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.
Instead of using brackish regions like river deltas, wouldn’t it make more sense to setup clusters of floating “lily pad” style DC power plants in tropical areas where the coast experiences regular heavy rainfall? The fresh water would sit atop the surface membrane due to its’ lower density and slowly filter through to the ocean surface. DC could be converted to AC onboard floating stations and transmitted to coastal regions using ocean floor cables.
Let's do the math. Rainiest spot on Earth sea level, say 1 inch freshwater per day, every day, and somehow consistent all year.
One square meter, 100% efficient, looks like it would produce less than 1 kW with a river feeding it. With just rain, figure on well under 100 watts. Probably under 10 watts.
Compare with a floating PV array, also 1 square meter, but real-world 20% efficient cells. In the same rainy rainy spot. Probably much higher than 10 watts, even factoring in nights and overcast skies.
Maybe you could combine them, but I bet you'd get better bang for your buck by just doubling the floating PV array size. PV doesn't need a freshwater (or saline) source either.
If you care about consistent power then you'll need a battery (or grid connection) either way.
I would think that you could combine the two. The solar collectors could also funnel rain water to a column of this membrane that is located within the salt water.
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.
As I understand they mean that a thermoelectric generator is much more useful in small scales of size and weight, where it can't operate very efficiently.
It's primary application being for spacecraft since it can have a decent power output for a very long time, which is what's typically required from a long standing mission.
Another non-obvious (to me) thing it needs is the ability to drain the brackish water you’re creating. At scale this probably involves at least a long canal or pipe, and makes the setup more difficult that “let’s just put this were the river meets the sea”
This had me thinking, any thermodyanmic (i.e. statistical) potential can be used to generate work, right? There should be all sorts of natural processes that can be hitched to to extract energy.
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.
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.
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.
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.
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. :^(
Indeed, I assume it has something to do with the second law of thermodynamics. Just like you can't make electricity out of heat alone but need a temperature difference, here you need different concentrations of salt.
If you want to read about some interesting stuff, you might wanna research "flux coupling". That is when one flux is used to pump another against a gradient, i.e. in the opposite direction as it would normally flow. That's pretty common in biological systems but there are physical cases as well when e.g. thermal and electric fluxes are coupled (Seebeck-/Peltier-effect).
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.
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.
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!
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.
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.
•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.