Water From the Air

The atmosphere contains about as much water as the terrestrial fresh water supply. That’s a lot of water! Wouldn’t it be nice if we could extract water directly from the air instead of having to depend on rainfall or groundwater?

98% of atmospheric water is invisible water vapor. The other 2% is condensed water that appears as clouds. The water vapor in the air is also known as humidity. For weather forecasting purposes they measure relative humidity.  Relative humidity is the comparison between the maximum amount of water vapor that the air can hold at the ambient temperature and the actual amount of water vapor present. For example, a relative humidity of 30% means that the air contains 30% percent of the maximum amount of vapor the air can hold without condensing out. Higher temperature air can contain more water vapor.

The temperature at which water condenses out of the atmosphere is called the dew point. The dew point temperature is higher as humidity increases.  Lowering the air temperature to below the dew point will cause the water to condense out. This is the basis for air wells and most atmospheric water generators.

Air wells are passive systems that need no external energy source and have no moving parts. There have been, historically, two kinds of air wells – the mass air well and the radiative air well.

The first ones were mass air wells and they did not perform very well. They consisted of massive stone buildings that, because of their mass, stayed cooler than the surrounding air thus accumulating condensed water on cool stone surfaces within the structure. The water would run down the stone surfaces into catch basins. A big one was built in the early 1900’s that was 45 feet high and made of stone. It only manages to capture five gallons of water per night. That’s really a lot of work to get only that small amount of water. The main problem with these, besides the fact that they are huge structures, is they also heated up during the day and could not cool down enough at night to produce more water.

Another ancient water collecting method was to create dew ponds. As it turns out, they produced most of their water from moisture in the soil or by collecting rainwater. Very little, if any, of the water collected in dew ponds actually came from dew.

The radiative air well uses night sky radiation to produce a cool surface for water to condense on. The thermal properties of night sky radiation are the reason frost sometimes occurs on roofs even if the air temperature is above freezing.  This phenomenon is now being explored for whole house cooling that doesn’t require air conditioners or evaporative coolers.

The material used to collect moisture has to be very low mass so it can’t hold heat. It also has to be insulated from the heat held by the ground. The material also has to be hydrophobic (water repellent) so that the water runs quickly off and into a storage container before it can evaporate. Typically, radiative air wells consist of a thin plastic film backed by insulation and placed far enough above the ground to mitigate heat coming from the ground. Most of the research and development on radiative air wells has occurred in the last 40 years. New thin-film materials are being designed that maximize the collection of the condensation. The best new materials have bumps on them to increase the surface area. 

You can make your own radiative air well using a sheet of polyethylene suspended at about a 30 degree angle with a trough at the lower end to collect the water and direct it into a storage container. I am thinking black polyethylene would work better than clear. That’s worth an experiment I think. You will want a filter between the collector and the storage container to keep out dust, etc.

How much water you can collect will depend on the humidity and how cold it gets at night. If you regularly get dew on your car windshield then a radiative air well could work for you.

There is a commercially available system that collects water from fog. The Warka Water system is being tested in Africa. http://www.vittori-lab.com/introductionwarka  Since the sides of the collecting fabric are vertical I don’t think it would work very well capturing water vapor.  They are interesting looking!

The radiative air wells are capable of producing more water than the mass air wells but not a lot of water. Larger surface areas produce more water. They can produce enough to provide drinking water under the right conditions - high humidity and a large temperature differential between day and night. Where I live, in central Arizona, humidity is very low most of the year so collecting water from the air would be difficult.

Atmospheric Water Generators (AWG) are active systems that use the same technology as air conditioners. They produce the most water of all and under a wider variety of atmospheric conditions. They contain cold surfaces and the water condenses on them. These are energy intensive appliances so are not practical in applications where cheap energy is not available. There are home systems that you can plug into house current. The use between 400 and 1000 watts which is a lot of you are off-grid. It may be worth it if clean water is not available and you have enough energy to run the equipment.

Some commercial AWG systems are being designed with their own renewable energy sources to provide the refrigeration necessary for atmospheric water generators to work. An example of this is the Eole Water Generator. http://www.eolewater.com/gb/our-products/range.html  This generator runs on wind power and also generates a considerable amount of extra electricity that can be used.

You could probably make your own AWG using an air conditioner. You would have to reconfigure the placement of the condenser so that water can be collected from it. I think you could use  a much slower fan, if you need a fan at all since the goal is to collect the condensation rather than deliver cool air into your house. To get a reasonable amount of water with AWG systems the humidity should be above 30% and the temperature should be above 65 degrees Fahrenheit during the day.

 

There is another method to get the water out of the air that involves hygroscopic liquid salts (desiccants) that absorb water vapor. These salts are then heated to boiling and the water evaporates and is collected and condensed.  It sounds like these might use less energy than the condensation systems although toxic salts don’t sound like something I want to fool with.

Even if collecting water from the air does not produce a lot of water via the above methods, the water produced is very clean. We really don’t use that much water for drinking so it might be enough for that purpose. It can also be a fun thing to experiment with. May someone will discover a new method to get more water from the air without using too much energy.