Carbon Nanotubes Excluding Salt from Seawater | Hacker News Reader

Carbon Nanotubes Excluding Salt from Seawater

sciencedaily.com

82 pointsmeri_dian17 comments

Perhaps this membrane is also interesting for electricity production. From the abstract:

CNTPs block anion transport, even at salinities that exceed seawater levels

High salinity levels that occur in some mining processes could produce electricity via pressure-retarded osmosis. Reaching adequate flow rates through membranes has been a barrier to this tech in the past [1].

As a slightly off-topic question: Does anyone know of any industrial processes where high volumes of fluids with high salinity concentrations occur? I am doing some research on where this technology could be applied.

[1] www.nature.com/articles/nature11477

Anyone read the paper to see what kind of flux this technology might have compared to materials used in existing membranes?

I often hear about desalination as an energy problem, since the plants use so much to run. I'd be interested to know if this technology would lower the energy requirements of desalination. Anyone read the paper?

The real problem is that clean drinkable water is often given away almost for free. California, for example, reserves vast amount of available water for specific groups, instead of having a market where a higher price would not only directly drive conservation, but would make things like desalination more attractive.

Why does everything have to be so economically driven? Clean drinking water should be totally free for everyone, we kinda need it to live

Clean drinking water for drinking, yes. Clean drinking water for industrial (or even irrigation) purposes, no – because putting a fair price on it would appropriately punish those who use it wastefully.

Many municipalities are switching to grey water irrigation in an effort to conserve.

And to build on that, only about 5% of California's water goes to indoor household use. It, or perhaps the first 50/100 gallons per person per day, could easily be free based on a tiny fee to the other 95%.

I wish it were so simple to remove economics from things. Unfortunately, the problem isn't the need (the demand), its the supply. We don't just have unlimited clean drinking water, and as soon as there is a constrained supply, we have to consider how we can divvy it up to meet demand.

Usually that means increasing price, but the what the commenter above was saying is that often we're making it artificially cheaper. Often by either subsidizing things w/ our taxes (hiding the cost), or by not adequately funding the collection, which will eventually cause failure or exhaustion of the supply.

The problem is that fresh water is a scarce resource. Give it away for free and it will be used inefficiently and you will run out. This is an area where markets are an efficient solution, so long as the scarcity problem does not become so pronounced that the poor are unable to afford water. If this happens it is a policy failure and an external force (the government) would have to step in to insure that everybody has enough water to at least survive, probably by shutting down some big industrial users to reduce demand and lower the price.

The government can also intervene by actually paying the price. Like food stamps for water.

In California, I believe the state actually does subsidize the price of water for farmers. However, as a result of water being significantly under market price, farmers are not incentivized to undertake water conservation efforts -- even though in times of drought, that aggravates the water supply.

"because osmosis is a simple matter of thermodynamics, it's possible to calculate exactly how efficient we can make the process. And, as it turns out, we're really quite close as these things go. A state-of-the-art facility is now within a factor of two of the theoretical energy minimum, and only 25 percent higher than the realistic minimum for the current reverse osmosis process."

Hm, I’m not sure I buy this argument, as it relies on the entire procedure being in equilibrium at all times. For the same reason that the human body can operate at much higher efficiency than a steam locomotive (or even the "perfect" Carnot engine), it should be possible to devise a desalination process which is much more efficient than the thermodynamic optimum by relying on catalysts and other non-equilibrium effects.

People are far less energy efficient than modern locomotives. (In terms of chemical energy to motion.) So, I am now not sure what you mean.

PS: You can also extract energy by mixing salt and fresh water, so the minimum energy to separate them must be greater than that.

> the human body can operate at much higher efficiency than ... the "perfect" Carnot engine

Can you expand on what you mean by this please? The ideal Carnot cycle describes the fundamental thermodynamic maximum for extracting work from heat.

I thought that applied across all state changes (i.e. not just in equilibrium situations)?

Of course it applies if you wish to extract work from heat. However, it does not apply when extracting work from chemical, nuclear or electrical energy, only if you first transformed that other energy into heat and then transformed the heat into work.

Haven't read the paper, but the large energy requirements for desalination right now come from the rough interiors of the channels creating large frictional forces which resist the flow of water. Carbon nanotubes don't suffer that drawback.

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