Why is desalination so difficult? • Hacker News Board

Re: Why is desalination so difficult?

#31

Post by cyberax » Wed, Jul 05, 2023, 5:39 PM UTC

Animats wrote: post #3

It's not difficult. It's just energetically uphill. Undoing entropy costs energy. Second Law of Thermodynamics.

The thing is, it doesn't have to be. The energy released by mixing salt into water is small, around 3.9kJ/mol.

Molar mass of salt is 58g/mol, and the average sea water salinity is around 3.6%

So a cubic meter of sea water will have 1000*0.036/0.058=620 moles of salt, and it'll require 2.4MJ of energy to remove the salt in a perfect desalinator.

In more common units, 2.4MJ is about 0.75 kWh. Around here electricity is ~10 cents per kWh, so the absolutely lowest price of one cubic meter of desalinated water would be around 8 cents.

Re: Why is desalination so difficult?

#32

Post by samstave » Wed, Jul 05, 2023, 5:40 PM UTC

josephjrobison wrote: post #24

zokier wrote: post #9

Probably very dumb idea, but would it be feasible to just pump and sprinkle sea-water on hot, dry desert and let it naturally evaporate, and then collect it as fresh rainwater back? i.e. how much water would you need to evaporate to have a noticeable increase in rainfall?

There is talk about pumping sea water into the Saharan desert in certain parts, which gets at that. https://www.youtube.com/watch?v=V2b7ztWvFOg

Was going to say this, there are/have been many ideas to do just this - one was to build a huge canal and just then let the sea flood the area...

Also - there are lots of un-earthed treasures to be found under the saharan which was once a lush environ and have been covered with sand - so prior to flooding it, we need to lidar and excavate it.

What if we vaccuumed up all the sand and built a new island/continent with the material and just revealed everything underneath - then flooded it. (I believe UAE is in the market for more sand-built-land-masses)

Re: Why is desalination so difficult?

#33

Post by djent » Wed, Jul 05, 2023, 5:41 PM UTC

SillyUsername wrote: post #30

Stupid question time, why not expose the sea water over a larger area and expose to sunshine? There's plenty of room in the UAE for this, somebody is even building a wall that could hold the water which could be dual purpose and act as cooling via evaporation...

Larger area than the oceans?

Re: Why is desalination so difficult?

#34

Post by nerbert » Wed, Jul 05, 2023, 5:42 PM UTC

fwlr wrote: post #29

Relatively high energy cost since you’re undoing an endothermic reaction, you need to do a lot of it since we use water in large quantities… but most of all, the planet naturally does a lot of desalination for us already through various geological processes, so our “price point” for desalination is $0 per liter (infrastructure to capture rain, dam rivers, or tap groundwater isn’t literally free, but it’s pretty close…

In 2022, 85% of the country's drinkable water was produced through desalination of saltwater and brackish water. If there is a real need, and a will to address it, we have everything we need to to it. https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in...

Re: Why is desalination so difficult?

#35

Post by luxuryballs » Wed, Jul 05, 2023, 5:42 PM UTC

Can I shower in salt water?

Then all I need to do is desalinate drinking water.

“Distributed” home desalination for drinking water seems like the best approach in my mind, then people can pay as much or as little they need, but I have no real data to back this up.

Re: Why is desalination so difficult?

#36

Post by philote » Wed, Jul 05, 2023, 5:42 PM UTC

jeffbee wrote: post #20

MarkusWandel wrote: post #12

Some years ago I toured a maple syrup operation that has the opposite goal: Concentrate the dissolved stuff in the water. Their first stage was reverse osmosis, but only to a point. Second stage is boil, but with aggressive heat recovery from the steam to preheat the incoming liquid. All this to keep the energy cost under control.

In an operation like that, do they heat the syrup under low pressure to reduce the boiling point of water?

I'd guess that if anything they'd increase the pressure to raise the boiling point. That way things dissolve in it faster and the water doesn't evaporate away.

Re: Why is desalination so difficult?

#37

Post by Terr_ » Wed, Jul 05, 2023, 5:42 PM UTC

zokier wrote: post #9

Probably very dumb idea, but would it be feasible to just pump and sprinkle sea-water on hot, dry desert and let it naturally evaporate, and then collect it as fresh rainwater back? i.e. how much water would you need to evaporate to have a noticeable increase in rainfall?

My guess is that no matter how much water you manage to evaporate, the scheme will be doomed by the original causes for why the area is a "hot dry desert", the wind and geography patterns.

So even if you evaporate a lot of water, it won't fall where you need it or where you can collect it.

Re: Why is desalination so difficult?

#38

Post by narag » Wed, Jul 05, 2023, 5:43 PM UTC

Tell that to israelis: https://www.israel21c.org/how-israel-used-innovation-to-beat...

Re: Why is desalination so difficult?

#39

Post by samstave » Wed, Jul 05, 2023, 5:43 PM UTC

zokier wrote: post #9

Probably very dumb idea, but would it be feasible to just pump and sprinkle sea-water on hot, dry desert and let it naturally evaporate, and then collect it as fresh rainwater back? i.e. how much water would you need to evaporate to have a noticeable increase in rainfall?

You know this is how the Phoenecians became the dominant culture, its also where the term "salary" comes from and "worth his weight in salt" -- as salt was the only known preservative of the massive amounts of Tuna the phoenecians were catching and shipping throughout the mediterrainian - and made them a super-power - they had control of the preservance of food over shipping distances...

Salt was used as money.

EDIT: Only a fn idiot without knowledge of history would downvote a comment... Jimminy Carter, what type of stupid are you trying to promote?

Re: Why is desalination so difficult?

#40

Post by tylerag » Wed, Jul 05, 2023, 5:44 PM UTC

philote wrote: post #36

jeffbee wrote: post #20

Earlier quoted context omitted.

In an operation like that, do they heat the syrup under low pressure to reduce the boiling point of water?

I'd guess that if anything they'd increase the pressure to raise the boiling point. That way things dissolve in it faster and the water doesn't evaporate away.

They're concentrating maple syrup. It comes out of the tree with the sugar and flavors already dissolved in it.