
Solar Powered De-Salination Is Being Explored by Several Countries - Osiris30
https://www.technologyreview.com/s/601419/to-make-fresh-water-without-warming-the-planet-countries-eye-solar-power/
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stcredzero
_" Desalination plants need to run 24 hours a day, requiring expensive battery
packs to supplement solar power when the sun’s not shining."_

Why? Electricity is hard to store cheaply. However, we've known how to store
water for millennia. Is there something about reverse osmosis that makes it
impossible to stop? Why not just turn off the pumps and let the pipes drain?

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Retric
Desalination infrastructure is expensive. If you get 54 million gallons from a
$1 billion plant, you need to spend twice as much if it only runs 12 hours a
day.

Storage costs are not free, but if you can save 1 billion then that buys a
_lot_ of battery's. It's ~15,000 kilowatt-hours of power for every million
gallons of fresh water that’s produced. If you assume 1/2 of your production
is at night that's 405Mega Watt Hours of storage which is less than 1 billion,
though at 2,000$ / kWh battery's not by all that much.

A better option is to just pump water up a hill to a lake, which could then
provide whatever pressure desalination needs at the bottom of a hill. But,
again that requires a tall hill. (~65 ATM * 33 feet = ~2,145 feet above sea
level.) Saudi Arabia actually does have a lot of land at that altitude or
above.

PS: This is also why you run them at night in the first place. We really don't
need a lot of water at night, but storing water is cheap.

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eloff
That seems to make a lot of sense, an we already know pumped hydro is one of
the most efficient "batteries" for renewables. But then the tradeoff is how
far do you have to pump the water from the ocean to get it up to a reservoir
above 2000 feet? You might lose all benefits (if there were any) of the
technique if you don't have a handy mountain nearby.

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jsmcgd
I assume this has been considered, but what is the reason for not using giant
solar stills? All countries that desalinate water have a coastline and have
hot climates so why isn't using solar energy directly in sea-based solar
stills economical?

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forkandwait
Hehe, i was writing my post when you submitted this.

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eloff
What I don't get is why they don't just run the desalinization process at
"full speed" when the sun is shining, and store the freshwater. Then you don't
need any batteries. Maybe the process is not amenable to scaling up and down
with a variable power supply. You could possibly use giant capacitors to
smooth the power supply, but maybe the economics still favor smaller
facilities that run 24/7, even though they need batteries.

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jeffdubin
The water can be the capacitor, or the battery for that matter, if you can get
gravity involved.

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aexaey
Interestingly enough, at the same time, several other countries are
experimenting with osmotic power generation [1], which is pretty much exact
opposite of electrically-powered desalination.

[1]
[https://en.wikipedia.org/wiki/Osmotic_power](https://en.wikipedia.org/wiki/Osmotic_power)

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dogma1138
Well it's only a suitable solution for a very few countries that do not have
water shortages. So not sure how interesting it is since those countries also
usually have good renewable sources such we wind, hydro and geothermal.

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eloff
Well I wonder if could be used near a river mouth? Since that water is going
to be "thrown away" into the ocean anyway.

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forkandwait
Why not make a big solar still thus: go to Saudi Arabia, dig pit near the
ocean, pile up the fill next to the pit, let solar evaporation carry
freshwater up to the top of the pile, store it on top so you have pressure?

There must be a reason why not make lots of these on the coast. If someone
knew how to run the numbers I would be very curious.

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ZeroGravitas
It's not made clear why they don't just add solar to the general grid and
power the plants from whatever the grid is providing. Presumably that would be
solar during the day, maybe some CSP after sunset, maybe some wind at night,
but if they really need to burn some oil during the night then it's not the
end of the world. Better than giving up because they can't figure out how to
run the machinary from one particular power source.

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nashashmi
Sometime ago, there was an article talking about symbiotic process of
desalination. Water would be drawn from the ocean, go into a data center for
data center cooling and go to a power plant for steam powered turbines and
then go to a water desalination plant because it is easy for steam to be
converted to water. Isn't that a better setup?

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ccallebs
Would a system where the liquid fresh water was stored at a significant
altitude be feasible? Via evaporation + condensation you could create a
battery during the day to use at night.

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mirekrusin
Isn't it going to create local ultra salty water?

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ck2
Curious how many gallons can be removed from oceans before it causes
significant impact.

I realize it would require a massive amount but given the insatiable need for
clean water, over a century it could add up?

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rconti
Currently the major concerns (as I understand them) are the death of marine
life near the intakes, and the consequences of discharging extremely salty
wastewater (again, into a relatively limited area).

If you could spread the wastewater across the entire ocean, probably not as
big an impact. Possibly none, as the desalinated water eventually makes its
way back to the ocean anyway.

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fredgrott
evaporating the waste water gives the side benefit of being able to mine both
salt and other minerals known to be in ocean water such as gold, etc.

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manicdee
What about reducing the demand for fresh water?

