New membrane unlocks energy from chemical difference between fresh and saltwater
sciencemag.org
sciencemag.org
The nanotubes were easy. Cetindag says the lab just buys them from a chemical supply company. The scientists then add these to a polymer precursor that’s spread into a 6.5-micrometer-thick film. To orient the randomly aligned tubes, the researchers wanted to use a magnetic field. The problem: BNNTs aren’t magnetic.
So Cetindag painted the negatively charged tubes with a positively charged coating; the molecules that made it up were too large to fit inside the BNNTs and thus left their channels open. Cetindag then added negatively charged magnetic iron oxide particles to the mix, which affixed to the positively charged coatings.
When the researchers applied a magnetic field, they could maneuver the tubes so that most aligned across the polymer film. They then applied ultraviolet light to cure the polymer, locking everything in place. Finally, the team used a plasma beam to etch away some of the material on the top and bottom surfaces of the membrane, ensuring the tubes were open to either side. The final membrane contained some 10 million BNNTs per cubic centimeter.
When the researchers placed their membrane in a small vessel separating salt- and freshwater, it produced 8000 times more power per area than the previous French team’s BNNT experiment.
For an idea of the scale of this water based power reactor that would produce this energy, I compare to the amount of space required for a large surface area reactor such as a CO2 chemical scrubber. These reactors have effective surface area of about 500sq meters of surface area per 1 meter3 of volume. This implies that you are looking at a reactor that has about 600 meters3 of volume. This is smaller than a typical olympic sized swimming pool, which is about 1700cm3.
I don't know if these comparisons are fair, but it implies that there are some questions about heat removal and how it might be as dense of a source as claimed.
> it produced four times more power per area than the previous French team’s
>*Correction, 6 December, 11:30 a.m.: This article has been corrected to accurately reflect how many homes a blue membrane could power and how much energy per area it produces.
I agree. You could use the process to power a desalination plant, and make even more freshwater!
In fact, I think you'll need that membrane for electricity generation too. Otherwise you will only get to make a single layer of it, what is underwhelming.
But it is a large breakthrough.
I'll just post here that it does not violate the second law of thermodynamics. It is a completely real phenomenon that one can verify at home with the ion exchange membranes used to make batteries. (Get some aluminum electrodes, on the scheme salty-fresh-salty water, and add some aluminum hydroxide on the negative side, so you'll have a neutral oxidation and reduction of aluminum on both sides.)
What those people created was just an incredibly better ion exchange membrane.
The Wikipedia article on osmotic power explains the physical principles better:
https://en.wikipedia.org/wiki/Osmotic_power
The Science article didn't explain very well why an estuary is required. The idea is apparently to have two bodies of water - with differing salinities - in close proximity. This happens near estuaries. The ocean provides water with high salt content. The outbound river provides water with low salt content. Those two sources of water in close proximity can either be used where they are or pumped into even closer proximity.
Place a membrane (like the carbon boron nanotube devices discussed in the article) between the two pools of water with different salt content. They may be housed inside a power plant or outside. Then capture the energy released from the movement of ions through the membrane. There appear to be different approaches for that last part.
In other words, the saltwater and freshwater sides of an estuary provide the two charge compartments of a very large battery.
The breakthrough here is a way to manufacture the high-performance membranes needed provide a path for the ions through this system.
In fact, the only place "estuary" is used in the article is in the image caption, and I assume they used it because it's tangentially related and it's a pretty picture. Really you just need a pool of salt water and a pool of freshwater, and those two things are easy to get where rivers flow to the sea.
Isn’t that an estuary?
I think of an estuary as that partially enclosed area, which is neither the river or the sea. But really all you need are those two things: the freshwater river, and the saltwater sea.
I am not confident enough with English to be sure though.
The article headline is pretty weird, by the way. So this technology can generate “thousands of nuclear power plants worth of energy”? But then in the third paragraph it sounds like they mean to say if you installed this membrane in every estuary in the world, then it’d beat 2000 nuclear plants.
It’s like saying there’s more water in a cup than in a bathtub, as long as the cup is the size of a car.
*Correction, 6 December, 11:30 a.m.: This article has been corrected to accurately reflect how many homes a blue membrane could power and how much energy per area it produces.
I’m not saying this isn’t a cool membrane. I’m making the point here that the comparison to 2000 nuclear power plants seems arbitrary.
Then you can get bigger stuff for your garage etc, you have 230V three phase in either 16A or 32A (blue connectors, physically incompatible across amperages) as well as 400V three phase in 16A and 32A (red connectors, again foolproof), i.e. 22 kW max power, generally only available if you have a new-ish house and you're running some sort of industrial equipment.
I understand that the energy is extracted from the movement of ions between reservoirs of different salinity. If you're just letting the two sides mix, as they do naturally, where does the energy go instead? Is it released as heat?
That's a huge "if." This type of development happens all the time and usually that "if" never comes true.
Not to be a downer, but this feels like it is in the "possibly long-term solution" department. Which is definitely important to investigate, don't get me wrong!
'Single-layer MoS2 nanopores as nanopower generators'
https://www.nature.com/articles/nature18593
'Electricity generated with water, salt and a three-atoms-thick membrane'
https://phys.org/news/2016-07-electricity-salt-three-atoms-t...
w/r/t the actual scientific discovery, i'm fascinated by how much of solving the overall energy/carbon problem is coming down to 'nano' scale material science and engineering. does anyone know of a better media venue for materials science developments that isn't just some loud guy reading wiki articles to a webcam?
A few refrigerators or electric ranges or electric heat pumps will blow that power budget right out the window.
For comparison, the sun delivers about 1 KW per square meter (at Noon, in Arizona).
Fig 11: https://www.newport.com/t/introduction-to-solar-radiation
What I do know is that the 400 home figure is way, way off. The average home uses 10 MWh/yr, or 1 kW [1]. This system is enough to power about three homes per square meter. Unless I am missing something, their math is totally wrong.
I think it's probably just the usual case of the scientists' actual claim getting garbled by pop-sci writing.
We generate energy from heat the same way. It is no good to just have heat. To generate electricity or motion, you also need cold. It is the transfer of heat from hot to cold, making the hot cooler and the cold warmer, that allows us to make use of it.
Practically speaking, this membrane lets positive ions through, but not negative ones. So, you put fresh water on one side, which has few ions, and salty water on the other side, which has plenty of both. Then, the positive ions flow through, and you end up with water full of positive ions on one side and water full of negative ions on the other side. That is electricity that can be directly tapped.
I mean, you could say solar is non-renewable because the sun die one day. After all, there is a finite supply of solar energy.
Of course, solar is nuclear too. So we're actually 100% nuclear powered.
The sea will remain salt and as long as rivers will flow the ion potential will exist along the coastline. Are you referring to other parts of the method?
When you think about it this way, all of our energy sources are just repackaged solar except for nuclear.
And nuclear is just repackaged supernovae!
The article was updated to correct this number, it was actually 3 instead of 400.
All that said, it's certainly miles above either coal or natural gas.
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If you didn't understand the implication, I think that in the future we will need the fresh water more than we will need to get power from making it more salty. The Colorado River has only rarely reached the ocean since 1960. It's like the Aral Sea. Human activity uses the entire river.
The cool thing about using membranes near rivers and the ocean is it uses the ocean as your solar energy plant.
You could also place this device near natural dead seas to produce power... but that's basically what the article is proposing.
Maybe it could be used at the Great Salt Lake, but otherwise the options are limited.