Hello,

Read this article:
http://phys.org/news/2016-07-electricity-salt-three-atoms-thick-membrane.html

Bertus

 

Wonder about the cost-benefits? We need desalinized water for life. Electricity is optional.

John

 

Wonder about the cost-benefits? We need desalinized water for life. Electricity is optional.

John

Yup, fresh water mixing with sea water is just wasteful. We should really build a storage tank for all the volume of the great rivers so they don't dump into the salty oceans - maybe we'll build it in your backyard.

 

Something seems fishy here.

"The potential of the new system is huge. According to their calculations, a 1m² membrane with 30% of its surface covered by nanopores should be able to produce 1MW of electricity - or enough to power 50,000 standard energy-saving light bulbs. "

At the end of the day, you have a certain amount of seawater that has been mixed with a certain amount of freshwater. This is a description of state so the system has less energy after the mixing than it had before. How much water passes through this 1 m² membrane in a second? I can't imagine the flow rate can be very high, but whatever it is that amount of saltwater has 1 MJ less energy after mixing with the freshwater. Because this is a description of state, it doesn't matter how this mixing occurs, which means that if you simply mixed that amount of seawater with that amount of freshwater, 1 MJ of energy has to be given off somehow. That seems like a very noticeable amount of energy, so why don't people notice all of this energy being given off as rivers flow into oceans?

 

Something seems fishy here.

"The potential of the new system is huge. According to their calculations, a 1m² membrane with 30% of its surface covered by nanopores should be able to produce 1MW of electricity - or enough to power 50,000 standard energy-saving light bulbs. "

At the end of the day, you have a certain amount of seawater that has been mixed with a certain amount of freshwater. This is a description of state so the system has less energy after the mixing than it had before. How much water passes through this 1 m² membrane in a second? I can't imagine the flow rate can be very high, but whatever it is that amount of saltwater has 1 MJ less energy after mixing with the freshwater. Because this is a description of state, it doesn't matter how this mixing occurs, which means that if you simply mixed that amount of seawater with that amount of freshwater, 1 MJ of energy has to be given off somehow. That seems like a very noticeable amount of energy, so why don't people notice all of this energy being given off as rivers flow into oceans?

Because that 1MJ is absorbed by the 1 metric ton of water that was used to generate that heat (about 0.25 °C).

Heat of mixing and heat of solution is very common in chemistry and much more dramatic than mixing pure after with a 3.5% salt water solution.

 

I suppose one could start with this sentence:

The concept is fairly simple. A semipermeable membrane separates two fluids with different salt concentrations. Salt ions travel through the membrane until the salt concentrations in the two fluids reach equilibrium. That phenomenon is precisely osmosis.

In fact, it is precisely not.

Thus, the factual accuracy of the remaining text is questionable, IMHO.

 

I suppose one could start with this sentence:



In fact, it is precisely not.

Thus, the factual accuracy of the remaining text is questionable, IMHO.

Blame the reporter, not the researcher.

 

Anybody here have access to the full Nature article? They're not known for publishing junk and the U of Illinois isn't either.

But I'm very skeptical that 1MW could be extracted by any practical means from a 1 square meter monolayer. Think of the current density. Where would that much current go?

You could let rain fall on one side of a sheet floating atop seawater, and get all the energy you could ever need. Too good to be true, I suspect.

 

Because that 1MJ is absorbed by the 1 metric ton of water that was used to generate that heat (about 0.25 °C).

Heat of mixing and heat of solution is very common in chemistry and much more dramatic than mixing pure after with a 3.5% salt water solution.

Where did it indicate that this was for one metric ton of water? All the article said (that I saw) was how water flowing, at some unspecified rate, through this 1 m² membrane could produce 1 MW of power.

Is it reasonable that one metric ton of water would flow through such a membrane each second? It seems like the flow rate (assuming it IS one metric ton of water) would be about 7 mph (through the pores). What would the pressure differential need to be across the membrane to support that flow rate and how well could a three atom thick membrane resist that pressure differential?

What IS the heat of mixing for fresh water and salt water? I did a quick Google search and wasn't able to find it (quickly). I would imagine it is a pretty well established quantity.

 

Where would that much current go?

Duh! The article said so: "Into 50,000 'standard' energy saving lightbulbs."

 

Where did it indicate that this was for one metric ton of water? All the article said (that I saw) was how water flowing, at some unspecified rate, through this 1 m² membrane could produce 1 MW of power.

I read it that the 'positive ions' were traveling, via 'osmosis', across the membrane.

This is just wrong on so many levels.

 

Blame the reporter, not the researcher.

Agreed. So the question here is just what the reporter screwed up. Is it an osmotic process that was misdescribed, or what it a properly described process that was misnamed?

 

Agreed. So the question here is just what the reporter screwed up. Is it an osmotic process that was misdescribed, or what it a properly described process that was misnamed?

Third possibility: neither.

 

The article says a single pore was studied. Not a square meter of pores.

Also, the pore is only large enough to carry sodium ions through (more likely a diffusion process than an osmotic one*) and the pore is small enough to prevent chloride ions from passing. I, therefore assume that, As the ions "slosh" to an equilibrium, the potential created is a separation of charges. Claiming that the effect from a single angstrom-scale pore can be scaled to an array of pores on a meter-scale is absurd.

* osmosis is the pressure created as more pure water tries to dilute less pure water. The side with less pure water builds pressure as pure water flows into the system.

 

Claiming that the effect from a single angstrom-scale pore can be scaled to an array of pores on a meter-scale is absurd.

Actually, only 30% of a meter-scale. Not so absurd anymore...

 

Also, MoS2 is a graphite-like 2-dimensional material that seems to exist in pretty small particles. Usually a component in Moly-greases. So, find me a 1 square meter sheet of MoS2 (single-layer) and I will make you king.

Once this all plays out, I think they will find it is less of a diffusion/ion flow issue and more of a reduction/oxidation of MoS2 membrane. If it only works with MoS2 and not a single graphite layer "membrane" (aka graphene), then the sulfur or Moly is playing a role.

 

I'm working on getting the Nature article but in the meanwhile I'm quite confident that it's far too good to be true.

 

The article says a single pore was studied. Not a square meter of pores.

From the article: "The potential of the new system is huge. According to their calculations, a 1m² membrane with 30% of its surface covered by nanopores should be able to produce 1MW of electricity - or enough to power 50,000 standard energy-saving light bulbs."

 

Also, MoS2 is a graphite-like 2-dimensional material that seems to exist in pretty small particles. Usually a component in Moly-greases. So, find me a 1 square meter sheet of MoS2 (single-layer) and I will make you king.

Why would should I find this sheet of material in hopes that you will make ME king? I'm not the one claiming that a one meter square membrane is going to produce 1 MW of power -- THEY are.

 

From the article: "The potential of the new system is huge. According to their calculations, a 1m² membrane with 30% of its surface covered by nanopores should be able to produce 1MW of electricity - or enough to power 50,000 standard energy-saving light bulbs."

Yes, that is their theoretical scale-up calculations. Read the next paragraph...

Until now, researchers have worked on a membrane with a single nanopore, in order to understand precisely what was going on. '' From an engineering perspective, single nanopore system is ideal to further our fundamental understanding of membrane-based processes and provide useful information for industry-level commercialization'', said Jiandong Feng.​

To me, that means it has failed with an array of 10 or 100 pores and they don't want to report it. Why would you not do that as soon as you get done studying one pore with success? Instead, the author mentions how important studying a single pore is.