside of the pipe. We can put a pump down 
there and pump up fresh water. Nothing 
tricky here. The energy to lift the water 
240 m is exactly the same energy that would 
be required to push the sea water through 
a membrane at ground level. 

Now suppose we keep pushing the pipe down to 
290 m. The pressure across the membrane will 
remain at about 2.4 MPa and this means that 
the fresh water level inside the pipe will 
rise about 50 m, so that the water surface 
inside the pipe will remain at about 240 m. 

We keep pushing the pipe further and further 
down, the fresh water column gets longer and 
longer, but the top of the fresh water column 
remains at about 240 m below sea level. 

No, that is too simple. Fresh water is slightly 
less dense than sea water, so to keep the 2.4MPa 
pressure differential across the bottom of the 
pipe the fresh water column needs to rise faster 
than the pipe sinks. Fresh water has a density 
of 1000 kg/m^3 while ocean waters is about 
1025 kg/m^3. If the membrane end of the pipe 
is down 1000m the freshwater column will be 785 m, 
so the fresh water will rise to within 215 m of the 
surface (25m higher than -240m). 

If we keep pushing the pipe down to 9600 the 
fresh water will come up to sea level. If we 
push the pipe below 9600 me fresh water will 
spew out of the top of the pipe above sea 
level. We could then hook up a water wheel and 
have a perpetual motion machine. Now we know 
something is seriously wrong, but what?

spike