Solar Powered De-Salination Is Being Explored by Several Countries | Hacker News Reader

Solar Powered De-Salination Is Being Explored by Several Countries

technologyreview.com

34 pointsOsiris3025 comments

"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?

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.

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.

I thought that maybe the volume is too large, but 25 million gallons (half of that plant in California) is just a cube with about 50 meters side length, so that seems totally doable. You still need electricity to pump the stored water around, but that should be a lot easier than pumping it through the reverse osmosis membranes.

Or you immediately pump it up into a water tower/elevated tank so that when electricity is no longer available, it can still have pressure.

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?

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

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.

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

Also, solar cells are not as efficient as direct sunlight evaporating the water to be condensed later.

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

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.

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

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.

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.

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?

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.

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

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?

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.

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.

Freshwater makes up a very small fraction of all water on the planet. While nearly 70 percent of the world is covered by water, only 2.5 percent of it is fresh. The rest is saline and ocean-based. Even then, just 1 percent of our freshwater is easily accessible, with much of it trapped in glaciers and snowfields. In essence, only 0.007 percent of the planet's water is available to fuel and feed its 6.8 billion people. http://environment.nationalgeographic.com/environment/freshw...

I don't think we need to stress about taking water from the ocean.

The water is not destroyed, it ends up back in the ocean.

The major concerns are the raised salinity and reduced oxygen content of the effluent. Sea life basically dies around the egress pipes for desalination plants.

What about reducing the demand for fresh water?

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