Evaluating a Design for Using Wind Turbines to Harvest Water

Small Wind Turbine Water Condenser

Is this design feasible?

https://m.facebook.com/story.php?story_fbid=10153855162601512&id=102515706511

The claim is that 37L of water can be condensed per day.

Let’s evaluate the claim based on a hot location, but one that’s also humid. If it doesn’t work there,

it’s unlikely to work anywhere that really needs it. Use Mumbai, India data:

https://en.wikipedia.org/wiki/Climate_of_Mumbai

We’ll use a reference temp T =  27.2C and humidity RH = 0.75 (averages over the whole year)

Using a dew point calculator:

http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html

This gives an average dew point of 22.4C. Multiple feet below ground, soil tends to be at a temperature of about 55F, which is below that dewpoint.

To see the upper limit of how much water might be pulled out of the air, calculate how much water

is in a reference air mass.

Use this graph http://www.engineeringtoolbox.com/water-vapor-air-d_854.html to find ratio of water to air in our Mumbai climate:

RH=75% at T=27C => 0.017 lbs H2O / lbs dry air.

37L H20 is 37kg H20.

37kg / Mair = 0.017

So, Mair = 2200 kg, and air + H2O mass = 2240 kg.

Using http://www.engineeringtoolbox.com/density-air-d_680.html to adjust for slightly less dense 

than standard air, gives a density ratio of this air of .96.

Using standard air density of (1.2 kg/m3) * .96 = 1.15 kg/m3

So, the volume of air that holds 37L of H2O is about 1950 m3.

Over one day (24*60 minutes), this equates to 1.4 m3 of air per minute. This is equivalent to

two refrigerators full of air.

This would be the amount of air the turbine needs to process if 100% of H2O was extracted.

Cooling two refrigerators worth of air per minute even 9F to the dew point seems very rapid.

Accounting for partial removal of H2O, we use the psychrometric chart:

https://en.wikipedia.org/wiki/Psychrometrics#/media/File:PsychrometricChart.SeaLevel.SI.svg

Recall that we start with a humidity ratio of 0.017, on the Right Axis.  Intersect with 75% RH

at our air temperature of 27.2C (Horizontal Axis). Cooling in the underground chamber moves 

us straight left (no moisture lost, yet) until Dry Bulb Temp hits the 100% RH red line. Now, we

start to condense out the water, moving along the curved 100%RH line down and to the left

until we hit 12.8C Dry Bulb Temp, which is our 55F ground temperature. Now, we are at a 

humidity ratio of about 0.0085.

Going from a humidity ratio of 0.017 to 0.0085 means we condensed half the water from the

air. So, our perfectly efficient rate of 1.4m3 of air per minute, would have to be doubled, to

2.8 m3 of air per minute, cooled from 77F to 55F. That seems like a tall order.