Passive Solar Water Desalination
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In the last 15 years large scale desalination has become practical. It's one of the great engineering feats of our time.
We can now generate it at scale for less than $.01 per 6.4 gallons. This includes the cost of the electricity, which is about half the expense.
San Francisco charges $.02/gallon to residents, 12 times that price. It's now cost effective for any coastal city in America to supply residential water this way.
Lots of people still think desalination is still impractical, because it was 20 years ago. They just haven't learned about the new tech.
The tech:
https://www.energymonitor.ai/tech/can-desalination-save-a-dr...
Residential rates:
https://sfpuc.org/accounts-services/water-power-sewer-rates/...
"Refuse to go to war with us? We'll... remove you from our cafeteria menu! Take that!"
"... what do you mean they're actually Belgian?"
It's a commentary on all the unique and interesting ways Americans appear to measure things.
Sometimes people will take it further by, for example, converting a length in meters into football fields or bald eagle wingspans.
https://www.nist.gov/system/files/documents/pml/wmd/pubs/201...
Can’t leave out our fellow beacons of freedom!
If you tell the average (US) person 1000 feet, they won't be able to envision it. But if you say "a little more than three football fields", they can visualize that.
The point of strange units of measure is just to make a quantity relatable.
Those that use the metric system. Those that landed a man on the Moon.
The country is a metric country (see: Official definition of the US foot).
Its citizens, however, largely still roll coal measured by chains to the hogshead.
I try to learn but the only thing I think I have retained is that if the number has a negative 6 its most likely a small number.
> My problem with the metric scale is that when values are expressed in scientific notion they lose all reference to me.
As if the two are connected. But my point is that scientific notation is not part of the metric system at all. If you problem is with scientific notation, that is a problem with scientific notation, not metric.
If anything, with metric units most people outside of the sciences will use the metric prefixes and never ever use scientific notation at all.
As said above, it has nothing to do with the metric system anyway, it's just about dealing conveniently with very big or very tiny value (1⋅10^-9 meter is 3.9⋅10^-8 inch, no matter the unit you're using you'd be using scientific notation to express things that small instead of dealing with 8 or 9 zeros).
I think my particular problem is when dealing with electricity and physics.
I never once blamed the metric system and identified the problem as my in ability to comprehend values in scientific notation. I appreciate I shouldn't be so dumb and try to read about things I don't understand. It's a life long struggle.
You may be getting confused over prefixes vs base units because some pairs of prefixes and base units are so common they sort-of end up treated as a unit in themselves.
E.g. "kilo" is not a metric unit, but sometimes used as short for "kilogram", where "kilo" is the prefix for 1000 and "gram" is the metric unit. These prefixes are consistent for all metric units.
Fun fact: the metric unit for mass is … the kilogram, not the gram because f*ck consistency.
I've heard people recommend _Dead Pool_ (James Powell) but I haven't read it.
I swear that SF water started tasting worse after they started adding groundwater a few years ago:
https://www.nbcbayarea.com/news/local/san-francisco-new-sour...
That’s not directly comparable to residential rates because you now need to pump that water up hill and through a distribution network to customers. Desalination occurs at sea level and getting that to anywhere but costal cities is even more expensive.
Further, 0.01 cents per 6.4 gallons is 500$ per acre foot which is horrifically excessive for agriculture where most water is used. Using it for Alfalfa would cost more than 10x what the crop is worth. Desalination is therefore still only viable in very niche areas without heavy subsidies.
Which suggests that something is egregiously wrong with residential water pricing.
The number they are quoting is 0.41 $/m^3. Per capita water withdrawals (including agriculture) in the US were ~1200 m^3 in 2015 [1]. That means the cost per person to convert all residential, industrial, and agricultural to desalination is only ~480 $/(person * year). Or approximately 6/1000 of US GDP. Even if we assigned all of that cost to agriculture and food, all that would mean is that the average person's food budget would increase by ~480 $/year. That is sizeable, but not even slightest bit infeasible. If we really needed to we could just assign 3% of the US federal government budget to preventing death by starvation and thirst by achieving full water independence.
Desalinated water is only uneconomical in comparison to unsustainable groundwater depletion. On a absolute basis it is very affordable and would not constitute a material problem for the US or any other developed country.
[1] https://www.statista.com/statistics/263156/water-consumption...
Anyway your numbers aren’t even close. Water needs to reach people not simply exist. Groundwater depletion really isn’t a thing in the east cost, it’s mostly a thing west of the Mississippi and mostly at fairly high elevations.
To offset water withdrawals of people living 2km above sea level that 1200 m^3 of sea water would need 6480 kWh before consideration inefficiencies. For much of the Midwest you’re spending more on pumping than desalination, but you also need pipes etc.
I am being a little flippant here. Transitioning to a desalinated water economy is a gigantic megaproject. I am not including the costs needed to add all the transport infrastructure, but we also do not need to convert over 100%. We only need desalination where we are consuming water in excess of water renewal rates. And if you need excess water you will need to move your business to where water resources are cheap. That or your business and people die of no water. I know what I prefer. Luckily, highly productive farmland which consumes the vast majority of the water is generally on flat, low elevation ground where transport costs will be low.
Desalination is a viable solution. Are there challenges? Sure. Can you do it without any sacrifices and without changing your lifestyle at all? Probably not. But if your alternative is insufficient freshwater resources desalination can solve that economically at scale at a modest cost and with relatively minor sacrifices.
Spending 8% of the US budget to subsidize exports would be lunacy. Ban Alpha exports from California largely solves the problem within the state as does a host of other possibilities like say charging a fee for using an aquifer.
Anyway it’s a self correcting problem, when farms can’t pump out water they will shut down reducing water use. The US is such a massive exporter that none of this will be noticed by US consumers.
I pointed out how even if we decided to support and subsidize the moronic use cases the US can still easily support a desalinated water economy if we had to.
In actuality, if we moved to bulk desalination we would see a reconfiguration of water usage to higher value usage since water would be immensely more expensive. However, despite being much more expensive, even if we massively overestimate water usage by including water wasteful export crops, desalination still ends up being viable including the usage that would almost certainly disappear if they did not get to defray their depletion externalities.
Massive overestimate comes out reasonable. Therefore correct estimate will also come out reasonable.
You need to fix the underlying issues or nothing changes. But by fixing those issues there’s no niche for desalination.
Desalination is a solution in search of a problem, not a useful tool here.
This seems like a false dichotomy.
Want to pump that cubic meter of water 100 km inland and the energy cost alone will be over $1/m^3. Ignoring pumps, piping, maintenence, land purchasing, cabling, planning permission, etc etc.
Where people live isn’t where people consume aquifers faster than they are replenished. Across much of the east coast farmers don’t bother with irrigation because even the cost of pumping water from a well and delivering it to their fields isn’t worthwhile.
West of the Mississippi the amount of rainfall drops and therefore the need for water skyrockets. https://us-canad.com/rainfall-usa-map.html The difference between 150 inches per year of rainfall and less than 25 is huge.
At the same time the average altitude above sea level also increases. https://gisgeography.com/us-elevation-map/
So if you want to get water where it’s needed in the US. That’s mostly up hill and a long way from the sea.
PS: Coastal cities can always out bid farmers for water, or use some desalination but that’s a tiny fraction of the overall water usage.
You know you cannot simply dump it back into ocean in one place. It is also a problem in Gibraltar or so to say they have it figured out but it costs them a lot of money.
This is not the reason the US doesn't desalinate. It is actually actively making things worse in the places where it can still extract water from aquifers. Removing the water allows salt water to penetrate those thus destroying the local ecosystems. It's also speeding up the desertification of other areas.
Desalinating ocean water (done right of course) is part of the solution, not the problem.
Solutions like discussed in the article are more interesting for smaller/rural setups. Interesting but desalinating at industrial scale is a solved problem already. Can it be done cheaper. Probably and that would be nice. But it can be done economically right now. The largest challenges here are bureaucratic, not technical.
https://www.yourgibraltartv.com/society/28113-tender-awarded...
Gibraltar apparently doesn't have sewage treatment because they have some salt-water flush system for toilets due to fresh water historically being so expensive there and they seem to have had difficulty getting a treatment system that works with that.
In one episode they discovered a way to use the transporter to become young again. You would think eternal youth would be a big deal, right? Nope, it was never mentioned again.
Eternal youth using a transporter would have been shouted from the rooftops.
I recall the CEO of terraformation (Yishan Wong) talking about using desal with brackish wells : https://www.terraformation.com/blog/solar-powered-desalinati...
so .01 * 300,000/6.4 is... $500.
How often do they need the water?
This is 2015 data. Not sure if the rapid rise in solar and wind has changed this, though it might over time.
But judging by the researcher names and the date I believe the actual paper is titled "Extreme salt-resisting multistage solar distillation with thermohaline convection" which appears to be available as a PDF at https://www.cell.com/joule/pdf/S2542-4351(23)00360-4.pdf
the blog post in question is written by steve novella, who also produces _the skeptics guide to the universe_, a fairly popular science and skepticism podcast. it reads like more than just a "restatement of an MIT press release" to me. i don't love the entirety of the message but it's not just a summary of the paper or a summary of the summary of the paper.
This system (better described here [0]) simply washes the brine out as it makes it into the ocean, so the hypertoxic concentrate isn’t an issue. (Original paper here [1])
“Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers.”
0 https://news.mit.edu/2020/passive-solar-powered-water-desali...
1 https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0...
The minuscule amount of salts being added into the ocean does nothing to the whole ocean. One river will have washed out more salt.
No, you can't just assume this salt burden is fine, and that we get good salt marshes from it, but it's also not entirely true that high salt level brine is going to "destroy" the ecology. It's going to alter it, for sure. That isn't always very good, it depends what else has also destroyed the ecology, and what upsides there are to making a salt marsh.
https://www.sciencedirect.com/science/article/pii/S034181622...
It's called the water cycle for a reason. Water doesn't disappear after being used.
In any event, the Greenland ice sheet is on track to melt over the next few centuries and add 2,850,000 cubic kilometers of water to the ocean. We're not in danger of making it too salty with desalination.
Or perhaps it's like drowning - sure, people can swim for a total period of hours a day, but they're never underwater for a continuous duration of more than a few minutes in a row.
The Mediterranean Sea once completely evaporated, leaving a large salt deposit. Later, the Gibraltar barrier broke, allowing water to flood in with new salts, forming a saltier-than-average sea: 3.8 g/100g vs 3.4 in the World Ocean. But that is no problem for marine fauna and vegetation.
Hold on, still parsing…
Very likely with cheap or negative prices for brine we will solve many other resource scarcity issues.
They claim that a box with a surface area of about one square meter could desalinate 5 L of water per hour, using nothing but solar energy.
To evaporate one liter of energy of water takes 2.30 MJ, and that's assuming that it's already boiling point (the most efficient temp to evaporate). Solar energy maxes out at about 4.97 MJ per hour per square meter. That’s just a bit over 2 liters – and that’s assuming best case scenario and the sea water is at the boiling point.
So how is this box with 1 m^2 surface area gather enough sunlight to evaporate 5 L water per hour?
On that note, there are also desalination proposals out there that rely on heat pumps, where great is transferred from the condensing side to the evaporating side. (Obviously additional outside energy is required.)
If that energy is from the sun, then it's part of the same calculation.
(And anyway, evaporation is faster under less pressure, but the same objections apply.)
Assuming you get around 8 hours of operations out of such a box per day, that would be 3m² of land usage for the German usage, or 7.5m² / 80ft² for the US, per person. Doesn't sound great, but not too terrible either, compared to current land use (for housing, cars, infrastructure, agriculture, ...)
That's of course ignoring other water sources, and also that the hours of operation will vary wildly with weather and season.
If I weigh 85 kg and measure 185 cm (BMI 24.8, right at the edge between normal and overweight), my body surface to wash is 2.09 m². Let's assume a 3-minute shower at 15 liters per minute, so 45 liters of water. So 21.53 liters per m².
If my weight is 120 kg (140% of above) and I still measure 185 cm (between Obesity I and II), then my body surface to wash is 2.42 m², an increase of 0.33 m². Applying the same ratio as above, at 21.53 liters per m², I would need 52.11 liters of water (115% of above).
So a weigh of 140% results in a water consumption of 115%.
Qualitative argument is right, but the effect is not linear.
Simple, solar-powered water desalination (2020) - https://news.ycombinator.com/item?id=27708411 - July 2021 (227 comments)
Simple, solar-powered water desalination - https://news.ycombinator.com/item?id=22269115 - Feb 2020 (192 comments)
Let’s see if the claim is possible from physics and math perspective.
In one hour the water is boiled to evaporation and then cooled (I presume with some added energy).
Raw sun energy is 1375W/m2 directly hitting earth but after clouds, tilt, material absorption we could say perhaps ~25% efficiency of input energy is captured so 350W. That is 0.35KWh in one hour.
1 hour solar energy is 0.65kWh.
To raise 1C of 1ml is 1 joule. So 100C from 20C of 1000ml is 80K joules, which is 0.02kwh. 5 liters would be 0.1kwh of energy.
0.1 < 0.35. The math checks out. Should be possible to evaporate 5L of water and have some energy leftover for cooling via heatpump mechanics.
How much land would be used?
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Suppose this was put in Tampa since it’s in the sunshine state. We could say 5 hours of sunshine so 25L of clean water / m2 / day.
Average person uses 70 gallons / day or 265L/day. https://water.phila.gov/pool/files/home-water-use-ig5.pdf
Per person we’d need (265/25)= 10m2, for a city of ~500,000, we’d need 5 million m2 or 5km2.
Land area of Tampa is 300km2. So 1.6% of land would be used to make water.
5km2 is a looooot of land in one big plot, but not a lot of divided in chunks across where people live.
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If we are serious about global warming and future compfortable survival of our species then we should build homes where the entire roof is harnessing the energy of the sun.
To desalinate, to capture solar into electricity, to heat water.
Shingle roofs are strong snd cheap but if every foot of roof in US sun belt region was covered with solar cells, we’d be net negative for residential energy (produce more energy than use - that energy could supply electric transport).
It would make total sense to live in a solar farm of homes and use energy close to where it is captured.
Our future very much depends on how efficiently we can harness the sun’s energy which is free and abundant.
A number of the threads here were discussed over there, including the particularly-interesting observation that concentrated salts are not that much of a concern if the (presumably-locally-consumed) water round-trips out via the same physical location, in the form of sewage / grey-water / etc.
Really nice innovation, you need holes of 2.5mm to avoid clogging. The tradeoff is between efficiency and maintenance.
[0] https://news.mit.edu/2022/solar-desalination-system-inexpens...
"It is estimated by 2030, 50% of Saudi Arabia’s local oil and gas will solely be used to meet the rapidly growing demand for water." -- source: https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in...
1 sqare meter of membrane is about 900 USD.
And you need 1 sqare meter for each stage. With a 10 stage device as developed in the article, things start to add upp.
If someone makes a product of this they would probabbly be able to push the price down. We will see. As it is right now PV pannels and an electric desalinator would give allot better returns.
A cubic meter is 1000 liters. If we assume (as you said) that 1m^2 can produce 50L per day, you would need 20m^2 (1000/50) to produce a cubic meter. Not 20,000m^2(!!).
So for 600k cubic meters of desalinated water you'd need 12 million m^2 (or 12km^2 or ~3000 acres), which sounds very doable.