SoylentNews is people
SoylentNews
Submitted via IRC for Fnord666
In desert trials, next-generation water harvester delivers fresh water from air
Scientists who last year built a prototype harvester to extract water from the air using only the power of the sun have scaled up the device to see how much water they can capture in arid conditions in Arizona. Using a kilogram of MOF[*], they were able to capture about 7 ounces of water from low-humidity air each 24-hour day/night cycle. A new and cheaper MOF could double that.
[...] "There is nothing like this," said Omar Yaghi, who invented the technology underlying the harvester. "It operates at ambient temperature with ambient sunlight, and with no additional energy input you can collect water in the desert. This laboratory-to-desert journey allowed us to really turn water harvesting from an interesting phenomenon into a science."
[*] Metal-organic frameworks (MOFs) are:
compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. They are a subclass of coordination polymers, with the special feature that they are often porous. The organic ligands included are sometimes referred to as "struts", one example being 1,4-benzenedicarboxylic acid (BDC).
Also at Berkeley News.
Practical water production from desert air (open, DOI: 10.1126/sciadv.aat3198) (DX)
(Score: 0) by Anonymous Coward on Monday June 11 2018, @12:35AM (5 children)
You can't harvest enough water from dry air to be worth anything. Extremely simple chemistry equations you probably learned in high school make this quite clear. There is no amount of cleverness that will make it possible to extract meaningful amounts of water from air that doesn't contain much water. Even if you were beside a rain forest these stupid things wouldn't generate enough water from the air.
(Score: 2, Offtopic) by realDonaldTrump on Monday June 11 2018, @12:46AM
It's solar. And maybe it wouldn't work in a rain forest. But it works in sunny Arizona. And maybe we'll build it into our magnificent Border Wall!!!!
(Score: 2) by frojack on Monday June 11 2018, @01:46AM (2 children)
Define "meaningful amounts". (See, I'm even giving you a chance to define your own weasel words.).
How bout 25 gallons of clean drinking water per day? Does that mean anything to you?
https://www.smithsonianmag.com/innovation/this-tower-pulls-drinking-water-out-of-thin-air-180950399/ [smithsonianmag.com]
No, you are mistaken. I've always had this sig.
(Score: 4, Touché) by suburbanitemediocrity on Monday June 11 2018, @03:05AM
The AC on my house in Arizona has been producing this much water per day at least since 1990. And hey, since I put up some solar panels and a big inverter ten years ago, it has been 100% solar powered.
I wouldn't drink the water without filtering it and running it through a UV sanitizer (legionaires and all), but that is way over engineering where it's not needed. A couple hundred meters of collection area and a big water tank is far less expensive, will collect more water even in the desert (600gallons/1000sqft/inch of rain (that's 5000 gallons in Phx, AZ/1,000sqft)), low tech (nothing to break), will last 100 years and can be built by grade schoolers.
(Score: 0) by Anonymous Coward on Thursday June 14 2018, @03:12PM
We can compare the operation of these systems with the cost of pumping water into a tank (where water is plentiful) and shipping it to where the device is proposed to be installed. We all know it is possible to condense water out of air. You can go to the store and buy a machine to do it: it is called a dehumidifier.
A "meaningful amount" means the device can produce a comparable amount of water, in a comparable amount of time, for a comparable amount of energy, as just shipping the water from a big lake somewhere else in the water.
However, turning water vapour into liquid water takes a tremendous amount of energy. The water is in a gaseous state because the water molecules have enough thermal energy to overcome the hydrogen bonds between different water molecules. To turn it into a liquid you need
to remove all that energy, which is approximately 2.2MJ per litre of liquid water.
A tanker truck holds about 50,000 L of water. Suppose we fill it in Chicago, IL and drive to Phoenix, AZ (about 3,000 km). According to wikipedia [wikipedia.org] a heavy truck requires about 2.5 kJ per tonne-kilometer. So the truck hauling 50 tonnes of water travelling 3000 km yields about 400 MJ of energy required to transport 50,000 L. The whole process might take 2 days. 400MJ in 2 days is about 2.5 kilowatts.
Let's now consider condensing water. At 2.2MJ per litre, condensing 50,000 litres will require removing 110 gigajoules of energy from the water vapour. To do that in 2 days means we need to remove energy from the water vapour at a rate of about 600kW.
A good heat pump could be about 200-300% efficient at removing energy from the water, provided we have a bigass heatsink that can dissipate that 600kW. That still gives us a power input requirement of about 200kW assuming there are no losses (and glossing over the fact that there might not even be enough water vapour in the air to do this), roughly 100 times the power requirement of just trucking it halfway across the continent.
Even air freight halfway around the world will be an order of magnitude cheaper.
The reason these devices are a dumb idea is not because they can't possibly work. The reason is they are simply too expensive to operate for the purposes of obtaining potable water, compared to the much simpler, cheaper, faster and safer methods available.
But I guess you can't raise millions of dollars on Indiegogo flexible funding just by driving a water truck across the country.
(Score: 2, Informative) by Anonymous Coward on Monday June 11 2018, @02:04AM
One missing piece of data -- the relative humidity in Arizona isn't all that low, see https://www.currentresults.com/Weather/Arizona/humidity-annual.php [currentresults.com]
Since the air temps in AZ are often high (increases ability to hold moisture), the absolute amount of moisture in the air is significant.
If you want dry air, go into any heated house in the winter when it's below freezing outside (assuming no added house humidification). Heating cold outside air (nearly independent of the RH% at the cold outside temp) winds up with very low RH % at room temps.