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PostPosted: Fri Jul 24, 2009 7:53 pm 
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New breakthroughs in generating energy from the mixing of fresh and salt water.
http://www.physicscentral.com/buzz/blog/index.cfm?postid=8192106608311312838
Areas where this would be applied are the points where fresh water poors into salt water, meaning we wouldn't be losing any fresh water to run these things.

Problems i see with doing it is estuaries are pretty awesome biomes because they have the nutrients of erosion pouring into the sea. So you'd have to be pretty choosy to avoid destroying very sensitive ecosystems. Still, has promise.

cue bad mouthing.

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PostPosted: Fri Jul 24, 2009 9:54 pm 
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Hmmm... seems to be the presumption that just because desalination uses up otherwise useful energy, so the reverse should provide useful energy. This isn't a given. It depends on the reversibility [magnitudes] of those 'potentials'.

Push a weight over a rough surface uses up energy. Pushing it back again doesn't generate energy!! I think if you do the sums then the amount of energy gotten out of this system is just way to small. This process isn't fully reversible, by a big margin. Happy to be wrong on that - I'd like to see some numbers from an experimental setup.


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PostPosted: Fri Jul 24, 2009 10:45 pm 
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Yes it is reversible.
I did a tiny demonstration as a senior chemistry project.
Two major problems as an energy source.
1) disruption of wetlands migrating fish etc.
2) it is a dilute power source i.e. you have to process many many gallons of seawater to get significant power.
Lots of smaller ones.
Ever designed for seawater contact and long periods of time? It is a corrosive, aggressively growing thing with hurricanes. Same problem as OTEC. Also you have to have one end far enough upstream to get truly fresh water and the other far enough from the river mouth to get fully saline. Extreme example: the Amazon makes the Atlantic drinkable 100 miles out. (I suppose those old-time square rig sailors were pretty desperate for water after a few months out and would tolerate much worse than we would put up with.)
Polarization issues and ion migration through the separator are going to cut into efficiency.
My take was that is was possible but not economically viable.

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PostPosted: Fri Jul 24, 2009 11:36 pm 
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Actually, you might be able to pull it off.

A couple of decades ago I worked with salt water and some perm-selective membranes designed to pass only selected ion species. These were available off-the-shelf. I had a membrane for sodium, another for chloride, in a cell in which I intended to test valve membrane properties (membrane systems which conducted better in one direction than in the other).

The membranes were incredibly conductive to the species involved.

I can envision allowing salt ions to migrate to fresh water via the two channels, and use that to create a charge separation.

Caution is needed doing this in estuaries. It is difficult to kill an ocean, but it is easy to kill an estuary (we've about wrecked the Chesapeake Bay, much to the dismay of crab and oyster lovers). Since about 80% of sea life either spends part of its time in estuaries, or depends on life that does, killing estuaries can kill the oceans. That said, it may be possible to harness the mixing process without significantly affecting life.


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PostPosted: Fri Jul 24, 2009 11:49 pm 
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Beautiful. Once it is started all you need to do to keep the electricity flowing is to keep pumping in fresh reactants. And of course the pump uses electricity from the process.

It all depends on the flows and delta p required.

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PostPosted: Sat Jul 25, 2009 3:48 pm 
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I have always suspected this could be a valuable energy storage method. Instead of pumping water up a high hill against the gravity gradient, pump it from a salty to a fresh pond against the salinity gradient. If seawater is about 700' of head, heavy brine would be a huge hill worth of gradient.

Also, the river running through Salt Lake City falls off a cliff that is about 3500' high, salinity-wise. That is one heck of a source!


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PostPosted: Sat Jul 25, 2009 6:23 pm 
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As mentioned, the effect on local fresh water, and conversion zones (like estuaries) would have to be minimal. Even if effective, I could not see it being used except where there is a very large surplus of fresh water. How much could the local enviornment tolorate(1% diversion, 10% diversion)?

The idea of using it as an energy storage system in a closed loop sounds interesting. How would it compare to average hydroelectric in terms of land use? Would it compete with other storage schemes? Would it have benifits as a dual purpose system- large storage (of solar or wind energy), modest direct generation. ie- have a closed system that stores intermittantly aviable energy like solar, combined with a smaller capacity, more enviormentally tolerable, open system to suppliment power when needed.

Dan Tibbets

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PostPosted: Sun Jul 26, 2009 6:14 am 
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Energy storage would be a better use.
Interesting idea.
Safer than high pressure tanks.
Less excavation and real estate than pipes and hills.
You would just need 3 tanks (salty water, pure water, concentrated brine) and the RO equipment between them.

I currently work with seawater reverse osmosis plants.
The usual working pressure is roughly 900 psi.
You could go much more saline than seawater with stouter equipment.

They use energy recovery only in situations where power is very limited, but it is an available option.

They normally work by having a sea water inlet, pumping it up to 900 psi, pushing some fraction (25% to 75%) through the membrane as fresh water at 0 psi and the rest is concentrated brine at 900 psi which is normally just expanded through an orifice wasting the energy.
Energy recovery is done by putting a pump running in reverse on the brine outlet and using it to drive the main high pressure pumps.

The main problems of clogging come from growing creatures (barnacles, mussels, algae etc.) and mineralization.
Both could be controlled by starting with pure water (which can be made with the unit itself) and keeping it pure.
The choice of salt need not be limited to NaCl.

It could be tested using mostly off the shelf equipment. Say for use on a sailboat.
Shooting from the hip I expect efficiencies to be on the order of water pumps.
Ideally look for one with energy recovery and reversible pumps turned by reversible motor/generators. It would need the controls redone.
The other way to get the energy out would be to set up a pair of electrolytic half cells to create dc low voltage directly.

I think I'll ask my boss for some hours to look into it. Maybe we can sell the local utility a demonstration plant to put next to their demonstration solar panel array.

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PostPosted: Sun Jul 26, 2009 3:05 pm 
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I was reading about forward osmosis desalination technologies recently and came upon this...which seems on topic for this thread.

http://www.yale.edu/env/elimelech/publi ... S_2007.pdf

Apparently some research is needed into creating appropriate semi permeable membranes for this sort of application.


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PostPosted: Mon Jul 27, 2009 4:21 am 
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It sounds kinda neat but horribly inefficient.


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PostPosted: Tue Jul 28, 2009 12:49 am 
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for what it's worth:

very long url

They may discuss efficiency but I can't afford to find out.

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PostPosted: Wed Jul 29, 2009 7:35 am 
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Thanks,
The above article says that efficiency is pretty good but the membrane cost needs to come down by a factor of 100 for the capital cost to be affordable.
And that is using mine brine that is almost 3 times the concentration of seawater.

Also
http://www.energyrecovery.com/
has a mechanical system that looks at first cut like the total efficiency for the round trip is ~60%.
As shown in this spreadsheet from their very informative web site:
http://www.energyrecovery.com/tools/downloads/80068-01-4%20PX%20Power%20Model%20Selector.xls
But, based on my experience with desalination plants, I expect capital costs to be the project killer here too.

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PostPosted: Sun Nov 29, 2009 11:46 am 
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Power of osmosis used to deliver eco-friendly energy: A Norwegian firm is testing a renewable and emission-free source of energy that harnesses the power of water through osmosis (guardian.co.uk, Wednesday 25 November 2009)

Before anybody gets too excited, this excerpt puts the news into perspective:
Quote:
The two-storey, tennis-court-size plant, situated next to a pulp mill, will generate little power. "It will produce two to four kilowatts. You might be able to run a coffee machine on it, if you are lucky," says Gotaas.


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PostPosted: Sun Nov 29, 2009 1:11 pm 
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So they're going to run a turbine. What is the "head equivalent" pressure between seawater and freshwater with the best osmotic membranes available? When they say "factors of 100" improvements, are they talking imaginary improvements? Do we have a "Maxwellian type Daemon" critter working a trap door saying "you're a salt, you're a water, you're a salt, You're a water....."

I suspect this is just another "lets ignore power densities" approach, that "works" because the public also ignores power densities. If anything it is just this ignorance that is going to leave the poorest people of the world short of energy.


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PostPosted: Sun Nov 29, 2009 2:37 pm 
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Helius wrote:
So they're going to run a turbine. What is the "head equivalent" pressure between seawater and freshwater with the best osmotic membranes available? When they say "factors of 100" improvements, are they talking imaginary improvements? Do we have a "Maxwellian type Daemon" critter working a trap door saying "you're a salt, you're a water, you're a salt, You're a water....."

I suspect this is just another "lets ignore power densities" approach, that "works" because the public also ignores power densities. If anything it is just this ignorance that is going to leave the poorest people of the world short of energy.

http://en.wikipedia.org/wiki/Osmotic_Pressure: "The osmotic pressure of ocean water is about 27 atm."
Makes about 270 m of head. 27 atm = 2.7e6 Pa = 2.7 MJ/m^3
Take, for example, the Rhine, with an average discharge of 2,290 m^3/s, putting it at number 30 in the world. 2,290 m^3/s * 2.7 MJ/m^3 = 6.2 GW. The Rhine is 400 m at its widest point, so I consider this to be a respectable power density, if it could be harvested.
You don't need to hire a demon to work a semipermeable membrane. Ideally, it lets every water molecule through that comes knocking and turns back every salt ion - in either direction. Whether it does this based on the size or the net charge or the electrical polarization, I don't know, but it doesn't matter in principle. The pressure builds up because the salt ions take up space, so it's harder for a water molecule on the salty side to get near a gate.


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