[extropy-chat] energy from osmosis

[Dan Clemmensen]

spike wrote:

>Ja.  My calcs show that if you have a 9600 meter pipe
>hanging vertically in the sea with a filter on the 
>bottom end and the pipe filled with fresh water, the
>pressure across the filter will be about 240 meters
>of water and the fresh water will reach up to the
>surface of the sea.
>
>Then by that reasoning, if the pipe end is deeper
>than 9600 meters, then fresh water would be raised
>higher than sea level.  Free fresh water, or free
>energy.  
>
>Mike your earlier comment made me realize this
>doesn't violate any cosmic principles.  You
>are dropping less dense fresh water at the surface
>of the sea, whereas the depths become more saline
>and thus denser.  In doing so, you slightly lower
>the center of gravity of the ocean, thus extracting
>a bit of potential energy.  Of course the tides and
>currents mix the sea, so what you are actually doing
>is very indirectly harnessing solar energy.
>
>  
>
To look at it another way:  create a closed system consisting
of an outer pips full of saline water and an inner pipe full of fresh
water, connected via an osmotic membrane at the bottom. start the pipes
bot full to the the same level. The saline water level will decrease
while the fresh water level will increase, If you then let the freshwater
from the top of the pipe fall into the outer pipe through a turbine,
you will extract energy, but the fresh water will float on top of the 
salt water
and the system will eventually stop. In some sense you are extracting the
gravitational potential of the weight of the salt as it sinks to the bottom
of the salt water side.