
‘Two-faced’ membrane can create electricity from salty and fresh water - flippyhead
http://www.sciencemag.org/news/2018/10/two-faced-membrane-can-create-electricity-nothing-salty-water
======
mchannon
There are a couple more things you need for this to make electricity:

•Wire. Whatever goop makes up this membrane is not going to be able to send
all that energy back to the desired destination. I'd expect you to have to
come with some kind of metallization and tabbing not unlike what you find on
solar cells.

•Electrical insulation. At least with solar cells the plus and minus sides are
pretty well insulated from each other. Wet goop on the other hand, not so
much, and with water occasionally flowing around the edges to shunt the power
produced.

•MPPT. I'd imagine this produces a highly variable low DC voltage, which is
guaranteed to not be even close to the highly stable high AC voltage you want
to provide, so you need these boppers. Luckily, PV has made them cheap.
Unluckily, they're not waterproof.

•Lots of pressurized freshwater, and lots of pressurized saline water.
Eventually, it'll all be saline, unless you have streams or rivers or other
naturally charged sources. When it's all saline, power output goes to zero.
When it's the dry season and the river dries up, power output goes to zero.
When sea levels go up a foot, well, you know.

In the end, the amount of available surface area, if we spent trillions on R&D
to make the perfect design of these, and resigned ourselves to killing all the
species at this halocline and making river mouths nonnavigable, would probably
be rated less than a couple conventional hydro plants. Efficiency is a red
herring, and many devices (thermoelectric generators in particular) become
more useful the less efficient they get.

~~~
warent
"Efficiency is a red herring, and many devices (thermoelectric generators in
particular) become more useful the less efficient they get."

I don't understand this part. How does a thermoelectric generator become more
useful with less efficiency?

~~~
mchannon
When getting down to the nuts and bolts of a Seebeck (thermoelectric)
junction, the efficiency is highest when the heat flow is minimized and the
temperature difference is minimized.

You get no power when those things are minimized, and in fact you want them
both as high as possible.

------
Jarmsy
*nothing but salty water... and fresh water. Pretty important distinction

~~~
kurthr
Yes, it's the fresh water that's rare, and becomes salty in the process so
it's sort of like stored solar energy from rain (mostly from the oceans)...
osmotic power.

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

The power density even at 40% efficiency is pretty high (300Wh/m3) so major
rivers (Mississippi 20,000m3/sec or 72Mm3/hr) would generate 20GW fully
exploited.

The problem has always been the growth of biofilms on the surfaces of the
membrane, which break them down and/or lower their efficiency.

~~~
chongli
20GW is basically nothing next to the environmental damage it would cause by
destroying the Mississippi delta in order to build this plant. 20GW gives you
480,000 MWh/d. The US as a whole is currently using over 13,000,000 MWh/d [1].
There is only one Mississippi! Not worth it, if you ask me.

Besides, it's not just biofilms that are the problem. With a river you're
going to have huge amounts of silt and very fine clay clogging up your
membrane. The maintenance costs for an installation like this would be
overwhelming.

[1]
[https://www.eia.gov/realtime_grid/#/summary/demand?end=20160...](https://www.eia.gov/realtime_grid/#/summary/demand?end=20160802&start=20160702)

~~~
Retric
There is far more than just one US river. Hudson for example is: 21,400 ft³/s,
Potomac is 10,810 ft³/s etc.

And the output is just mild salinity water, so you could probably extract form
1/2 the flow without causing that much environmental harm as long as you pipe
the output to an area with similar salinity.

~~~
chongli
It's not the salinity I'm worried about, it's the installation.

~~~
kurthr
Don't worry, it won't be installed and it's not practical. Also, the delta
will be submerged by rising oceans soon enough so that won't be a worry
either. :^(

------
_Microft
_If the membrane performs as well in “the wild,” the new membranes could be
used to power remote communities with no other sources of renewable energy in
just a few years, the researchers say._

I wonder what communities they have in mind where neither wind turbines nor
solar power are viable options but fresh and salt water are abundant.

~~~
Pxtl
Northern Canada? Lacks the transportatjon for the massive weight of the
concrete needed for wind, lacks the sunlight for solar. Has fresh water... But
question is how much of that water is flowing in winter. I assume this thing
fails at sub zero temperatures.

------
punnerud
Our try in Norway on a large osmosis plant:
[https://en.m.wikipedia.org/wiki/Statkraft_osmotic_power_prot...](https://en.m.wikipedia.org/wiki/Statkraft_osmotic_power_prototype_in_Hurum)

Crossing fingers that this could reignite the view on osmosis.

------
ph0rque
I wonder if this can be reversed to use electricity to make fresh water. Then
you can have a giant battery in the ocean that also produces fresh water as
needed!

~~~
perlgeek
The reverse process is called desalination, and there are a few approaches to
desalination that use membranes:
[https://en.wikipedia.org/wiki/Desalination](https://en.wikipedia.org/wiki/Desalination)

Typically, you don't use electricity directly, but pump in salt water with
high pressure, and some of the water is pushed through, while the salt
remains.

------
robg
Useful any where near a coast where fresh water costs less than electricity.

------
hannob
The idea isn't new. It's been tried before in Norway, unfortunately not very
successful:

[https://en.wikipedia.org/wiki/Statkraft_osmotic_power_protot...](https://en.wikipedia.org/wiki/Statkraft_osmotic_power_prototype_in_Hurum)

------
solarkraft
I am no expert, but I think I have seen such a thing about 10 years ago and
this article doesn't explain how it would be any different.

Also, here's why this thing, even with the technology completely perfected
(100% efficiency), would be absolutely terrible: You're turning valuable fresh
water into mostly unusable salt water.

------
danans
What,if any, are the side effects of generating electricity this way, other
than lowering the speed of the water going through the membrane?

Also, the article didn't mention the power density of the membrane, which
matters for making this work without covering river mouths.

------
markhahn
any differential can generate power - what am I missing here?

