07 September 2018

"Water From Air" is the best applied science I've seen all year

I first heard about this development in a BBC podcast of Science in Action where Dr. Varanasi was interviewed.
A new system devised by MIT engineers could provide a low-cost source of drinking water for parched cities around the world while also cutting power plant operating costs.

About 39 percent of all the fresh water withdrawn from rivers, lakes, and reservoirs in the U.S. is earmarked for the cooling needs of electric power plants that use fossil fuels or nuclear power, and much of that water ends up floating away in clouds of vapor. But the new MIT system could potentially save a substantial fraction of that lost water — and could even become a significant source of clean, safe drinking water for coastal cities where seawater is used to cool local power plants.

The principle behind the new concept is deceptively simple: When air that’s rich in fog is zapped with a beam of electrically charged particles, known as ions, water droplets become electrically charged and thus can be drawn toward a mesh of wires, similar to a window screen, placed in their path. The droplets then collect on that mesh, drain down into a collecting pan, and can be reused in the power plant or sent to a city’s water supply system.

The system, which is the basis for a startup company called Infinite Cooling that last month won MIT’s $100K Entrepreneurship Competition, is described in a paper published today in the journal Science Advances, co-authored by Maher Damak PhD ’18 and associate professor of mechanical engineering Kripa Varanasi. Damak and Varanasi are among the co-founders of the startup, and their research is supported in part by the Tata Center for Technology and Design.
Most people are aware of the way fog or mist condenses into water droplets on a mesh or screen (or on the bodies of insects in the Namib Desert or on redwood trees).  A power plant tower is basically creating distilled water - all you have to do is harvest it.  The principle has been demonstrated in a laboratory setting; now it's being scaled up.  It's cheaper by a log power than fashioning new desalination plants.

Reposted to add an image and text from a more recent BBC report:
If the emerging ‘water from air’ (WFA) devices can crack it, it could go a long way towards solving the world’s freshwater problem...

There are several companies adapting dehumidifier technology for drinking water. Mechanical dehumidifiers contain chilled metal coils filled with a refrigerant gas, much like a kitchen fridge-freezer, which create an artificial ‘dew-point’ (the temperature at which water vapour in the air saturates, turning from a gas into a liquid, like the beads on the side of your ice-cold drink). Water vapour entering a WFA machine condenses on a chilled coil in the same way, but once collected it is filtered, sterilised by UV light, mineralised, and stored in a food-grade tank ready to drink...

There are, however, some important conditions for many of these devices to function at their best. The efficiency, for instance, often depends on the relative humidity – the amount of water present in air, as a percentage of the amount needed to reach saturation. For most of the devices, that figure is above 60% for optimal functioning...

But a new UK company, Requench, is entering the market later this year (2018) with a unit literally the size of a shipping container, and it can reportedly function at a relative humidity of just 15%. The prototype produces 2,000 litres a day in humid conditions and no less than 500 litres even in dry climates.

Another solution may come from entirely different WFA technology. Instead of refrigeration coils, a ‘desiccant’ material absorbs water from the air like a chemical sponge, needing no energy to do so... “You need something that absorbs water at ultra-low humidity, even 5% humidity. For example, when you leave the lid off the sugar bowl, it gets kinda clumpy. Sugar is a natural desiccant...

But there is one more WFA approach that requires zero electricity – solar or otherwise – and is designed for the poorest regions of the world. In rural Ethiopia, Togo, and soon Haiti, stands the near 10-metre tall ‘Warka Tower’. Looking like something from Glastonbury Festival, a giant vase-shaped bamboo frame supports hundreds of square metres of fine polyester mesh. The mesh collects the morning mist and drips down into an underground tank via a stone-based filtration system... Such dew harvesting, however, relies on very high humidity and fog.


  1. And all of this because power plants have been getting water for nearly free. If there was a serious price on water they'd have figured this out years ago.

  2. Interesting. It was my understanding that cooling water for a power plant was all returned to its source, unchanged but a couple degrees warmer. And that the "smoke" stake was just for the dispersal of combustion by-products (particulates, water, carbon oxides, nitrogen oxides, etc) . I would think that distilling water from a smoke stack plume would give you rather contaminated water.

  3. It takes about 2.5 kilowatt hours to convert a gallon of water at almost 100c into steam. If US power plants are sending “hundreds of billions” of gallons of water into the air every year, then conservatively (assuming 200 billion gallons), they’re pissing away 500 gigawatt hours of power every year. Emmett Brown’s Delorean aside, this seems like a poor way to make fresh water from already fresh water. The whole point of nukes and fossil power plants is to convert heat into electricity. Optimally, you don’t want to cool a power plant, you want to convert every last bit of heat into electricity.

    1. Not cooling the power plant isn't an option, thermodynamically; it's the difference from the hot side to the cool side (which you then have to keep cool) where the work is extracted.

      A really good modern natural-gas power plant converts 62% of the heat from burning the fuel into electricity, and has to dispose of the remaining 38% as heat into the environment (using the water being talked about here). A typical coal or nuclear power plant is 25-33% efficient, thermally, and is "wasting" most of the heat energy.

      Some data here: https://www.eia.gov/tools/faqs/faq.php?id=107&t=3

    2. Entropy is like unfair randomness, like a casino, the house always wins more. But it doesn't just take a cut! Like a casino sooner or later in the end the house takes it all.

      The formula is also really simple using Temperature in Kelvin: Efficiency = 1 - (T cold / T hot)

      Your 62% seemed incredibly high. That link shows up as first search result, but also highest stated average efficiency is in that table is 3412/7870 = 0.43.

      The heat engine Wiki-article has a table at the very bottom comparing theoretical 0.64 (Carnot) and observed efficiencies of 0.36


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