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

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.

[1] https://www.eia.gov/realtime_grid/#/summary/demand?end=20160...

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.

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

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. :^(

Interesting ... curious what the replacement cost of these membranes are as compared to installation and ongoing maintenance of wind turbines.

Also, as always, curious how having these membranes in the water would affect the local ecosystem.

Have robots constantly cleaning surfaces. Then humans clean the robots.

Then use the salty water to clean the humans. The circle of life!

Amazing new steam engine creates power from nothing but water![1]

Still a pretty cool thing if they can just install these in a river mouth and get free electricity out of 'em.

[1] And an external heat source such as a coal fire.

Maybe even a sea/river buffer dam. Hydro + Osmotic

Ok, we'll add fresh water to the title above.

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).

This reaction is also commonly demonstrated in handheld heat pouches and military MRE heating packs.

No that’s different. That is caused by superheating the salt/water mixture.