Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Simple, solar-powered water desalination

news.mit.edu

Post reply on HN
Print view View original

41–50 of 205 posts

Re: Simple, solar-powered water desalination

#41

Post by logfromblammo »

jolmg wrote: post #3

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…

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.

Re: Simple, solar-powered water desalination

#42

Post by kragen »

This is a well-known approach known as 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 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.)

Re: Simple, solar-powered water desalination

#43

Post by KaiserPro »

quotha wrote: post #7

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

During the day, the salt collects in the capillary material. If it was working 24/7, then it would continuously increase in salinity until you had to do something about it.

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.

Re: Simple, solar-powered water desalination

#44

Post by rstupek »

101404 wrote: post #28
DanTheManPR wrote: post #23

Earlier quoted context omitted.

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

What's that in bathtubs per football field?

well an NFL football field is approximately 5351 m2 so about 8000 gallons. not sure how big your bathtub might be though

Re: Simple, solar-powered water desalination

#45

Post by kragen »

asdfman123 wrote: post #30
DanTheManPR wrote: post #23

Earlier quoted context omitted.

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

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

What, per millifortnight? cf. https://news.ycombinator.com/item?id=22270160

Re: Simple, solar-powered water desalination

#46

Post by rdiddly »

101404 wrote: post #28
DanTheManPR wrote: post #23

Earlier quoted context omitted.

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

What's that in bathtubs per football field?

Layman's Explanation Greatest Hits right there. Except I was actually curious so I converted it. Assuming a 30-gal tub, it's about 267 tubs/field-hour.

Re: Simple, solar-powered water desalination

#47

Post by VikingCoder »

101404 wrote: post #28
DanTheManPR wrote: post #23

Earlier quoted context omitted.

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

What's that in bathtubs per football field?

EDITED:

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!

Re: Simple, solar-powered water desalination

#48

Post by riazrizvi »

hinkley wrote: post #20
riazrizvi wrote: post #2

> 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.

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…

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.

Re: Simple, solar-powered water desalination

#49

Post by kragen »

jfengel wrote: post #39
worik wrote: post #9

"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!

I guess it's not really surprising for American engineers. Food and drink are measured in Imperial units. Technical doodads are measured in metric. 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"…

The engineers don't enter into it; they used metric, of course.

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

Re: Simple, solar-powered water desalination

#50

Post by VikingCoder »

rdiddly wrote: post #46
101404 wrote: post #28

Earlier quoted context omitted.

What's that in bathtubs per football field?

Layman's Explanation Greatest Hits right there. Except I was actually curious so I converted it. Assuming a 30-gal tub, it's about 267 tubs/field-hour.

Hrm - "To start, the standard bathtub will hold roughly around 80 gallons (302 liters) of water. Much smaller bathtubs can only hold around 40 gallons (150 liters) of water, which typically are more suited for smaller children or function more as a shower space."

30 gal seems too small.

Post reply on HN