Simple, solar-powered water desalination
news.mit.edu
news.mit.edu
https://www.powershow.com/download/3b604b-ZGQwN/Pressure_Exc...
Read the history, including how the then-current prototype (out of series of improved versions developed over the 12 years it took to get to a market ready design) and all the inventor's personal belongings were lost in the invasion of Kuwait.
>One particularly efficient type of pressure exchanger is a rotary pressure exchanger. This device uses a cylindrical rotor with longitudinal ducts parallel to its rotational axis. The rotor spins inside a sleeve between two end covers. Pressure energy is transferred directly from the high pressure stream to the low pressure stream in the ducts of the rotor. Some fluid that remains in the ducts serves as a barrier that inhibits mixing between the streams. This rotational action is similar to that of an old fashioned machine gun firing high pressure bullets and it is continuously refilled with new fluid cartridges. The ducts of the rotor charge and discharge as the pressure transfer process repeats itself.
This system gets approximately 6 liters/hour/sq m... so it makes sense if we can manufacturer it for less than 40 dollars per square meter and space is cheap.
I have had these feelings since I saw the Egypt map showing how much of it is just not usable.
Not for a large desalination project. You will pay close to the spot price. Which is 30-45 cents.
Your biggest cost will be labor anyway. And the osmosis membrane, which needs to be regularly replaced, unlike the panels.
For large scale desalination, you need to attach tiny magnetic particles (think nano-scale) to the salt, and then extract it with magnets. And as a byproduct, you'll get Lithium.
I can't imagine why you'd use batteries at all. Storing fresh water efficiently is cheap, storing power efficiently is not.
---
I’ve heard on the grapevine that facilities are now being designed which work at under 100% capacity in order to soak up excess/cheap power.
I believe this is coming about because: 1) the high cost of power and presence of sporadically cheap power makes it economically viable, and 2) designing equipment with lower duty cycles can actually provide substantial cost savings. Ie a facility which only runs 50% of the time is much cheaper to build & run than once which runs 100% of the time.
Sorry I cannot cite sources right now.
Imagine doing shift planning at such a site though. Eight days from now, there's a 60% chance of wind in excess of 5m/s between 22.00 and 06.00. But labour costs would be 130% higher than 06.00 to 14.00 on that day. On that morning shift there's a 50% chance of clear skies ...
This means that you just staff it at a consistent level and plan the work around the energy levels.
You either have losses in:
* Underutilized solar generation
* Underutilized plant capacity
* Overhead of energy storage
Design your system to minimize that cost. Engineering systems of all sorts are full of this kind of optimization problem: "Should I add a subsystem to recover loss x?" There is always more you can do to recover losses, but each extra system you add suffers diminishing returns a little more.
A long time ago, as a kid i read about a experiment in a GEO-magazine harvesting water directly from air. The used idea was a hygroscopic chemical (Silicagel) collects the moisture from the air. Sunlight heats the silica gel- which releases the moisture as steam- which then condenses in a destil. It was very little water for a lot of effort, but the complete lack of moving parts and self-containedness of such a system deeply impressed me, being young and with Frank Herberts Dune on my mind.
To drop such a contraption into the deep dessert, where it could keep a plant alive, become a oasis with no basis ..
Its hard describing this fascination with self-contained or only slowly expanding pockets of life.. imagine a glass bubble , filled with small plants, beatles and life, thriving in the midst of a frozzen over wasteland like mars or a dessert like the death valley.
Its like a https://en.wikipedia.org/wiki/Bottle_garden but without the surrounding walls..
https://www.youtube.com/watch?v=XPSYzLZ7xKU
https://www.sciencemag.org/news/2019/09/crystalline-nets-har...
From the panel to the outlet, you only get out from 10 to 15% of input solar power, while thermal desal can use nearly all of it right away, and it leaves more concentrated brine. Multiple effect desal will probably move the efficiency up a bit. It's nowhere near clear cut.
Also if you have low potential reject heat from power plants, it's idea for desal use.
If you have nuclear as your heat source, then economy-wise thermal desal will beat electric many times over.
I recently read about thermochemical, and electrochmical desalination methods which can be added atop a thermal plant, further increasing thermal desalination efficiency.
I wonder if RO based desalination has any learning rate associated with it and whether a lot of CAPEX costs are related to some proprietary IP. I know that a lot of OPEX is energy costs and hopeful that solar will get even more cheaper with time and even more scale. However if the same can be done with scaling of RO technologies and reducing costs to say ~20 % of existing a great many opportunities will arise.
Really hope it will be lower than 0.1 $ / m3 before 2030.
It does mean much larger service water tanks as you need to buffer demand but if you have lots of "free" solar power then it may well be worth it.
Units, people. Units. Be imperial. Be metric. Hey! Be both!
30 gal seems too small.
bathtub ≈ 302 liters
football field = 5351 square meters
They achieved 5.78 L/m^2/h
So 5351 m^2/football field * 5.78 L/m^2/h = 30928.78 L/h/football field
30928.78 L/h/football field / 302 L/bathtub = 102.4 bathtubs/football field/hour
h/t to brudgers for the correction about football field surface area!
That's 0.062 cubic Flemish ells, for those who were wondering.
https://en.wikipedia.org/wiki/Smoot
https://www.google.com/search?q=barn%20megaparsec&oq=barn%20...
> 5.78 L/m^2/h
Whoever wrote this PR is responsible for the mix and match units.
You don't really need them to be compatible in this case: even though it appears that you're talking about multiples of units of length, you don't really compare them. You're not going to do the calculation that this is "1.58 micrometers per second" or ".447 feet per day", which is what this is.
I mean, I guess if it helps you visualize a half-foot of water growing on top of the object every day, great. But I feel like "volumes of water" and "areas of solar panel" are quite different units, and it's kinda helpful that the system distinction helps make that clear.
I think it is important to use SI units, because it allows you to do comparisons between different approaches to desalination more easily, as I did in my comment https://news.ycombinator.com/item?id=22270160
Maybe if you don't mind losing your global Mars orbiter you don't:
http://edition.cnn.com/TECH/space/9909/30/mars.metric.02/
_everything_ is measured in metric, period.
I'm more confused by the efficiency numbers exceeding 100%, which seems wrong. A theoretical efficiency of 100% would find the solar irradiance energy of 1 m^2 and then find the volume of fresh water per hour that produces an equal amount of energy when its salinity is increased to the mean salinity of seawater (about 3.5%). So if you can get 5000 Wh/m^2/day from insolation, and the energy difference between salt and fresh water is 0.810 W * h/L, 100% efficiency would be a 1 m^2 area producing 6173 L/day of fresh water. You're just dividing the daily energy of sunlight on your panel in Watt-hours by that 0.810, to get L/day.
The units in the article are all wrong anyway. They say "a rate of 5.78 liters per square meter", but there is no time factor mentioned whatsoever. An MIT roof gets mean 4.59 kWh/m^2/day of solar energy, so 100% efficiency would be 4590 * 1000/810 = 5667 L/m^2/day (1 m^3 = 1000 L). If the number given was per day, that's 0.1% efficient. If it's per hour, that's 2.4% efficient.
The soda industry went to 2 liter bottles back when the country tried to go metric, and it stuck. But most things haven't, including the individual serving sizes. So people never really got the feel for anything smaller than a liter.
Bottled water is often in metric too, even for single serving sizes.
Not always. I would wager that you are using an electronic device whose circuit board was dimensioned in 'mils', which are not millimeters but thousands of an inch.
Sort-of related: https://what-if.xkcd.com/11/
For architects of 21st century cities, most of these sustainable technologies would benefit from a cleanslate design, rather than trying to compete with builtup 20th century infrastructure.
I think Austrailia is postured to be the biggest beneficiary of all of these "leaps" in technology this century. Most of the desert area is the perfect canvas to start building indoor cities that will be the models/prototypes for the spacestations that the next generation will use to explore urban life outside of our atmosphere. Even building a cruise ship on land would be a good starting point for a 21st century city if they could support several thousand residents with enough freshwater for self-sustaining agriculture and energy to take root.
Even though we won't have saltwater in space, most of these advancements in sustainables fit the roadmap to space more than they're currently reported. Hopefully they'll break ground on new cities using these new resources sometime soon and change the narrative appropriately.
Cities like Detroit and other areas past their prime did not devolve into madmax scenes, but instead are left with a bunch of toxic funk floating in a cesspool.
Definitely not somewhere you'd want to settle down.
I was part of a team that suggested using geothermal heat for desal with MED technology in Perth, Western Australia a bit more than a decade ago!
The advantage over work based (i.e. electrical/Reverse Osmosis) systems is that you do not lose heat via Carnot (or real world, for that matter) efficiency. You utilize more of the original heat, and via the multiple stages ("effects") you can re-use the latent heat of vaporization over and over.
MEDs are really cool gizmos!
So a football field could collect around 8,000 gallons of water per hour. Let's say you can get 5 hours of good sunlight per day = 40,000 gallons of water.
>> The team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.
>> Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent
So now we're looking at 80,000 gallons of water per day.
>> Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater.
>> In production, they think a system built to serve the needs of a family might be built for around $100.
[1] https://data.cityofnewyork.us/Environment/Water-Consumption-...
* How much of that water need be drinkable?
* How much of that water can be salinated (e.g. for flushing)?
* How much of that water can be partially desalinated (e.g. for showering)?
And it would simplify plumbing a lot to put everything in the same place, vs pumping brine up many roofs, and water down again.
In any event, NYC gets its water from upstate, not locally: https://en.wikipedia.org/wiki/New_York_City_water_supply_sys...
"One of its largest watershed protection programs is the Land Acquisition Program, under which the New York City Department of Environmental Protection (DEP) has purchased or protected, through conservation easement, over 130,000 acres (53,000 ha) since 1997"
In the end, I’m not sure if it wouldn’t just be easier to make sure that all the water is potable.
Based on this random article I found from 2015 [1], the average cost per build able sq ft in Manhattan was getting to $1200.
So, if you magically found 5 sq miles in NYC, it’d cost around $167.3B just for the land [2].
I’m guessing you’d want to build it outside the city and pump it in via aqueduct.
Following the article, an array of off-shore desalinated that pumped water back to the city would probably be the most economically viable solution and could probably be built with other kinds of offshore utility systems to try and share costs.
[1] https://therealdeal.com/issues_articles/486631/ [2] https://www.wolframalpha.com/input/?i=%241200+per+square+ft+...
Of course it probably makes more sense economically to do it offshore or just have a giant field somewhere outside of NYC and pump the water in, but you could find the space inside the city if you really wanted to.
How many swimming pools of water is that?
So around 0.15 Olympic pools — roughly, a football field would fill an Olympic pool in a week.
Of course, once we factor in cleaning uses of unsalted water, the picture is less pretty. Still, not bad.
https://en.wikipedia.org/wiki/Multistage_flash_distillation
It's surprising they didn't mention this in the article. (It is, of course, mentioned in the paper, because its authors are decent and honest people.)
> more than 1.5 gallons of fresh drinking water per hour
I don't understand. Are those imperial gallons, US gallons, US dry gallons, or Irish gallons, which are all different sizes? https://en.wikipedia.org/wiki/Gallon#Sizes_of_gallons How many hamburgers is that the same weight as? How many ngogns is that? Or maybe acre-feet, pony shots, or Olympic swimming pools? MIT should be ashamed of their News Office.
The abstract of the paper https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0... does give the result in modern units, and to a much higher precision:
> a production rate of 5.78 L m⁻² h⁻¹ under one-sun illumination
In Dercuano http://canonical.org/~kragen/dercuano I wrote a note about multistage distillation, which I didn't realize was already a known technique until later; see notes/recycling-distillation.html or p. 1776 in the PDF. The basic summary is that modern reverse-osmosis distillation plants require on the order of 7 kJ/ℓ to get freshwater from seawater (which works out to about 50 kJ of sunlight per liter with a cheap 16%-efficient solar panel) while multistage distillation requires a few hundred kJ/ℓ.
The number given by the paper abstract works out to 623 kJ/ℓ if we assume one sun is 1000 W/m² (a common figure for the solar constant at the surface). So if you use those square meters to run photovoltaic panels which you then use to drive a reverse-osmosis machine, you can get dozens of times as much fresh water from the same solar energy. But you need to build or buy a reverse-osmosis machine, and then you need to defoul it, so this device might be a good choice in some circumstances. (You might need to defoul the distillation apparatus as well, though.)
Imperial and US gallons would both make sense, and you're right it's ambiguous.
Dry gallons would not make sense, since this is a liquid quantity, and Irish gallons are long obsolete.
The word "Irish" in this sentence is unnecessary. Gallons are long obsolete.
I need an explanation with pictures for that, because it seems like the author is using ‘efficiency’ incorrectly.
> the solar-to-vapor conversion efficiency, defined as the ratio of total vaporization enthalpy to total solar energy input,
So ya, they are just comparing to a really low baseline...
With a heat pump, for instance, you might be able to produce 5BTUs per watt-hour, because you're extracting heat from the environment (a very large heat source), which would be 140% efficient.
The paragraph you excerpted has your answer:
> The key to the system’s efficiency lies in the way it uses each of the multiple stages to desalinate the water. At each stage, heat released by the previous stage is harnessed instead of wasted. In this way, the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.
You can't drink steam (and even bottling it is futile), so that heat has to go somewhere.
With counterflow systems, like ventilation systems for houses, the inlet and outlet temperatures are pretty close to each other, and the temperature on the 'inside' of the system is either much higher or much lower.
When distillation came up a month or so ago, several people pointed out that modern systems do not operate at ambient air pressure, so the temperature delta may be less than you'd imagine.
[edit: pedants gonna pedant]
If we're talking about distillation, the civil engineer probably wants to think about rates (liters per minute per $1000), but for a civilian just trying to figure out how this could work in a laboratory situation?
I find quantities easier to comprehend (and relate). Tell me how many AA batteries or days of full sun it would take to convert a big beaker of salt water into a smaller beaker of distilled water.
Good luck.
As a reminder, "Be kind. Don't be snarky." [1]
The specific heat of water is one calorie per gram per kelvin, so a BTU works out to one pound, times a calorie per gram, times a degree Frankenstein per kelvin.
A calorie has been defined as having various different values, since water's specific heat varies with temperature (and pressure!) but they're all about 4.18 to 4.19 joules, except for the food "calorie", which is actually a kilocalorie.
Pounds have also been defined as having many different values, even avoirdupois pounds; the values used include 6992 grains, 7000 grains, 7002 grains, and 6999 grains. (Troy grains, not metric grains, which are different.) The currently most popular pound is 7000 grains, but by international agreement it is now defined as 453.59237 grams, previous definitions in terms of the metric system having included 453.59265 grams and 453.59243 grams.
Finally, one degree Frankenstein is precisely defined as 5/9 of a kelvin, although Dr. Fahrenheit's original definition was rather different.
Working all of this out, a BTU turns out to be about 454 g · 4.18 J/g · (5/9)K/K, which is about 1054.3 J. A watt hour is of course 1 W hour · 60 s/min · 60 min/hour = 3600 W s = 3600 J. 3600/1054.3 is about 3.41.
I'll be here all week. Don't forget to tip your servers.
I would personally define desalination efficiency of 100% as a perfectly-reversible reaction that establishes an equilibrium between fresh water and oceanic salt water.
Since adding sea salt to fresh water until it has oceanic salinity represents a theoretical maximum of 0.810 Wh/L (a maximally efficient osmotic power plant, situated where a river empties into the ocean, could get about 0.75 Wh/L). So 100% efficiency would be adding 0.810 Wh to one liter of seawater to get one liter of fresh water back. 100% is an unachievable goal, thanks to the laws of thermodynamics.
So to figure your solar desalination efficiency, from a solar panel that receives X Wh/m^2/day of insolation energy, you divide by 0.810 Wh/L to get L/m^2/day. Whatever fresh water you can produce per day, divide by that number to get your efficiency.
An MIT roof gets mean 4.59 kWh/m^2/day of solar energy, so 100% efficiency for them would be 4590/0.810 = 5667 L/m^2/day. By the numbers given, their process is about 2% to 3% efficient (by my definition).
They could be a lot more efficient if they didn't have to overcome the huge heat of vaporization that water has, which is exactly why reverse osmosis is so much more efficient than multistage flash distillation. They are stacking so that the evaporation energy can be recovered from the condensation, which deposits the same amount of energy on the next layer, but it's better off all around to just never evaporate in the first place.
It surely would require fairly specific conditions to work well; a wet ground and low humidity.
After a while, condensation will form on the underside of the film, the drops will run downhill to the center, and drop off into the cup.
Voila! Distilled water.
See for instance this article of the Guardian about a region in Chile where the majority of drinkable water comes from a desalination plant built in 2003: "The salt they pump back in kills everything'" https://www.theguardian.com/cities/2020/jan/02/the-salt-they...
I was a little kid when I watched it and wondered why he would drink his own urine when he was surrounded by such vast amounts of water. With this type of technology, maybe he wouldn't have had to do something so gross! XD
But that assumes the scriptwriter would have thought of that, which may be a stretch, given that Earth has insufficient water to cover all landmasses to a depth of 7 km. That would require about 3.6 billion cubic kilometers more water than already exists in the oceans, and there are only about 1.4 billion cubic kilometers of water on Earth to begin with, . Certainly there isn't that much locked up in glacial ice. That is only 26.5 million km^3, which is enough to raise sea level by a maximum of 52 m, far short of the 8 km it would take to cover everything but Mt. Everest.
Anyone who would skip that bar-napkin math was probably only putting that scene in specifically for the gross-out factor. It's just like Mad Max eating the can of dog food. Which is an apples-to-apples comparison, because Waterworld was written specifically to be an ersatz Mad Max.
The only hard sci-fi explanation is that someone towed Europa to Earth and slowly lowered it down via a cable or pipe. Terraforming by an alien aquatic species. Hyperintelligent space whales.
But on the other hand, society had collapsed along with most of the stored knowledge of the world, so maybe it was just the people being uneducated and highly insular. They don't even know how long a kilometer is anymore.
There was a point in history a few hundred million years ago where the entire American midwest was a shallow water ocean; we could easily be heading back to that scenario in just a few centuries.
>> 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.
> 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.
Seriously though, far out to sea away from any landmass it's pretty much a desert. Any salt dumped there would disperse very quickly and not pose a problem to wildlife.
Almost every part of the world ocean has been described as a desert, before people went under and looked. It turns out there's a huge amount of life everywhere you look, even miles down.
But there is also a lot of room, so anything localized, unpleasant, and temporary can be swum away from.
However he sun goes down, which means that osmosis works in its favour. The less salty water wicks into the matting, the matting over saturates and leaks back down, equalising the salinity.
I am not to sure about it on a metropolitan scale though, that is a lot of surface area per person. One sq kilometer is a pretty big patch for serving up water for about a million people. Call it 800K for losses due to transport.
But again, I doubt that any investor will be interested, as they will probably not get good enough ROE with this.
This would probably get the attention of government of some poor countries, or maybe some humanitarian organizations, or even the military.
I would probably be confident that a small portable kit could be marketed to survivalists and it would sell extremely well.
I don't care how good you think you are, you're not getting 3.85kW per square meter from sunlight.
Several pollutants I have in mind are VOCs and fluorine compounds.
Hopefully there is someone here that is qualified to answer this question.
edit: I just missed them :(
It seems like a 100 year old tech they are trying to optimise. I feel like random optimisations would mean there are no advances to get here.
But it is possible using computer simulations they could do something of value. Have they?
The Dive boat fire in San Diego, where 34-persons perished, is one example.
I'm surprised boats are already moving to lithium. I wonder if it would be worthwhile to tow the batteries in a separate buoy behind the boat?
An interesting aspect of the history of water jugs is that the humans were cultivating them for thousands of years before they started cultivating food crops: https://phys.org/news/2014-02-mystery-bottle-gourd-migration...
When the alternative is a bank of lead-acid batteries with effectively ~50% usable capacity and poor charge efficiency, at least for cruisers effectively living at sea off solar power, it's not surprising at all.
Charge efficiency is a big one, but it's also just maximizing energy capacity in the available volume.
"Lead-acid is still better, it's 1/10 the cost for equivalent usable amp-hours. Most boaters use deep cycle golf cart batteries, they're produced at such scale that they're cheap as hell. Lithium is a waste of money and dangerous to boot. They only make sense if you're extremely space limited, which most bigger boats aren't."
Edit: apparently enough people vouched for the comment, it's back from the dead. But this definitely should not be happening to people like gnaritas who have devoted so much time and effort to making HN better.
Lithium ion batteries contain lithium in an ionic (non-metallic) form. Lithium ion batteries contain flammable electrolytes and polymers, so they can burn, but these are not metal fires calling for a Class D fire extinguisher.