[HN Gopher] Why don't we get our drinking water by taking salt o...
___________________________________________________________________
Why don't we get our drinking water by taking salt out seawater?
(2008)
Author : LinuxBender
Score : 68 points
Date : 2023-07-23 16:07 UTC (6 hours ago)
(HTM) web link (www.scientificamerican.com)
(TXT) w3m dump (www.scientificamerican.com)
| AshamedCaptain wrote:
| In addition, voters complain about the taste.
| josh_fyi wrote:
| In Israel they add minerals back in because the desalinated
| water is too pure.
| politelemon wrote:
| I've vaguely known that what the taste is related to
| minerals. Does anyone know what kind of minerals they're
| adding back in?
| niemandhier wrote:
| Are state of the art systems more efficient than the ion-pumps in
| cell membranes?
|
| I remember reading about light driven sodium and chloride pumps,
| if we could make these transport ions across some biomembrane we
| could desalinate water.
| parentheses wrote:
| Knowing that the vast majority of water use is not the water
| flowing to homes for drinking, plants and washing, the government
| (local or federal) could subsidize it for individuals while
| taxing industrial water consumers.
|
| As mentioned in another post. Basic utilities can easily become
| affordable with proper regulation.
| shaunxcode wrote:
| Yet. Why don't we yet.
| superb-owl wrote:
| It'd be helpful to know the minimal energy requirement to
| desalinate a gallon of water. With perfect technology, how
| expensive (in terms of energy) would it be? What's our current
| level of efficiency?
| Enginerrrd wrote:
| Depends a lot on what you start with and on some of the other
| assumptions you make, but for seawater, the theoretical limit
| is probably ~3 kJ/kg, and the absolute state of the art full
| scale desalination is probably 11-14 kJ/kg
| nsenifty wrote:
| Or about $0.20 per US person per day.
|
| Given average water use per person in US is 310 liters/day
| (82 gallons) [1] and average electricity cost/kwh is $0.17
| [2], and assuming 14 kJ/kg. 310 * (14 / 3600) * $0.17 =
| $0.20.
|
| [1] https://www.epa.gov/watersense/statistics-and-facts
|
| [2] https://www.bls.gov/regions/midwest/data/averageenergypri
| ces...
| digdugdirk wrote:
| Holy smokes, 82 gallons per day is wild. Is this inclusive
| of all tangential water usage as well? (industrial reasons,
| lawn watering, etc)
| fbdab103 wrote:
| I need to see a median number because 82 gallons indeed
| sounds outrageous for a consumer. Unless it is all-in
| calculation (agriculture + industry + consumers): America
| uses XXX units of water per year / # Americans.
| hamandcheese wrote:
| > Theoretically, about 0.86 kWh of energy is needed to
| desalinate 1 m3 of salt water (34 500 ppm). This is equivalent
| to 3 kJ kg-1. The present day desalination plants use 5 to 26
| times as much as this theoretical minimum depending on the type
| of process used.
|
| That translates to about 3.25 watt-hours/gallon as the
| theoretical limit, if my math is correct.
|
| Source: https://www.desware.net/Energy-Requirements-
| Desalination-Pro....
| RagnarD wrote:
| New technology is making it easier and cheaper. There's also the
| fact that environmentalist suppression of nuclear power has made
| electricity much costlier, intermittent, and much less reliable.
|
| https://scitechdaily.com/new-device-purifies-saltwater-over-...
| drbojingle wrote:
| Same reason as always: cost. If it was easy we'd be doing it long
| ago kn every boat and every coastal city.
| YesThatTom2 wrote:
| We need to figure out how to make cell phones out of the refuse
| salt / salty sludge.
|
| We'd find new ways to desalinate in a week!
| ChrisArchitect wrote:
| So many newer attempts since then it feels like, different
| approaches
|
| Like this _Creating Drinking Water Using Ocean Wave Power_
| (https://news.ycombinator.com/item?id=10522700)
|
| But what happens to them? Initiatives never seem to go anywhere
| far....... energy requirements.... no financial
| incentive/interest? Same sort of tepid interest as general
| climate change initiatives?
| admax88qqq wrote:
| Lack of drinking water is sadly mostly an issue in poor
| countries, so there's little money to invest/spend on new ways
| of creating it.
| wheelerof4te wrote:
| Where's that guy advocating for the desalination plants in
| Phoenix now?
| runjake wrote:
| Spoiler: desalination requires a lot of energy.
| konschubert wrote:
| Solar is cheap
| grej wrote:
| Solar is cheap _when active_ , and the good news this is one
| of the cases where intermittency doesn't really matter. We
| should really explore more use of intermittent power for
| cases like this where the limited reliability of wind and
| solar is less significant.
| nwiswell wrote:
| It's still significant if it means that your desalination
| plant is only active 50% of the time, since it means you
| need twice the capacity.
| [deleted]
| Groxx wrote:
| If solar was _cheap enough_ , it would be used.
|
| In a handful of places, it is. In the rest, it's so much more
| expensive than alternatives that it's borderline insanity to
| use it.
|
| Like ice: ice is cheap in Antarctica! But importing Antarctic
| ice to a desert would be absurd.
| AnotherGoodName wrote:
| Although it shouldn't. Adding salt to water is endothermic, it
| absorbs energy to add salt and releases it to separate them.
| The article talks about the trouble of breaking the bonds but
| this is wrong. It's harder to break the na-cl bond to make salt
| water in the first place.
|
| We just haven't found a good process. That's it. Cheap
| desalination is not physically impossible. The deep ocean
| separation that someone here mentioned seems quite promising
| and gives off energy which is expected. Salt has weak bonds in
| water and when they reform the na-cl bond there's a lot of
| energy to be had.
| jacooper wrote:
| TLDR too expensive.
|
| It depends on the country though, countries like Saudi Arabia get
| most of their water though desalination.
| reaperman wrote:
| Qatar has no groundwater or surface water anymore AIUI. But
| they have an incredible oversupply of natural gas that they
| can't build enough export capacity for. So it mashes sense to
| use the gas they can't export to desalinate water, and they do.
| politelemon wrote:
| It would be ironic if they become a supplier of water to
| other countries too.
| mhb wrote:
| Also big cruise ships.
| _0ffh wrote:
| IIRC Israel even produces enough fresh water to establish a
| flourishing export business.
| nir wrote:
| I don't know about exports but for certain there's quite a
| lot of water Israel transfers to Jordan as part of the 90s
| peace accords. That's a huge deal in the Mid East.
|
| For decades, the weather report in Israel would include the
| water level of the Sea of Galilee, the only real lake in the
| country and the source of most of the nation's drinkable
| water. It would be a major concern during droughts. Now
| there's so much desalinated water that some are actually
| being pumped into the lake, for environmental reasons.
| bagels wrote:
| Article puts the cost to produce a cubic meter of water at $1-$2
|
| In the bay area, I'm paying ~$2.80 per cubic meter.
|
| I don't see why it's not feasible.
|
| Yes, there are transmission/pumping costs. Assuming the water
| we're getting now is free, that's still only ~50-75% increase in
| cost over current costs.
| labster wrote:
| Article is 15 years old, energy and labor prices have increased
| a lot. We need a new cost estimate.
| pstuart wrote:
| Solar prices have plummeted, and that use case can skip
| storage issues (just run when there's power available).
| Obviously land, labor, and infra cost are not so simple.
| nerdawson wrote:
| If you use solar to generate electricity then the cost of
| that electricity is whatever you're missing out on by not
| selling it to the grid.
| Libcat99 wrote:
| On the other hand, you could have a system that pumps out
| water or power, depending on current needs and market.
| fbdab103 wrote:
| As a consumer, falling solar prices have not led to a
| noticeable decrease in my electrical bill.
| cronix wrote:
| As a recent convert to solar, we no longer have any
| electric bill, period.
| Xeoncross wrote:
| Why don't we...
|
| money.
|
| Think of everything good in the world we could have but don't
| because we and everyone else want to keep as much money as
| possible.
| [deleted]
| the_snooze wrote:
| It's not a matter of greed as you seem to be implying. In this
| case, the money is really just a proxy for how practically
| difficult desalinazation is vs. the alternatives. It's the same
| reason why we don't ship parcels via ICBM; the alternatives are
| way easier on multiple dimensions. The juice isn't worth the
| squeeze.
| xcv123 wrote:
| That's not how money works.
|
| Otherwise you could solve these problem by printing money.
| konschubert wrote:
| Isreal does. California also has desalination, no?
|
| If power costs are the driver then solar will be the answer
| retrocryptid wrote:
| Because it's expensive. There, saved you a click.
| jokoon wrote:
| What about that small cheap device invention thing that can be
| used by people who live on the coast to produce enough drinking
| water?
| whatshisface wrote:
| A kettle.
| Arnt wrote:
| The article sidesteps something that seems important to my naive
| eye: Water is easy to store if you don't have very much of it, so
| this is the kind of thing you can do when the sun is shining and
| store for nights and rainy days. Storing water for a year or two
| would be hard, of course.
| sidewndr46 wrote:
| Most of Texas consists of "build a dam at this convenient
| chokepoint and create a huge reservoir" for its freshwater. So
| I disagree. We don't do anything special. The biggest issue is
| invasive species taking up residence in the newly formed
| reservoirs.
| [deleted]
| flangola7 wrote:
| Like birds? Or fish?
| paulcole wrote:
| Mussels like Zebra and Quagga are common invasive species
| in reservoirs like this. Asian carp and other fish species
| as well.
| ianburrell wrote:
| Places with intermittent water have lots of water storage in
| reservoirs. The problem is that reservoirs are up in the hills
| away from the coast so it would cost a lot to pump it up.
|
| But running the desalination when energy is cheap makes a lot
| of sense. Especially when not providing all the water but
| topping up when supply is low. The only problem is the capital
| costs of desalination plant that isn't used all the time.
| slv77 wrote:
| Return on capital and deprecation tend to exceed energy input
| costs for most plants so it makes sense to run them 24x7x365
| rather than over-size them and run them when energy costs are
| lower.
| Arnt wrote:
| They tend to exceed energry costs because energy costs are
| dominated by the cost of purchasing oil, which is as easy to
| store as water. We're moving away from that now, towards a
| world where solar power plants produce power at stunningly
| low prices _some of the time_ (like EUR0.01114 /kWh on the
| top google hit just now) and those who need a lot of power
| and can adapt their usage to match solar power plants can
| make use of those prices.
| nonethewiser wrote:
| > They tend to exceed energry costs because energy costs
| are dominated by the cost of purchasing oil
|
| The energy costs are tied to oil because thats the cheapest
| option. Well, besides coal. If solar were cheaper wed be
| using it.
| quertered wrote:
| There are also times where energy cost is negative and
| you'll actually get paid to consume it. Consumers don't
| typically see this of course
| quertered wrote:
| Question: why is desalinated water costs calculated based on as
| if the water was immediately thrown back into the ocean? Wouldn't
| it rather be recycled after once desalinated?
| IshKebab wrote:
| Because it's easier to desalinate salt water than sewage?
| quertered wrote:
| How would salt end up back in sewage?
| [deleted]
| e_y_ wrote:
| Recycled water is usually considered non-potable, so while it
| can be reused it's usually not available for drinking water and
| needs separate pipes.
| quertered wrote:
| Though there are processes that turn it back into potable
| water, just reusing it even once in agriculture would bring
| huge cost savings compared to having to desalinate it again,
| which is why I wonder why there doesen't seem to be any
| studies of the realistic likely costs of desalinated water
| when accounting for reasonable reuse
| klaustopher wrote:
| Great video explaining this in some more detail:
| https://youtu.be/mxqOPdEUNTs
| JumpCrisscross wrote:
| Is there a video of a physicist answering this question? _Why_
| is this problem so difficult?
| seu wrote:
| Indirectly related: https://seawatergreenhouse.com/
|
| "The idea behind the process is simple. It combines two unlimited
| resources - sunlight and seawater - to provide ideal growing
| conditions for crops in hot, arid environments.
|
| The innovation utilises the cooling and humidifying power of
| water vapour produced from evaporating salt water. Using modeling
| and simulation techniques developed in collaboration with our
| partners at Aston University, we are able to process local
| climate data to predict greenhouse performance and inform the
| design. The combined effect of reducing temperature and
| increasing humidity, together with providing a protected
| environment for crops, results in up to 90% reduction in
| evapotranspiration. This greatly reduces irrigation requirements,
| which can be provided by desalination, and improved growing
| conditions.
|
| As a result operating costs are lower, yields increase, and
| farmers can benefit from year-round production of high-value
| horticultural produce. "
| freitzkriesler2 wrote:
| We do in some places. But in other places like California NIMBYs
| and greens decided putting extra salty brine back into the ocean
| was worse than taking fresh water run off from the inland,
| worsening desertification
|
| Really mostly nimbys. Greens have relented.
|
| Nuclear desalination would solve a lot of the fresh water
| problems but we can't have solutions now can we?
| Gibbon1 wrote:
| > Really mostly nimbys. Greens have relented.
|
| Head in the Sandies is probably even more encompassing. From
| people that tink the solution is drilling for oil in national
| parks to people thinking suing developers and separating their
| trash is all you need to do. They're stupid people. Neither
| should be influencing policy.
|
| Seems to me that the usual historical options aren't
| acceptable. Business as usual looks bad. The traditional
| solution, genocidal warfare seems like a really bad idea for
| someone living in an industrial civilization. We could
| degrowth. But the Khmer Rouge tried that in the 1970's and back
| to the rice fields didn't work very well for most people.
|
| So seems like technological mitigation is all we can hope for.
| Metacelsus wrote:
| See also: a way to potentially do this with no energy input
| (exploiting density differences):
| https://www.science.org/doi/10.1126/science.183.4121.157
|
| Basically you put a reverse-osmosis membrane at least 231 meters
| deep in the ocean. If you put it deeper, you could potentially
| extract both freshwater and energy.
| bell-cot wrote:
| That article's subtitle: "In principal, _but probably not in
| practice_ , fresh water can be extracted from our oceans for no
| expenditure of energy" [Italics mine]
|
| Also - 231 meters deep is where, in theory, you'll just start
| getting fresh water coming through your membrane. Which is 231m
| deep. Fresh water that you'll spend plenty of $energy to pump
| up to the ocean surface.
|
| (For reference - Hoover Dam, when brim-full, has a slightly
| _lesser_ drop from the water surface in the reservoir to the
| output of the hydroelectric turbines. You 'll be pumping
| $energy _in_ , to lift each ounce of your "free" fresh water up
| to where it is useful.)
|
| [Edit - from some quick calculations, and the density of sea
| water relative to fresh water...it looks like the "don't need
| to pay to pump the fresh water back up" version of this scheme
| will need to run its osmotic membranes at the bottom of the
| Challenger Deep (~11,000m), or something pretty close to that.]
| eslaught wrote:
| If I built an ocean base 231 meters below the surface, does
| that mean I'd get free fresh water by sticking a membrane on
| the outside of my base? (Ignoring the impracticality of
| building an underwater base, of course.)
|
| Sounds like a cool sci-fi premise.
| thaumasiotes wrote:
| Adding an open window to your suboceanic base sounds like
| an awful idea, frankly. It's not difficult to see the
| obvious problem.
| idontwantthis wrote:
| Delightfully close to "a screen-door on a submarine".
| brilee wrote:
| Almost. You would have to maintain your underwater base at
| 1atm and you would indeed get free freshwater. The catch is
| that the internal volume of your underwater base would
| decrease and you would eventually have to eject wastewater
| by pushing against the deep sea pressure. Nothing's free in
| physics!
| nonethewiser wrote:
| But what would take more energy? Pushing that wastewater
| out or desalinating?
| hwillis wrote:
| Pressure (N/m^2) times volumetric flow (m^3/s) equals
| power (N-m/s). At 231 meters the pressure is ~340 psi or
| ~6.3 MPa. 1 liter would take ~6.3 kJ to pump.
| jl6 wrote:
| Also depends on the energy/materials needed to make and
| maintain that membrane, I guess.
| basseed wrote:
| *principle
| iancmceachern wrote:
| It seems there is one use case that would be viable.
|
| If we use the water at depth. I think the way to go is
| underwater living pods.
|
| If we go this route, I propose we also kickoff a solid
| Merpeople program (Mermaids, Merman, etc). The goal woukd be
| to gradually move the human race to be Merpeople. One day we
| will be viewed as the Neanderthals and Merpeople the people.
| As everyone knows, the hallmark of any reputable Merpeople
| program I'd breathing underwater, indefinitely, in Salt
| water. If we've been able to achieve that it's reasonable to
| assume we'd also be able to engineer the ability to "drink"
| seawater directly.
|
| All this to say, I guess there isn't really an application
| for this.
| darkclouds wrote:
| > If we use the water at depth. I think the way to go is
| underwater living pods.
|
| We need way more daylight than most people realise, look at
| what goes into a submarine who are already living in a what
| is effectively a cramped mobile blacked out cave.
|
| Living under water in pods has been done before but damp
| was the biggest issue from what I remember.
| irrational wrote:
| Haven't there been experiments with using wave motion to
| generate electricity? If practical, maybe wave motion at the
| top could be used to generate electricity for pumps at the
| bottom.
| barney54 wrote:
| Yes and they are expensive and unsightly. People don't like
| to look offshore and see the ocean industrialized.
|
| I like the idea of wave energy but the economics are really
| difficult in large part because the ocean is a difficult
| environment.
| geysersam wrote:
| Still, pumping water from 230 m is not that expensive energy
| wise compared to other approaches.
|
| Best case scenario, pumping 1 kg of water 230 meters costs
| 2.3kJ.
|
| Elsewhere in this thread it was mentioned that typical
| desalination approaches need 10-15kJ/kg.
| jrockway wrote:
| Yeah, 230m sounds like a lot but skyscrapers are taller
| than that and have running water at the top. So it's
| definitely possible to do.
| chongli wrote:
| I think you're still going to get killed on maintenance
| costs. When the membrane gets clogged, how do you replace
| it cheaply? How do you repair/replace damaged/worn out
| pumps? What about sections of pipe and the fittings?
|
| All of these sorts of repairs are also required for land-
| based water treatment facilities, of course, but now you
| have to perform them all at 230m underwater. That's gotta
| be mega-expensive. Furthermore, all of your equipment needs
| to be able to survive at those depths, with saltwater
| attacking exposed surfaces and seals, requiring much more
| expensive materials and tighter tolerances.
|
| It really doesn't seem very economical to me!
| irrational wrote:
| Is economical important? If we really are desperate for
| fresh water, does the cost become immaterial?
| chongli wrote:
| It's not just that it's expensive, it's also ridiculously
| complex. About the only thing I can think of that would
| be more complex is launching rockets into space in order
| to collect water from comets.
| stereo wrote:
| When your straw gets clogged with ice cream, do you dip
| your fingers into your milkshake to unclog it?
|
| You add your osmosis filter at the end of a long flexible
| hose. For maintenance, you reel it back in.
| chongli wrote:
| Straws don't work beyond 10 metres. You need all your
| pumping equipment either at the bottom or distributed
| along the pipe. Either way, that's going to put a ton of
| strain on your "flexible hose" when you try to reel it
| back in. And what if it breaks? Oops!
| Enginerrrd wrote:
| I mean, we do it with oil. ...At least sort of. Usually
| the wells are pressurized so pumping isn't a concern, but
| there's a surprising amount of infrastructure involved
| down at depth and the maintenance of that infrastructure
| is significant but largely a solved problem. Obviously
| economics probably favor it when oil is $75/barrel
| compared to ~$3 maybe for the fresh water, but still.
| chongli wrote:
| Your estimate for the water is way too high. I pay $2 USD
| (as of today's exchange rates) for a cubic metre of water
| from my local utility. That works out to about 32 cents
| per "barrel" of water. A $75 USD barrel of oil is 234
| times as expensive as the same amount of water.
|
| From what I can gather, oil rigs have profit margins
| below 20% (at best) and margins decline as the pressure
| in the well head drops, until they become totally
| unprofitable and shut down to wait for oil prices to go
| up. Water prices would have to increase by more than 2
| orders of magnitude to make this strategy begin to
| approach profitability!
| GhostVII wrote:
| Wouldn't pumping from 230 meters be exactly equivalent to
| just pumping through the membrane at sea level?
|
| If you have a pump 230 meters below sea level trying to
| push water up a pipe, to get water to the surface that pump
| must be exerting the same amount of pressure as you'd be
| experiencing 230 meters below sea level (since otherwise it
| wouldn't be strong enough to push up the water column). So
| you might as well just take the pump up to sea level and
| have it pump water through the membrane, since it would be
| exerting the same amount of pressure.
|
| Or alternatively, you could just find a hill that is 230
| meters high, pump sea water to the top of it, and then have
| the sea water come down in a pipe and go through a membrane
| in the bottom. Should be totally equivalent without
| requiring anything underwater.
| Natsu wrote:
| Just dig a ditch from California to Arizona, then use the
| sunlight to evaporate the water. Easy, right? :)
| jrockway wrote:
| That's the most common way of collecting drinking water,
| right? Sun evaporates water from the ocean, then it condenses
| in clouds, and the clouds rain into a river which we suck the
| water out of. Boom! Plus you can get electricity out of it.
| cma wrote:
| How much energy does it take to pump it back up from 231
| meters, compared to traditional methods? That would be 2.25kJ
| of energy per liter. Normal desalination I believe is 3kJ per
| thousand liters.
| greesil wrote:
| You can power it by pumping down an equal amount of surface
| water
| hwillis wrote:
| If you had a 231 meter dam, with a membrane filter at the
| bottom, fresh water will come out at atmospheric pressure.
| It then has to be carried back up to sea level. You could
| power that pump with another hole in the dam, but then the
| non-sea side of the dam would constantly be filling up with
| water. Once it's full, there's no longer any pressure
| difference to drive a pump.
| paulcole wrote:
| My idea is to not pump it back up but to let it drain down to
| Australia.
| MarkMarine wrote:
| That is amazing! I'd never thought of leveraging the pressure
| of ocean depth for this, it's a great idea. For anyone reading,
| the energy that OP is talking about would actually force the
| fresh water above the surface of the ocean without pumps.
|
| I didn't get to the whole paper, but I imagine the challenges
| of keeping that membrane clean are remarkable.
| IshKebab wrote:
| It's not amazing, it's obviously delusional.
|
| Think about it - if it really worked you have invented a
| perpetual motion machine, since you could just feed the fresh
| water through a little water wheel back into the ocean.
|
| You don't need to do any maths at all to know that his maths
| has gone drastically wrong somewhere.
| MostlyStable wrote:
| In the absence of the sun (which is what ultimately drives
| ocean mixing), eventually this would lead to a layer of
| pure fresh water on top of a layer of salt. The energy is
| being driven by osmotic pressure of salt water. Once you
| have extracted the water out of the salt, that pressure is
| exhausted. Thus it is not perpetual.
| MarkMarine wrote:
| This isn't a closed system. It's not perpetual motion,
| there is an EXTRAORDINARY amount of energy added to the
| system by the sun.
|
| Saying "maths" instead of just math sounds quite odd, why
| are you making it plural? Is solving one equation "a math"
| and solving multiple equations "maths"?
| xdex wrote:
| "Math" is used in USA and Canada. "Maths" is used by the
| rest of the English speaking world.
| dpratt wrote:
| Yet another example of how the American education system
| is falling behind the rest of the world. Students in the
| US only learn one kind of math, but apparently the rest
| of the English speaking world is taught multiple kinds of
| maths.
| rhtd wrote:
| I disagree to disagree, I henceforth declare this a
| mathses space for all mathstronauts
| MarkMarine wrote:
| Sounds ridiculous
| devindotcom wrote:
| No one has mentioned it but maths is short for
| mathematics. Also, your judgment of a very common figure
| of speech comes off as provincial.
| MarkMarine wrote:
| Fair, I was being trite. The idea that the ocean is a
| closed system that doesn't include all the solar energy,
| currents, mixing and everything else added to it plus an
| osmotic membrane would represent a perpetual motion
| machine just sounded so laughably stupid I was just
| trying to be equally annoying and dumb.
| AlexAndScripts wrote:
| "Math" sounds funny as a brit - it's just what you're
| used to, although I will admit "math" is probably more
| technically correct.
| vorticalbox wrote:
| Reminds me of "school of rock"
|
| > So, get off your ath, let's do some math.
| jaapbadlands wrote:
| You do
| boxed wrote:
| "The rest" being Australia, New Zeeland, UK and Ireland?
|
| Still a minority in people right?
| shaky-carrousel wrote:
| Well, I guess a billion flies can be wrong.
| bipop5000 wrote:
| +1.6 billion indians
| Keyframe wrote:
| Maths is common in UK
| slashdev wrote:
| Where's the part of the system where the energy from the
| sun comes into play?
|
| The sun isn't what causes pressure in the ocean...
| MarkMarine wrote:
| Lol. Oh sun isn't what causes pressure? Shit I better go
| back to my thermo teacher and let him know.
| MostlyStable wrote:
| Not an expert, but from what I understand looking at the
| paper, the sun is what drives ocean mixing. Without ocean
| mixing, this method would eventually fully extract all
| the water out of the ocean water resulting in fresh water
| oceans on top of a floor of precipitated salt. Once that
| happened, the osmotic pressure would be exhausted and the
| system would stop working.
| The_suffocated wrote:
| "Maths" is the contracted form
| (https://www.imperial.ac.uk/brand-style-
| guide/writing/grammar...) of MATHematicS. It had been in
| use in English-speaking countries since more than a
| century ago. As a matter of fact, it was also used in the
| US during the 19th century. Yet, for some curious reason,
| the Americans had stopped using contractions from some
| time onwards. So, "maths" became "math" and "PhD" became
| "Ph.D.".
| devindotcom wrote:
| I think the justification is that because maths isn't
| plural in practice (I've never heard anyone say
| "mathematics are"), it's pointless to retain the s, and
| only leads to an awkward megasyllable like the end of
| "wasps."
|
| The Anglo-American linguistic divide is always fun to
| examine. We don't always come out ahead, but now and then
| we get winner like "acclimate."
| jrockway wrote:
| I think "mathematics are" is a normal construction. "We
| have a new widget that does this, but the mathematics of
| mass production are troubling."
|
| If you don't like something coming between "mathematics"
| and "are", how about "I can prove Fermat's Last Theorem,
| but the mathematics are pretty high level." One could
| argue that many people would say "the math is pretty high
| level", but I think I've heard each before. It doesn't
| feel odd to write at all.
| IshKebab wrote:
| Their claim does not involve energy any input from the
| sun.
|
| > maths
|
| That's what it's called in the UK (and maybe Australia &
| NZ). Shortening of mathematic _s_. I could say the same
| about "legos". :)
| Metacelsus wrote:
| Yeah, I thought of it a few years ago, then realized I'd been
| scooped by ~40 years. But given the advances in technology
| maybe it's possible today?
| tinus_hn wrote:
| Technology will never advance enough to enable the
| perpetuum mobile device, which is what this is,
| unfortunately.
| gadilif wrote:
| Not exactly, the pressure is an external force working on
| the system (so, open system). Perpetuum mobiles is a
| closed system with no energy loss.
| IshKebab wrote:
| No he's right, the idea of extracting work from the
| pressure of the ocean is as impossible as perpetual
| motion.
| quertered wrote:
| It is clearly not impossible, the paper clearly states
| that it's thermodynamically feasible, even if not
| practical at 1970s
| waveBidder wrote:
| where are we extracting the energy from? is it
| effectively geothermal power?
| postalrat wrote:
| Salt falling though gravity? It won't work after the
| upper layers of the ocean are depleted of their salt.
| fbdab103 wrote:
| There are 1e20 kgs of salt in the ocean. I am going to
| put that as a somewhat low concern.
| MarkMarine wrote:
| It's solar powered.
|
| Ok, I'm being brief but you seem genuinely curious, so,
| lemme explain what I mean:
|
| In thermo, you would draw a line around a closed system,
| and say basically conservation of energy applies in this
| system. Energy in = work out + losses.
|
| In a system where one input is the ocean, you basically
| just consider that infinite because of the size
| differences. One dinky little pipe compared to the mass
| flow into the system from the ocean, that math would be
| silly to bother with. That said, to swat down the stupid
| "this is perpetual motion" comments from people with
| Reddit engineering degrees, you also have to add in the
| energy input into the system of the sun, which is keeping
| the water from freezing over the course of 1 million
| years as it runs someone's sarcastic water wheel.
| hwillis wrote:
| No it isn't. This is just like putting a filter at the
| bottom of a dam, except the filter also blocks salt. The
| top of the dam is at sea level, so you have to bring the
| water back up to the surface.
| motohagiography wrote:
| If you want anything to be economical or "worth it," you create
| the conditions where you are providing something to external
| parties, and the money will come in to sustain it. The economics
| of desalination are trivially viable if you can set up a stable
| financial regulatory environment in the region that has
| competitive taxation internationally.
|
| Desalination is economical in the middle east because they export
| oil. In Israel it's economical because they export tech knowhow,
| and have foreign income sources from their diaspora networks.
| Some people will moan about "tax havens! for the filthy rich!"
| But really it's just viable habitation for locations without a
| lot of resources to export. Favourable capital rules are a
| valuable export. We do this in customs-free zones around the
| world today. If you have those rules in an area that needs a
| desalination plant to survive, I guarantee you it's going to work
| really, really well.
| dukeyukey wrote:
| Same in Australia - you've got a very wealthy population,
| living on the coastal outskirts of a wind/solar/coal heavy
| desert. Basically the perfect combo for desalination.
| AnotherGoodName wrote:
| The coastal areas of Australia have literally some of the
| highest rainfall anywhere in the world. Sydney has more
| annual rainfall than 'rainy' Seattle or London.
|
| For Australia the correct thing is to store the damn (or
| should I say dam) water that falls where people live.
| golemiprague wrote:
| [dead]
| thunkshift1 wrote:
| All comments seem to focus on the cost/energy requirements of the
| filtration process.. but the elephant in the room is the waste.
| How to safely dispose off the waste water after filtration
| completes? That water is virtually useless
| samtho wrote:
| Run long tubes (1-3mi) out into then ocean with perforations so
| you can diffuse the highly salty water back into the ocean
| without disrupting local ecosystems.
| lovemenot wrote:
| Ocean-going vessels could be fitted to take on brine as ballast
| in port and gradually replace it with sea-water during their
| voyage.
| blurker wrote:
| It's surprisingly difficult to separate salt from water to the
| point that it's drinkable. That difficulty translates to it being
| very costly and that cost is so prohibitive that most places will
| choose another option. Like even building massive pipelines to
| transport water across 1000's of kilometers is probably going to
| be a better option. It's pretty much always going to be more
| economical to use the water that the earth is naturally
| desalinating for us.
| Obscurity4340 wrote:
| Doesn't Israel (have the tech to) do it for like nothing or
| extremely nominal cost?
| josh_fyi wrote:
| Not nothing or nominal, but cheap enough that it is feasible,
| and about the same as other options. We also recycle 80% of
| the water, first in the world. The second-place country does
| 15%,
| slashdev wrote:
| Actually it's quite cheap, and Israel does it at scale, among
| other places.
|
| It's just still much more expensive that just pumping free
| fresh water from rivers, lakes, and ground water. So you'd only
| do it if water were actually scarce.
|
| I think this is why the scare-mongering over fresh water
| availability (agriculture excluded) is mostly just hot air.
| Fresh water is only scarce at ridiculously low prices. Raise
| the price and the market will solve the problem quickly. That
| matters for agriculture, but not for human consumption.
|
| Even many dry regions that are far from the ocean, like the
| southwest US, only have water problems because most of it is
| used for agriculture at ridiculously low prices. Price it
| appropriately and that will end very quickly and there won't be
| any shortage.
| zbrozek wrote:
| Though the southwest is a great place to farm if you have the
| water to leverage the sun and the relative dearth of pests.
| Given the economic potential of a huge fraction of the
| country, I'd wager that getting water there will be
| worthwhile within my lifetime.
|
| I'd love to see an aqueduct covered in solar panels roughly
| along I-40. Drain the too-wet southeast to irrigate the too-
| dry southwest.
| thaumasiotes wrote:
| > Though the southwest is a great place to farm if you have
| the water to leverage the sun and the relative dearth of
| pests. Given the economic potential of a huge fraction of
| the country, I'd wager that getting water there will be
| worthwhile within my lifetime.
|
| The reason for the dearth of pests is that it's a desert.
| If you change that, the pests will arrive just like
| everything else does.
| zbrozek wrote:
| It's true, but that'll take quite a long time and the
| advantage is unlikely to evaporate completely even if it
| erodes. And there's probably no amount of water use that
| humans can realistically achieve in that same time period
| which will take away the sunlight.
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