A Clever New Approach To Desalination 128
jbeaupre writes "The Economist reports on progress by a company called Saltworks on using saline gradients to do the heavy lifting of desalination. In essence, Saltworks uses solar energy or waste heat to concentrate sea water. They then use the ionic gradient between the concentrated brine and two sea-water streams to pull ions from from a 3rd sea-water stream. It appears to work with entropy by trading the reduced entropy of the desalinated water against the increased entropy of 'mixing' the brine and the other sea-water streams. The article only discusses Na and Cl, but even just removing these ions is a step in the right direction."
Re:Anyone else think... (Score:5, Informative)
Yeah, pretty much, for all practical purposes, but not quite, because sooner or later the fucking sun will in fact burn out.
You didn't need to read TFA. It's in the summary. Second sentence.
Re:Maybe (Score:5, Informative)
what if they collected the fresh water vapour that is evaporating off the salt water as well?
I'm guessing this would require active refrigeration unless they're in a colder climate?
TFA is a bit light on details: why do Na+ ions go to one stream and CL- to the other? Have they got membranes that are impervious to CL- and NA+?
Yes. From TFA:
Re:Maybe (Score:5, Informative)
Electricity is the flow of charge, not electrons.
If your statement was accurate, your computer would not work as it depends upon semiconductors which function in part based on the flow of positively charged holes in the electron structure of the material. (see p-n junctions, etc.) The Hall effect can be used to verify the charge of the moving carrier within a current. It can be either positive or negative.
Note that this desalinization mechanism works very similarly to a fuel-cell which also involves ion flow as part of an electric circuit.
Re:Maybe (Score:3, Informative)
Re:Maybe (Score:2, Informative)
A stream of alpha particles would have a well-defined current, despite the lack of electrons.
Re:ion bridges cost? Consumable? (Score:3, Informative)
The energy was input by the sun before the different solutions were brought together.
delta S > 0 for a closed system as a consequence of the second law of the thermodynamics. No need for additional energy. The entropy of a the concentrated solution is less than the entropy of the dilute solution, hence dilution happens spontaneously, much like osmosis.
Re:Maybe (Score:5, Informative)
Be that as it may, atoms are not ions, which is what the attempt at an article states.
The article doesn't state that atoms are ions. Rather, it states that ions are electrically charged atoms, which is totally correct. Here is the exact quote, in context:
I don't find any incorrect statement in the above quote regarding ions.
Re:ion bridges cost? Consumable? (Score:4, Informative)
More important than the cost is the question of effectiveness.
In their diagram, they have this schematic in the critical location:
[Salt water]<----(+)----[Brine]----(-)----->[Salt water]
Chemically, that "equation" just doesn't balance without an input of energy.
The article DOES explain this, the salt imbalance makes
a kind of battery.
It's brilliant! Solar energy concentrates a brine, which
then (just as dissimilar metals make a thermocouple)
causes current and builds an electric potential
when connected via a membrane (impermeable
except to Na+ ions) to a less-concentrated brine.
So, the difference in concentration of ions between two
channels results in a diffusion from more-concentrated to
less-concentrated, OF A CHARGED ION. That means
electric current flows, until the charge buildup raises
the electric potential enough to stop the diffusion.
The solar input concentrates the brine, the resulting
(small) voltage then is electrically applied to the to-be-desalinated
channel, and (in the absence of a concentration difference)
the electric field causes the ions to leave the
to-be-desalinated stream.
Thus, it's a solar-concentration-of-salt that makes
the desalination occur. The electricity caused by the
diffusion is active ALL NIGHT until the concentration
of salt goes down, so the concentrated brine is
an effective load-leveling device for the whole plant.
The 'electric input' part of the process is entirely for
pumping the brines around, so it can be a small fraction
of the brute-force desalination energy requirement.
Heck, you could use wave or wind power for that.
Solar collectors for this kind of gizmo are just open-air
trays of brine. Can't get any more cost-effective than
THAT.
Re:ion bridges cost? Consumable? (Score:5, Informative)
You missed the other side of the reaction. It is charge neutral because the +/- charges in the two side pools are then balanced with -/+ charges from the water to be desalinated.
The charges flow apart in the first place because the central pool is highly concentrated - so it contains far more + and - charges than anything else in the system.
This kind of approach would never yield completely drinkable water, but that isn't the point. The goal is to get rid of a lot of the ion load before using more expensive processes to get rid of the rest.
Desalination is a marvel of process optimization. Multiple stages of purification are used - each one being more expensive than the last but more effective. The early steps get rid of a huge mass of dissolved matter for dirt cheap, so even if their product isn't drinkable it GREATLY reduced the cost of the later stages.
If you don't care about cost then desalination is trivially easy. Just run any kind of water you like through a H+ exchange resin followed by an OH- exchange resin, and then run it trough activated charcoal. The resulting water will be as clean as clean can be and the system would be remarkably simple. The catch is that those resins cost a small fortune to make, and if you run seawater into them then they're probably going to last all of 5 minutes. It might be a good approach for a camper to use to obtain water (the resin is a lot lighter than the amount of water that it could clean), but it is not a cost-effective method overall. Also - the purity it would achieve would be massive overkill. This is drinking water - we're not manufacturing CPUs.
Re:ion bridges cost? Consumable? (Score:5, Informative)
No, the key piece of work is the idea. Ion bridges have been around forever.
This has to permit the travel of one kind of ion but not the other, i.e. Na+ or Cl-. Looks like this material could be expensive.
So you use, for instance, a polymer electrolyte (ionomer) with negatively charged side-chains for one bridge and a polymer with positively charged side-chains on the other. Only the counterions are mobile. The article says they're using modified polystyrene. This is not new, or terribly expensive. Similar things are already being used in industrial desalination technology for ion exchange columns.
It might plug up need to be periodically replaced.
Plug up with what? You naturally would have a mechanical filter to keep the crap out. It's not a major problem.
How expensive these are? How non toxic these are? What is needed to manufacture them? These are the questions we need to ask.
No, they're the questions asked by someone who doesn't know s--t about chemistry/chemical engineering. I happen to have a degree in the subject, but damnit, I learned about (used, even) polymer ion exchange columns in high school. If you want answers to your questions, go get Coulson & Richardson or some other chemical engineering textbook, and find the relevant section.
This technology is certainly very clever, but it does not make use of any new technology. The only question I think is worth asking here is whether or not it turns out to be more efficient or not.
Re:ion bridges cost? Consumable? (Score:4, Informative)
[Salt water]<----(+)----[Brine]----(-)----->[Salt water]
Chemically, that "equation" just doesn't balance without an input of energy. It doesn't matter what kind of "ion bridges" they put into place between the brine and salt water reservoirs, or what the concentration of salt exists in the brine or salt water, it will require some energy to offset the entropy increase.
This is exactly backwards; energy input is required in order to decrease entropy of (part of) a system. Entropy increases come for free. Consider if you fill one half of a fish tank with fresh water and the other half with brine, do you get a fish tank full of somewhat salty water or do the fresh and salt water separate out?