[HN Gopher] Simple, solar-powered water desalination (2020)
___________________________________________________________________
Simple, solar-powered water desalination (2020)
Author : drran
Score : 580 points
Date : 2021-07-02 06:33 UTC (1 days ago)
(HTM) web link (news.mit.edu)
(TXT) w3m dump (news.mit.edu)
| raarts wrote:
| [Feb 2020]
| rorykoehler wrote:
| Would it be possible to re-green deserts with this technology?
| Does anyone know of any studies on using this technology to
| combat climate change?
| thinkcontext wrote:
| Not likely. This is much less cost efficient at scale than
| desalination through reverse osmosis (RO). And RO itself is
| generally too expensive to be used for large scale irrigation.
| underwater wrote:
| Yishan Wong, of Reddit CEO infamy, has been working on this for
| a few years: https://www.terraformation.com/
| kevmo wrote:
| I seriously considered taking a job there (I did not for
| personal reasons, namely not wanting to move) and would
| recommend looking at them to others.
|
| I think they are poised to be a key player in the forestry
| and reforestation space in the next 2 decades.
| justshowpost wrote:
| But... the reforestation occurs naturally for _last_ 2
| decades (at least, more like for 3 or even 4)
| jarenmf wrote:
| Greening deserts is not a good solution for climate change. In
| fact it might worsen climate change due to albedo changes. A
| nice video about this:
| https://www.youtube.com/watch?v=lfo8XHGFAIQ
| youngtaff wrote:
| Challenge with that video is it takes a very industrial
| approach to green deserts - mono cultures for trees, massive
| desalination plants to provide water etc
|
| Some of the original ideas for greening deserts are
| permaculture based
| (https://www.youtube.com/watch?v=sohI6vnWZmk) which are a way
| more sustainable approach but won't bring around carbon
| reduction fast enough.
| carl_dr wrote:
| This device "could" produce 1.5 gallons per day, per square metre
| of solar collecting area. The researchers estimate a system
| suitable for a family might be built for $100, so double or
| triple that after profits etc.
|
| I assume that the 1.5 gallons was the highest yield they would
| have had from a single day, had they had a 1 square metre solar
| panel, and the expected average yield would be less (accounting
| for cloud cover etc.)
|
| But even with reduced yield, for very small communities of a few
| dozen people, this definitely could work out.
| tromp wrote:
| The article says "per hour", not per day. Although that would
| seem able to support dozens of users rather than just 1.
| kingsuper20 wrote:
| The average US citizen uses 82 gallons of potable water per
| day.
| finiteloops wrote:
| Sounded astonishingly high, but a quick search from a few
| sources (one cited [1]) checks out. Now to go down a rabbit
| hole of how this 82 gallons breaks down and what I can do
| to reduce my consumption...
|
| [1] https://www.usgs.gov/special-topic/water-science-
| school/scie...
| carl_dr wrote:
| Thanks for spotting that.
|
| I'm a little confused then, later in the article it says "The
| team estimates that a system with a roughly 1-square-meter
| solar collecting area could meet the daily drinking water
| needs of one person." So something doesn't add up.
|
| [Edit: the paper is available at https://pubs.rsc.org/en/cont
| ent/articlehtml/2020/ee/c9ee0412.... It says, "To meet the
| average daily water intake for one adult ([?]3.2 L),49 100
| TMSS devices can be placed into a 10 x 10 array, filling an 1
| m2 area, which would provide approximately 10-20 L of clean
| water every day depending on the weather condition."
|
| The lower bound of 10l is outdoor performance on a partly
| sunny day. So I think they are just being conservative by
| saying it would meet the requirements of one person - coastal
| areas are frequently cloudy, and in some locations, there
| might be little sun for extended periods of time.]
| londons_explore wrote:
| 'drinking water' often doesn't mean the water a human
| literally drinks, but all the clean water necessary for
| living everyday life (bathing, flushing toilets, washing
| dishes, cleaning clothes, cooking etc.).
| celticninja wrote:
| I am pretty sure if you had to get your water from one of
| these devices you would not be using it to flush toilets
| or other non-essential use.
|
| And actually it does seem like 'drinking water' is
| actually drinking water and not for all those other uses
| you mentioned. As 3.7 litres is the amount required for a
| human male each day.
|
| https://www.mayoclinic.org/healthy-lifestyle/nutrition-
| and-h...
| lyschoening wrote:
| From Wikipedia:
|
| > Drinking water, also known as potable water, is water
| that is safe to drink or use for food preparation.
|
| 40-50L per day would seem to about cover one person's
| essential uses of potable water.
| kingsuper20 wrote:
| >I am pretty sure if you had to get your water from one
| of these devices you would not be using it to flush
| toilets or other non-essential use.
|
| Are you suggesting that toilets, showers, washing
| machines use salt water?
| celticninja wrote:
| Is there a problem with flushing a toilet with salt
| water?
| kingsuper20 wrote:
| Offhand I'd say that you are doubling the plumbing system
| plus you are using a much more corrosive fluid.
|
| It would be an interesting experiment, since toilets use
| a great deal of water. Take a beach town, build a
| separate pressure water and sewer system, see if it's
| worth it. It's certainly more complicated than a
| sailboat's set up.
| maximus-decimus wrote:
| If their toilets used clean water, then I have a much
| more efficient way to produce clean water : drink from
| the toilet and shit in a hot house.
| Covzire wrote:
| As I understand desalination, a huge bottleneck besides cost is
| what to do with the brine. Why can't they build a gigantic
| pool/lake out in the desert and pump the brine there and let it
| dry into salt and other minerals. A 10 square mile pit/pool with
| a clay or other barrier to prevent it from sinking into the
| aquifers would allow for a massive amount of brine to be safely
| contained in one place wouldn't it?
| okareaman wrote:
| brine disposable seems like a made up problem to me. We're not
| going to raise the salt content of the world's oceans by
| pumping the salt back from whence it came. I understand we
| don't want to put the high salt discharge near sea life, but
| that is not a hard problem.
| travisporter wrote:
| > We're not going to raise the salt content of the world's
| oceans by pumping the salt back from whence it came.
|
| Sorry, maybe dumb question but why not? We're taking out
| water and leaving behind salt
| e12e wrote:
| Partly we're taking out small amounts of water (compared to
| the size of oceans) - secondly some of the water is likely
| to come back via eg sewer treatment plants.
| okareaman wrote:
| I meant the salinity content per liter of the world's
| oceans would rise an insignificant amount, especially since
| fresh water flows back to the sea for the most part. If
| someone can tell me otherwise, I'm ready to be educated.
| mixmastamyk wrote:
| It takes a while to disperse. Meanwhile salinity in the
| region has gone up, say 2%. Apparently sea life is very
| sensitive to it.
|
| I agree there are a number of solutions though. Adding to
| treated sewage is a good one.
| okareaman wrote:
| I grew up in a town with a gray water pipe to the ocean.
| When the environmentalist movement took hold, it was
| decided the pipe should be extended further into the
| ocean so as not to disturb the ecosystem. They got it
| done, so I know even a small town can do this.
| dntrkv wrote:
| Seriously, so many possible solutions.
|
| You can sell it.
|
| You can give it away for free.
|
| You can turn it into something else and use it/sell it.
|
| You can mix it with treated sewage water flowing into the
| ocean.
|
| You can mix it with fresh water flowing into the ocean.
|
| You can discharge it over a larger area.
|
| It's not a "real" problem.
| philwelch wrote:
| The logistics would be a challenge. You'd have to transport the
| brine from the coast to the desert. In some places this would
| be a relatively short trip (Israel, North Africa, California)
| but in other places you'd be shipping a lot of heavy fluid over
| long distances. And you don't even have someone buying the
| stuff on the other end; you're just disposing of it.
| dsign wrote:
| This reminds me of how many places we have on the planet which
| are beautiful but inhospitable. Fixing the inhospitable part will
| come its own environmental impact, and the places will end up
| being less "beautiful". I vote we go to live elsewhere and leave
| the planet as a giant touristic resort :) .
| crackercrews wrote:
| For those curious what the 385% refers to
|
| > 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.
|
| Honestly I still don't know what that means, or how efficiency
| can be over 100%.
| ordu wrote:
| _> Honestly I still don 't know what that means, or how
| efficiency can be over 100%._
|
| They explain it in their article[1]:
|
| "the solar-to-vapor conversion efficiency, defined as the ratio
| of total vaporization enthalpy to total solar energy input, for
| most previous studies has been limited to below 100% as the
| vaporization enthalpy is lost to the ambient environment."
|
| [1]
| https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0...
| est wrote:
| I guess water evaporates naturally, adding sunlight speed up
| the process.
| OJFord wrote:
| And my perpetual motion machine uses naturally running
| hamsters.
| raxxorrax wrote:
| Exposing a deep body of water to sunlight is probably the
| reference.
|
| edit: deep as in practically having unlimited heat capacity and
| the heat conductivity of water.
| legulere wrote:
| My guess is that they get 3.85 times as much water evaporated
| as the energy used to just normally evaporate water. They did
| that by also using the energy from condensation to be put in
| the process again.
| kbenson wrote:
| I think maybe it means "3.85 times as efficient as sunlight in
| evaporative filtering" ?
| [deleted]
| ComodoHacker wrote:
| It's an example of a good old scientifically flavored
| clickbait.
|
| The energy efficiency of anything cannot exceed 100% (until I
| missed something groundbreaking in physics).
| hans_castorp wrote:
| How can it be "click bait" if neither the title nor the
| headline contains that number? You have to read at least the
| first three paragraphs of the article to stumble about that
| number. The focus of the article is on the inexpensive
| design, not on its efficiency
| crackercrews wrote:
| The HN title has been changed. It used to refer to the 385
| percent claim. Probably GP thought this was from the title.
| trainsplanes wrote:
| It very much can exceed 100% and it's not a measure invented
| for click bait purposes.
|
| Look up heat pumps. You can make things hotter by moving heat
| than you could by directly heating it by burning fuel.
| stephen_g wrote:
| The 'more than 100%' is only in comparison to a less
| efficient process. A heat pump itself is operating below
| 100% efficiency - it's just kind of "cheating" by using an
| external source of external energy as one of its inputs in
| addition to the electricity running it.
| MrsPeaches wrote:
| As another comment pointed out, it's usually called the
| Coefficient of Performance in this case.
|
| Which is to say this is click bait, because saying it has a
| COP of 3.85 isn't anywhere near as sexy, whilst being
| technically more accurate.
| TheSmiddy wrote:
| I think it means 3.85x more water is evaporated compared to
| just leaving the water outside in the sun.
| OJFord wrote:
| I'm not even sure what 'the energy of water evaporation' means?
| Gravitational potential of the mass of water evaporated (and
| condensed at some height)?
| fuzzybear3965 wrote:
| I think they mean latent heat/enthalpy of evaporation.
| eigart wrote:
| My understanding: As the water condenses onto the next surface
| layer in the stack, the solar heat is recycled. This is because
| the transition from gas to liquid releases heat.
|
| Really clever stuff!
|
| Edit: mixed up evaporate/condense
| ppf wrote:
| For that to work, each successive layer will need to remain
| cool enough for water to condense on it.
| Retric wrote:
| It's due to changes in pressure. https://upload.wikimedia.o
| rg/wikipedia/commons/thumb/3/33/Ph...
|
| At 5ATM water condenses at a higher temperature than it
| boils at 4ATM.
| TeMPOraL wrote:
| Others already explained where the >100% efficiency comes from,
| but I want to point out that a good 1/4th of the article is
| repeatedly explaining how this works, over a couple of
| paragraphs.
| amelius wrote:
| Reading it in the comments was still more efficient than
| reading the article.
| achn wrote:
| 220% more efficient.
| ajb wrote:
| Its disingenuous. We know what the theoretical best efficiency
| of desalination is, by thermodynamics. You can calculate it by
| assuming a completely ideal reverse osmosis setup. Compared
| with that, the efficiency would be less than 100%. This article
| takes 'efficiency' as compared to just evaporating the water
| and not reclaiming any energy on condensation.
| 867-5309 wrote:
| I thought it would be something impressive like somehow using
| the obtained salt for further powering of the device but nope,
| another clickbait trip into bullshitland
| wcoenen wrote:
| The 100% level refers to a system where all energy goes to
| heating up water in order to evaporate it, and then letting it
| cool down to condense it. All the energy that was spent to heat
| up the water, is lost to the environment in the "cooling down"
| step.
|
| If some of the heat is instead recovered during condensation to
| heat up the next batch of water, then you have >100%
| efficiency.
| raducu wrote:
| Does it have to be batches?
|
| Can't it be like the reverse of a rocket engine where they
| use regenerative cooling from the fuel to cool the rocket
| nozzle, but just in reverse.
|
| Or like they way my grandpa was doing moonshine -- the
| alcohol vapors pass through a serpentine in a water tank,
| condense, at the end you obtain alcohol, the water in the
| tank gets warmer -- instead, heat water coming from the water
| cooling tank that is preheated by the vapors of water that is
| condensing in the serpentine pipe.
| alpaca128 wrote:
| Unless you ignore energy sources from the environment you
| cannot exceed 100% efficiency. And that would be incorrect,
| applying that same standard to photovoltaic panels would
| result in infinite efficiency. That doesn't make any sense.
| Edit: Also, when you take heat/energy from a previous step in
| the process, you also need to account for the energy put into
| that previous step. In the end that will again be <100%.
|
| As someone else already pointed out, this would be called
| Coefficient of Performance. Efficiency is clearly defined and
| cannot exceed 100% without breaking laws of physics. Call it
| "3.85 times more efficient than before" or something along
| those lines and it won't sound like a free energy claim.
| afiori wrote:
| the full sentence is:
|
| the [..] device can achieve an overall efficiency of 385
| percent in converting the energy of sunlight into the
| energy of water evaporation.
|
| it seems quite clear how the 3.85 ratio is obtained
| Dylan16807 wrote:
| > applying that same standard to photovoltaic panels would
| result in infinite efficiency
|
| No it wouldn't. The equivalent for panels would be like...
| you want to run some number of watts through a diode, and
| you're using solar panels to collect this power. The diode
| happens to give off waste light. By aiming this light at
| your panels, you can recapture most of it back into
| electricity, and reuse it 2.85 more times.
| readflaggedcomm wrote:
| It quantifies the heat re-used between stages. In the
| supplement, they note 600% is the maximum. The derivation is at
| the bottom of p834 from the journal pdf.
|
| It's basically vapor produced at the measured average
| temperature divided by energy input.
| crackercrews wrote:
| If 600% is the maximum, then what does this mean?
|
| > Theoretically, with more desalination stages and further
| optimization, such systems could reach overall efficiency
| levels as high as 700 or 800 percent, Zhang says.
| readflaggedcomm wrote:
| I think it tracks how well the materials they're using move
| heat between stages or lose it to the atmosphere. Their
| modelling (supplement figure 2, mentioned on page 4)
| depends on their specific construction. I wonder what it
| would do with gold as the plate material and aerogel
| everywhere else...
| goodcanadian wrote:
| Usually, numbers over 100% mean that you are putting in less
| energy than is needed for the process. In this case, that would
| mean putting in a little over a quarter of the needed energy.
| That does not imply free energy or anything. The rest of the
| energy has to come from the environment.
|
| A heat pump is another common example with efficiency numbers
| in the same ballpark. With a heat pump, the heat is being moved
| from outside to inside (or vice versa for air conditioning and
| refrigerators). In that case, it requires, for example, 1kW of
| electricity input to move 3.85kW of heat.
| kwhitefoot wrote:
| Efficiency is still the wrong name. What you are describing
| is usually referred to a _coefficient of performance_ ,
| abbreviated to _COP_.
|
| See https://en.wikipedia.org/wiki/Coefficient_of_performance
| faceplanted wrote:
| The Deja vu sensation of this conversation happening almost
| identically 2 days ago (everything from wondering how >100%
| efficiency works, someone explaining it's from the
| environment, and then someone else explaining efficiency is
| inappropriate and COP exists) is kinda wild.
| adolph wrote:
| Nothing to worry about. Its people alpha-testing the
| Copilot internet commenting plugin.
| SamBam wrote:
| Can't wait for all our text input boxes on the web to be
| GPT-3-enabled, and then all comments on HN and Reddit and
| Twitter will be just people accepting the defaults, and
| it will end up just being GPT-3 talking to itself, and we
| can all go back to doing something productive.
| Scoundreller wrote:
| Reminds me of a post here where it was some neural
| network chat/excuse generator. Just two people back and
| forthing about why they couldn't meet up.
|
| I lost it when one of the excuses was, 3 weeks later "oh,
| I can't meet for lunch on July 29th, I have to go to my
| mother's funeral"
| SamBam wrote:
| Amy way you can link to that conversation? I tried to
| find it on Google.
| Scoundreller wrote:
| I think the convo is different each time. You'd have to
| search here for the chat itself. It was 2-3 months Ago
| adolph wrote:
| Pretty sure FB has several instances of itself with all
| the users played by GPT-3 trained on the users' previous
| activities so they can monte carlo various changes, like
| pre-A|B testing or estimating impact of various new ad
| types or congressional testimonies.
| mrfusion wrote:
| Relevant xkcd: https://xkcd.com/810/
| m463 wrote:
| I wonder if concentrating solar cells should have similar
| nomenclature.
| theiasson wrote:
| The paper[0] goes into more detail about how the efficiency is
| calculated (scroll down a bit to see the actual formula).
|
| [0]
| https://pubs.rsc.org/en/content/articlehtml/2020/ee/c9ee0412...
| dmos62 wrote:
| I think of it as taking the combustible fuel energy required
| for a car's motor to drive it somewhere, divided by the energy
| required to turn the ignition key and press the pedal.
| timonoko wrote:
| You need exactly 3 litres per day. Or rather: I do and I did it
| for several years. This would mean tiny portable and durable
| device and it would make many places totally habitable. Not just
| Baja California.
| walrus01 wrote:
| > 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
|
| I would be very interested to see data on this vs. sunlight and
| climate conditions, in what weeks/month of the year they tested
| it. I think its effectiveness would be highest at MIT's location
| from late April to end of September and considerably less in
| colder/overcast/less sunny weather and winter.
| kzrdude wrote:
| Is the water pumping also passive? I'm just assuming that you'll
| have salt water on a lower level, and you'll need to lift it up
| to this installation to process it.. with a pump.
| h0l0cube wrote:
| The article suggests it could float:
|
| > 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.
| sonicggg wrote:
| What's the point here? Is electricity even the bottleneck of
| water desalination? I thought the biggest issue with these plans
| is the brine. That is the major pollutant, and no amount of solar
| panels will solve that.
| hnhg wrote:
| The article directly addresses your point.
| Johnythree wrote:
| It doesn't. Any salt extraction creates brine which must be
| disposed of somehow.
| neilwilson wrote:
| The trick is to flow seawater past the still and take only,
| say, 1% of the water from each litre that flows past, which
| you can do with a still as opposed to a filter.
|
| It's a bit like slingshotting a space probe past venus.
| Technically to move Venus a bit closer to the sun, but not
| enough to make a difference.
| shoto_io wrote:
| Reading the article might help:
|
| _> 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._
| Johnythree wrote:
| Your rely is not helpful. The problem with any desalination
| system is that it creates brine. And brine in large amounts
| is toxic to the marine environment.
|
| The only difference with this example is its small scale.
| Once it is scaled up it will have the same problem as any
| existing desalinating plant.
| ghshephard wrote:
| But you would never scale this system up - it's entirely
| inefficient compared to reverse osmosis, on a cost basis.
| This is the type of system that you would use for just a
| few people, and as such - brine would never be an issue.
| andromeduck wrote:
| It makes no difference how concentrated the output brine is
| if it's the diluted to same concentration at the outlet and
| the same amount of fresh water is extracted.
| Johnythree wrote:
| You completely miss the point. Extracting fresh water from
| salt water creates brine and in a large scale plant it is
| extremely difficult to dilute it sufficiently.
|
| The only answer is to pipe the brine into areas with strong
| currents, or dilute it by distributing it over a very large
| area.
| spockz wrote:
| Yes. So it is not an issue for the device. However, the salt
| does end up back in the ocean where at scale the salt levels
| will be higher leading to the (edit: local) environmental
| issues mentioned by the GP.
| kitd wrote:
| Is the water cycle not a thing any more?
| pa7x1 wrote:
| Not an expert but I could imagine some local negative
| effects. Salt concentration may get higher in the area
| where the brine is returned. Depending on volume,
| currents, etc...
| shellfishgene wrote:
| A large part of the fresh water generated may be used for
| watering crops or gardens, and would thus evaporate in
| part. The evaporated water in desert areas would probably
| rain back down far way, and take long to mix with the sea
| near the plant again. For example, the Red Sea has much
| higher salinity that other oceans due to high evaporation
| and narrow connections to the other oceans. I doubt the
| effect is large though.
| EricE wrote:
| Indeed - the dead sea has stupid high salinity because
| the only way water leaves is via evaporation :)
|
| It is a very odd sensation to "swim" in the dead sea.
| Even floating is pretty hard because you are so buoyant.
| It's probably the closes to true weightlessness I will
| ever get.
| FartyMcFarter wrote:
| The sea is a big place. I could imagine perhaps some very
| localised effects near the plant, but even this isn't
| obvious.
| Cthulhu_ wrote:
| > imagine > perhaps > but even this isn't obvious
|
| So basically you're admitting you don't know anything
| about the subject, but you're making a conclusion anyway.
| It's OK to admit you don't know enough about a subject,
| and it's OK to not comment or theorize based on no
| knowledge besides an imagination.
| [deleted]
| andai wrote:
| A man once said, "imagination is more important than
| knowledge."
|
| That man's name? _Albert Einstein._
| pjerem wrote:
| I didn't know that making assumptions while clearly
| saying you are making assumptions is forbidden on HN.
| Arnt wrote:
| Brine is the biggest issue with some desalinators, not this
| one.
|
| Power can be a constraint, too. It depends on how much power
| you have available or can pay for, how much fresh water you
| need and how pure, how much you can buffer, etc. In short,
| power/cost is a constraint sometimes.
| connorproctor wrote:
| This doesn't use electricity at all, the "solar power" is heat
| from the sun causing the water to evaporate. It's a still, not
| a filter. According to the article the salt/brine freely flows
| back into the body of water.
| Johnythree wrote:
| Which by definition will increase the salinity in the local
| environment.
| tenfourwookie wrote:
| I imagine a trillion of these floating along the coast of the
| western United States, little landing pads (and solar powered
| charging stations) for a trillion drones that can move the
| freshly desalinated water from the ocean to the nearest forest
| fire. You need a drone cloud 500 million strong to deliver 100
| million gallons of fresh water to X location immediately? No
| problem. Need a drone cloud to water your 20,000 acre vineyard?
| Coming right up. Who needs clouds?
| afterburner wrote:
| A trillion? Well I guess efficiency doesn't concern you...
| tenfourwookie wrote:
| It's wild ass guess, but imagine a drone fleet capable of
| delivering that quantity of water on demand.
|
| We will see this in our lifetimes. Rain on demand.
| jvanderbot wrote:
| I don't think we will. Except in the case of fighting fires
| in remote areas, if you want water in some area, you use
| pipes.
|
| For fighting remote fires, we already have piloted vehicles
| and in some cases uncrewed vehicles delivering water in
| bursts ... so perhaps.
| snek_case wrote:
| Water is very heavy though. For watering crops I think you
| would do better in terms of energy efficiency with land-based
| infrastructure. At the very least, if you're going with drones,
| you ideally want something more energy-efficient than
| quadcopters.
| [deleted]
| ppf wrote:
| It's incredibly disappointing to see MIT put its name to some
| apparently PhD-level research that claims over-unity efficiency,
| and apparently (according to the picture) using a few baking
| trays and tinfoil.
|
| It also does not explain how to deal with the increased
| salination of the water source, nor how water can continue to
| condense on the successive layers of the device as they are
| heated due to that condensation.
| linschn wrote:
| This is for small scale, family-sized units, where power and
| cost are the limiting factor, so brine will not be an issue at
| this scale.
|
| As for condensation, the plate is hot, but colder that the
| vapor, and heated by the vapor only, so when it is too hot,
| condensation stops, the temperature drops quickly due to
| evaporation on the other side, and condensation can continue.
|
| At equilibrium, each successive plate is colder than the next,
| the last one using the sea water as a heat sink.
|
| This is actually innovative and the optimization of the design
| parameters (e.g. The distance between the plates) is not
| trivial.
|
| I think you are overly dismissive of this solid piece of
| engineering.
| einpoklum wrote:
| > The system delivered pure water
|
| That's toxic! We can't drink pure water. You have to maintain
| _some_ of the minerals in there, you just want to take most of
| the NaCL out.
|
| Does the system avoid this over-distillation? i.e. is the water
| properly potable, or is it just not-salty?
| 6nf wrote:
| I drink distilled water all the time, it's not toxic in normal
| quantities.
| einpoklum wrote:
| Ok, fair enough, but if that's all the water you drink than
| you will have to get a bunch of minerals elsewhere in your
| diet.
|
| Here: https://naturalhealthfundamentals.com/is-distilled-
| water-saf...
|
| it says:
|
| Those who may need to be cautious when drinking it:
|
| * Anyone who is already deficient in minerals. You may be
| someone who would benefit from the little extra minerals
| water can give you. Although if you are already deficient in
| minerals, and have health issues, make sure to get your water
| from a good source to avoid all the chemical additives from
| most tap waters.
|
| * There also isn't much information on how well we absorb
| inorganic minerals from water. There is also the possibility
| that the minerals present in the water are not doing us much
| good. But since it could be helpful it probably doesn't hurt
| to have the minerals there.
|
| * Someone who has health issues or malabsorption problems.
|
| * Anyone who has an extremely poor diet and doesn't get many
| minerals from their food should remineralize distilled water
| if they are drinking it.
| c618b9b695c4 wrote:
| It is my understanding that it is incredibly difficult to keep
| water pure. Transporting it any amount of distance from the
| source is likely to leech quite a few minerals from the
| available pipes.
| isthisnametaken wrote:
| Sorry, what? Pure water isn't toxic. And you can get minerals
| from other sources.
|
| That's like saying tap water is toxic because it doesn't
| contain a balanced diet of vitamins.
| regularfry wrote:
| Distilled water isn't toxic per se, but it's definitely got
| harmful effects. It'll leach minerals out of your teeth, for
| instance.
| jokoon wrote:
| Just add some artificial minerals, it can come in some powder
| form I guess.
| regularfry wrote:
| A few years back Coca-Cola got in trouble by doing exactly
| this. What they were doing was taking tap-water, and
| purifying it so they could sell it as "Dasani" - not a
| mineral water product, exactly, but a "lifestyle drink" they
| called it.
|
| The thing about water when it's pure enough is that it's
| remarkably odd. It's unpleasant stuff: it'll weaken concrete
| if it leaks onto it, apparently it'll damage brass and steel,
| and it'll leach the minerals out of your teeth, but only
| until the water is no longer pure enough to do so. So what
| people do is, as you say, add buffer minerals back into the
| purified water to take the edge off.
|
| What Coca-Cola got wrong was the dosing: they put ten times
| the amount of buffer minerals into the water as they should
| have, and _made_ it toxic. So they 'd taken a perfectly safe
| tap-water supply and ruined it.
| throwaway894345 wrote:
| The comments here are all upset about the efficiency number
| presented in the article, but I was hoping for more commentary on
| the 1.5 gallons per square meter thing with respect to improving
| lives in the developing world.
| eterevsky wrote:
| > 1.5 gallons of fresh drinking water per hour for every square
| meter
|
| 7 litres per m^2 per hour.
| femto wrote:
| A comparison with reverse osmosis:
|
| 2.46kWh/m^3 (energy consumed per unit of water produced) is
| claimed for reverse osmisis [1]. This equates to 8.86MJ/m^3.
|
| Output for this still is 7L/(m^2.h)
|
| Assume a solar flux of 1kW/m^2.
|
| Energy consumed per unit of water is 1kW/m^2 / [7L/(m^2.h)] =
| 3.6MJ/0.007m^3 = 514MJ/m^3.
|
| Assuming the above is correct (check anyone?), the still uses
| 58 times more energy than reverse osmosis. The solar energy may
| be "free" but with a 20% PV cell efficiency a reverse osmosis
| system would produce about 11 times more water per unit area of
| solar collector?
|
| [1] http://www.ijesd.org/papers/243-B20001.pdf
| jeroenhd wrote:
| > In production, they think a system built to serve the needs
| of a family might be built for around $100.
|
| I think this is much more important to a great many people
| than the theoretical efficiency of reverse osmosis.
| Percentages greater than 100% always do well in media reports
| about these topics, but they're not necessarily the point of
| the exercise. If the goal was to produce a system that's as
| efficient as possible, the researchers wouldn't have used
| household-style supplies but more expensive, advanced
| materials.
|
| Reverse osmosis is great for a central area such as a large
| city in a place with reliable distribution, but in many
| places around the world, an independent, affordable system
| that can turn seawater into drinking water for a family or
| two has much more value.
| londons_explore wrote:
| Nothing about reverse osmosis can't be scaled down in size
| or cost.
|
| The actual membranes are $9 for enough for 100 gallons per
| day (retail prices, [1]).
|
| High pressure pumps and hoses scale linearly. Solar panels
| scale linearly. Filters scale linearly.
|
| In fact, using this solar fountain [2] as the basis for the
| design, and switching the pump impeller for a high pressure
| version, and the nozzle for an RO membrane and hose, you
| immediately have a drinking water machine for a few people
| for $20. The fountain already has a pre-filter built in.
|
| [1]: https://www.aliexpress.com/item/32669709750.html [2]:
| https://www.aliexpress.com/item/1005002883892948.html
| SiempreViernes wrote:
| Ah, so what you are saying is that this is already
| deployed on a large scale to solve distributed water
| issues?
| SV_BubbleTime wrote:
| What he's actually saying is he found some residential
| grade nonsense from China that only seems economically
| feasible because of the slave labor and subsidized
| transport.
| kortex wrote:
| I have first-hand experience that a) you need positive
| displacement pumps of a certain quality and power for RO
| and the cheap Ali ones self-destruct in minutes/hours b)
| you still need housing, plumbing, receptacles, etc c)
| fouling is a non-trivial issue.
|
| Expect to pay around $300 minimum for a small RO solar
| plant, and a couple bucks a month for upkeep. And
| logistics. Granted that's USA prices (upstate NY, not SV)
| but that prices out a lot of developing regions.
| londons_explore wrote:
| Fouling can be dramatically reduced by putting a small
| electrical current through the saltwater, freeing
| Chlorine ions which quickly kill biological things before
| they can adhere to your membrane.
|
| That combined with reverse flow flushing will probably
| last many years. And it can all be controlled by a 10
| cent microcontroller, one pump, one valve, and a pressure
| sensor.
|
| I don't doubt that high pressure pumps are bad... but
| that's just a design issue - there is nothing
| theoretically expensive about them.
| jkqwzsoo wrote:
| Commercial RO membranes (based on the
| poly(m-phenylenediamine trimesic acid), a.k.a. polyamide)
| chemistry have zero tolerance to free chlorine and will
| rapidly degrade under such conditions. The chlorine
| tolerance is listed as "nil" on many manufacturer's spec
| sheets. Even using tap water on a polyamide membrane
| without some kind of prefilter (like GAC) will cause
| noticeable reduction in both the permeance and rejection
| of the membrane.
|
| There was a big push towards developing chlorine-
| resistant chemistries a few years ago, but as far as I
| can tell, that has fallen into a "researchers don't know
| what they're doing, the plants are already designed
| around this problem" narrative. Of course, those plants
| are huge investments, and maybe it's correct that one
| wouldn't be able to take advantage of chlorine-resistant
| membranes until a new plant was built.
|
| Cellulose triacetate RO membranes have chlorine
| tolerance, but have inferior chemical stability,
| productivity and selectivity to polyamide membranes, so
| it is sometimes used where chlorine tolerance is an
| issue. CTA membranes are also available in hollow fiber
| format, while polyamide membranes are essentially
| exclusively found in spiral wound configuration (some
| operators want fibers for higher fouling/solids feed).
| CTA is limited to a much smaller pH window (like 4-6
| versus 2-12), and are not suitable for more aggressive
| cleaning methods, so I'm not sure if it overall provides
| a benefit versus polyamide RO with more aggressive
| cleaning cycles.
| kortex wrote:
| Heh, this is reminiscent of the infamous HN dropbox
| comment.
|
| Free chlorine (technically assorted chlorine oxides like
| hypochlorite) attacks RO membranes, so now you have to
| deactivate your reactive species first. Usually UV lamps,
| you use sun, but now you need UV-clear tubing, not easy.
|
| Reverse flow flushing can be done with the components you
| mention, that's another $20/50 plus sourcing logistics.
|
| Yes, the theoretical expense in quality pumps is quality.
| There are tighter tolerances, beefier components, better
| polymers, and more QA. It all adds up.
|
| It's still all fairly cheap by Western standards, but
| it's a tall ask for a lot of places that barely have
| potable water.
| msandford wrote:
| Even these numbers are too small if you're going to RO
| seawater. I suspect what you have in upstate NY isn't
| access to seawater but access to some kind of less than
| amazing mostly-not-saltwater.
|
| The osmotic pressure you need to overcome to perform
| desal is directly proportional to the concentration. So
| 500-1000ppm water that you don't love is a lot less
| challenging than seawater which is about 3.5% or
| 35000ppm.
|
| You can do "I want extra pure drinking water from
| 'normal' water RO" for a few hundred, sure because the
| pressures are in the range of 20-60 psi. Reasonably
| efficient seawater starts somewhere around 400-600psi and
| I've heard of plenty of systems that work at more like
| 800-1000psi. Different pumps, membranes, membrane
| housings, piping, all of it. It's a couple thousand
| dollars plus a fair amount of energy unless you buy even
| more expensive energy recycling pumps that use the
| pressure in the brine output to help on the input.
| kortex wrote:
| Exactly, there's a pressure/flow/salinity tradeoff
| triangle. Higher salinity needs higher pressure and/or
| waste bypass. This was running around 100-150 psi, was a
| few years back, don't quite remember.
|
| But a few hundred bucks will still get you from either
| "potable but not great" to fairly pure, or brackish to
| potable. If you have effectively infinite seawater and
| power, you can still pump at lower pressure and get pure
| permeate, it's just less efficient per unit pump and
| filter lifetime. If you had a super reliable pressure
| system (low friction ceramic pumps for example, not cheap
| but last forever) and cheap first-pass filters, you can
| run it for quite some time. But you're spot on in that
| you get to a point of engineering-give-a-mouse-a-cookie
| and it just makes sense to optimize the whole stack.
| londons_explore wrote:
| The MIT system appears to output clean water from each
| 'stage' of the system at variable temperatures. Thats energy
| lost - if a counterflow heat exchanger could be devised such
| that the output water was all cool, the final efficiency
| would be much higher.
|
| They also don't seem to keep good control of salinity within
| the stages - I suspect after a few hours operation the
| efficiency will drop as the salinity gets higher and higher
| within the paper towels.
| TeMPOraL wrote:
| That's a good point, but! I wonder if it couldn't be more
| efficient overall, if you look at it globally. Is an
| equivalent reverse osmosis installation cheaper or more
| expensive to produce, in terms of embodied energy that goes
| into manufacturing and maintaining all equipment, including
| control equipment?
|
| Also, I wonder which system would be easier to slap together
| in a garage out of leftover junk. Not everyone can rely on
| access to commercial-grade, turn-key solutions to a problem.
| A design that could be reasonably DYIed could be better in
| certain contexts, even if less efficient.
| midasuni wrote:
| American gallons are smaller than U.K. gallons.
| midasuni wrote:
| American gallons are smaller than U.K. gallons, which is where
| I suspect you got the 7l (6.8) from
| vagrantJin wrote:
| Thank you.
| im__so__meta wrote:
| 5.78 liters per square meter actually, per the article.
| hexane360 wrote:
| 96.3 um/s! That's enough to drain a 6 foot deep swimming pool
| in 5.27 hours.
| sdoering wrote:
| So in ideal conditions one could expect to provide about 12
| to 15 people with drinking water per square meter. For me
| this is an impressive number.
|
| If I calculate one unit with 300 dollars this would mean 5.5
| cents per person per day for one year for fresh drinking
| water. And after that only a little bit for maintenance
| probably.
| _0ffh wrote:
| Hmmm, yes it is. It's nearly _too_ impressive!
|
| From the article "The team estimates that a system with a
| roughly 1-square-meter solar collecting area could meet the
| daily drinking water needs of one person.", which sounds
| very different.
|
| On this Q&A page [1] Zhang is quoted "Our current strategy,
| for example, is to use an assembly of 100 of these devices
| to achieve an area of 1 m2, which will increase the total
| production by 100 times to create 10-20 liters of clean
| water per day."
|
| I don't want to sound too negative, but something doesn't
| add up here. Still, enough water for one person per square
| meter of desalination plant is an impressive result I
| think. The oceans are big.
|
| [1] https://www.techbriefs.com/component/content/article/tb
| /feat...
| ghshephard wrote:
| Back of the envelope - say that you could get 7-8 hours of
| sunlight a day that could destill water. That would mean ~50
| liters/day. ~20 Days to get 1m^3. Reverse Osmosis costs approx
| $0.50/m^3 water, so your payback on a $100 system would be ~200
| * 20 Days or 4,000 Days to equal what you could get for
| spending $100 on buying water from a reverse osmosis system.
| The objective here isn't large scale economics, but self
| sufficiency.
| MauranKilom wrote:
| Looking at a 10+ year time frame, I feel that maintenance
| cost would become the dominating factor. For both.
| jillesvangurp wrote:
| I think the difference here is that the materials used are
| cheap and easy to procure. That sounds like maintenance
| should be straightforward. The difference between producing
| water locally and shipping/trucking it in is probably huge
| in places without water pipes. A couple of gallons a day
| goes a long way. Scale it up a little and now you are
| irrigating your vegetable garden as well.
| ghshephard wrote:
| The Reverse osmosis cost estimate was based on a 10 year
| committed Hyflux Desalination commercial contract with the
| PUB in Singapore (from about 5 years ago, so might be less
| expensive now, actaully) - so includes maintenance costs.
|
| Regardless - this isn't a scale able solution, but doesn't
| need to be - should work fantastic for a small family with
| negligible environmental impact.
| mianos wrote:
| There are a lot of quite liveable islands that are deserted
| only due to lack of fresh water. This could change that.
| tonyhb wrote:
| Or maybe we don't have to spread humans everywhere like a
| malignant cancer, leaving at least these Islands to nature.
| betwixthewires wrote:
| Maybe don't spread your Malthusian bullshit like a
| malignant cancer. Some of us like being human.
| tonyhb wrote:
| Hey, me too! Never said I didn't. Just that it's nice to
| leave some parts of the world to itself.
| civilized wrote:
| >> malignant cancer
|
| > it's nice
|
| come on dude, have some self-awareness
| tonyhb wrote:
| The only thing I was saying is - just because humans can
| live everywhere doesn't mean we need to.
|
| If you look at any map of the modern world, there is
| almost nowhere that we haven't transformed into farmland,
| cities, etc, and it's pretty good (nice) that we have
| left the uninhabitable islands to nature. We've consumed
| most of the land, which is both pretty amazing and also
| quite bad for biodiversity, as everyone knows.
|
| I'm not debating the "cancer" metaphor which is
| definitely harsh - it has been catastrophic for other
| life, but great for us. Maybe that wording can be less
| harsh, but IMO that's not the point.
| [deleted]
| sMarsIntruder wrote:
| Really thanks for the conversion.
| I_complete_me wrote:
| Assuming 1.5 US gallons, this converts to 5.678 litres.
| nxpnsv wrote:
| Now we have 3 numbers... US units are great...
| dr_dshiv wrote:
| https://en.wikipedia.org/wiki/United_States_customary_units
| Jenk wrote:
| Nearly 2ml per second?
|
| (My faith in my own maths is weak ..)
| tsjq wrote:
| California is building the largest desalination plant in the
| US.(2015) Is this ready yet ??
|
| https://news.ycombinator.com/item?id=10343296
| midnightclubbed wrote:
| Largest SOLAR desalination plant in the US. The article doesn't
| seem to list the numbers but this article
| https://www.businessinsider.com/california-waterfx-solar-des...
| seems to indicate the next step up from their demo plant was a
| plant capable of 2M gal/day.
|
| Hopefully they are still developing the technology, it seems
| like it could be a life-changer for small farming communities
| around the world.
|
| By contrast the biggest 'traditional' desalination plant in the
| US makes 50M gal/day and the biggest plant in the world makes
| around 260M gal/day (US gallons).
| [deleted]
| specialist wrote:
| How can we couple this with carbon sequestration?
|
| Use byproducts (sodium, chloride, calcium, etc) as feed stock for
| useful compounds. eg Phosgene COCl2 is a valuable industrial
| product.
|
| Create artificial salt water marshes, which are pretty good at
| carbon capture.
|
| Everyone's trash is someone else's gold. People are clever. They
| can find good use for the salt.
| hinoki wrote:
| It sounds like they figured out how to make the distillation
| version of a compound steam engine [0]. Like others, I'm confused
| by the efficiency number. Are they producing 385% more water for
| the same energy compared to the best single state result?
|
| [0] https://en.m.wikipedia.org/wiki/Compound_steam_engine
| andromeduck wrote:
| That's already a thing.
|
| https://en.wikipedia.org/wiki/Multi-stage_flash_distillation
| bigbizisverywyz wrote:
| "Tests on an MIT building rooftop showed that a simple proof-of-
| concept desalination device could produce..."
|
| So doesn't look like they've even hooked it up to the sea yet, so
| still a bit to go for a real implementation, but still sounds
| interesting.
|
| I harbour a small dream that I could buy some seaside land
| someday on a Greek island and hook one of these desalination
| devices up to the sea and build myself a small oasis.
| gregoriol wrote:
| The device itself would not be "on" the sea, it would just get
| water from it with a pipe/pump. I don't really see any reason
| it wouldn't work the same way by the sea: the main problem
| seems to be how to remove the salt, efficiently, not how to get
| water into the device, which is easier. Getting enough sun
| might even be harder than getting sea water.
| bigbizisverywyz wrote:
| Yep, and that's what I basically meant by 'hooking it up'.
|
| I guess you would need a pump to bring the seawater to the
| device, something to pump it elsewhere (for storage, or
| irrigation), and also some mechanism to dump the brine back
| in the sea when you're no longer desalinating.
|
| And for maximum ecological efficiency that could be powered
| by solar panels.
|
| I wonder how maintenance free you could build such a device,
| and just leave it to work away by itself for months on end.
| stubish wrote:
| I like the idea that brine is disposed over time at night, which
| will greatly reduce the disposal problems.
| lvs wrote:
| Does anyone actually believe a word of this?
| azinman2 wrote:
| Yes. Why wouldn't you?
| defend wrote:
| A healthy exercise in skepticism.
| shapefrog wrote:
| I have a healthy skepticism about your skepticism
| wesleywt wrote:
| An exercise in skepticism is actually asking a specific
| question that bothers you about the article. And not just a
| random "I don't believe this".
| [deleted]
| hiroshirama wrote:
| We can now have enough salt to last forever!
| neilwilson wrote:
| Presumably you could use the same technology on liquid effluent
| to create a 'toilet to tap' cycle
| [deleted]
| dang wrote:
| Discussed at the time:
|
| _Simple, solar-powered water desalination_ -
| https://news.ycombinator.com/item?id=22269115 - Feb 2020 (192
| comments)
| bjackman wrote:
| Does anyone know of any designs for DIY passive desalinators? My
| mum has a house on an island that doesn't have a natural source
| of fresh water (except rain of course), and it just occurred to
| me that a desalinator would be a fun thing to build there.
| Normally if there's a long drought she would have to have
| drinking water delivered by sea.
| ratsforhorses wrote:
| I'm sure you'll get better answers but I really liked the idea
| of ventilating greenhouses with salt water.... could be a good
| way to get the extra liquid needs..a quick google found me this
| https://www.sciencedirect.com/science/article/pii/S001191641...
| Cthulhu_ wrote:
| The most simple ones ("survival" type, but it won't produce
| enough water unless you scale up) is spanning clear plastic
| over a pool of (shallow) salt water, weigh it down with a rock
| in the middle, and put a collector in the center underneath so
| the evaporated water runs down it.
|
| But I'm sure there's commercial solutions out there.
| pomian wrote:
| Another interesting use for this system, and I didn't see it
| mentioned, is for water purification. That may be even more
| useful across the world especially in remote regions.
| pfdietz wrote:
| An issue with desalination systems of this type (that have water
| vapor in a carrier gas, so called HDH or Humidification-
| Dehumidification desalination systems) is the retardation of mass
| transfer to the cold surface by the carrier gas. Multieffect
| distillation (MED) systems usually operate without a carrier gas
| for this reason, but this means they operate below atmospheric
| pressure, so they have to be sturdy to resist pressure loads.
|
| A few years ago, some people at MIT solved this problem with a
| bubble heat exchanger: the humidified carrier gas is bubbled up
| through trays of water (at progressively lower temperatures). The
| surfaces of the bubbles provide a very large surface area for
| heat/mass transfer.
|
| https://news.mit.edu/2013/produced-water-cleanup-0205
| https://dspace.mit.edu/handle/1721.1/86334
| http://web.mit.edu/lienhard/www/papers/reviews/HDH-Desalinat...
|
| Some of those involved went on to form a company to commercialize
| the technology, which has been used to recycle/purify brine from
| natural gas wells and other industrial tasks. I think they ended
| up being bought out after several years.
| Gravityloss wrote:
| The opposite of bubbles, mist, might also work. Extremely high
| surface area.
| kumarvvr wrote:
| The 385% seems to refer to obtaining de salination in lesser
| solar collection area requirements.
| fabbari wrote:
| I had the "stupid moment" of the day. The thermal effects of
| evaporation on a surface is that the surface actually cools down
| -- so I could not get the "it produces heat" part.
|
| After drinking my morning coffee I realized that the heat
| transfer is from the surface to the water droplets/vapors that
| then carry it to the next layer of this still.
|
| Ergo: coffee makes you smarter and I shouldn't be on HN so early
| in the morning.
| [deleted]
| The_rationalist wrote:
| Nuclear desalination is more efficient though
| https://www.forbes.com/sites/jamesconca/2019/07/14/megadroug...
| Ice_cream_suit wrote:
| " Using a low-cost and free-of-salt accumulation TMSS
| architecture, we experimentally demonstrated a record-high solar-
| to-vapor conversion efficiency of 385% with a production rate of
| 5.78 L m-2 h-1 under one-sun illumination, where more than 75% of
| the total production was collected through condensation. "
|
| Abstract is available here:
|
| https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0...
|
| The original journal article is available here as a pdf:
|
| https://pubs.rsc.org/en/content/articlepdf/2020/ee/c9ee04122...
| reizorc wrote:
| There's still the issue of what you do with all that salt
| konschubert wrote:
| put it back into the ocean where it came from?
| Johnythree wrote:
| Brine is toxic unless it can be diluted. Which ends up being
| a huge problem in large plants.
| [deleted]
| gumby wrote:
| Over a decade ago I saw simple solar stills installed in India
| with the cascades built of concrete and a glass panel. This was
| at Maharana Prtap university in Udaipur.
|
| So "proud" of my Alma mater university for developing an exotic
| technology that uses aerogels and such ... sure, it's more
| efficient, but those folks In Udaipur, without a big PR office
| like MIT's, built something that could be built by anyone using
| everyday material.
| ohgreatwtf wrote:
| These are all extremely cool approaches to the problem but
| unfortunately they are all high tech and most require a
| substantial amount of complex component fabrication to work.
|
| I have been thinking about a passive, low cost system. It would
| use series of rows of shell-like structures with fresnel
| attributes and coatings that simultaneously cause water to
| condense on heated surfaces and then drain into subsequent rows
| of shells, a process that passively cycles with the heat of the
| sun. A large fraction of each row's output collects in a tank
| that is periodically raised to backflush the row and reduce the
| saline buildup.
| vr46 wrote:
| This is the winning sentence for me:
|
| _Unlike some desalination systems, there is no accumulation of
| salt or concentrated brines to be disposed of._
| creshal wrote:
| Or is it?
|
| > _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._
|
| This is fine for a small scale demonstration unit, but with
| bigger plants you will again run into the problem of over-
| salinating seawater, destroying the environment (and reducing
| your still's efficiency).
|
| So once you get past a certain scale, you'll again need to
| redirect the wick into some waste brine tank and figure out
| logistics for disposing it.
| hoseja wrote:
| That would have to be a very small sea indeed.
| creshal wrote:
| I'll assume the device isn't using an infinitely long wick,
| so you face the same outflow problems as traditional
| plants: https://en.wikipedia.org/wiki/Desalination#Outflow
| Chris2048 wrote:
| Can salt be compressed and then just buried?
| creshal wrote:
| Sure, it's just that in this regard it has no advantage
| over other desalination technologies. You always have to
| deal with the excess salt, MIT is dishonest in waving away
| these concerns.
| baybal2 wrote:
| The article in question:
| https://pubs.rsc.org/en/content/articlepdf/2020/ee/c9ee04122...
| NiceWayToDoIT wrote:
| How much would it cost on industrial scale? (for irrigation
| system for farming purposes traditional/hydroponic/aquaponics
| (considering that aqua systems use 1/10th of the water of soil-
| based gardening)
|
| https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4483736/
| NiceWayToDoIT wrote:
| Thinking about California drought problem and what scale of
| investment would it be needed to fix it.
| owaisin wrote:
| How much cost?
| bondolo wrote:
| When the $100 system can run for 5 years of continuous production
| get back to me. I would be surprised if the system, as described,
| would work beyond a couple of days. Other than for emergency
| situations this isn't useful.
| jokoon wrote:
| What is this aerogel thing, and this "capillary wick"?
|
| Not bad for $100, but I'm more interested in durability, if parts
| needs to be replaced and maintained, and if yes what is required
| to make those parts.
| MauranKilom wrote:
| The capillary wick is literally a paper towel.
| marco_craveiro wrote:
| Amazing. I wonder if there has been an update since 2020, in
| particular with regards to mass-production...
| inglor_cz wrote:
| This article is 17 months old. Has anything happened in the
| meantime? (Aside from Covid, of course.)
| drawkbox wrote:
| This is essentially a solar still that uses the natural water
| cycle to produce fresh water.
|
| Learning about desalinization should be taught in all schools for
| more innovations in this area.
|
| It would be a cosmic joke to run out of water on a water planet,
| we'd look like universal dunces.
|
| USGS site has a great overview of desalinization that is a good
| place to start, you can even try your own solar still in your
| backyard.
|
| USGS Desalination site [1]
|
| Build your own backyard desalinization system (solar still) [2]
|
| > _You can make your own personal desalination plant_
|
| > _Remember looking at the picture at the top of this page of a
| floating solar still [3]? The same process that drives that
| device can also be applied if you find yourself in the desert in
| need of a drink of water._
|
| > _The low-tech approach to accomplish this is to construct a
| "solar still" which uses heat from the sun to run a distillation
| process to cause dew to form on something like plastic sheeting.
| The diagram to the right illustrates this. [2] Using seawater or
| plant material in the body of the distiller creates humid air,
| which, because of the enclosure created by the plastic sheet, is
| warmed by the sun. The humid air condenses water droplets on the
| underside of the plastic sheet, and because of surface tension,
| the water drops stick to the sheet and move downward into a
| trough, from which it can be consumed._
|
| > _You can try this at home!_ [2]
|
| > _- Dig a pit in the ground_
|
| > _- Place a bowl at the bottom of the pit that will be used to
| catch the condensed water_
|
| > _- Cover the pit loosley with a plastic sheet (you can use
| stones or other heavy objects to hold it in place over the pit_
|
| > _- Be sure that the lowest part of the plastic sheet hovers
| directly over the bowl_
|
| > _- Leave your water "trap" overnight and water can be collected
| from the bowl in the morning_
|
| We need to put tons of money in desalinization. California is
| already a leader in that but we need more. Israel and Saudi
| Arabia are also pretty good at desalinization due to more dire
| water situations.
|
| Additionally we need geoengineering in terms of helping create
| moisture/rain in areas that feed the Colorado.
|
| The better bet is desalinization that uses the nature water
| cycle, it makes for cleaner water as well. Saudi Arabia is doing
| a solar dome to test this [4], we need more of this.
|
| [1] https://www.usgs.gov/special-topic/water-science-
| school/scie...
|
| [2] https://www.usgs.gov/media/images/how-build-your-own-
| solar-s...
|
| [3] https://www.usgs.gov/media/images/a-floating-solar-still-
| des...
|
| [4] https://wired.me/science/environment/desalination-solar-
| dome...
| squarefoot wrote:
| FTA: "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."
|
| The capabilities are freaking interesting, but let's say someone
| builds a big enough settlement on a coastline or island in which
| every building has one or more of these devices on its rooftop.
| Would this release back enough salt so that the surrounding
| seawater becomes hostile to its previous lifeforms? Also, since
| then the water used in the process would contain more salt, how
| much would that render the device less efficient?
| bruce511 wrote:
| There are two counterpoints to the issue of excess salt being
| an environmental hazard.
|
| A) brine can be introduced back into the ocean combined with
| waste water. In the Cape Town drought of 2017 some small
| desalination plants were brought into service very quickly, and
| the brine was expelled in the same pipe as the outflow from a
| (treated waste) sewerage plant.
|
| B) the ocean is big - very big - and at least by our coast
| seldom "calm". So outflow of anything would disperse very
| quickly. Hot-water outflow from a nuclear power station
| dissipates very quickly for example - typically within tens of
| metres of the outflow.
|
| C) the natural salinity of the ocean varies a fair bit at the
| very local level - think river mouths - storm water -
| evaporation etc. Outside of specifc closed bodies of water
| (Dead Sea etc) we'd need desalination on a massive scale to
| even measure the impact.
| dystroy wrote:
| Salt release is already a huge environmental concern with
| exiting desalinization systems. Energy consumption and
| installation costs aren't the sole (and probably not the main)
| problems. Desalinization is inherently a pollution.
| throwaway894345 wrote:
| Why not release the salt back to the sea? Presumably it would
| not increase the salt content of the ocean very much
| considering the vastness of the ocean and the sheer quantity
| of fresh water that is being added to it by melting glaciers,
| etc? What am I missing?
| kevstev wrote:
| Why not just collect the salt, and sell it as... salt? Even
| if its not ok for cooking, it should be fine for roads? I
| am sure there are other industrial uses for salt.
| SV_BubbleTime wrote:
| Actual salt is rarely used on the roads in the USA
| anymore. Majority use magnesium chloride now. Don't know
| about the rest of the world.
|
| Lots of reasons, pros and cons. I work in automotive and
| hate it.
| EricE wrote:
| Salt from roads is seriously contaminating our
| environment and significantly impacting farming in more
| and more areas.
|
| Ever hear of the Romans salting the earth of people the
| conquered? It was so they couldn't grow crops - yet we
| routinely do this to ourselves - pretty daft.
| throwaway894345 wrote:
| We also farm in a way that is almost optimized for
| erosion of topsoil. We've already lost a third of it and
| it only renews on geological timescales. Daft is too
| polite.
| dystroy wrote:
| The salt you release isn't instantly diluted in the whole
| ocean and we're speaking, for a plant, of millions cubic
| meters of salt enriched water per day. The release area is
| just dying.
| shellfishgene wrote:
| Can you provide a source for that? I dove near a
| desalination plant exit pipe in Saudi, the impact on the
| marine live did not seem to big in the area. But it was a
| small plant for a few thousand people and we did not do a
| proper assessment.
|
| I don't doubt there is an effect, but am not convinced
| that the "release area just dying" is correct.
| EricE wrote:
| https://www.keiken-engineering.com/en/brine-disposal-how-
| eff...
|
| Brine disposal is indeed a complex issue. Should be noted
| the above URL is from a desalinization industry proponent
| so I would say it's probably a little more optimistically
| biased but it was one of the better summaries I found so
| it still has value from that perspective. If you do more
| digging around each of the solutions they discuss you can
| find more pro's/con's for each and you will quickly find
| out that it is a pretty significant issue.
| shellfishgene wrote:
| The link does not really show any evidence that marine
| ecology at brine outlets is disturbed. It does however
| suggest that pouring the brine on land or roads (for de-
| icing) are alternative solutions, which I doubt ;).
|
| I was more thinking of proper studies, such as reviewed
| in [0], which says in the abstract: "Ecological
| monitoring studies have found variable effects ranging
| from no significant impacts to benthic communities,
| through to widespread alterations to community structure
| in seagrass, coral reef and soft-sediment ecosystems when
| discharges are released to poorly flushed environments.
| In most other cases environmental effects appear to be
| limited to within 10s of meters of outfalls."
|
| So, impact yes, but "release area is just dying",
| probably not.
|
| [0] https://www.sciencedirect.com/science/article/abs/pii
| /S00431...
| throwaway894345 wrote:
| This makes sense to me. I wonder how feasible it would be
| to release the saltier water over a broader area to
| reduce the salinity in any one area? Also, there are at
| least some ecological benefits to increasing the amount
| of fresh water. An extreme example is the lake that
| appeared in the middle of a desert in the United Arab
| Emirates. I have no idea how we weigh one against
| another.
| SV_BubbleTime wrote:
| Now you're paying for salt transportation again.
| throwaway894345 wrote:
| Not if you have many small desalination plants
| distributed over a larger area. This would work well for
| less-densely populated areas.
| dfilppi wrote:
| That is acceptable
| Someone wrote:
| A small part of it can be avoided by combining this with a
| salt evaporation pond
| (https://en.wikipedia.org/wiki/Salt_evaporation_pond)
|
| Problem will be that we'll get way too much salt (a few grams
| of salt per liter of water), so it won't help much.
| dsr_ wrote:
| This chunk from the article is, unfortunately, an outright lie.
|
| By definition, removing salt from seawater leaves you with
| water and salt. The article handwaves the salt away by
| supposing that the desalinator will float on top of the ocean.
|
| In actual production configurations other than towing it behind
| your sailboat, you will end up with a brine pool that needs to
| be disposed of, or a concentration in the wicking material that
| prevents low-concentration seawater from entering, or a pumping
| system.
|
| This is not just a common negative externality, but a casual
| lie about it.
| EricE wrote:
| Water in open ocean is not still - there are currents. I
| suppose if you had a large enough operation you could create
| a gradient of sufficient size to be self-sustaining without
| further mechanical agitation (such as your example of towing
| behind a ship). It would be an interesting exercise to see a
| large collection of these in operation. Heck even if you had
| to pump ocean water on land and back out to dilute the brine
| this process would still use significantly less energy
| overall than current desalinization plants.
|
| Speaking of current plants, a recently constructed one in
| Carlsbad CA leverages the cooling outflow of a power plant to
| handle the dilution. The power plant was already there and so
| was the outflow - these things can be planned to leverage
| overlapping uses to further increase efficiency.
| KaiserPro wrote:
| > supposing that the desalinator will float on top of the
| ocean.
|
| I think that as its solar, you only have 50% duty cycle (more
| or less) so the idea is that you "just" slow down the water
| supply to remove the salt over night.
| adrianmonk wrote:
| I don't think they are talking about the impact on the
| environment. I think they are talking about the procedures
| for operating the equipment.
|
| The way I read it, they mean whoever operates this device
| does not have to periodically go empty some bin/tank of salt
| or brine.
|
| An analogy would be to a frost-free refrigerator. When you
| say the freezer compartment of your fridge is frost-free, you
| don't mean that it never generates frost. You mean it does
| generate frost, but it also automatically removes it. It's
| "free" of frost in the sense that you are free of doing a
| chore that you have to do with a freezer that lacks this
| feature.
| dsr_ wrote:
| If you operate it directly on the open ocean, towing it
| behind your sailboat, you don't have to do anything
| special.
|
| If you operate it in a commercial, protected context where
| you pump water through it, you will generate brine.
|
| If you operate it en-masse in the ocean, the things that
| are insignificant at the scale of 1 and 10 square meters
| may become significant at the scale of 1 and 10 square
| kilometers.
|
| All of these things are true for other desalinization
| systems, too. Claiming "you don't have to worry about
| brine" because it floats on the water surface is misleading
| at best. You can put any system you want on the water and
| make it float -- we have concrete and steel hulls, no
| problem -- but that doesn't solve the concentration
| problem.
| jonas21 wrote:
| It sounds like they're targeting small deployments in the
| developing world, at the scale of 1 to 10 square meters,
| not large commercial systems.
|
| From the article:
|
| > _In production, they think a system built to serve the
| needs of a family might be built for around $100... The
| hope is that it could ultimately play a role in
| alleviating water scarcity in parts of the developing
| world where reliable electricity is scarce but seawater
| and sunlight are abundant._
|
| The problem with brine from desalination plants is that
| it's released at high concentration in one spot. This
| would not suffer from that problem.
| lucbocahut wrote:
| I wonder if the fresh water used in such a way wouldn't
| eventually be released back into the ocean thus somewhat
| mitigating the effect.
| amn79 wrote:
| Why release the salt back into sea water? This is salt
| production alongside desalination.
| jedberg wrote:
| We already have a massive oversupply of salt from existing
| desalination. The cost of table salt is basically all
| transportation at this point.
| mixmastamyk wrote:
| Can be buried also.
| e12e wrote:
| > The cost of table salt is basically all transportation at
| this point.
|
| Using the salt on site would save the transport, though?
| EugeneOZ wrote:
| In large enough systems some kind of "salt removal" mechanism
| might be added - I think it wasn't added to a PoC just to make
| it as simple and cheap as possible.
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(page generated 2021-07-03 23:02 UTC)