MIT has improved the passive solar still
February 10, 2020 7:10 PM Subscribe
"...at a rate of 5.78 liters per square meter(per hour)" for the rooftop prototype" "The team settled on a 10-stage system for their proof-of-concept device, which was tested on an MIT building rooftop. The system delivered pure water that exceeded city drinking water standards, at... (given rate)".
(snip)
Adding more layers increases the conversion efficiency for producing potable water, but each layer also adds cost and bulk to the system. The team settled on a 10-stage system for their proof-of-concept device, which was tested on an MIT building rooftop. The system delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters per square meter (about 1.52 gallons per 11 square feet) of solar collecting area. This is more than two times as much as the record amount previously produced by any such passive solar-powered desalination system, Wang says.
(snip)
Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of.
(snip)
Adding more layers increases the conversion efficiency for producing potable water, but each layer also adds cost and bulk to the system. The team settled on a 10-stage system for their proof-of-concept device, which was tested on an MIT building rooftop. The system delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters per square meter (about 1.52 gallons per 11 square feet) of solar collecting area. This is more than two times as much as the record amount previously produced by any such passive solar-powered desalination system, Wang says.
(snip)
Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of.
It's a good idea and a tidy piece of engineering, but this is bullshit:
And sure, a little proof-of-concept prototype is not going to make consequential amounts of brine. But if this thing does prove amenable to scaling up until it's a real alternative to currently available commercial reverse osmosis tech, it's going to be making brine in similar quantities and the idea that you don't have to care about the effects of that is just flat wrong.
posted by flabdablet at 7:24 PM on February 10, 2020 [20 favorites]
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.If you're taking fresh water out of seawater, you're increasing the salt concentration of what's left behind. That's just not avoidable. The salt has to go somewhere.
And sure, a little proof-of-concept prototype is not going to make consequential amounts of brine. But if this thing does prove amenable to scaling up until it's a real alternative to currently available commercial reverse osmosis tech, it's going to be making brine in similar quantities and the idea that you don't have to care about the effects of that is just flat wrong.
posted by flabdablet at 7:24 PM on February 10, 2020 [20 favorites]
*proof-of-concept*
Now the Engineering tradeoffs start.
posted by aleph at 7:24 PM on February 10, 2020 [3 favorites]
Now the Engineering tradeoffs start.
posted by aleph at 7:24 PM on February 10, 2020 [3 favorites]
Expense is solvable.
Paper used to be ungodly expensive. Like only Kings and Church officials could afford it.
Now it’s trash that blows down the street.
posted by Everyone Expects The Spanish Influenza at 7:25 PM on February 10, 2020 [7 favorites]
Paper used to be ungodly expensive. Like only Kings and Church officials could afford it.
Now it’s trash that blows down the street.
posted by Everyone Expects The Spanish Influenza at 7:25 PM on February 10, 2020 [7 favorites]
Yes. Aerogel-like might be worked out. Maybe something less expensive/less-effective but good enough.
posted by aleph at 7:27 PM on February 10, 2020
posted by aleph at 7:27 PM on February 10, 2020
Also seems to me that this project is a good illustration of the idea there are massive potential energy savings to be had in redesigning existing industrial processes to make better use of recovered waste heat.
Given enough attention paid to physical positioning of process stages with respect to one another and integration of contra-flow heat exchangers, I can see no reason why processes with stages that need to run at very high temperatures inherently require energy input many multiples of what ends up locked into the chemistry of whatever is being manufactured.
posted by flabdablet at 7:35 PM on February 10, 2020 [12 favorites]
Given enough attention paid to physical positioning of process stages with respect to one another and integration of contra-flow heat exchangers, I can see no reason why processes with stages that need to run at very high temperatures inherently require energy input many multiples of what ends up locked into the chemistry of whatever is being manufactured.
posted by flabdablet at 7:35 PM on February 10, 2020 [12 favorites]
a panel seems to be in the low hundreds, and times 10 stages, maybe not for the folks that need it most
If I'm understanding the design correctly it doesn't need aerogel for every stage. The entire point of the multi-stage design is to use rejected heat from one stage as input for the next; the aerogel is only there to help trap solar heat inside the first stage. It's essentially a less fiddly alternative to the kind of vacuum-insulated borosilicate glass commonly found in tubular rooftop solar hot water heaters.
posted by flabdablet at 7:42 PM on February 10, 2020 [1 favorite]
If I'm understanding the design correctly it doesn't need aerogel for every stage. The entire point of the multi-stage design is to use rejected heat from one stage as input for the next; the aerogel is only there to help trap solar heat inside the first stage. It's essentially a less fiddly alternative to the kind of vacuum-insulated borosilicate glass commonly found in tubular rooftop solar hot water heaters.
posted by flabdablet at 7:42 PM on February 10, 2020 [1 favorite]
Also, from the paper:
The total estimated material cost of this prototype based on retailers was E$1.54 including the AR-coated glass, silica aerogel, solar absorber, paper towel evaporators, aluminum condensers and nylon frames. Since nylon frames account for more than 70% of the cost, it is possible to further reduce the cost by replacing the existing partially-hollow frame design with fully-hollow structures to save material or switching to other cheaper alternatives.posted by flabdablet at 7:57 PM on February 10, 2020 [1 favorite]
If you're taking fresh water out of seawater, you're increasing the salt concentration of what's left behind. That's just not avoidable. The salt has to go somewhere.
The sun's already evaporating moisture out of any body of water and leaving behind the salt.
Reverse osmosis pumps are sucking in huge amounts of sea water from a single inlet and have the brine to dispose of at a single site.
This, if deployed in scale, would effectively be leaving the salt in situ, and skimming freshwater from across a broad area.
The ecological concern would be how the reduced sunlight due to the devices' shadows impacts the life below. This won't be creating a briny dead zone, it'll be blotting out the sun.
posted by explosion at 8:06 PM on February 10, 2020
The sun's already evaporating moisture out of any body of water and leaving behind the salt.
Reverse osmosis pumps are sucking in huge amounts of sea water from a single inlet and have the brine to dispose of at a single site.
This, if deployed in scale, would effectively be leaving the salt in situ, and skimming freshwater from across a broad area.
The ecological concern would be how the reduced sunlight due to the devices' shadows impacts the life below. This won't be creating a briny dead zone, it'll be blotting out the sun.
posted by explosion at 8:06 PM on February 10, 2020
The sun's already evaporating moisture out of any body of water and leaving behind the salt.
Sure. And if you massively increase the efficiency at which that evaporation happens, and you do that on the city-supplying scale commonly seen with existing commercial RO desalination, you're going to generate brine at a rate where figuring out how to deal with it becomes a real consideration, just like it does with a city-scale RO desalinator.
And sure, if you've chosen to deploy these things on the ocean surface instead of pumping water to a large array of them housed in a better-sheltered onshore desalination plant, then you get to trade off brine generation against ocean shading considerations. But my point is that brine generation is not something that an alternative desalination technology is ever going to let you just ignore. If you're pulling a city's worth of fresh water out of a body of seawater then there will be a localised increase in brine concentration that will need to be modelled and understood and dealt with. Free drinks are for marketroids and politicians, not engineers.
posted by flabdablet at 8:33 PM on February 10, 2020 [6 favorites]
Sure. And if you massively increase the efficiency at which that evaporation happens, and you do that on the city-supplying scale commonly seen with existing commercial RO desalination, you're going to generate brine at a rate where figuring out how to deal with it becomes a real consideration, just like it does with a city-scale RO desalinator.
And sure, if you've chosen to deploy these things on the ocean surface instead of pumping water to a large array of them housed in a better-sheltered onshore desalination plant, then you get to trade off brine generation against ocean shading considerations. But my point is that brine generation is not something that an alternative desalination technology is ever going to let you just ignore. If you're pulling a city's worth of fresh water out of a body of seawater then there will be a localised increase in brine concentration that will need to be modelled and understood and dealt with. Free drinks are for marketroids and politicians, not engineers.
posted by flabdablet at 8:33 PM on February 10, 2020 [6 favorites]
I wonder how it would work to pull clean water out of wastewater.
posted by clew at 8:57 PM on February 10, 2020 [1 favorite]
posted by clew at 8:57 PM on February 10, 2020 [1 favorite]
In boy scouts we did survival campout weekends, and we lived in southern NM which meant one of our campout weekends was desert survival and building a solar still (you were given black plastic and a compact shovel as part of your kit) was sort of de rigueur. Chopping up moisture bearing cactus like barrel cactus, urinating in the pit, setting up the still, then going off looking for vague edibles while the sun baked everything.
The idea that they've created something that works 10x better and that feeds its heat into its own efficiency is actually sort of thrilling to me.
posted by hippybear at 8:58 PM on February 10, 2020 [2 favorites]
The idea that they've created something that works 10x better and that feeds its heat into its own efficiency is actually sort of thrilling to me.
posted by hippybear at 8:58 PM on February 10, 2020 [2 favorites]
clew: I wondered not so much about using it to purify waste water (the volume needed would seem too large though it could always be just a feed source). I wondered about using it to purify other types of "problem" water, like contaminated with heavy metals (arsenic, mine tailings/contamination) or other toxins like that.
posted by aleph at 9:21 PM on February 10, 2020
posted by aleph at 9:21 PM on February 10, 2020
Maybe a home kit for people in situations like Flint? Or a Disaster prep kit?
posted by aleph at 9:22 PM on February 10, 2020
posted by aleph at 9:22 PM on February 10, 2020
A solar still isn't going to do much in Flint unless you have maybe a bathtub in your backyard and even then it's yielding not enough for your family for a day? This is a "floating on a body of water" sort of device from my understanding.
posted by hippybear at 9:31 PM on February 10, 2020
posted by hippybear at 9:31 PM on February 10, 2020
using it to purify other types of "problem" water, like contaminated with heavy metals (arsenic, mine tailings/contamination) or other toxins like that
Should work great for that. Contamination with volatile organics like benzene, perhaps not so much; RO should yield better results there.
it's yielding not enough for your family for a day?
The nice thing about a very low cost solar-powered device is that you can just deploy enough of them to match any desired level of output.
Seems to me that the most likely failure mode for these things is going to be biological fouling of the evaporator wicks and condensation surfaces. These would need to be made easily accessible for regular cleaning.
posted by flabdablet at 9:37 PM on February 10, 2020
Should work great for that. Contamination with volatile organics like benzene, perhaps not so much; RO should yield better results there.
it's yielding not enough for your family for a day?
The nice thing about a very low cost solar-powered device is that you can just deploy enough of them to match any desired level of output.
Seems to me that the most likely failure mode for these things is going to be biological fouling of the evaporator wicks and condensation surfaces. These would need to be made easily accessible for regular cleaning.
posted by flabdablet at 9:37 PM on February 10, 2020
I wonder if you could run a small direct current through them to prevent/slow-down fouling? Seems like I've heard of efforts like that.
posted by aleph at 10:23 PM on February 10, 2020
posted by aleph at 10:23 PM on February 10, 2020
This place says they are selling a system vaguely like that in the description. It runs a current and it's for both bio-fouling and cathodic protection. Don't know. Not a lot of details though there is a downloadable paper.
posted by aleph at 10:30 PM on February 10, 2020
posted by aleph at 10:30 PM on February 10, 2020
That looks more like a poison-it-with-copper thing than an unseat-it-with-zappage thing. And biofilms are pretty bloody tenacious, and absorbent mats seem kind of delicious.
Interesting to see whether some kind of deliberate microecology could be engineered into the absorbent mats to make them self-maintaining.
posted by flabdablet at 10:52 PM on February 10, 2020
Interesting to see whether some kind of deliberate microecology could be engineered into the absorbent mats to make them self-maintaining.
posted by flabdablet at 10:52 PM on February 10, 2020
And sure, if you've chosen to deploy these things on the ocean surface instead of pumping water to a large array of them housed in a better-sheltered onshore desalination plant, then you get to trade off brine generation against ocean shading considerations.
Build it on top of the Pacific trash vortex. Done and dusted.
posted by They sucked his brains out! at 11:12 PM on February 10, 2020 [1 favorite]
Build it on top of the Pacific trash vortex. Done and dusted.
posted by They sucked his brains out! at 11:12 PM on February 10, 2020 [1 favorite]
Paper used to be ungodly expensive.
Considering the environmental impact of paper, "used to be," seems to me to be a relative term. Plundering our forests may have been economically effective for the people cutting down trees, but there were a lot of costs that were kept out of the profit equation. We're still paying them today.
posted by Chuffy at 11:28 PM on February 10, 2020
Considering the environmental impact of paper, "used to be," seems to me to be a relative term. Plundering our forests may have been economically effective for the people cutting down trees, but there were a lot of costs that were kept out of the profit equation. We're still paying them today.
posted by Chuffy at 11:28 PM on February 10, 2020
MIT has improved the passive solar still
A solar still, you say?
"...at a rate of 5.78 liters per square meter(per hour)" for the rooftop prototype"
That seems like a lot! Wow!
The system delivered pure water that exceeded city drinking water standards, at... (given rate)".
Water? Oh, uh, that's...that's good, I guess. Uh, never mind.
posted by Huffy Puffy at 4:52 AM on February 11, 2020 [9 favorites]
A solar still, you say?
"...at a rate of 5.78 liters per square meter(per hour)" for the rooftop prototype"
That seems like a lot! Wow!
The system delivered pure water that exceeded city drinking water standards, at... (given rate)".
Water? Oh, uh, that's...that's good, I guess. Uh, never mind.
posted by Huffy Puffy at 4:52 AM on February 11, 2020 [9 favorites]
If you have enough mash, this rig should produce more white-lightning than the claims for clean water (though would need an attendant for proper fractionation - this is basically just a dumb pot-still).
Of course, very shortly clean water will be more valuable.
posted by pompomtom at 5:10 AM on February 11, 2020
Of course, very shortly clean water will be more valuable.
posted by pompomtom at 5:10 AM on February 11, 2020
Seems to me that the most likely failure mode for these things is going to be biological fouling of the evaporator wicks and condensation surfaces. These would need to be made easily accessible for regular cleaning.
Tell me about it. I've pretty much given up on humidifying my apartment this winter. I just can't get the damn things clean. The hilarious manufacturer's recommendation is to clean them every 3 days!
posted by srboisvert at 8:10 AM on February 11, 2020 [1 favorite]
Tell me about it. I've pretty much given up on humidifying my apartment this winter. I just can't get the damn things clean. The hilarious manufacturer's recommendation is to clean them every 3 days!
posted by srboisvert at 8:10 AM on February 11, 2020 [1 favorite]
Just thought of putting these on roofs and pump (small amount of) the "problem" water up to them. Why pay extra to do that? Well, shade *can* be good. Then I'm fascinated by the re-use of heat as it goes *up* the successive distillation stages (the prototype used 10). It isn't a "dumb pot still" if I understand them correctly. This reminds me of a (crude) fractional distillation column I saw back in Organic Chemistry. Besides shade, putting these on the roof would re-use the heat that diffuses upward from the living spaces below.
(snip)
Fractionating columns help to separate the mixture by allowing the mixed vapors to cool, condense, and vaporize again in accordance with Raoult's law. With each condensation-vaporization cycle, the vapors are enriched in a certain component. A larger surface area allows more cycles, improving separation.
posted by aleph at 10:35 AM on February 11, 2020
(snip)
Fractionating columns help to separate the mixture by allowing the mixed vapors to cool, condense, and vaporize again in accordance with Raoult's law. With each condensation-vaporization cycle, the vapors are enriched in a certain component. A larger surface area allows more cycles, improving separation.
posted by aleph at 10:35 AM on February 11, 2020
""used to be," seems to me to be a relative term"
It was a random example off the top of my head. Not a moral parable.
posted by Everyone Expects The Spanish Influenza at 10:39 AM on February 11, 2020
It was a random example off the top of my head. Not a moral parable.
posted by Everyone Expects The Spanish Influenza at 10:39 AM on February 11, 2020
That's about 0.0024 gpm/sf. An off the shelf conventional RO system, not designed for desalination or optimized for brackish water, and designed for low flows (not the large scale that most desal plants are) can do 0.24 gpm/sf.
Is the SF in this square feet of solar collection area?
I ask because my first thought about this was as a survival tool for life rafts and I have heard of hand pump reverse osmosis for life boats. Are you saying that a small RO pump with a meter of solar cell would provide enough water for a person to survive?
posted by Pembquist at 11:02 AM on February 11, 2020
Is the SF in this square feet of solar collection area?
I ask because my first thought about this was as a survival tool for life rafts and I have heard of hand pump reverse osmosis for life boats. Are you saying that a small RO pump with a meter of solar cell would provide enough water for a person to survive?
posted by Pembquist at 11:02 AM on February 11, 2020
Wonder if these, as roof top systems, can provide appreciable cooling? That could be important in the not-too-distant future. And use the heat sucked away from the living quarters below to process/distill "waste" water to recycle that?
posted by aleph at 11:22 AM on February 11, 2020 [1 favorite]
posted by aleph at 11:22 AM on February 11, 2020 [1 favorite]
Making heat do work usually acts like insulation, so I'm guessing it won't provide cooling. One might be able to use the concentrated heat at the end of `sucked away' to run one of these in a small space, but that would make the `sucking' take more energy.
posted by clew at 11:57 AM on February 11, 2020
posted by clew at 11:57 AM on February 11, 2020
In this case it would just be preventing a solar gain rather than doing any work. Assuming the materials can handle the temperatures and they are mounted so air can flow between the building and the panels they would have a significant impact on A/C loading in many cases. Most residences in the US/Canada are, IMO, under insulated for the solar gain they have. Something that will be exasperated by rising temperatures.
posted by Mitheral at 12:35 PM on February 11, 2020 [1 favorite]
posted by Mitheral at 12:35 PM on February 11, 2020 [1 favorite]
The press release quotes "an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation". I have to think that I'm obviously not understanding something about this because I interpreted an efficiency >100% as something that would imply free energy and perpetual motion machines, which is obviously not what they would be claiming.
I looked into the cited article, and it seems that they're defining "efficiency" as:
posted by mhum at 12:54 PM on February 11, 2020
I looked into the cited article, and it seems that they're defining "efficiency" as:
(water vapor production rate * enthalpy of water) / (solar flux * absorbing area)Working out the units, I get that the numerator and denominator are both in watts, so we're essentially looking at the comparison of two measures of power (i.e: energy per unit time). So, I'm guessing the part I'm missing is the relationship between solar flux and maybe how the heat is stored up and/or propagated through the system? It seems that one of the key advancements in this design is how they capture otherwise wasted heat from one stage and reuse it in the next stage. But, for now, it just seems a bit odd to me that for 1 watt of sunlight, you can get 3.85 watts worth of water evaporation. I mean, where did the other 2.85 watts come from?
posted by mhum at 12:54 PM on February 11, 2020
Something that will be exasperated by rising temperatures.
The occupants of poorly insulated buildings do indeed become exasperated by the inability to cool their space with even unreasonably large amounts of energy. This exasperation will certainly be exacerbated by rising average temperature.
posted by wierdo at 1:48 PM on February 11, 2020 [1 favorite]
The occupants of poorly insulated buildings do indeed become exasperated by the inability to cool their space with even unreasonably large amounts of energy. This exasperation will certainly be exacerbated by rising average temperature.
posted by wierdo at 1:48 PM on February 11, 2020 [1 favorite]
mhum, I agree that it's a weird way to define efficiency, and I think it'd be better to say that it's 285% more efficient than a simple solar still. It takes some heat energy to turn water to vapor, but then the vapor condenses to water again, releasing that heat back into the system. So the same 1 watt of power is used up to 10 times over the whole process. It's not a free energy machine because you can't produce 3.85 watts of power by mixing the distilled water with the brine.
posted by ectabo at 2:39 PM on February 11, 2020 [1 favorite]
posted by ectabo at 2:39 PM on February 11, 2020 [1 favorite]
> ectabo: "So the same 1 watt of power is used up to 10 times over the whole process."
Indeed. For my own sanity, I've constructed a mental model where you have, say, one joule entering the system per second but they can keep that one joule hanging around for 3.85 seconds (by being used over and over again), so it essentialy turns into 3.85 watts worth of evaporation. This is probably grossly inaccurate, but this mental fiction will at least help me sleep tonight.
posted by mhum at 4:52 PM on February 11, 2020
Indeed. For my own sanity, I've constructed a mental model where you have, say, one joule entering the system per second but they can keep that one joule hanging around for 3.85 seconds (by being used over and over again), so it essentialy turns into 3.85 watts worth of evaporation. This is probably grossly inaccurate, but this mental fiction will at least help me sleep tonight.
posted by mhum at 4:52 PM on February 11, 2020
for now, it just seems a bit odd to me that for 1 watt of sunlight, you can get 3.85 watts worth of water evaporation. I mean, where did the other 2.85 watts come from?
Heat of condensation, less heat of solution, less losses.
I agree that dividing the total heat of internal evaporation by the total heat power input is kind of a weird way to calculate an "efficiency" because it completely ignores the system's end products. It's a bit like saying that a fire achieves billions of percent efficiency by dividing the energy required to evaporate gasoline by the energy output of a flaring match.
posted by flabdablet at 6:05 PM on February 11, 2020
Heat of condensation, less heat of solution, less losses.
I agree that dividing the total heat of internal evaporation by the total heat power input is kind of a weird way to calculate an "efficiency" because it completely ignores the system's end products. It's a bit like saying that a fire achieves billions of percent efficiency by dividing the energy required to evaporate gasoline by the energy output of a flaring match.
posted by flabdablet at 6:05 PM on February 11, 2020
FTFA:
Wang emphasizes that the team’s key contribution is a framework for understanding how to optimize such multistage passive systems, which they call thermally localized multistage desalination. The formulas they developed could likely be applied to a variety of materials and device architectures, allowing for further optimization of systems based on different scales of operation or local conditions and materials.
Yes they used aerogel, yes there are a lot of areas for improvement, and they know this. What is the net new thing is they’ve developed a model that also for swapping out parts and measuring efficiency. Maybe add more stages of cheaper more common available material instead of needing aerogel.
It’s a vertical still, place the bottom end in a pool of salt water and each successive chamber does an evaporation and filtration. The key here is it’s continuous - you aren’t cooking the chambers dry, you’re just extracting cleaner water at each stage as you go up the still. These folks have put together a method to help inform how to best design, stack, and measure each chamber and see how it effects the others. It’s a pretty big deal even if it’s not going to be next years Lifestraw product, it’s going to make it easier to build other systems.
posted by mrzarquon at 7:19 PM on February 11, 2020 [1 favorite]
Wang emphasizes that the team’s key contribution is a framework for understanding how to optimize such multistage passive systems, which they call thermally localized multistage desalination. The formulas they developed could likely be applied to a variety of materials and device architectures, allowing for further optimization of systems based on different scales of operation or local conditions and materials.
Yes they used aerogel, yes there are a lot of areas for improvement, and they know this. What is the net new thing is they’ve developed a model that also for swapping out parts and measuring efficiency. Maybe add more stages of cheaper more common available material instead of needing aerogel.
It’s a vertical still, place the bottom end in a pool of salt water and each successive chamber does an evaporation and filtration. The key here is it’s continuous - you aren’t cooking the chambers dry, you’re just extracting cleaner water at each stage as you go up the still. These folks have put together a method to help inform how to best design, stack, and measure each chamber and see how it effects the others. It’s a pretty big deal even if it’s not going to be next years Lifestraw product, it’s going to make it easier to build other systems.
posted by mrzarquon at 7:19 PM on February 11, 2020 [1 favorite]
So how is this different from a packed reflux column (I'm guessing it's something about the wicking-away of the brine....)? Anyone got a diagram?
posted by pompomtom at 7:45 PM on February 11, 2020
posted by pompomtom at 7:45 PM on February 11, 2020
Anyone got a diagram?
The paper is linked from TFA and available in both HTML and PDF formats.
you aren’t cooking the chambers dry, you’re just extracting cleaner water at each stage as you go up the still
The stages don't extract progressively cleaner water, just progressively less. The only thing that moves from stage to stage is heat; all stages are wick-fed from the same salt water source.
Also, the only thing preventing salt buildup inside the evaporation chambers over time is the day and night cycle. If the still wasn't given a chance for concentrated brine to diffuse back out through the wicks and into the source seawater overnight, it certainly would supersaturate at the far end of the wicks and crystallize out inside the chamber. There is a section in the paper about testing the speed at which that happens and demonstrating that an overnight rest period is enough to correct it.
posted by flabdablet at 8:40 PM on February 11, 2020 [1 favorite]
The paper is linked from TFA and available in both HTML and PDF formats.
you aren’t cooking the chambers dry, you’re just extracting cleaner water at each stage as you go up the still
The stages don't extract progressively cleaner water, just progressively less. The only thing that moves from stage to stage is heat; all stages are wick-fed from the same salt water source.
Also, the only thing preventing salt buildup inside the evaporation chambers over time is the day and night cycle. If the still wasn't given a chance for concentrated brine to diffuse back out through the wicks and into the source seawater overnight, it certainly would supersaturate at the far end of the wicks and crystallize out inside the chamber. There is a section in the paper about testing the speed at which that happens and demonstrating that an overnight rest period is enough to correct it.
posted by flabdablet at 8:40 PM on February 11, 2020 [1 favorite]
I have a question about desalination: Does this not, over time and in aggregate, with increasing scale given changing water supply for municipalities, increase the salinity of the ocean? Would that not have dangerous effects for sea life and humans alike?
Something I don't understand about water supply problems is: where has all the water gone? When Australia becomes more arid, does the water previously present dissipate and drift all the way to other continents? Does it wash into the ocean? Are now-polluted water sources also a part of these problems with maintaining adequate supply? Can anyone with relevant expertise enlighten me?
posted by constantinescharity at 11:43 AM on February 12, 2020
Something I don't understand about water supply problems is: where has all the water gone? When Australia becomes more arid, does the water previously present dissipate and drift all the way to other continents? Does it wash into the ocean? Are now-polluted water sources also a part of these problems with maintaining adequate supply? Can anyone with relevant expertise enlighten me?
posted by constantinescharity at 11:43 AM on February 12, 2020
I have a question about desalination: Does this not, over time and in aggregate, with increasing scale given changing water supply for municipalities, increase the salinity of the ocean?
Ultimately essentially all fresh water returns to the Oceans eventually (a tiny fraction gets pumped underground). Using desalinization to produce fresh water in a location from sea water results in more salty sea water (brine) local to the plant but that effect is unnoticeable when talking about the average salinity of the whole ocean. It still can be bad or really bad locally and brine is a major consideration when siteing desalination plants. And the fresh water is eventually returned to the sea either directly as waste water or indirectly via evaporation and rainfall.
posted by Mitheral at 2:48 PM on February 12, 2020
Ultimately essentially all fresh water returns to the Oceans eventually (a tiny fraction gets pumped underground). Using desalinization to produce fresh water in a location from sea water results in more salty sea water (brine) local to the plant but that effect is unnoticeable when talking about the average salinity of the whole ocean. It still can be bad or really bad locally and brine is a major consideration when siteing desalination plants. And the fresh water is eventually returned to the sea either directly as waste water or indirectly via evaporation and rainfall.
posted by Mitheral at 2:48 PM on February 12, 2020
Wikipedia's introduction to the water cycle is pretty good. Also worth bearing in mind is that the oceans are not some kind of final resting place for dissolved salts, all of which have their own cycles (see also: subduction).
posted by flabdablet at 4:01 PM on February 12, 2020
posted by flabdablet at 4:01 PM on February 12, 2020
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At about $23,000 per pound, aerogel is currently more expensive than gold ... well ok, that's a smidge ingenious as it's very light, no one buys an ounce let alone pounds, but a panel seems to be in the low hundreds, and times 10 stages, maybe not for the folks that need it most. Cool project though.
posted by sammyo at 7:21 PM on February 10, 2020 [3 favorites]