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These types of systems are generally not competitive with PV for electricity, since they require a lot of concentrated solar energy thus limiting the options for deployment. However, they are great for generating heat. And with heat you can trivially desalinate water. Since trees can be a nice carbon sink, I've often wondered if this type of scheme might make sense:

- Plant heliostat molten salt reactors in desert coastal areas (the concentrated solar kind)

- Use generated heat to desalinate water (e.g. Multi-stage flash distillation)

- Use desalinated water to provide irrigation

- Plant (potentially GMO'ed) trees and other bootstrapping organisms to "green" the desert

- Rinse repeat.

The net effect is that you can potentially turn large swaths of desert into forests, with minimal external energy requirements (and potentially build an agricultural community around them, more or less self contained in terms of water/energy)

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> These types of systems are generally not competitive with PV for electricity, since they require a lot of concentrated solar energy thus limiting the options for deployment.

> However, they are great for generating heat. And with heat you can trivially desalinate water.

Not trying to be a PITA, but to your first point, suggest you provide a citation if you're going to make a sweeping claim. I'm actually interested in this claim.

To your second point, while desalination at scale isn't 'rocket science', it isn't exactly trivial. Having designed and worked on desalination plants for commercial ships, yes, you can buy an off the shelf evaporator. However, you still need to carefully design the heat supply system and pressure regulation.

Also, you have to carefully manage the highly corrosive brine discharge. When I say corrosive, I mean this stuff is NASTY. At sea, it's a little easier, because you are constantly moving can kust discharge it overboard (although the environmental impacts are becoming increasingly apparent, so I'm not sure that direct discharge option will last much longer). However, you still need thick/expensive corrosion resistant brine discharge line piping/valves/etc. (or you will be replacing pipe often). Ashore, you need to manage to transport the brine away and somehow dilute it to a point where it's non-toxic.

You also have to carefully manage scaling inside of the evaporator, and I'm telling you this scaling is not something you can just brush off. This stuff is like a mollusk she'll coating on all of your internally heat transfer surfaces. If you can remove it with a chisel and hammer and not damage your system, you're a better person than I.

I'm not saying it's not doable, there are land-based desalination plants all over the place. I'm just saying it's not as simple as running some heating coils in a tub of water and collecting the evaporated water.

Lazard's Levelized Cost of Energy Analysis for 2018 has utility scale PV at $36-$46/MWH vs solar thermal w/ storage at $98-$181. Haven't seen an estimate that attempts to add storage to the PV to make it more directly comparable to thermal + storage.

https://www.lazard.com/media/450784/lazards-levelized-cost-o...

This is a fascinating comment.

In a municipal environment, would standard remediation of sewer water be good enough to dilute the brine? It seems like you could get something close to a closed loop. Obviously some water would be lost to evaporation, but in a city a lot of water comes out of the tap, over dishes or people or laundry, and then right back down the drain. It's not drinkable anymore, but we've got lots of tools to make it reasonably cleanish enough to put in a river. Would using that gray water to dilute brine alleviate some of that pain?

I'm not remotely an expert in this area, just spitballing here. But you clearly have some experience. I'm curious if closing the loop might be enough to make such a system workable.

Just to clarify, the brine is just concentrated sea water, that's left over after evaporating as much pure water as possible?

It seems weird that that could damage ocean life in any substantial way. I imagine i gets diluted back to "regular strength" fairly quick. At least if we're talking about the open seas.

Hypothetical (impossibly efficient) desalinator for consideration, turns 101L seawater into 100L potable water and 1L brine effluent. If you diluted the brine with another 100L of seawater, the diluted effluent would have twice the salinity of natural seawater. Dilute that 1L of effluent with 1000L and it would still be 10% above background (3.8% salinity, rather than 3.5%), which I think would affect sea life in the discharge area. Would have to dilute it with 10,000 litres to get it back down to 1% of background level (3.53% instead of 3.5%). I'm sure some saltwater aquarium people would have comments here :)

More mixing and dilution will happen after discharge, which would likely make it not a big deal for moving brine source. But a land based desalinator would need a very detailed impact assessment to prove it wouldn't have an impact on marine life at the discharge site

Back in the '80s, Grant Callin wrote a couple of pretty decent hard science fiction novels. In one, the aliens had a magic quantum mechanical technology that enabled flying cars and such, but generated very large amounts of heat as a side effect. (A Lion on Tharthee, https://archive.org/details/lionontharthee0000call)

"What do you do with the heat?"

"We store it temporarily on the vehicles and then radiate it into the ocean."

"Doesn't that cause an ecological problem?"

"Hah! That's the joke! We could do this for hundreds of thousands of years before we raised the ocean temperature 0.1 degree."

My question: How do you distribute gigawatts of energy uniformly through the ocean? Or are you simply boiling a bay somewhere and calling it good?

> My question: How do you distribute gigawatts of energy uniformly through the ocean? Or are you simply boiling a bay somewhere and calling it good?

Assuming this is your question, and not a continuation of the quote: thermal energy is definitely a concern. It could be solved with dilution, like other pollutants.

In the particular case we're discussing, of using heat from a nuclear power plant to desalinate water: Every thermal power plant already creates waste thermal heat. They exhaust this heat either through evaporation (the stereotypical hyperboloid cooling tower of a nuclear power plant) or discharge into a body of water (thermal plants around me do this). The latter is definitely a sticking point in gaining modern environmental approval.

If you disperse it into an ocean current, you’d have much of the mixing done for you.

(How that would affect the ocean current, given that they’re thermally driven, is less clear and unlikely to be good.)

Why in the world would you concentrate seawater to 100:1, and then dilute it, instead of just reducing it to 2:1? If it is a matter of heat efficiency, surely a long heat exchanger can recover it all on the way back to the sea, absorbing it into the incoming water.

It was a hypothetical to illustrate how much dilution is needed to get effleunt close to background level salinity. It isn't even possible to extract that much fresh, since seawater is 3.5% salt. I was not talking about heat at all.

To illustrate: the effluent dilution requirement in your case isn't actually much better. If you took in 100L of seawater, extracted 50L fresh, that leaves you with 50L brine at twice the background concentration. To get that effluent back to 1% above background, you have to dilute that 50L brine with 4950L of seawater.

The infrastructure to get the salt water to the desert and then to distribute the fresh water to the trees will also be a challenge.

There are other challenges that you might not have considered, like sand dunes ruining your progress.

China has been working to turn some of it deserts into forests with pretty good success.

For the sand dunes issue, they’ve come up with a webbed system that’s fairly easy to deploy. The material is cheap and it’s easy for unskilled labor to deploy.

Putting aside the horrible human rights issues with China, it’s amazing to see what they’re doing with infrastructure.

Lots of videos on the desert projects, just search for “china desert to forest”.

Here’s one of them https://youtu.be/7Nur35fnjDA

In the US Southwest there is the Salton Sea which is an already artificial saline reservoir. Just pipe more ocean into it and use that as the salt water source.

> The infrastructure to get the salt water to the desert and then to distribute the fresh water to the trees will also be a challenge.

The proposal was for "desert coastal areas".

Doesn't matter. Has been done the other way around already, so it could be reversed.

[1] https://en.wikipedia.org/wiki/Great_Man-Made_River

> Putting aside the horrible human rights issues with China, it’s amazing to see what they’re doing with infrastructure.

Mussolini got the trains running on time, and China has hundreds of thousands of Uyghur and Muslim people locked away in concentration camps.

Do you consider what side of history you're on when you "put aside" such behavior?

1) This is not a constructive comment. Author did not minimize PRC's conduct, or justify it. Rather he negated it from the equation. If PRC's de-desertification strategies were reliant on the prison camps, or in any way contingent, you might have a point.

2) There is no right side of history; there is only what gets remembered and how that memory is portrayed. Consider that America is quite accurately describes as a White Nationalist government for the duration of WWII. One could then frame the war as Nazis and Imperialists being defeated by Communards and White Nationalists with the assistance of Colonizers.

The role of the US in WWII is always grossly exaggerated.

WWII was a slugfest between two totalitarian regimes, Germany and USSR, conducted using troops that were, in the majority, neither Russian nor German, but slaves of one and then the other, and then, typically, executed.

The Western front was a sideshow. Stalin could easily have rolled over all of Europe, but stopped short for reasons known only to him.

The US accounted for significant portions of the USSRs vehicle fleet as well as provisioning. The USSR fought Germany, but the US defeated Germany. It wasn't fair; war never is.

The US sent 400,000 trucks, 350,000 tons of explosives, 3m tons of petroleum, 4.5m tons of food, 3000 ship engines, and 2000 locomotives in through Iran, and corresponding amounts of other supplies. But Russia had already pushed Germany much of the way out by the time the stuff arrived in any volume.

For all that, it added only 20% to their own production, food possibly excepted. Russia made a point of acknowledging the food, and the trucks were maintained for long afterward.

There's the old saying: WW2 was won with British intelligence, American steel, and Russian blood.

To be fair, the Soviets are directly responsible for arming the Third Reich. They created the monster and then they had to put it down. People always forget the cross border training and massive amount of raw and finished goods transfered from Russia to Germany.

That is said, but without both British intelligence and American steel, the result would be largely the same. And the blood wasn't mostly Russian, although it was sent into combat by Russians.

Maybe the British intelligence and American steel bought western Europe. Or maybe the success of the Manhattan project -- which Stalin was kept briefed on, in detail, by his spies -- gave him pause.

I should add that a very, very large fraction of the Soviet blood that was spilled was done by Russians. Stalin killed maybe 10-15 million before formal hostilities began, including every educated person in the east half of Poland, before 1941. (The Germans killed them in "their" half at the same time.) A large fraction of that number weren't Russian, but maybe more than half were.

You could say that killing 15 million people seems pretty hostile, but that's not how historians count. Anyway the US and Britain weren't evidently bothered by it.

Mussolini did not get the trains running on time. Rather, he had the people complaining about them not running on time shot. There's a difference.

Perhaps it's subtle.

Tangent - is there a systems reason (aside from the "base ecological ethics" of wanting to preserve biodiversity and not eradicate any unique ecosystems) why humans would not want to get rid of deserts?

Obviously having more arable or at least livable land is useful to human society, but I generally assume that messing with any large scale climates or biomes has adverse effects. I'm not yet aware of what that would be for deserts though. I suppose they have fairly high albedo, unlike most land formations that can exist in lower latitudes.

(To be clear my own views aren't necessarily so utilitarian and I like deserts. Just curious about a rational geoengineering civilization might do.)

Some deserts are quite natural. It doesn't matter what you do to them short of knocking down the mountains upwind of them, they're going to be deserts.

Some are not.

I consult my mental image of deserts and I observe that the Sahara is almost devoid of life, whereas the deserts of the American Southwest are rich and diverse despite their lack of substantial rainfall. (The latter I've also visited and spent some time in. I've never been to the Sahara to know if that's correct.) If my mental image is accurate, fixing the unnatural, almost entirely uninhabited desert would be nothing but a win. "Fixing" the natural kind would be relatively destructive of an existing ecosystem.

(Then you get into the question of "how do you valuate and compare two different ecosystems?". I've thrown that gauntlet a couple of times and I don't think I've even seen anyone engage with that question properly in a non-knee-jerk way. The heuristics people have been taught to use about environmentalism do not admit of that question even existing.)

Also note that some deserts are variable, and human activity can tip the balance [1].

especially considering the global carbon balance, it seems that tipping some parts of the desert back to forest would be a net plus. Humans are already doing massive uncontrolled experiments with bad results, it may help some to do some deliberate experiments with good foundations with the goal of creating good results?

[1] https://www.smithsonianmag.com/science-nature/what-really-tu...

>Tangent - is there a systems reason (aside from the "base ecological ethics" of wanting to preserve biodiversity and not eradicate any unique ecosystems) why humans would not want to get rid of deserts?

Getting rid of deserts could actually be quite bad for other ecosystems. The Amazon is fertilized by the Sahara for example https://www.nasa.gov/content/goddard/nasa-satellite-reveals-...

That's incredible. I assumed arrogantly it was mostly inert silica, not phosphorous.

Yeah the earth is a very complex system. When YC came out and proposed the idea of proposed turning the Sahara into a bunch of algae ponds [1] I cringed. First the plan requires more power than we currently produce just to pump all the water but then you would drastically reduce how much gets blown over to the Amazon AND you would suddenly introduce a tremendous amount of moisture to the atmosphere which would almost certainly change global weather patterns.

All of that moisture could cause a considerable increase in rainfall for somewhere (presumably the Amazon, I don't know anything about weather modelling though and am just guessing) which could be catastrophic for life there and even result in tremendous economic losses.

Even relatively small scale geoengineering like damns can have profound negative impacts on ecosystems with some being very predictable and some being completely wildcard. We know the Sahara fertilizes the Amazon but what level does it effect marine life by feeding stuff at the bottom of the food chain (plankton for example) and what would removing that mineral/nutrient source do to the food chain, would it increase rains (and cloud cover) over that area of ocean that might change temperatures subtly enough to make an impact on currents, etc. We're at the point now as a species where we have the power of gods but can't begin to understand the implications of what we are capable of doing.

[1] http://carbon.ycombinator.com/desert-flooding/

We know the Sahara was green, not too long ago. What do we know about the Amazonas from that time? Was it barren because it lacked the input? Was it agricultural land, worked by long overgrown cities by some sunken civilization? I've read about that somewhere, where LIDAR found signs of that, similar to what the Aztecs had.

I've read multiple times that the dust from the sahara makes it over the atlantic and works as fertilizer for the amazonian rain forests. That is relatively new knowledge, since about a decade or so. Has been featured in nature documentations also. What is not known is how much dependence there is on this input. Whereas i'm asking myself what about the other sand deserts then, like the Namib, or Gobi?

If we wait long enough, the planet is going to get rid of the desert.

> For several hundred thousand years, the Sahara has alternated between desert and savanna grassland in a 20,000 year cycle[8] caused by the precession of the Earth's axis as it rotates around the Sun, which changes the location of the North African Monsoon. The area is next expected to become green in about 15,000 years (17,000 AD).

Humans have been creating deserts for thousands of years, by denuding forests. Grow trees there again, and the forests will bring their own water, after a time.

Petra used to be forest. So did much of the North American southwest.

If you're displacing groundwater based desert agriculture, that's one thing, but it's worth keeping in mind that in general, desert != wasteland. Deserts are fragile ecosystems with many rare and specialized plants and animals.

Absolutely, but then again (almost) everything is a fragile ecosystem. And if man-made climate change is real (I'm inclined to believe that), then it's less of an absolute and more of a trade-off ... a tricky one certainly, but already we're seeing deserts expand (https://en.wikipedia.org/wiki/Desertification) and there are efforts to prevent it (https://en.wikipedia.org/wiki/Great_Green_Wall) so it could potentially be another tool.

For sure, I just don't think we should gloss over the nature of such ecosystems, since many people really don't know.

Deserts are fragile ecosystems with many rare and specialized plants and animals.

Climate change may push the temperature of some of those deserts too high, wiping out the specialized plants and animals that live there. Perhaps we could use a solar installation to provide shade in areas that are getting too hot.

I'm well aware, just wanted to point out a fact about an underappreciated ecosystem. I feel the same way when people complain about swamps.

Hmmm we have multiple CSP plants in southern Spain, I wonder if we could do that, because it has been predicted that Spain will desertify with coming climate changes.

My issue is what do we do with the byproducts of desalination? If I did understand it there's no really use for that, and I've seen that other countries just dump it into the water, which is a big no no for local ecosystems (and I guess for oceans ph levels if this becomes widespread).

Straight up dumping it in the ocean is generally considered bad. The standard trick is to wash/mix the brine with fresh ocean water to reduce the concentration of salt before dumping it back. Another interesting byproduct that is often overlooked are the metals and minerals. Things like lithium (and even gold) are relatively abundant in sea water and could be recovered from the brine (at the cost of complexity of course).

That looks like a large scale operation. CSP is struggling to compete in Spain with PV, but I can see it happening with companies like Acciona or some other spanish utility behemoths.

Gold is not abundant in seawater: ~10 parts per trillion by mass.

Coastal areas (even desert ones) are often plagued by clouds, and clouds are a big problem for concentrated solar. This doesn't make your idea bad but it's an obstacle.

Edit: Now that I think more about it, who cares if the heat is less reliable if all you're doing is distilling water? I need to take off my electric generation hat sometimes.

Yeah it does limit it a bit, and if the goal is really to plant trees, then eventually it will be somewhat self defeating as forests cool the air and will generate clouds. On the other hand, desalination doesn't require extremely high temperatures, so it also kinda depends on the heat capacity of the molten salt. It probably also doesn't really have to be 24/7/365. But that's I guess a numbers game, so obviously it has to be modeled first.

You should check first that lowering the desert's albedo with trees is not worse than keeping the CO2 in the air.

Pretty soon we won't even need all these parts. If Metal-Organic Frameworks (MOF) take off like these reports describe, we'll be able to irrigate deserts passively.

https://www.youtube.com/watch?v=XPSYzLZ7xKU

https://www.sciencemag.org/news/2019/09/crystalline-nets-har...

Something like this? https://www.sundropfarms.com/facilities/

Yep! but then with fewer greenhouses so it has more of a https://en.wikipedia.org/wiki/Desert_greening effect

This would only lead to a constant shift in total CO2 emissions because there is only so much space you can utilize. When a plant dies it releases the CO2 back to the atmosphere. For the money, you can likely get higher CO2 abatement (i.e. close a coal fired power plant and replace it with renewables subsidy).

In the first case, you save the CO2 roughly equivalent to the total weight of the plants. In the pther case, you save x tonnes of CO2 every year.

True, but planting trees as a method for carbon sequestering is not a novel idea, and currently seems one of the better short-term options. Long term it's not that great for the reason you mentioned, and there is quite a lot of debate about it going on right now.

https://science.sciencemag.org/content/365/6448/76

https://www.nature.com/articles/d41586-019-00122-z

for two more or less competing views.

These types of systems are generally not competitive with PV for electricity

You should tell that to the people who built at least a dozen of these plants in the American west, with more to come.

Right now there are at least four that I've seen in Nevada exporting electricity to California.

There is a good map on the wiki of Concentrated Solar [1] showing the Direct Normal Irradiation. The US West coast is in somewhat of a unique position (as is Australia, South Africa, and a couple of others I suppose): high density population centers in areas with a lot of solar radiation. In those areas it might very well be the best method available (dunno, could be). Transporting electricity at distance is non-trivial however. Plans to put these installations in the Sahara and transport it to Europe via high-voltage DC lines have pretty much gone nowhere (https://en.wikipedia.org/wiki/Desertec). So non-competitive was more a reference to the constraints that it imposes, but I could've been clearer for sure.

[1]: https://upload.wikimedia.org/wikipedia/commons/c/c7/Global_M...

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