[HN Gopher] Desalination system could produce fresh water that i...
___________________________________________________________________
Desalination system could produce fresh water that is cheaper than
tap water
Author : geox
Score : 169 points
Date : 2023-09-27 15:14 UTC (7 hours ago)
(HTM) web link (www.eurekalert.org)
(TXT) w3m dump (www.eurekalert.org)
| pfdietz wrote:
| A simple evaporation system without heat recovery would use about
| 100x the energy of reverse osmosis per unit of fresh water
| produced (which is somewhere around 5 to 8 kWh per cubic meter of
| fresh water.)
|
| PV-driven RO, even without energy recovery, is going to produce
| much more water than this scheme.
| galangalalgol wrote:
| Don't RO systems need regular maintenance, and have consumable
| parts? It sounds like this was designed to work unmaintained
| for years. That could balance out the efficiency.
| jjk166 wrote:
| Any system that has saltwater flowing through it is going to
| need regular maintenance to deal with buildup and corrosion.
| galangalalgol wrote:
| I also think that seems likely, just reading their
| announcement though they claim eddies prevent buildup and
| the thing looks like a glass solar still, so the brine may
| not contact anything it can corrode. Even normal countertop
| distillers come out cheaper per gallon than an RO system
| depending on your electricity prices, though that is not
| with saltwater for either.
| xkcd-sucks wrote:
| More that the important wear component in a RO system is a
| membrane that depends on access to sophisticated
| manufacturing, while plumbing is relatively easy to DIY in
| the field, and the big innovation in this system is 2.5mm
| perforated polyurethane which can also be remade with
| simple tools (and access to polyurethane of course but
| that's simpler to make or salvage than RO membranes).
|
| Kind of analogous to ICE vs electric cars; electric cars
| are simpler in principle but the most important components
| require access to advanced semiconductor and battery
| manufacturing, while all the components, _in theory, of an
| ideal "ICE car"_ can be recreated with early 20th century
| machine shop technology
| rootusrootus wrote:
| A modern electric car is probably comparably difficult to
| a modern ICEV. If you want to draw a comparison with
| early 20th century machine shop technology, then compare
| with an EV of that era. They existed. Lead acid batteries
| are _much_ simpler than lithiums.
| mixedmath wrote:
| This sounded pretty strong to me, so I did a quick napkin-math
| check of feasibility (in short - it passed the napkin check).
|
| On a sunny day, total solar energy hitting the top of the
| atmosphere gives about 1360 watts per square meter (source - top
| result from google, something something NASA). It takes about
| 4.184 joules to raise one ml of water by 1 degree celsius. Let's
| suppose that the "suitcase" proposed in the article has 1 square
| meter of space (a pretty large suitcase) and is able to capture
| 100 percent of solar energy (unreasonably effective). Then pure
| solar energy-wise, there is enough energy to raise 325 ml of
| water by one degree of celsius per second in a square meter. If
| the water is, say, at 16 degrees (the temperature of the ocean
| near me), in would take 84 seconds to bring 325 ml of water to
| boiling temperature. To transform boiling water to steam, each ml
| of water takes an additional 2257 joules. In 540 seconds, the 325
| ml of water is now converted to steam.
|
| In total, it has taken 624 seconds to steamify 325 ml of water
| (under very optimistic circumstances). Morally one could do this
| almost 6 times in an hour, giving approximately 2 litres of water
| per hour.
|
| (Edit): Looking a bit closer, I see that the article's
| description of their work suggests that they recapture some of
| the energy from condensing the steam back into water. I didn't
| account for this. Even a moderate amount of recapture changes the
| math very favorably.
|
| This makes me suspect that their claim of 4-6 liters per hour
| would require a rather large "suitcase"-sized device, but is
| within the ballpark of reason.
| JackFr wrote:
| > On a sunny day, total solar energy hitting the top of the
| atmosphere
|
| Aren't they all sunny at the top of the atmosphere?
|
| > gives about 1360 watts per square meter
|
| I assume that's very much latitude dependent
| gmm1990 wrote:
| should be the same at all latitude
| laurencerowe wrote:
| But only if you point angle it towards the sun, so at 45
| degrees latitude a 1x1m panel would take up 1x1.41m of
| ground area.
|
| There's also more atmosphere in the way the further you go
| from the equator which will affect how much solar radiation
| reaches the earth. Overall only about half of solar
| radiation reaches the surface.
| [deleted]
| badrabbit wrote:
| I had this same idea on an HN comment early this year lol.
|
| Part of my idea is to use desalinated water to build artificial
| lakes and rivers. Specifically, a river system along interstate
| highways, especially in the US west and alongside that infra
| build highspeed railways. These two things along with
| interstate highway will allow new towns and cities to flourish
| which will help with economic activity recouping some of the
| cost but also solve homelessness, climate goals and even social
| unrest and instability. It passed my poorly done napkin math.
|
| If hitler had plans to dam the mediterranean and turn the
| sahara into an fertile land and generate crapton of energy wth
| are we doing today with all the peace, economic health and
| insane amounts of technical progress? $300B in subsidizing ev
| car chargers (car makers should pay for) instead of high speed
| railways makes me sad.
| japanuspus wrote:
| The fundamental limit to energy expenditure is not the heating
| and boiling of water (as others have pointed out, that energy
| can all be captured). Rather, the limit is set by the
| requirement for the entropy of the universe to not decrease
| when we reduce the local entropy in the salt-water system.
|
| According to GPT4, the numbers come out to 760 J/L for seawater
| with a salt concentration of 35 g/L. That would mean a limit of
| around 2 L/s*m2 for full intensity sunlight.
| PaulKeeble wrote:
| Solar heaters are remarkably efficient, well above 95%. Solar
| water heaters are really quite efficient so there is no
| particular reason why with the right coatings such a device
| wont get quite close. Its actually surprisingly easy to make a
| solar water boiler and they aren't expensive. People heat pools
| with DIY water heaters.
|
| If heating water is the goal then going directly to heating the
| water is more than 4x the power capture of a solar panel.
| Dropping one solar panel on your roof for a water heater pays
| off its just that the solar heaters are kind of expensive
| dealing with all the other aspects, like if the pump fails or
| its too cold and the water would freeze etc etc. Those other
| aspects are probably going to dominate the complexity and cost
| of the devices.
| bertil wrote:
| I've always wondered how hard it would be to have standard
| solar panels water-cooled and use that heat for hot water
| storage.
| colechristensen wrote:
| The problem is you want your hot water much hotter than
| your solar panels so there may be limited use for this
| compared to a traditional solar water heater.
| hinkley wrote:
| Heat pump water heaters exist. I would think running a
| warm water source past one would increase the efficiency
| since it lowers the delta-T
| PaulKeeble wrote:
| The main problem is the expected lifetime (and payout
| periods) of the two parts. Cooling solar panels brings more
| efficiency so it feels like a reasonable fit, cool the
| panels and get hot water. Its less efficient water heating
| because the panels aren't the ideal coating but they do
| still get quite hot and a few of them can definitely do the
| job. The big mismatch is in life.
|
| A solar thermal heater typically lasts 10 years, its a
| pretty harsh environment and they have pumps and expansion
| vessels and water in the heat range of -25 through +80 C is
| just damaging over time. Where as a solar panel has a rated
| 80% output at 25 years. That mismatch is problematic for
| Solar panel payoff, 10 years is only just past break even
| point. Whereas the solar thermal heaters typically pay off
| within a few years so the reduced lifetime isn't such a big
| problem (but both are driven by local gas and electrical
| prices).
|
| Companies are trying to do this better and products exist
| to do it. The added installation complexity and reduced
| expected lifetime are all a bit of an issue. Currently I
| suspect most people are better off with a dedicated solar
| heater and solar panels separately, at least for now until
| the water channels in the panels at least are reliable
| enough that they meet the panel lifetime and then the pump
| is installed in the roof cavity so its cheap and easy to
| replace. Work still needs to be done to work out the
| details and make the parts that fail easily swappable.
| azornathogron wrote:
| You can buy such things. Eg,
| https://dualsun.com/en/products/dualsun-spring/
|
| (Not affiliated, no idea if that particular one is any
| good, it was just one of the first results)
| adrianmonk wrote:
| Can't you boost the efficiency by using heat recovery?
|
| The steam is very hot, and the condensed fresh water is also
| pretty hot. You can use a heat exchanger to transfer this heat
| into your 16degC ocean water, preheating it so the sun doesn't
| have to do all the work.
|
| This wouldn't mean magically getting free energy. Your output
| fresh water would otherwise be really hot, and that's where the
| energy comes from.
|
| Obviously there are limits. If you have 100degC fresh water
| coming out and 16degC ocean water going in, a heat exchanger
| will at best average their temperatures to 58degC. You could in
| theory overcome this with a heat pump, but that's pretty
| elaborate.
|
| (I'm not trying to design the perfect desalination system here.
| The point is there may be some easy efficiency wins.)
| m463 wrote:
| if you can do it at lower pressure, boiling and evaporation
| happen at a lower temperature.
| bertil wrote:
| Good idea, but it probably makes a very cheap system more
| expensive -- kitchen-equipment expensive.
| e44858 wrote:
| A countercurrent heat exchanger could transfer up to 99% of
| the heat from the steam to the ocean water:
| https://en.m.wikipedia.org/wiki/Countercurrent_exchange
| hinkley wrote:
| It sounds to me like the 'condenser' is perhaps wrapped
| around the water intake.
| bertil wrote:
| That's obviously the way to go, but... Where does the 99%
| come from?
| Faaak wrote:
| The theoretical efficiency could almost reach 1 with an
| infinite tube. In practice I've seen ~80%.
| delecti wrote:
| A simple system can do much better than the average. Picture
| the water flowing in a "U", with the hottest water at the
| bend. The coldest incoming water is warmed just a bit by the
| coldest outgoing water, then the slightly warmer incoming
| water is warmed by water a bit further upstream from the
| output. Carry that logic through and you recapture the heat
| with a fairly simple system (give or take longer pipes to let
| enough heat exchange happen).
| Dylan16807 wrote:
| A multi-stage or continuous heat exchanger can transfer a lot
| more than half the heat.
| Gibbon1 wrote:
| There are desalinization systems for boats. They are expensive
| but everything about boats is. I saw one model that produces
| 20gph and seems to use about 1200 watts.
|
| Doesn't feel like there is a gross thermodynamic reason it
| wouldn't work.
| grandchild wrote:
| further down the article:
|
| > if each stage were scaled up to a square meter, it would
| produce up to 5 liters of drinking water per hour
|
| so 1m2, times the number of layers.
| lta wrote:
| I think the layers are on to of each other if the total
| surface is a square meter, it doesn't bring any more solar
| energy to stack layers, right ?
| o0banky0o wrote:
| I don't think they are boiling it. It read to me like they're
| just evaporating the water in some efficient way, rather than
| boiling.
|
| What would the math look like if they aren't necessarily going
| to boil?
| bertil wrote:
| They have to "boil" but not get the water to 100oC. Water
| evaporates in the air at any temperature; it's faster when
| the water is warm, and the air is warm and dry. Technically,
| that's boiling, even if it's not exactly like how your kettle
| does it.
|
| Essentially, they find an equilibrium between the cold water
| coming in, warming in the sun, an increasing amount
| evaporating into the warm, damp chamber, and the remaining
| brackish water being cooled by the new water.
| fatnoah wrote:
| > I don't think they are boiling it. It read to me like
| they're just evaporating the water in some efficient way,
| rather than boiling
|
| There are definitely efficiencies to be had, though I don't
| know enough of the math to judge one vs. the other. During my
| brief patent career, I wrote the patents for a distillation
| system where the main elements involved heating water that
| was distributed across rotating blades (heat + surface area +
| air movement) to evaporate the water. When the water was
| collected, it passed through a heat exchanger that exchanged
| heat with the in-flowing water. The result was a very
| efficient system on a small scale, at least.
| genewitch wrote:
| the swamp cooler panel seems more durable than rotating
| blades, maybe*. If i were going to desal it'd be with
| solar; which seems inefficient but one could precipitate
| "CO2" out of the water as calcium carbonate during the same
| process. Emergency water supply for tropical weather
| aftermath, during the quiet season park upstream from a
| coral reef that's in danger.
|
| *edit: although window and wall unit HVAC use the blades to
| fling water around so the condenser gets the coolest
| possible air
| Izkata wrote:
| From some quick searches, it looks like 2257 J the energy
| used by the phase transition, so that part will probably be
| the same.
| AlexandrB wrote:
| Boiling is just evaporating water rapidly. No matter what you
| do, you need to put the same amount of energy in to convert
| some mass of water to vapor. Any efficiency gains would be in
| how you get this energy into the water (electric element vs.
| directly heated by the sun) not in the energy required.
| PicassoCTs wrote:
| Ultrasound would do it without boiling?
| AlexandrB wrote:
| I don't think it matters whether you "boil" the water or
| not, you still need to put in enough energy to cause a
| state transition in the water you're evaporating. I see
| some papers on using ultrasonics to increase the
| efficiency of energy _transfer_ from a heating
| element[1], but I don 't think the second law of
| thermodynamics allows for a free lunch here.
|
| [1] https://www.sciencedirect.com/science/article/abs/pii
| /S13594...
|
| Edit: If you're thinking of something like an ultrasonic
| humidifier, I don't think these actually evaporate the
| water[2]. The mist these produce would still contain salt
| if you tried to use them for desalination.
|
| [2] https://en.wikipedia.org/wiki/Humidifier#Ultrasonic_h
| umidifi...
| mensetmanusman wrote:
| That will give you bone lung
| hinkley wrote:
| https://en.wikipedia.org/wiki/Legionnaires%27_disease
|
| I didn't know that legionella can kill macrophages from
| the inside. That's nasty.
| abakker wrote:
| Ultrasonic "steam" tends to aerosolize the total
| dissolved solids. Google "white dust" in regards to
| ultrasonic humidification. I suspect that this would not
| work for desalination.
|
| Aside:
|
| I have a home-built ultrasonic humidifier. If I run it
| with Boulder, CO tap water that is low in TDS, it only
| takes a day or so to have a PM2.5 >600 in my house. For
| this to work I had to install an RO filter in order to
| humidify with ultrasonic and not degrade air quality.
| hinkley wrote:
| Also the total undissolved bacteria and ameoba.
| chipsa wrote:
| Ultrasonic humidifiers need to be run with distilled or
| equivalent purity water, yes. Not just for the lack of
| salts being aerosolized, but also because anything that
| may incidentally grow in the water also will be
| aerosolized. Distilled water helps minimize growth.
| jjk166 wrote:
| Technically evaporation takes less energy than boiling as
| your product is ambient temperature water vapor instead of
| hot water vapor. You also don't have to replace heat lost
| to the environment by the hot water and steam during the
| boiling process. The difference can be lessened by good
| insulation and heat regeneration, but they still can't be
| perfect.
|
| Of course on the flip side, your goal is fresh liquid
| water, so you need to condense the vapor. Condensing hot
| vapor is easy, just expose it to cooler ambient conditions.
| Condensing ambient vapor is harder, and will require you to
| run something like a refrigeration cycle or a chemical
| desiccant system which will need energy to be regenerated.
|
| Most commercial systems use vacuum distillation which boils
| water at low temperatures and pressures, which has its own
| drawbacks but is generally more efficient.
| AlexandrB wrote:
| Fair enough. I guess what I'm getting at is that the heat
| of vaporization sets a lower bound for how much energy
| you need to add, regardless of whether you boil the water
| or evaporate it.
| DoctorOetker wrote:
| You are correct that at ambient temperatures there is an
| ultimate lower bound for energy.
|
| You are incorrect that at ambient temperature the lower
| energy bound is set by the latent heat of vaporisation,
| as others have pointed out this is theoretically
| recoverable.
|
| At ambient temperature there is however still a
| fundamental physical limit: the solvation energy of the
| salt in the water: https://en.wikipedia.org/wiki/Solvatio
| n#Solvation_energy_and...
|
| Now the whole globe does not have the same ambient
| temperature, and as you know about global warming it
| would be great to shed some energy in the form of heat.
|
| There are many forms of desalination. Another way to
| desalinate is freezing: when salty water freezes, it
| pushes out the salt, so while desalinated water ice
| forms, the liquid water surrounding the ice will increase
| in salinity and become brine. One could then use simple
| nets or grills to separate ice from brine.
|
| Suppose one has a space elevator, or even a tether from a
| balloon, but capable of carrying significant weight.
|
| The temperature falls roughly adiabatically with height.
| Above the tropopause the atmosphere is essentially cloud-
| free, CO2 free and below freezing point (say -60 deg C).
| Hence the latent heat of fusion (freezing) can be shed to
| outer space. So it should be possible to lift salty water
| up an elevator, allow it to freeze over, separate brine
| from ice at the top, then lower the separated brine and
| ice.
|
| The energy required to lift the brackish water is
| compensated by the energy released by lowering the
| freeze-distilled water and brine. What comes up must go
| down, so simplistically speaking a pulley in equilibrium,
| so that the only energy intentionally exerted is lost to
| pulley and air friction. Then one would be cooling the
| planet and receiving frozen freeze-distilled water at the
| same time.
|
| The law of conservation of misery is typically not a
| fundamental law of nature, but imposed by reluctance to
| study of those who dictate artificial laws.
| hinkley wrote:
| If your water source is cooler than ambient air, is that
| still a problem?
|
| If it works less well on cool days, well you don't need
| as much water on cool days as you do on hot ones.
| mannykannot wrote:
| I just found out that the latent heat of evaporation
| itself _decreases_ with increasing temperature and
| disappears at the critical temperature [1].
|
| With regard to the energy expenditure for reaching that
| temperature: if we were merely raising the temperature of
| the water and then cooling it down again, I think it
| would be correct to say that with a completely efficient
| contra-flow heat exchanger, perfect insulation, and no
| pumping losses, the steady-state heat input could be
| arbitrarily low.
|
| If we now modify this system to evaporate and then
| condense some of the water at the point of highest
| temperature, we would have to supply, and then extract,
| the latent heat at whatever temperature the evaporation
| is performed at. Once that has been performed, the
| outflow would comprise of the same amount of water as
| before (and at the same temperature), and it would be
| equally available to warm up the incoming stream as in
| the initial scenario (though now we would need two heat
| exchangers in order to keep the fresh water separate.)
|
| Of course, both the heat exchanger and the insulation
| will have losses, but we are rejecting quite a bit of
| heat in the condenser, and it is at the highest
| temperature in the system. Would that, in principle, be
| available to make up for any losses elsewhere? This makes
| me wonder if, counter-intuitively, it could be more
| efficient to do the distillation at higher temperatures,
| at least up to the point where the diminishing latent
| heat can no longer compensate for the losses of running
| at a higher temperature?
|
| I'm leaving out some considerations that I don't know how
| to handle (and probably others that have not occurred to
| me.) For one thing, there's the question of what happens
| if the evaporation occurs into a chamber containing some
| air, rather than just steam (my guess is that the
| relevant temperature is determined by the water vapor
| partial pressure.) For another, what difference does
| having salt dissolved in the water make? And this may all
| be moot, as this system has no moving parts, so the
| pressure is probably atmospheric (or somewhat below, if
| the condensation can be exploited to create a partial
| vacuum.)
|
| I have no idea if any of this makes the slightest bit of
| sense, and it's probably wrong - as you say, most systems
| run at reduced pressure.
|
| [1] https://www.engineeringtoolbox.com/water-properties-
| d_1573.h...
| scotty79 wrote:
| Thermodynamics can't be cheated. If you want to turn some
| liquid into gas at given pressure you need to deliver
| specific amount of energy regardless of how you do it.
| mitthrowaway2 wrote:
| However, you also need to turn the same amount of gas back
| into liquid, just somewhere else without the salt. It does
| seem like there's good potential for recovering and reusing
| that energy.
| gwbas1c wrote:
| So 1 suitcase-sized device can generate about 1-1.5 gallons of
| water an hour. (A gallon is a little less than 4 liters.)
|
| The average American uses 82 gallons of water a day.
|
| Assuming 10 hours of sunlight, that requires 8-12 of these per
| American.
|
| I wonder how practical that is?
| yurishimo wrote:
| How much of that is shower water though? Drinking water, I
| think is probably closer to 2-3 gallons (on the high end) per
| day for most Americans.
|
| We also shouldn't forget that after disposing of the water, the
| local municipality can recycle it! For island nations, a few
| strategic water reserves could be cleaned and stored for future
| use.
|
| And then obviously, for more developing nations, they already
| use a lot of grey water for daily activities, so this becomes a
| source of consistent clean drinking water.
| danw1979 wrote:
| > The researchers estimate that if the system is scaled up to the
| size of a small suitcase,
|
| Go on then.
|
| Yet another MIT paper on desalination jumping the gun and
| announcing a breakthrough, that could be scaled up to modestly
| useful sizes if only they had the time and the meagre budget to
| do so.
|
| Here's[1] a previous announcement from Feb 2022 of a device that
| could produce water for a small family for only $4 of materials,
| that they didn't bother building.
|
| [1] https://news.mit.edu/2022/solar-desalination-system-
| inexpens...
| _a_a_a_ wrote:
| A naive question but genuine. Reducing atmospheric pressure
| reduces boiling point, so would a pressure pump mixed in with
| heating be beneficial?
|
| Relatedly, Howard energy efficiency be affected if you didn't
| heat the water to be desalinated, but just evaporated it using
| low pressure? Would it be less, more, or equally efficient than
| using heat alone?
| Gibbon1 wrote:
| I think the energy needed to vaporize water goes up slightly as
| you reduce the pressure. To condense the vapor you need to
| remove energy and that represents a loss in a single stage
| system. You can use multiple stages each at a lower and lower
| pressure to increase the efficiency. Using the heat of
| condensation to vaporize the water in the next stage.
| NegativeLatency wrote:
| How destructive is hot dense brine (the waste from) desalination
| systems? Seems like it would sink to the floor and create a dead
| zone.
| gooseus wrote:
| It's not good, desalination plants have to mitigate this with
| long diffusers that regulate the rate and concentration of the
| brine that is being put back into the ocean.
|
| Here is one for sale:
| https://www.jains.com/Pipefittings/JainPEPipes/spacial%20fit...
| westurner wrote:
| Desalination: https://en.wikipedia.org/wiki/Desalination
|
| Brine > Uses: https://en.wikipedia.org/wiki/Brine :
|
| > Culinary, Chlorine generation, Refrigerating fluid, Water
| softening and purification, De-icing, Quenching
|
| Uses for Brine / NaCl not listed on Wikipedia:
|
| Hypochlorite generation. Hypochlorite is the sanitizing
| primary component of household bleach. Hypochlorite can be
| made with a 5V USB Hypochlorite generator, salt, water, and
| watts of electricity.
|
| Salt-based cleaning products; "Non-Toxic Cleaners and EPA
| Disinfectants" https://saltbased.com/
|
| Energy storage; thermal battery (as heated by concentrated
| solar, for example)
|
| Energy storage; /? brine NaCl batteries:
|
| Sodium-ion Battery: https://en.wikipedia.org/wiki/Sodium-
| ion_battery
|
| /? Proton battery brine / sodium
|
| Not brine, but if you're already processing seawater:
|
| Diesel can be made by processing lots of seawater
|
| Hydrolysis and Electrolysis; [Green] Hydrogen production
|
| Nuclear Fusion; to extract D, T, He3, and He4 from (Helion,)
|
| What can be made with Brine and/or NaCl with modern
| sustainable production processes involving e.g. lasers and
| fusion heat?
|
| Salt belt: https://en.wikipedia.org/wiki/Salt_Belt :
|
| > _The Salt Belt is the U.S. region in which road salt is
| used in winter to control snow and ice._
|
| Nebraska roadways are treated with brine to pre-treat and de-
| ice roadways (instead of rock salt, which corrodes many
| metals).
|
| Though listed as a DIY weed killer ingredient, sodium is a
| dessicant which dries and prevents plant growth, so salt on
| the lawn will kill weeds but then leave a dead patch.
|
| Does discharge of fresh water into the ocean by desalination
| plants, for example, affect the thermal content of the water
| due to formation of halocines and other thermochemical
| effects?
|
| Solar pond: https://en.wikipedia.org/wiki/Solar_pond :
|
| > _A solar pond is a pool of saltwater which collects and
| stores solar thermal energy. The saltwater naturally forms a
| vertical salinity gradient also known as a "halocline", in
| which low-salinity water floats on top of high-salinity
| water._
| Robotbeat wrote:
| Sounds like it's easily solved with a diffuser.
| mywittyname wrote:
| It's a very difficult problem to solve at human scale. And
| getting it wrong can have catastrophic impacts.
|
| Even if 100% of grey water runoff is mixed with the brine,
| the resulting concentration is still high enough to cause a
| localized collapse in the ocean if it isn't properly
| regulated.
| SoftTalker wrote:
| If it's discharged with household sewage, it should be close to
| original salinity, assuming most of the household water usage
| eventually goes out the sewage pipe.
| JackMorgan wrote:
| That's.... remarkably clever. But I think the system loses
| quite a bit due to evaporation.
| DoctorOetker wrote:
| and gains quite a bit due to rainfall
| nomilk wrote:
| For most cities, the cost of the water _infrastructure_ (building
| and maintaining pipes) is _much_ more expensive than the cost of
| the actual water.
|
| In Melbourne, Australia, since tap water mostly comes from
| catchments in national parks, water requires very little
| treatment (some fluoride and chlorine is added), so it works out
| cheap: ~$25 AUD per million liters. Desalinated water, by
| contrast, cost about 24 times more (~$600 AUD per million
| liters).
| golemiprague wrote:
| [dead]
| grecy wrote:
| Melbourne is the perfect example of "what are we going to do
| when the water runs out". Remember when the storage was down to
| single digits?
| DoctorOetker wrote:
| do you have a reference handy for this storage time series?
|
| from the historical net loss rate approaching the lows one
| can estimate the required backup desalination flow rate.
| Ekaros wrote:
| Also here disposal of water cost nearly the same as water
| itself. 1,5EUR/m^3(water) vs 1,41EUR/m^3... And here it is
| sourced from artificially rained groundwater. So not most
| expensive way.
| bcrosby95 wrote:
| $600 AUD per million liters is still dirt cheap. That comes out
| to about an extra $3.5 per person per month (assumes 50 gallons
| of water/day/person). Would you be willing to pay an extra $3.5
| per month to never have to worry about a drought again?
|
| I know I would.
| birdyrooster wrote:
| Wouldn't it make the cost of anything using power nearby more
| expensive too?
| nwiswell wrote:
| > Would you be willing to pay an extra $3.5 per month to
| never have to worry about a drought again?
|
| Yes, you'll never have to worry about your basic supply of 50
| gal/d/person being threatened in a drought, but just think
| about this for a second: the population of Melbourne is
| around 5 million people. In the 2020-2021 year, Melbourne
| Water delivered 439 billion liters (116 billion gallons) of
| fresh water[1]. That is roughly 318 million gallons of water
| consumption on an average day. What kind of drought would
| reduce the available water from 318 million gallons to 25
| million gallons per day?
|
| Personal consumption (drinking, showering, household washing)
| is not the primary driver of water use. In general, it is
| dominated by agriculture, and to a lesser extent by industry.
| The economics of these activities would not permit an order
| of magnitude increase in the cost of fresh water.
|
| [1]: https://www.melbournewater.com.au/media/18976/download
| Veserv wrote:
| You missed a zero. 5 million people by 50 gallons/day is
| 250 million gallons/day. So they only missed by ~25%.
|
| Australia uses a estimated ~16T liters of water per year
| [1] for all uses including personal and agricultural. That
| is 16M ML * 600 AUD/ML for a total of ~10B AUD.
|
| Australia has a population of ~25M. So that is ~400
| AUD/person to completely replace all water usage in
| Australia. As Australia is a island, I assume they do most
| of their own agriculture, so for additional 400 AUD on
| their food bill a year they never need to worry about a
| drought ever again.
|
| The Australian government appears to have a budget of
| ~500B/year which is around 25% of GDP [2]. So, for ~2% of
| the government budget or ~0.5% of GDP the economics are
| completely managed.
|
| [1] https://www.worldometers.info/water/
|
| [2] https://www.aph.gov.au/About_Parliament/Parliamentary_D
| epart...
| eikenberry wrote:
| Are you saying that the water is so cheap that switching to
| desalination will be a non-issue OR are you saying that this
| will make the water a large enough portion of the cost to make
| it significant?
| bsder wrote:
| > Are you saying that the water is so cheap that switching to
| desalination will be a non-issue
|
| For human consumption: this is pretty much true.
| Desalinization works just fine for people. The biggest issue
| is where to put the extra salty waste products.
|
| When people talk about the "water crisis" in the US, for
| example, it's _always_ about big agribusinesses doing
| farming.
| genewitch wrote:
| any time there's a fish storm in the atlantic, just dump
| the salt in that area?
| selimthegrim wrote:
| New Orleans could use this right about now.
| AlexandrB wrote:
| > The resulting water vapor can then be condensed and collected
| as pure, drinkable water.
|
| Article doesn't mention how this part would work. Peltier cooler?
| Heat pump? Is that included in the "suitcase sized" concept? What
| powers it?
| Yeul wrote:
| You underestimate how cheap tapwater is. I'm paying EUR1.21 for
| 1m3 and that includes taxes.
| mdhen wrote:
| _The researchers estimate that if the system is scaled up to the
| size of a small suitcase, it could produce about 4 to 6 liters of
| drinking water per hour and last several years before requiring
| replacement parts. At this scale and performance, the system
| could produce drinking water at a rate and price that is cheaper
| than tap water._
|
| This seems like it would be extremely useful if it pans out.
| dylan604 wrote:
| scale up to a small suitcase...how small is it now?
| Kon-Peki wrote:
| ... and what is preventing them from scaling up to a small
| suitcase?
| Animats wrote:
| That's a very good question.
|
| We see these articles all the time, and they somehow don't
| get scaled up. You'd think they'd at least scale this up to
| the size of a typical rooftop solar panel before issuing a
| press release. So what's the problem? Uses some expensive
| material? Hard to fabricate at scale? Doesn't actually hold
| up in bright sunlight?
|
| Go back and read their previous "breakthrough announcement"
| from 2020.[1] That has more useful info. That system used
| some expensive aerogel. The process is a bit clearer,
| During the daytime it evaporates salt water and condenses
| the vapor. During the night it back-washes the wicking
| material. Whether this can actually work with some simple
| device floating in a pool of salt water is not clear. The
| experimental systems all have plumbing, pumps, and
| instrumentation.
|
| If this is for real, please scale it up to at least kiddie-
| pool size before turning on the PR department.
|
| [1] https://news.mit.edu/2020/passive-solar-powered-water-
| desali...
| dylan604 wrote:
| these premature announcements always make me think of a
| little kid desperate for attention to have mom/dad hang
| their drawing on the fridge. it's cute from a developing
| child, but as grown as adults, it's just sad really.
|
| oh, you had a clever idea, but can't do anything
| practical with it? here's your gold star. now go have
| some milk and cookies and get ready for your nap.
| trey-jones wrote:
| Instead of a gold star, they probably would like to have
| some funding.
| walleeee wrote:
| It's a wicked and pervasive problem in academia. Put
| otherwise capable people in a pathological incentive
| matrix and this is what happens.
| imbusy111 wrote:
| Or a large suitcase? Or a thousand large suitcases?
| tomcar288 wrote:
| there's one big problem. It looks like they're creating distilled
| water. The WHO has stated that distilled water without the
| minerals that they normally contain puts people at increased risk
| of health problems. and you can't just get minerals from another
| source: that's not the problem. the problem is the water itself
| lacking in minerals creates problems for the body.
| DoctorOetker wrote:
| > there's one big problem.
|
| Bombastic.
|
| > It looks like they're creating distilled water.
|
| "distilled water" can mean many similar things. It is not a
| binary property of water:
|
| Consider a water still that has 3 ports: one intake port of
| seawater, an output port of brine and an output port of water
| with a lower salt concentration than seawater.
|
| Distilled can in this case refer to the water exiting the third
| port, even though it still contains some salts.
|
| Distillation and purification in general is a process with
| diminishing returns: to get ever lower ppm's of mineral content
| requires ever more patience or energy.
|
| In the context of seawater desalination, "distilled water"
| typically means water with sufficient salt removed to be now
| potable. Either way even if the third port was effectively
| Sigma-Aldrich 100% pure H20, the simple fix would be to dilute
| a tiny amount of the brine into the distilled water to restore
| the mineral content to potable levels.
|
| You did not reference the study or page of the supposed WHO
| conclusion.
|
| When I was studying at university I recalled the concensus
| basically being that the adverse health effects are a myth,
| since 95% of our salt intake comes from food.
|
| Remember sailors have been drinking distilled water for over a
| hundred years.
|
| Steamboats needed the ability to desalinate water anyway.
|
| It is quite conceivable, that this myth starts as a counter-
| myth for a prior myth.
|
| Scientifically different types of desalinated, deionized and
| distilled waters are closer to pure H2O than normal waters.
|
| Linguistically pure is the antonym of impure.
|
| The mere existence of distilled or pure water in the scientific
| literature conceivably caused purity zealots to opt for
| drinking distilled waters, falsely claiming health benefits.
|
| Just like taking 10x vitamin C doses will not make you 10x
| healthier, neither would drinking distilled water. But it
| "sounds" purer.
|
| In order to counteract obsessions about drinking distilled
| water, and to counter the false and disinformative claims of
| health benefits, some started opting to use the same tactics
| and weapons to stop the "pure water zealots", and the countery-
| myth is born.
|
| That was my interpretation at university. So it surprises me to
| learn that WHO would recommend against drinking distilled
| water.
|
| Of course only drinking distilled water would not be healthy
| indeed: you also need to eat, breathe, etc.
|
| So I first did the easy check: wikipedia.
|
| On wikipedia the same vague statement of the WHO concluding
| something along the lines of it being unhealthy is reiterated,
| but with a [citation needed] appended. Recall anyone can edit
| wikipedia. Such citation was never provided.
|
| So next I tried to find the source myself, and downloaded the
| 312 page Drinking Water and Health: Volume 4 1982.
|
| What page am I supposed to be looking at?
|
| As I said at the start, its a non problem, because adding back
| in some mineral salts to the desired ppm is easy to do.
| jl6 wrote:
| Perhaps stir in some mineral powder before serving?
| kortex wrote:
| Citation needed. The best source I could find (in 30s) suggests
| the WHO neither recommends no discourages drinking distilled
| water.
|
| https://svalbardi.com/blogs/water/distilled-safety
|
| A casual application of math to the concentrations of minerals
| in tap water would show you that simply eating vegetables would
| provide much more in the way of minerals (namely calcium and
| magnesium). If you drink 2L of 60mg/L Ca and 25mg/L Mg, that's
| only 12% daily value.
|
| "Unsafe to drink DI water" is a myth. If your diet is that
| marginal that +/-12% matters, take a supplement.
| tomcar288 wrote:
| here's the citation: https://timesofindia.indiatimes.com/read
| ersblog/ezhil/danger...
| mongol wrote:
| Practical Engineering recently had a video on the topic "Why is
| desalination so difficult"
|
| https://youtu.be/mxqOPdEUNTs?feature=shared
| jack_riminton wrote:
| You'd think for such a breakthrough they'd at least provide a
| video. I couldn't find one elsewhere, has anyone else?
| Someone1234 wrote:
| Sounds interesting.
|
| Seems like it uses energy from the sun, but also requires a
| location with ocean currents. So finding good installation
| locations along the shoreline where it is both deep enough to
| work while being shallow enough to maintain may be a limit on how
| much capacity you could have.
|
| Hopefully they are able to try a scaled up version in a LESS
| controlled environment to see if additional problems arise.
| mcpackieh wrote:
| "Cheaper than tap water" is an inane meaningless claim. Cheaper
| than tap water _where?_ Tap water isn 't one price, the price of
| fresh water is hyper local. I doubt this technology will make
| water cheaper even for most people living along a coast.
|
| > _From these tests, the researchers calculated that if each
| stage were scaled up to a square meter, it would produce up to 5
| liters of drinking water per hour_
|
| So 1) They haven't yet tried this at the scale of one square
| meter. So this is a lab project that is nowhere close to
| commercialization but that doesn't stop them from making grand
| vague pronouncements about its potential. Classic MIT tbqh.
|
| 2) 5 liters per square meter per hour is trash. An average
| American uses a bit more than 300 liters of water a day, so you
| need 60 square meters of this to support one average American. So
| you'd need more than 5000 square _kilometers_ of this to provide
| water to NYC. Does this sound like it 's going to be cheaper than
| the tap water they already have?
| addaon wrote:
| > 5 liters per square meter per hour is trash. An average
| American uses a bit more than 300 liters of water a day, so you
| need 60 square meters of this to support one average American.
|
| Unit check. 5 liters per hour is 120 liters per day. So you
| need 2.5 m^2 per American (ignoring whether the hourly rate is
| sustainable, just correcting units).
|
| This is about a third of the projected area of a typical car,
| so in terms of surface-area-per-person we're at about half that
| of cars (given about 2/3 of a car per American).
| mcpackieh wrote:
| Good catch.
| terramars wrote:
| you can only account for 6hrs of sun per day, so it's more
| like 30liters per day. that gives you 10m^2 per American.
| however serving Americans is not a practical use case. i
| agree these kinds of systems are not going to be competitive
| with large scale PV driven RO (i build medium scale solar RO
| plants in africa and they're pretty good and surprisingly
| affordable). however having a mobile one you can put on a
| vehicle or deploy in a small scale distributed manner with
| little to no expertise would be extremely useful in low
| population density or nomadic context.
| nonfamous wrote:
| I guess one problem is that by definition, sea water is at sea
| level, and any desalinated water would have to be pumped to
| deliver it anywhere. Pumping costs. By contrast, isn't most
| fresh water delivered by gravity, for free?
| jjk166 wrote:
| Nowhere in either this article or the paper is an estimate given
| for cost, nonetheless an analysis justifying the estimate.
|
| Desalination has a fundamental energy cost, and solar energy is
| fundamentally limited by what is received. The real question is
| whether it is cheaper to set up a solar-thermal desalinator, or
| set up solar panels that can power more efficient desalination
| methods.
|
| Solar panels are cheap and getting cheaper quickly as there is an
| incredible economy of scale. While desalination systems could
| likewise be mass produced, there will never be as large a demand
| for that one specific application of solar power as for general
| solar power. Further, photovoltaics are reasonably simple solid
| state devices which are well suited to mass production. I am
| highly skeptical a priori that a solar thermal system could
| compete on economic terms. Perhaps in a space confined situation
| it makes sense, but it's tough to run out of space in the ocean.
| PaulKeeble wrote:
| A Solar thermal system will definitely out perform using solar
| panels to produce heat. A typical commercial solar panel is
| only 23% efficient whereas solar heaters are above 95%. With 4x
| the energy for the same area and a lot less sophisticated
| technology there is little doubt heating the water directly is
| better. But like with all desalination its a relatively slow
| process that leaks a bunch of material behind that damages
| everything and the costs will be dominated by all those other
| aspects not just where it gets its energy from.
| awei wrote:
| Awesome! It should even be cleaner than tap water with heavy
| metals and other contaminants removed by the evaporation. Just
| need to add some minerals back into it afterward.
| mint2 wrote:
| Title makes no sense. It says tap water is cheaper than tap water
| bilekas wrote:
| Not a water expert here but surely all tap water is not
| desalinated.. That would be just outrageous expensive with
| current pricing and technologies.
| mint2 wrote:
| Once desalinated, it becomes what is known as tap water.
| (Except only a few liters per day per unit)
|
| But yes. You've hit on the one of the absurdities of the
| headline: drinking water costs vary wildly based on a large
| number of factors including the source and treatments.
| nico wrote:
| This reminds me of the first ever solar desalination system,
| which was built by a Swedish engineer, for the saltpeter mines in
| Chile, over 100 years ago
|
| It would be great if they can scale it up and make it widely
| available
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(page generated 2023-09-27 23:01 UTC)