23 ms·
Passive Solar Water Desalination
- wgd 3y agoThis is some blog's restatement of an MIT press release, neither of which appear to name or link to the actual paper or other useful writeup. 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 https://www.cell.com/joule/pdf/S2542-4351(23)00360-4.pdf
- beardicus 3y agothe first link in the sidebar of the MIT writeup is to the paper you linked. it'd be nice if they linked it in the the body copy as well but here we are. 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.
- jayrot 3y agoI appreciate Novella’s bent towards realistic pragmatism in scientific skepticism (as opposed to idealism)
- wgd 3y agoAh, I stand corrected. I overlooked the PDF link over in the sidebar and am less disappointed by the MIT News writeup now (although I do still wish they could have copy-pasted the diagram from page 1 of the PDF into their photo carousel, reading those several paragraphs of text attempting to describe the device's construction was downright painful and the reason I gave up and went looking for the paper).
- joshe 3y agoAdvances are always welcome, but it's worth noting that desalination works economically right now. 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-drying-world/?cf-view https://www.energymonitor.ai/tech/can-desalination-save-a-dr... Residential rates: https://sfpuc.org/accounts-services/water-power-sewer-rates/2023-rates-frequently-asked-questions https://sfpuc.org/accounts-services/water-power-sewer-rates/...
- selimthegrim 3y agoWould having a nuclear plant nearby not help power/support this tech or the MIT one?
- rexreed 3y agoThe MIT one is completely passive only using solar energy, so no need for any power
- depressedpanda 3y agoFor the international audience who don't use freedom units, 6.4 (US) gallons is ~24.2 liters.
- fnordpiglet 3y agoMy understanding the big drawback of desalination is the residual substances. They trivialize it in this article as salt, but it’s actually a slurry of heavy metals and other toxic stuff, with lots of salt. 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-desalination-0207 https://news.mit.edu/2020/passive-solar-powered-water-desali... 1 https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee04122b#!divAbstract https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0...
- dopa42365 3y agoWhat's the actual difference between not taking the non-water out of the ocean vs putting the non-water back into the ocean?
- voat 3y agoThe sea becomes more salty over time when just the salt is added back.
- chii 3y agoNot the sea, but the localized area might have a brine lake, which is a deadzone for marine life there. If diffused over a large area, it will not have an impact. The only problem is cost. The minuscule amount of salts being added into the ocean does nothing to the whole ocean. One river will have washed out more salt.
- ggm 3y agoSalinas or salt marshes are highly productive for native species adapted to high salt levels. Brine shrimp feed Flamingoes (if you are lucky) or other aquatic bird species. 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/S0341816222004155 https://www.sciencedirect.com/science/article/pii/S034181622...
- pcl 3y agoThis is a dupe from last week, I think. 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. https://news.ycombinator.com/item?id=37681004 https://news.ycombinator.com/item?id=37681004 https://news.ycombinator.com/item?id=37675831 https://news.ycombinator.com/item?id=37675831
- adrianmonk 3y agoPrevious thread: https://news.ycombinator.com/item?id=37675831 https://news.ycombinator.com/item?id=37675831
- simbolit 3y agoMost people have no idea how much power current large scale desalination methods consume: "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_Saudi_Arabia#Desalination https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in...
- photonbeam 3y agoId have assumed they had the perfect climate for giant solar installations directly feeding desalination plants
- SamBam 3y agoThis was brought up by someone else in another forum, but do the MIT desalination numbers make sense to anyone else? 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?
- mitthrowaway2 3y agoDisclaimer: I haven't looked into this at all. But on a first-principles basis, it should be possible as long as the desalination process recovers most of the evaporation energy during the condensation process.
- SamBam 3y agoThat could make sense. 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.)
- tomcam 3y agoGreat transferring is. Efficiency you must have.
- mycall 3y agoWhat if that box is put under pressure? Would that reduce the MJ necessary for the boiling point?
- SamBam 3y agoHow is the box coming under pressure without additional energy being introduced? 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.)
- blacksqr 3y agoWithdrawn. Time to go to bed.
- fastball 3y agoOne of us has done their math terribly wrong. 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.
- dang 3y agoRelated? Others? Simple, solar-powered water desalination (2020) - https://news.ycombinator.com/item?id=27708411 https://news.ycombinator.com/item?id=27708411 - July 2021 (227 comments) Simple, solar-powered water desalination - https://news.ycombinator.com/item?id=22269115 https://news.ycombinator.com/item?id=22269115 - Feb 2020 (192 comments)
- perlgeek 3y agoTo get an idea of the scale: the average residential water usage is around 120 liter per person and day in Germany, around 300 in the US. 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.
- barsonme 3y agowe’re fatter so we need more water to bathe :)
- Rygian 3y agoLet's do the math (based on [1]): 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. [1] https://stats.areppim.com/glossaire/body_mass_def.htm https://stats.areppim.com/glossaire/body_mass_def.htm
- mtreis86 3y agoThat assumes time to wash is linear to surface area. It is harder for heavy people though to wash themselves tho, harder to reach everything including tools, more tools required, more time spent not actively washing while the water runs.
- is_true 3y agoI need more time to shower when I exercise, probably could save a lot of water by shaving my head.
- phero_cnstrcts 3y agoHow can I build one myself?
- j-a-a-p 3y ago> After careful calculations and experiments, the researchers determined the optimal size for the holes drilled through the perforated material, which in their tests was made of polyurethane. [0] 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-inexpensive-0214 https://news.mit.edu/2022/solar-desalination-system-inexpens...
- deleted 3y ago[deleted]
- michaelteter 3y agoThis is just a fluff "article" that wraps the MIT publication: https://news.mit.edu/2023/desalination-system-could-produce-freshwater-cheaper-0927 https://news.mit.edu/2023/desalination-system-could-produce-...
- deleted 3y ago[deleted]
- nojvek 3y ago> For a box a square meter in size it can produce 5 liters of drinkable water per hour. 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? —- 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 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. —- 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.
- beembeem 3y agoRoofs should do more than just capture the energy of the sun, it needs to capture and store the water that hits it, because diffusing that water is enormously expensive at a city level. Hint: shingle roofs inject things into otherwise fresh water.
- reliablereason 3y agoThe ptfe membranes with 1 µm pore size that is used in this are very expensive. 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.