Luke Skywalker's adopted family were moisture farmers. I have no
idea what he actually did on that farm, but those things looked cool.
Perhaps our society might never be able to produce real blasters,
Star Destroyers, hyperdrives, cool droids, or lightsabers.
But what about the moisture farms? Can we at least get those?
The methods and machines for extracting water out of the air
have long been around. The real question is whether they make commercial sense.
In this video, I want to look at commercial-scale atmospheric water generation.
## Water Water in the Air
There is an estimated 1.4 quintillion tons of water on the earth.
Just 2.5% of that water is freshwater.
And almost 70% of that 2.5% is locked up in glaciers, ice caps, and icebergs.
We already talked about the problems with tapping
that water source. Check out my video about dragging icebergs to Australia.
The majority of the other 30% is stored in underground lakes and reservoirs.
The remaining freshwater - representing about 0.4% of the world's total - is distributed
across lakes, rivers, swamps, our soils … and in the atmosphere.
It is estimated that 50,000 cubic kilometers of freshwater exists in the atmosphere in the form
of water vapor. That is about 6 times the water of all the rivers on earth.
Due to its nature, this water is essentially inexhaustible. Vapor is regularly replenished
as part of the water cycle, which annually moves some 45.5 trillion tons of the stuff.
And unlike with desalination, we aren't sucking up fish eggs along
with the seawater and we don't have to deal with super-saline waste water afterwards.
Sounds great! The big question of course is whether or not we can actually get our hands
on this atmospheric water in a way that is both economically and industrially scalable.
## Fog Collection
The first attempts to collect water from the air were based on fog.
Fog is comprised of tiny cloud water droplets suspended in the air.
The goal of fog collection is to accelerate the formation and growth of these small water
droplets. When they reach a critical size, they roll and fall off into a collector.
Fog and dew collection isn't new or original
by a long shot. Nature has been doing it since time immemorial.
There is a beetle living in the Namib Desert of southern Africa with special
slippery bumps that collect water and funnel it towards the beetle's mouth.
Fog collectors - sometimes also called fog catchers - are inspired by this concept. They are
basically meshes suspended in the air by cables or poles and positioned perpendicular to the wind.
For the mesh, most people use a specially-knit Raschel-weave
high-density mesh because it offers a high ratio of mesh openings for collecting fog droplets.
These collectors can be quite cheap. Depending on the location and the material,
it costs about $100-200 USD to build about 1 square meter of fog collector.
## Foggy Outlooks
The big practical issue with this type of atmospheric water generation is obvious.
The technology is highly dependent on how much fog passes through the
collector's mesh openings. That depends on location and local weather conditions.
Fog collection is regularly used around the world. One case study in Oman found
relatively high collection rates - 30 liters of water per square meter.
However the majority of that came during the monsoon season - just two months in
the year. People need water all year around, so we need something more.
Furthermore, the speed of collection is not that great, and the water isn't always clean to drink.
Fog collectors were given to three villages in the Namib Desert, and it did generate water from fog.
However, the collectors also collected salt and dust from the air, which turned
the water brackish and made it unsuitable for drinking without some additional treatment.
Fog collection is a passive method of collection so it uses very little energy. The downside
however is that you do not have much control over how much water you can end up collecting.
So I don’t really see Fog collection as a scalable solution for alleviating water shortages.
## Materials-based Water Generation
The Materials-based water generation extends on the fog collection idea a
bit. I have also seen it referred to as "sorption-based" or "desiccant-based".
Here, you use a chemical - called a "sorbent" - to
absorb water vapor from the air until it becomes fully saturated with water.
Those packets are like the "Do not eat" silica crystals that you find
in your ramen or other dried foods sometimes.
After that, you heat up the sorbent material - some people use solar thermal
energy to do this - which causes it to release all of its water.
## Sorbents
The most critical element in the whole process is obviously the sorbent material.
The ideal sorbent material has a lot of surface area, heats up relatively easily,
stays stable over multiple cycles, and is of course cheap.
For instance, Silica gel granules and powders. They are cheap - sold commercially at high
volume - and very stable over multiple cycles. But their water absorption powers are not that strong.
Other candidates include microporous
aluminophosphate zeolites, activated alumina, and so on.
One of the key issues is how fast you can perform this cycle. Depending on the conditions,
it can take up to the entire night for the sorbents to absorb the water.
So the only way to produce water at scale would be to buy a lot more
sorbents. At some point this makes the whole venture a bit unwieldy.
## Active Refrigeration
The reality is that when most people think about moisture farms,
they aren’t thinking of fog nets and sorbents.
So let us talk about the last and most interesting
approach to atmospheric water generation - active refrigeration.
This is where we cool atmospheric air to below the
temperature at which it condenses into liquid water - called the "dew point".
Like sorption methods, you can run active refrigeration water
generation methods virtually anywhere, assuming that you have the energy.
However, the local climate does have a significant impact on
whether the technology can generate an economically feasible amount of water.
Exactly how much water actually comes out depends on the ambient temperature,
air's relative humidity, and how low of a temperature the chiller can chill that air.
The less humid the air is, the colder the air has to get before it condenses into water. So if the
air is already very cold then we have to invest a lot more energy to chill that air even further.
For this reason, you shouldn't be running active refrigeration in a cold desert location
like Antarctica. You are probably better off using the energy to melt ice to get your water.
It also cuts out certain arid desert areas like Arizona. The air is very dry there,
which means we need to chill it to a very low temperature to get anywhere
near enough water to be economically competitive.
But tropical, coastal cities with lots of warm humid air can possibly
generate significant amounts of water using active refrigeration methods.
This describes coastal cities like New Orleans in the US, Mombasa in Kenya,
Perth in Australia, or any of the major cities in the Middle East.
Despite being near the ocean, many of these Mediterranean weather cities
ironically have issues with finding enough fresh water.
## Vapor Compression Devices
The dominant active refrigeration devices work like dehumidifiers and air conditioners.
I have a little dehumidifier in my apartment,
which I run from time to time. It dries out my air - leaving behind a condensed liquid.
My little dehumidifier guy works on the principle of
vapor compression refrigeration. It passes humid air over an evaporator
in order to chill that air below its dewpoint to condense vaporized water.
There exist a few machines more specifically geared to generate water this way. Small portable
devices can produce about 20 liters of water a day - equivalent to about 10 milk cartons.
And there are larger industrial-level devices from companies like Skywater,
Gr8 Water, EcoloBlue, and Airdrop Irrigation that can do up to 200,000
liters a day. The cost of these devices can range from $300-$16,000 before operation.
## HVAC Condensate
A slightly related method of atmospheric water generation are water reclamation
systems integrated into modern HVAC systems.
Most people need to run AC anyway. As part of their operations,
these systems generate water - called condensate. That water
condensate is oft free of solids, but is nevertheless unsafe to drink.
The first reason why is that this water captures dust, contaminants and
potentially dangerous pathogens from the outside air as it passes through
the machine's cooling - or evaporator - coils. This is despite a set of filters.
The second reason comes from the metal cooling coils themselves. The water constantly forming on
that metal is slightly acidic, reacting with the metal to create chemical contaminants.
And lastly, the whole environment is wet and warm,
which encourages microbial growth anywhere the water is being collected, distributed or stored.
If this condensate water is to be used for drinking, then it needs to be treated accordingly.
This means reverse osmosis, ozonation, filtration, UV exposure, chlorination, and so on.
All of this is expensive and I guess a lot of people are going to ask,
"If we are going to buy and install this expensive equipment,
might as well do it for ocean water rather than air water."
That's fair. That being said, we use water for more than just drinking. HVAC
condensate can be for flushing toilets, garden irrigation, cooling towers, and cleaning roofs.
I am particularly partial to using roof ponds as a way to passively heat and cool a building. And
you can put fish in them! None of this water touches human lips and as a result does not
have to be so intensely treated. Depending on local regulations and requirements of course.
## Peltier Effect Devices
One significant technical drawback with these vapor compression-style
active refrigeration devices is that the machine itself is not all that reliable.
Such machines are technically complicated and have a whole lot of moving parts. The
more moving parts, the higher the chances those parts can go wrong.
So in recent years, there has been some water generation machine experiments using
the Peltier Effect - sometimes also called the Peltier-Seebeck Effect.
The machine starts with a device called a thermocouple. This device has two sides joined
together. When you pass direct current through it, one side gets hotter while the other gets cooler.
The basic concept of a Peltier-based atmospheric water generator would be
to move humid, hot air towards the cool side of a thermocouple.
Theoretically, such a machine - lacking those moving parts - would be smaller
and more reliable than those devices relying on vapor compression. They
also wouldn't use potentially toxic and climate-impacting refrigerants.
There are a few drawbacks. One being that it would take some time to start producing water. And while
the thermocouple itself is fine, auxiliary components like fans and such can still break.
There are a few companies with products based on this concept,
though it can be a little difficult to determine this from the outside.
## Energy Hungry
Active refrigeration devices are industrially scalable,
but regardless of how they operate, they also use a lot of energy.
It is estimated that some 90-95% of the cost of the water is directly tied to energy cost.
The range is very broad, but research reports and informal estimates find
that we need about 500 to 850 watt-hours of electricity to generate 1 liter of
water. More likely the lower end of that range than the higher.
Depending on how we generate this electricity,
this can make atmospheric water very expensive both financially and carbon-wise.
A 2021 economic study found that atmospheric water generation cannot
compete with tap water. But it might be a compelling alternative to bottled water.
Especially in certain water-strained locations like Dubai, San Diego, and Aruba Island.
One possible way to ameliorate this would be to attach the system to renewable energy sources
like wind, nuclear or solar. But that applies to desalination too, a competing technology.
## Conclusion
Perhaps the biggest challenge facing atmospheric water generation's widespread
adoption - in addition to the energy cost - has been scale.
Depending on the specific local temperature and climate,
different atmospheric water generation solutions stand out over others.
It is definitely not as scalable as something like flash desalination
technology. For that reason, the technologies developed to suit one
country or region's needs cannot be easily mass-exported to many others.
But I do think that this type of water technology can help augment supply in the right locations. I
am particularly struck by how AC condensate water can be repurposed for non-potable use.
There is something there for sure. Or maybe I’m trying too hard to make something out of thin air.