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posted by mrpg on Sunday June 10 2018, @10:40PM   Printer-friendly
from the O,N,Ar dept.

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)


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  • (Score: 2) by frojack on Monday June 11 2018, @01:46AM (2 children)

    by frojack (1554) on Monday June 11 2018, @01:46AM (#691247) Journal

    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]

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  • (Score: 4, Touché) by suburbanitemediocrity on Monday June 11 2018, @03:05AM

    by suburbanitemediocrity (6844) on Monday June 11 2018, @03:05AM (#691272)

    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

    by Anonymous Coward on Thursday June 14 2018, @03:12PM (#692951)

    Define "meaningful amounts". (See, I'm even giving you a chance to define your own weasel words.).

    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.