
New membrane unlocks energy from chemical difference between fresh and saltwater - vo2maxer
https://www.sciencemag.org/news/2019/12/rivers-could-generate-thousands-nuclear-power-plants-worth-energy-thanks-new-blue
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aedron
So many jaded comments! This is incredibly cool. The 'breakthrough' (if you
can talk about that at such an early stage) is a few simple materials
engineering hacks:

 _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._

~~~
zozbot234
> So many jaded comments! This is incredibly cool.

I agree. You could use the process to power a desalination plant, and make
even _more_ freshwater!

~~~
marcosdumay
You need a membrane permeable to negative ions too for that.

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.

~~~
chongli
I think the GP was making a joke. What they suggested strongly implies the
existence of perpetual motion machines, contradicting the laws of
thermodynamics.

~~~
marcosdumay
Indeed he is.

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.

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aazaa
The article unfortunately does not use the more common term for the "blue
energy" principle: "osmotic power."

The Wikipedia article on osmotic power explains the physical principles
better:

[https://en.wikipedia.org/wiki/Osmotic_power](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.

~~~
asdfman123
Do you need an estuary? Can't you just divert river water, put it next to salt
water, and then you can produce power?

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.

~~~
aaronblohowiak
> those two things are easy to get where rivers flow to the sea.

Isn’t that an estuary?

~~~
asdfman123
"An estuary is a partially enclosed coastal body of brackish water with one or
more rivers or streams flowing into it, and with a free connection to the open
sea"

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.

~~~
riffraff
I think where "estuary" was used the correct term might have been "mouth",
i.e. a river gets to the sea through a delta or estuary mouth.

I am not confident enough with English to be sure though.

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aljungberg
One wonders how quickly these pores will become clogged over time. If the
membrane deteriorates fast, the energy cost of producing more membranes
continuously needs to be factored in.

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.

~~~
aedron
_the researchers estimated a single square meter of the membrane [...] could
[...] power more than 400 homes._

~~~
pjc50
.. or in real units, 30MWH/year = 3.425 kW. Which is quite a lot. That's what
I get from about 10 square meters of solar panels, or one electric kettle
running continuously. Seems implausibly high, I would move the decimal point
over by one and naively guess it has the same energy density as solar panels.

~~~
raisedbyninjas
That is really high. However I think you could run a few kettles off this. In
the US most tea kettles are 1-1.5kW, as are other high power appliances like
microwaves, hair dryers, vacuum cleaners etc. 15A circuit breakers are common
for general purpose home power outlets which max out at 1.6kW. I think the NEC
recommends 2 20A circuits to supply kitchen's non-dedicated power outlets.

~~~
laurencerowe
Kettles in the US are underpowered and slow. 3kW is pretty normal for a kettle
in Europe as everything runs at 230V.

~~~
bdamm
It isn't the voltage that improves the time, though, it's the power. So are
you saying that wiring in Europe uses the same or better circuit breakers
(e.g. 15A or 20A) at the higher voltage, thus providing more power? Or that
the wires in European homes are of equivalent gauge to wiring in US homes, but
due to the higher voltage, can therefore carry more power?

~~~
pjc50
Both in UK. 32A ring mains at 240V powering sockets up to 13A each.
[https://www.tlc-direct.co.uk/Book/6.3.3.htm](https://www.tlc-
direct.co.uk/Book/6.3.3.htm)

~~~
laurencerowe
Running multiple kitchen appliances at once without tripping a breaker is a
convenience I miss now living in the US.

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marcan_42
Where does the energy go when it is not being extracted with this method?

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?

~~~
marcosdumay
It converts thermal energy into usable electricity (that is, the water gets
cooler), at the cost of the increased entropy in the mixing of fresh and
saline water.

~~~
Accujack
Also, the environmental cost of keeping river estuaries blocked by impermeable
membranes so you can maintain the gradient of salt/fresh water.

~~~
sigmaprimus
Also why bother with a membrane, the fact is river water for the majority of
the year is either warmer or cooler than the typically constant ocean
temperature. Two bodies of water at different temperatures plus a heat pump
can be quite effective at producing energy, no?

~~~
semi-extrinsic
Unfortunately heat pumps are not very good at producing power from small heat
differences. In industrial plants, any waste heat stream that's below 100C is
considered "low grade" and mostly used for heating office buildings etc.

~~~
sigmaprimus
Ahh, I did not know that. My experience is through my HVAC course where it
only covered heating applications not power generation. Still the way a heat
pump in a HVAC system works you turn a small temperature difference into
larger difference through the "A" coil, essentially compressing the warm air
to make it hot, but I guess this does not scale well.

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driverdan
> If researchers can scale up the postage stamp–size membrane in an affordable
> fashion

That's a huge "if." This type of development happens all the time and usually
that "if" never comes true.

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vanderZwan
> _If researchers can scale up the postage stamp–size membrane in an
> affordable fashion_

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!

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8bitsrule
Following are original paper and phys.org article about (a similar project?)
at EPFL in 2016. They use a molybdenum disulfide membrane.

'Single-layer MoS2 nanopores as nanopower generators'

[https://www.nature.com/articles/nature18593](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...](https://phys.org/news/2016-07-electricity-salt-three-atoms-thick-
membrane.html)

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cagenut
at a meta level, these types of articles with a ridiculous headline and
without any meaningful pictures or diagrams are infuriating.

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?

~~~
rkangel
What I find infuriating is units like '30 megawatt hours per year'. Why not
just tell us how many megawatts?

~~~
dienciebsiwbsi
It's 3.4 kW, enough for two toasters. So not nearly enough for 400 homes;
their math is wrong.

~~~
yetihehe
But do people run their toasters continuously 24h/365 days a year? THAT's why
they used yearly average.

~~~
jandrese
No, but running 400 homes on that level of power only works if every home is
only intermittently using a single high efficiency lightbulb or something.

A few refrigerators or electric ranges or electric heat pumps will blow that
power budget right out the window.

------
martimarkov
Can anyone explain why the fresh water is needed? The article mentions that
the ions in water are not bound to one another so why can’t we just use salt
water entirely?

~~~
mnw21cam
There has to be a difference in the water on either side of the membrane. The
energy is being extracted from the entropy of the system. Currently, fresh
water flows into the ocean, where it mixes with salty water, and goes from a
highly ordered system (separate fresh and salty) to a disordered system.

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.

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kitsuac
It sounds like a non-renewable energy source which changes the natural flow of
the water permanently. Isn't there bound to be negative environmental effects?

~~~
aedron
I dunno, the article said that one square meter of the membrane could power
400 homes. And they believe the material can be made even more effective with
a proper manfucaturing process. That would not seem to disrupt waterflow. I
personally think this sounds incredibly promising.

~~~
orbifold
If you use a small membrane there will be basically no way that a sufficiently
large gradient between the two sides exist to produce meaningful amounts of
electricity. This article is total bullshit even if the material science is
nice.

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logfromblammo
Imagine how much power could be generated at the place where the Colorado
River empties into the Gulf of California!

\--

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.

~~~
TheSoftwareGuy
This is exactly what I was thinking. There are those who believe that the next
world wars will be fought over access to fresh water. This technology seems to
make that future even more likely

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bcatanzaro
Industrializing our most environmentally sensitive areas by controlling all
the water flow in wetlands... What could possibly go wrong?

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dwiel
I wonder if this could work in places with no ocean nearby just using rain
water and rain water with added salt. Would you need an obscene amount of new
salt/rain water?

~~~
asdfman123
One potential problem: you derive energy from converting freshwater to salt
water. What do you do with all that salt water? You could cook it in the sun
to evaporate out the water and start the process over again, but you've
basically made a solar energy plant.

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.

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Teknoman117
2.6 TW/h with 100% global resource utilization, which amounts to 1/60th of the
world's energy usage in 2013...

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alistproducer2
Can anyone provide a link to the paper? I've googled and haven't been able to
find anything.

