Comment by eldr | Hacker News Reader

Comment by eldr | Hacker News Reader

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Thanks for the insight. Some quick (wikipedia) research would seem to indicate that osmotic pressure (27atm for seawater) is independent of membrane construction, at least under ideal conditions. Supporting the membrane during operation would seem to be a huge challenge, especially as they indicate that it has a tendency to tear during handling.

This article claims a 100x reduction in required pressure for the graphene membranes, while the Science article claims that state-of-the-art plants are already operating at 2x the theoretical minimum. These links would seem to support the claims of the Science article, that RO plants currently run at about 54atm (i.e 2x the theoretical minimum): [1] http://www.lenntech.com/processes/desalination/reverse-osmos... [2] http://www.remco.com/ro_quest.htm

I do hope that I'm missing something here though, and that fresh water for all is around the corner. I think it would go a long way towards improving the lives of many millions of people.

Theoretically, I think the only energy spent is on the resistance inside the membrane.

Maintaining vessel pressure is not work and hence, not energy spent (in the same way as lifting a heavy weight and putting it down again does not do work)

Practically, maintaining pressure will take energy, but that may be recoverable (that first link talks about a Energy Recovery Device that helps with that)

So, I guess the article is wrong about the '99% less pressure needed' claim and is theoretically right about the '99% less energy' claim. I also guess that, in practice, that 99% will be a lot lower.

Corrections welcome.

But it's not just about maintaining pressure. Osmotic pressure can do work, and pushing against it requires work.

Here is an experiment you can do in your kitchen to demonstrate this: Take an egg, and peel a bit of shell off one end so the membrane is visible but unbroken. Push a straw into the other end , and put the egg (straw upwards) so the membrane is in contact with water. Osmotic pressure will force the contents of the egg upwards against gravity out through the straw.

The osmotic pressure has done work (lifted the contents of the egg) and water has become mixed with the egg contents. To obtain the water back, you would need to put in at least the same energy.

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