
Simple, solar-powered water desalination - chmaynard
http://news.mit.edu/2020/passive-solar-powered-water-desalination-0207
======
sacred_numbers
Efficiency in desalination is measured by comparing the energy expenditure to
the enthalpy of vaporization of water (basically the energy required to
distill water by boiling it). The current state of the art reverse osmosis
desalination plants use about 3.2 KWh of electricity per cubic meter of fresh
water
([https://pdfs.semanticscholar.org/d4d7/821d585699719289dddd10...](https://pdfs.semanticscholar.org/d4d7/821d585699719289dddd1025a582253e3314.pdf)).
This technology uses about 173 KWh of solar energy per cubic meter of fresh
water. The advantage of this method is lower capital costs and not having to
convert solar energy to electricity. For large-scale desalination, however, it
is almost certainly more cost-effective to use solar panels, batteries, and
large scale reverse osmosis systems. This is still a useful project for making
drinking water in remote locations, though.

~~~
pfdietz
One fascinating (well, to me) component of modern RO desalination systems is
the rotary pressure exchanger. Such a simple device, but so important. It
looks like something from the 19th century, but the base patent was issued in
1988.

[https://en.wikipedia.org/wiki/Pressure_exchanger](https://en.wikipedia.org/wiki/Pressure_exchanger)

~~~
steamer25
I found this video which seems to animate the concept nicely:

[https://www.youtube.com/watch?v=udffed4Pq3g](https://www.youtube.com/watch?v=udffed4Pq3g)

~~~
timonoko
Bloody Hell I did not know that. I have parts of broken Katadyn PUR-06. I was
thinking of rebuilding it from better materials. The pressure recovery is the
worst part. I can totally fathom now a hand-held desalinator with only
rotating parts running from solar or Li-On batteries.

~~~
timonoko
PUR-06 lasts 300 days, not "indefinitely":
[https://youtu.be/dNjdgfGgPdE?t=208](https://youtu.be/dNjdgfGgPdE?t=208)

------
worik
"than 1.5 gallons of fresh drinking water per hour for every square meter of
solar collecting area."

Units, people. Units. Be imperial. Be metric. Hey! Be both!

~~~
DanTheManPR
It comes out to about 0.4 liters per square foot, for those of you who are
confused.

~~~
asdfman123
That means for every square rod of surface area, this unit produces over
11,000 hogsheads of water! Impressive.

~~~
kragen
What, per millifortnight? _cf._
[https://news.ycombinator.com/item?id=22270160](https://news.ycombinator.com/item?id=22270160)

~~~
markdown
How much water can I get in a moon?

~~~
8bitsrule
If by 'moon' you mean our Moon, it has a volume of 2.1958×10^10 km3. Although,
how much of that will hold water, we don't know. Anyway, a jerrycan is
probably a lot more convenient ... holds 4.4 imp gals.

~~~
markdown
I really meant a month, but that was good :)

------
aSplash0fDerp
The combination of hydrogen production and desalination using simple
techniques are an exciting prospect for this decade.

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.

~~~
wffurr
Landships sound like the plot of some dystopian future film; ecosteaders fight
off the scavenger bands.

~~~
Nasrudith
It is already - in the my opinion deeply silly book/movie series(?) Mortal
Engines. I haven't read or watched them but saw a video ad and thought that it
was the stupidest thing I had ever seen. Anyway it is effectively landships
except they are massive steampunk mobile cities which grind up and harvest
other sessile settlements and smaller mobile ones. I get the extended metaphor
involved of colonalism and enrichment by exploitation of the weak and
pillaging instead of their own production but even for YA Dystopian it is just
too dumb logistically for me to suspend disbelief.

~~~
roywiggins
The books are much better than the movie, but it's not exactly hard sci-fi.
They predate the big surge in apocalyptic YA and are much better than they
deserve to be. They're not _less_ plausible than "Cities in Flight" or
"Marooned in Real-time" or anything with warp drives and laser pistols.

------
fghorow
It's a solar thermal Multi-Effect-Distillation (MED) plant!

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!

------
symplee
>> 1.5 gallons of fresh drinking water per hour for every square meter of
solar collecting area.

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.

~~~
numbsafari
If I am reading [1] correctly, NYC needs approx. 1 billion gallons of water
per day. So, they would need an array of 12,500 football fields to meet their
needs.

[1] [https://data.cityofnewyork.us/Environment/Water-
Consumption-...](https://data.cityofnewyork.us/Environment/Water-Consumption-
In-The-New-York-City/ia2d-e54m)

~~~
zamalek
> 1 billion gallons of water per day

* 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)?

~~~
ebg13
Indeed. If you do only the drinking water, 2L per person per day for 20
million people is only 10566000 gallons or 132 football fields.

~~~
MayeulC
And for those who wonder like I did, I counted around fifty of those on that
page:
[https://www.nycgovparks.org/facilities/football](https://www.nycgovparks.org/facilities/football)

~~~
ebg13
Ok, but if you use those then you don't have anywhere to play. I think more
importantly, 132 football fields is about the same as the total floor area of
the Pentagon.

~~~
taneq
Install the solar panels over the football fields as roofing.

~~~
MayeulC
If you can afford it, and/or land/actually mounting the panels is the most
expensive part, I'd recommend to install photovoltaic panels instead, though,
as they are more versatile.

And it would simplify plumbing a lot to put everything in the same place, vs
pumping brine up many roofs, and water down again.

------
kragen
This is a well-known approach known as multistage flash distillation:

[https://en.wikipedia.org/wiki/Multistage_flash_distillation](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](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...](https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee04122b#!divAbstract)
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](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.)

~~~
jefftk
_> I don't understand. Are those imperial gallons, US gallons, US dry gallons,
or Irish gallons, which are all different sizes?_

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.

~~~
kragen
> _Irish gallons are long obsolete._

The word "Irish" in this sentence is unnecessary. Gallons are long obsolete.

------
driverdan
The full paper:
[https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee0...](https://pubs.rsc.org/en/content/articlelanding/2020/ee/c9ee04122b)

------
riazrizvi
> 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.

I need an explanation with pictures for that, because it seems like the author
is using ‘efficiency’ incorrectly.

~~~
hinkley
It takes 1 watt-hour to produce about 3.5 BTUs.

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]

~~~
riazrizvi
I really enjoy artful displays of mental gymnastics, so thank you. A _watt_ is
a unit of _power_ , a _BTU_ is a unit of _energy_ , and a _watt hour_ is also
a unit of _energy_. If you can show me how solar watt hours are converted into
BTU's at an equivalent rate of 3.85 to 1, I will short sell all the energy
stocks using my children as collateral.

~~~
hinkley
So that's how this is gonna be.

Good luck.

------
WalterBright
When I was in Boy Scouts loooong ago, there were instructions and diagrams for
making solar stills out of a sheet of plastic, a cup, rocks and dirt.

~~~
emmelaich
Same. I wonder how well they worked; I've never seen one made.

It surely would require fairly specific conditions to work well; a wet ground
and low humidity.

~~~
WalterBright
Just get a large bowl or pail, put an inch of water, or plants, in the bottom.
Put a cup in the center. Cover the top with clear plastic film. Weight the
center of the plastic. Place it in sunlight.

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.

Found it!
[https://en.wikipedia.org/wiki/Solar_still](https://en.wikipedia.org/wiki/Solar_still)

~~~
emmelaich
Sure, I know the setup. Just never made it nor seen it made.

------
idoubtit
This is not only about efficiency. As noted in the article, it would reduce
the pollution of highly concentrated brine rejected into the ocean.

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...](https://www.theguardian.com/cities/2020/jan/02/the-salt-they-pump-
back-in-kills-everything-is-the-cost-of-chiles-fresh-water-too-high)

~~~
MertsA
It only does that by virtue of being much less efficient and spread over a
larger area than traditional desalination plants. At the end of the day unless
you're storing that brine or trucking it off somewhere you have to dump it
back in the ocean. That "highly concentrated brine" isn't all that
concentrated to begin with as it's less than a 50% increase in solutes coming
out of the discharge pipe and gets mixed and diluted in the ocean relatively
quickly. That article is just a fisherman blaming an unrelated problem on the
closest thing around.

------
jolmg
This topic always reminds me of the opening scene[1] from Waterworld where a
guy on a raft out in the sea passes his urine through some kind of filter that
turns it into drinkable water, and drinks it.

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

[1]
[https://www.youtube.com/watch?v=16txiqRoVNQ](https://www.youtube.com/watch?v=16txiqRoVNQ)

~~~
logfromblammo
Given the agricultural issues in that movie, perhaps the purification device
was intended more to collect and concentrate the urea than to purify the
water.

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_.

~~~
J5892
So the only possible explanation of the cause of the Waterworld apocalypse is
earth being hit by millions of giant ice meteors.

~~~
jandrese
IIRC the movie explicitly says "due to global warming" at the start or
something to that effect.

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.

------
ta1234567890
Pretty cool, seems like a modern version of what some people were doing before
electricity and solar panels:
[http://www.solaqua.com/solwatdis1.html](http://www.solaqua.com/solwatdis1.html)

------
quotha
A big question is what do they do with the salt?

~~~
spikels
Kinda explained in the 9th graf:

> 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.

~~~
quotha
So the salt is just going back into the ocean. How is that different from
other systems, they end up dumping the salt somewhere.

~~~
jvanderbot
There's no difference. Except that the _processing_ and _shipping_ of ions is
passive / much easier. Contrasted with large permeable membrane plants that
have to burn power to push all that brine out into the ocean.

~~~
i_am_nomad
But the processing and shipping isn’t the problem with desalination. The
problem is that dumping bribe or salt into the ocean can kill local wildlife.

~~~
markdown
They dump the salt outside the environment.
[https://www.youtube.com/watch?v=3m5qxZm_JqM](https://www.youtube.com/watch?v=3m5qxZm_JqM)

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.

~~~
ncmncm
Since the brine is denser than water, a long hose could deliver it to the
ocean bottom for just the cost of the hose. There are lakes of dense brine on
the sea floor already.

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.

------
ChuckMcM
This is pretty cool. It also could solve one of the seasteading problems
(water production) in a way that is more scalable. Certainly if they can
produce a robust version of this you could pack it into lifeboats so that the
first thing you did after abandoning ship would be to toss this overboard and
unfold it so that it could be filling/refilling the boat's water tanks.

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.

------
jokoon
I guess this an awesome tech for places where there is no clean water, as long
as those places are close enough to the shore.

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.

------
contingencies
See
[https://en.wikipedia.org/wiki/Watermaker](https://en.wikipedia.org/wiki/Watermaker)
and [https://www.cruisingworld.com/desalination-decisions-
waterma...](https://www.cruisingworld.com/desalination-decisions-watermakers/)

------
ineedasername
I have a stupid question (well, not to me because I don't know the answer):
isn't desalination tech fundamentally similar to alcohol distillation? I know
that's energy intensive too, I'm just curious, because I know a little about
alcohol distillation and an wondering if the concepts are transferable.

------
taneq
> the team’s demonstration device can achieve an overall efficiency of 385
> percent

I don't care how good you think you are, you're not getting 3.85kW per square
meter from sunlight.

------
forgotmypw
Would this process also be useful for filtering polluted water?

Several pollutants I have in mind are VOCs and fluorine compounds.

Hopefully there is someone here that is qualified to answer this question.

~~~
i_am_nomad
Probably depends on the vapor pressureS of the VOCs; if less than water, then
no.

------
mrfusion
Is humidity usually high by the ocean? Would it be more efficient to pull
water out of the air? Maybe take advantage of low night temps to collect dew?

------
deegles
Why do these articles never have pictures or diagrams?

edit: I just missed them :(

~~~
kilovoltaire
There is a photo and a diagram in the carousel at the top of the page. (Though
I agree it's easy to miss.)

------
Animats
Nice. When can I buy one at the camping equipment store?

------
cvaidya1986
This is great! We need more solar powered projects!

------
jd_gadus
A thunderfoot video waiting to happen

~~~
aaron695
While we wait are there any sites where people actually intelligently discuss
this article?

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?

------
aazaa
This would make a great DIY project.

------
dr_dshiv
Seems good for a seastead.

------
SCAQTony
A lot of lithium batteries are making their way onto both sailboats and
yachts. With significant advantages come equal disadvantages, and those
disadvantages are the risk of potential battery fires, which only a Class D
fire extinguisher can snuff out.

The Dive boat fire in San Diego, where 34-persons perished, is one example.

~~~
kragen
You don't need batteries to run a reverse osmosis desalinator from solar
panels; you can run the desalinator when the sun is shining and store the
water.

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?

~~~
newnewpdro
> I'm surprised boats are already moving to lithium.

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.

~~~
kragen
The lead-acid batteries still cost a lot less per joule of capacity than the
lithium batteries do. They just weigh more. Or maybe charge efficiency is the
reason?

~~~
newnewpdro
Space tends to be at a premium for cruisers, and modern cruisers are
increasingly embracing electricity for powering everything.

Charge efficiency is a big one, but it's also just maximizing energy capacity
in the available volume.

