15 ms·
Simple, solar-powered water desalination
- ChuckMcM 7y agoThis is pretty cool. It also could solve one of the seasteading problems (water production) in a way that is more scalable. Certainly if they can produce a robust version of this you could pack it into lifeboats so that the first thing you did after abandoning ship would be to toss this overboard and unfold it so that it could be filling/refilling the boat's water tanks. I am not to sure about it on a metropolitan scale though, that is a lot of surface area per person. One sq kilometer is a pretty big patch for serving up water for about a million people. Call it 800K for losses due to transport.
- Animats 7y agoNice. When can I buy one at the camping equipment store?
- riazrizvi 7y ago> the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation. I need an explanation with pictures for that, because it seems like the author is using ‘efficiency’ incorrectly.
- flatiron 7y agoMy guess is 385% better than the sun just shining on the water and and evaporating it out in the open
- riazrizvi 7y agoThat makes more sense. Maybe 'magnification effect' is better to describe what sounds like possibly a solar-panel-greenhouse system?
- czbond 7y agoExamples like this are why I like HN as a community of "engineer and similar minded people". We pick up details like this and call them out.
- deleted 7y ago[deleted]
- gpm 7y agoDefined in the paper as > the solar-to-vapor conversion efficiency, defined as the ratio of total vaporization enthalpy to total solar energy input, So ya, they are just comparing to a really low baseline...
- hinkley 7y agoIt takes 1 watt-hour to produce about 3.5 BTUs. With a heat pump, for instance, you might be able to produce 5BTUs per watt-hour, because you're extracting heat from the environment (a very large heat source), which would be 140% efficient. The paragraph you excerpted has your answer: > The key to the system’s efficiency lies in the way it uses each of the multiple stages to desalinate the water. At each stage, heat released by the previous stage is harnessed instead of wasted. In this way, the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation. You can't drink steam (and even bottling it is futile), so that heat has to go somewhere. With counterflow systems, like ventilation systems for houses, the inlet and outlet temperatures are pretty close to each other, and the temperature on the 'inside' of the system is either much higher or much lower. When distillation came up a month or so ago, several people pointed out that modern systems do not operate at ambient air pressure, so the temperature delta may be less than you'd imagine. [edit: pedants gonna pedant]
- r-bryan 7y agoYou mean BTU/hour.
- deleted 7y ago[deleted]
- hinkley 7y agoYeah yeah yeah. If we're talking about distillation, the civil engineer probably wants to think about rates (liters per minute per $1000), but for a civilian just trying to figure out how this could work in a laboratory situation? I find quantities easier to comprehend (and relate). Tell me how many AA batteries or days of full sun it would take to convert a big beaker of salt water into a smaller beaker of distilled water.
- riazrizvi 7y agoI really enjoy artful displays of mental gymnastics, so thank you. A watt is a unit of power, a BTU is a unit of energy, and a watt hour is also a unit of energy. If you can show me how solar watt hours are converted into BTU's at an equivalent rate of 3.85 to 1, I will short sell all the energy stocks using my children as collateral.
- logfromblammo 7y agoThey are. I would personally define desalination efficiency of 100% as a perfectly-reversible reaction that establishes an equilibrium between fresh water and oceanic salt water. Since adding sea salt to fresh water until it has oceanic salinity represents a theoretical maximum of 0.810 Wh/L (a maximally efficient osmotic power plant, situated where a river empties into the ocean, could get about 0.75 Wh/L). So 100% efficiency would be adding 0.810 Wh to one liter of seawater to get one liter of fresh water back. 100% is an unachievable goal, thanks to the laws of thermodynamics. So to figure your solar desalination efficiency, from a solar panel that receives X Wh/m^2/day of insolation energy, you divide by 0.810 Wh/L to get L/m^2/day. Whatever fresh water you can produce per day, divide by that number to get your efficiency. An MIT roof gets mean 4.59 kWh/m^2/day of solar energy, so 100% efficiency for them would be 4590/0.810 = 5667 L/m^2/day. By the numbers given, their process is about 2% to 3% efficient (by my definition). They could be a lot more efficient if they didn't have to overcome the huge heat of vaporization that water has, which is exactly why reverse osmosis is so much more efficient than multistage flash distillation. They are stacking so that the evaporation energy can be recovered from the condensation, which deposits the same amount of energy on the next layer, but it's better off all around to just never evaporate in the first place.
- jolmg 7y agoThis topic always reminds me of the opening scene[1] from Waterworld where a guy on a raft out in the sea passes his urine through some kind of filter that turns it into drinkable water, and drinks it. I was a little kid when I watched it and wondered why he would drink his own urine when he was surrounded by such vast amounts of water. With this type of technology, maybe he wouldn't have had to do something so gross! XD [1] https://www.youtube.com/watch?v=16txiqRoVNQ https://www.youtube.com/watch?v=16txiqRoVNQ
- logfromblammo 7y agoGiven the agricultural issues in that movie, perhaps the purification device was intended more to collect and concentrate the urea than to purify the water. But that assumes the scriptwriter would have thought of that, which may be a stretch, given that Earth has insufficient water to cover all landmasses to a depth of 7 km. That would require about 3.6 billion cubic kilometers more water than already exists in the oceans, and there are only about 1.4 billion cubic kilometers of water on Earth to begin with, . Certainly there isn't that much locked up in glacial ice. That is only 26.5 million km^3, which is enough to raise sea level by a maximum of 52 m, far short of the 8 km it would take to cover everything but Mt. Everest. Anyone who would skip that bar-napkin math was probably only putting that scene in specifically for the gross-out factor. It's just like Mad Max eating the can of dog food. Which is an apples-to-apples comparison, because Waterworld was written specifically to be an ersatz Mad Max.
- J5892 7y agoSo the only possible explanation of the cause of the Waterworld apocalypse is earth being hit by millions of giant ice meteors.
- logfromblammo 7y agoThe era of the movie is 2500 AD. To add nearly 3 oceans worth of water in that time frame by cometary bombardment would boil the existing ocean. The only hard sci-fi explanation is that someone towed Europa to Earth and slowly lowered it down via a cable or pipe. Terraforming by an alien aquatic species. Hyperintelligent space whales.
- deegles 7y agoWhy do these articles never have pictures or diagrams? edit: I just missed them :(
- kilovoltaire 7y agoThere is a photo and a diagram in the carousel at the top of the page. (Though I agree it's easy to miss.)
- kbenson 7y agoIt has a carousel at the top with a picture and a diagram.
- deleted 7y ago[deleted]
- quotha 7y agoA big question is what do they do with the salt?
- 52-6F-62 7y agoUndercut the existing suppliers of sea salt?
- itronitron 7y agoThey'd have enough salt to last forever! >> https://youtu.be/jmzJXVEnmKc https://youtu.be/jmzJXVEnmKc
- symplee 7y agoFrom the article: >> 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.
- spikels 7y agoKinda explained in the 9th graf: > 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.
- quotha 7y agoSo the salt is just going back into the ocean. How is that different from other systems, they end up dumping the salt somewhere.
- jvanderbot 7y agoThere's no difference. Except that the processing and shipping of ions is passive / much easier. Contrasted with large permeable membrane plants that have to burn power to push all that brine out into the ocean.
- worik 7y ago"than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area." Units, people. Units. Be imperial. Be metric. Hey! Be both!
- jerf 7y agoHow about 5.67812 liters for every 10.7639 square feet?
- DanTheManPR 7y agoIt comes out to about 0.4 liters per square foot, for those of you who are confused.
- 101404 7y agoWhat's that in bathtubs per football field?
- rstupek 7y agowell an NFL football field is approximately 5351 m2 so about 8000 gallons. not sure how big your bathtub might be though
- rdiddly 7y agoLayman's Explanation Greatest Hits right there. Except I was actually curious so I converted it. Assuming a 30-gal tub, it's about 267 tubs/field-hour.
- VikingCoder 7y agoHrm - "To start, the standard bathtub will hold roughly around 80 gallons (302 liters) of water. Much smaller bathtubs can only hold around 40 gallons (150 liters) of water, which typically are more suited for smaller children or function more as a shower space." 30 gal seems too small.
- VikingCoder 7y ago
- WalterBright 7y agoWhen I was in Boy Scouts loooong ago, there were instructions and diagrams for making solar stills out of a sheet of plastic, a cup, rocks and dirt.
- emmelaich 7y agoSame. I wonder how well they worked; I've never seen one made. It surely would require fairly specific conditions to work well; a wet ground and low humidity.
- WalterBright 7y agoJust get a large bowl or pail, put an inch of water, or plants, in the bottom. Put a cup in the center. Cover the top with clear plastic film. Weight the center of the plastic. Place it in sunlight. After a while, condensation will form on the underside of the film, the drops will run downhill to the center, and drop off into the cup. Voila! Distilled water. Found it! https://en.wikipedia.org/wiki/Solar_still https://en.wikipedia.org/wiki/Solar_still
- emmelaich 7y agoSure, I know the setup. Just never made it nor seen it made.
- deleted 7y ago[deleted]
- symplee 7y ago>> 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area. So a football field could collect around 8,000 gallons of water per hour. Let's say you can get 5 hours of good sunlight per day = 40,000 gallons of water. >> The team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation. >> Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent So now we're looking at 80,000 gallons of water per day. >> 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. >> In production, they think a system built to serve the needs of a family might be built for around $100.
- numbsafari 7y agoIf I am reading [1] correctly, NYC needs approx. 1 billion gallons of water per day. So, they would need an array of 12,500 football fields to meet their needs. [1] https://data.cityofnewyork.us/Environment/Water-Consumption-In-The-New-York-City/ia2d-e54m https://data.cityofnewyork.us/Environment/Water-Consumption-...
- deleted 7y ago[deleted]
- zamalek 7y ago> 1 billion gallons of water per day * How much of that water need be drinkable? * How much of that water can be salinated (e.g. for flushing)? * How much of that water can be partially desalinated (e.g. for showering)?
- ebg13 7y agoIndeed. If you do only the drinking water, 2L per person per day for 20 million people is only 10566000 gallons or 132 football fields.
- jd_gadus 7y agoA thunderfoot video waiting to happen
- aaron695 7y agoWhile we wait are there any sites where people actually intelligently discuss this article? It seems like a 100 year old tech they are trying to optimise. I feel like random optimisations would mean there are no advances to get here. But it is possible using computer simulations they could do something of value. Have they?
- sacred_numbers 7y agoEfficiency in desalination is measured by comparing the energy expenditure to the enthalpy of vaporization of water (basically the energy required to distill water by boiling it). The current state of the art reverse osmosis desalination plants use about 3.2 KWh of electricity per cubic meter of fresh water (https://pdfs.semanticscholar.org/d4d7/821d585699719289dddd1025a582253e3314.pdf https://pdfs.semanticscholar.org/d4d7/821d585699719289dddd10...). This technology uses about 173 KWh of solar energy per cubic meter of fresh water. The advantage of this method is lower capital costs and not having to convert solar energy to electricity. For large-scale desalination, however, it is almost certainly more cost-effective to use solar panels, batteries, and large scale reverse osmosis systems. This is still a useful project for making drinking water in remote locations, though.
- pfdietz 7y agoOne fascinating (well, to me) component of modern RO desalination systems is the rotary pressure exchanger. Such a simple device, but so important. It looks like something from the 19th century, but the base patent was issued in 1988. https://en.wikipedia.org/wiki/Pressure_exchanger https://en.wikipedia.org/wiki/Pressure_exchanger
- steamer25 7y agoI found this video which seems to animate the concept nicely: https://www.youtube.com/watch?v=udffed4Pq3g https://www.youtube.com/watch?v=udffed4Pq3g
- onceUponADime 7y agoThank you, that was fascinating.
- jpuderer 7y agoAwesome. The video made it crystal clear.
- timonoko 7y ago
- SCAQTony 7y agoA lot of lithium batteries are making their way onto both sailboats and yachts. With significant advantages come equal disadvantages, and those disadvantages are the risk of potential battery fires, which only a Class D fire extinguisher can snuff out. The Dive boat fire in San Diego, where 34-persons perished, is one example.
- aaronax 7y agoMost boats (recreational "cruisers" at least, which is what I am most familiar with) are using LiFePO4 chemistries which are much less likely to start fires. ReLiOn, Victron, BattleBorn, and MasterVolt for example as some of the leading vendors.
- kragen 7y agoYou don't need batteries to run a reverse osmosis desalinator from solar panels; you can run the desalinator when the sun is shining and store the water. I'm surprised boats are already moving to lithium. I wonder if it would be worthwhile to tow the batteries in a separate buoy behind the boat?
- hinkley 7y agoOr to put it another way, any device with intermittent power only requires one storage reservoir, not multiple. Water jugs are close to the oldest technology we have.
- kragen 7y agoRight. You still need batteries on your boat, just not for desalination. An interesting aspect of the history of water jugs is that the humans were cultivating them for thousands of years before they started cultivating food crops: https://phys.org/news/2014-02-mystery-bottle-gourd-migration-americas.html https://phys.org/news/2014-02-mystery-bottle-gourd-migration...
- newnewpdro 7y ago> I'm surprised boats are already moving to lithium. When the alternative is a bank of lead-acid batteries with effectively ~50% usable capacity and poor charge efficiency, at least for cruisers effectively living at sea off solar power, it's not surprising at all.
- driverdan 7y agoThe full paper: https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee04122b https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0...
- kragen 7y agoThis is a well-known approach known as multistage flash distillation: https://en.wikipedia.org/wiki/Multistage_flash_distillation https://en.wikipedia.org/wiki/Multistage_flash_distillation It's surprising they didn't mention this in the article. (It is, of course, mentioned in the paper, because its authors are decent and honest people.) > more than 1.5 gallons of fresh drinking water per hour I don't understand. Are those imperial gallons, US gallons, US dry gallons, or Irish gallons, which are all different sizes? https://en.wikipedia.org/wiki/Gallon#Sizes_of_gallons https://en.wikipedia.org/wiki/Gallon#Sizes_of_gallons How many hamburgers is that the same weight as? How many ngogns is that? Or maybe acre-feet, pony shots, or Olympic swimming pools? MIT should be ashamed of their News Office. The abstract of the paper https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee04122b#!divAbstract https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0... does give the result in modern units, and to a much higher precision: > a production rate of 5.78 L m⁻² h⁻¹ under one-sun illumination In Dercuano http://canonical.org/~kragen/dercuano http://canonical.org/~kragen/dercuano I wrote a note about multistage distillation, which I didn't realize was already a known technique until later; see notes/recycling-distillation.html or p. 1776 in the PDF. The basic summary is that modern reverse-osmosis distillation plants require on the order of 7 kJ/ℓ to get freshwater from seawater (which works out to about 50 kJ of sunlight per liter with a cheap 16%-efficient solar panel) while multistage distillation requires a few hundred kJ/ℓ. The number given by the paper abstract works out to 623 kJ/ℓ if we assume one sun is 1000 W/m² (a common figure for the solar constant at the surface). So if you use those square meters to run photovoltaic panels which you then use to drive a reverse-osmosis machine, you can get dozens of times as much fresh water from the same solar energy. But you need to build or buy a reverse-osmosis machine, and then you need to defoul it, so this device might be a good choice in some circumstances. (You might need to defoul the distillation apparatus as well, though.)
- jefftk 7y ago> I don't understand. Are those imperial gallons, US gallons, US dry gallons, or Irish gallons, which are all different sizes? Imperial and US gallons would both make sense, and you're right it's ambiguous. Dry gallons would not make sense, since this is a liquid quantity, and Irish gallons are long obsolete.
- cvaidya1986 7y agoThis is great! We need more solar powered projects!
- aSplash0fDerp 7y agoThe combination of hydrogen production and desalination using simple techniques are an exciting prospect for this decade. For architects of 21st century cities, most of these sustainable technologies would benefit from a cleanslate design, rather than trying to compete with builtup 20th century infrastructure. I think Austrailia is postured to be the biggest beneficiary of all of these "leaps" in technology this century. Most of the desert area is the perfect canvas to start building indoor cities that will be the models/prototypes for the spacestations that the next generation will use to explore urban life outside of our atmosphere. Even building a cruise ship on land would be a good starting point for a 21st century city if they could support several thousand residents with enough freshwater for self-sustaining agriculture and energy to take root. Even though we won't have saltwater in space, most of these advancements in sustainables fit the roadmap to space more than they're currently reported. Hopefully they'll break ground on new cities using these new resources sometime soon and change the narrative appropriately.
- wffurr 7y agoLandships sound like the plot of some dystopian future film; ecosteaders fight off the scavenger bands.
- RosanaAnaDana 7y agoClearly you've not passed through northern New Mexico.
- Nasrudith 7y agoIt is already - in the my opinion deeply silly book/movie series(?) Mortal Engines. I haven't read or watched them but saw a video ad and thought that it was the stupidest thing I had ever seen. Anyway it is effectively landships except they are massive steampunk mobile cities which grind up and harvest other sessile settlements and smaller mobile ones. I get the extended metaphor involved of colonalism and enrichment by exploitation of the weak and pillaging instead of their own production but even for YA Dystopian it is just too dumb logistically for me to suspend disbelief.
- aazaa 7y agoThis would make a great DIY project.
- dr_dshiv 7y agoSeems good for a seastead.
- zahreeley 7y agoThe key to the system’s efficiency lies in the way it uses each of the multiple stages to desalinate the water. At each stage, heat released by the previous stage is harnessed instead of wasted. In this way, the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.
- mrfusion 7y agoIs humidity usually high by the ocean? Would it be more efficient to pull water out of the air? Maybe take advantage of low night temps to collect dew?
- ta1234567890 7y agoPretty cool, seems like a modern version of what some people were doing before electricity and solar panels: http://www.solaqua.com/solwatdis1.html http://www.solaqua.com/solwatdis1.html
- fghorow 7y agoIt's a solar thermal Multi-Effect-Distillation (MED) plant! I was part of a team that suggested using geothermal heat for desal with MED technology in Perth, Western Australia a bit more than a decade ago! The advantage over work based (i.e. electrical/Reverse Osmosis) systems is that you do not lose heat via Carnot (or real world, for that matter) efficiency. You utilize more of the original heat, and via the multiple stages ("effects") you can re-use the latent heat of vaporization over and over. MEDs are really cool gizmos!
- ineedasername 7y agoI have a stupid question (well, not to me because I don't know the answer): isn't desalination tech fundamentally similar to alcohol distillation? I know that's energy intensive too, I'm just curious, because I know a little about alcohol distillation and an wondering if the concepts are transferable.
- contingencies 7y agoSee https://en.wikipedia.org/wiki/Watermaker https://en.wikipedia.org/wiki/Watermaker and https://www.cruisingworld.com/desalination-decisions-watermakers/ https://www.cruisingworld.com/desalination-decisions-waterma...
- ByzantineO6 7y agoummm..... rain?
- forgotmypw 7y agoWould this process also be useful for filtering polluted water? Several pollutants I have in mind are VOCs and fluorine compounds. Hopefully there is someone here that is qualified to answer this question.
- i_am_nomad 7y agoProbably depends on the vapor pressureS of the VOCs; if less than water, then no.
- taneq 7y ago> the team’s demonstration device can achieve an overall efficiency of 385 percent I don't care how good you think you are, you're not getting 3.85kW per square meter from sunlight.
- jokoon 7y agoI guess this an awesome tech for places where there is no clean water, as long as those places are close enough to the shore. But again, I doubt that any investor will be interested, as they will probably not get good enough ROE with this. This would probably get the attention of government of some poor countries, or maybe some humanitarian organizations, or even the military. I would probably be confident that a small portable kit could be marketed to survivalists and it would sell extremely well.
- idoubtit 7y agoThis is not only about efficiency. As noted in the article, it would reduce the pollution of highly concentrated brine rejected into the ocean. See for instance this article of the Guardian about a region in Chile where the majority of drinkable water comes from a desalination plant built in 2003: "The salt they pump back in kills everything'" https://www.theguardian.com/cities/2020/jan/02/the-salt-they-pump-back-in-kills-everything-is-the-cost-of-chiles-fresh-water-too-high https://www.theguardian.com/cities/2020/jan/02/the-salt-they...
- MertsA 7y agoIt only does that by virtue of being much less efficient and spread over a larger area than traditional desalination plants. At the end of the day unless you're storing that brine or trucking it off somewhere you have to dump it back in the ocean. That "highly concentrated brine" isn't all that concentrated to begin with as it's less than a 50% increase in solutes coming out of the discharge pipe and gets mixed and diluted in the ocean relatively quickly. That article is just a fisherman blaming an unrelated problem on the closest thing around.