Water from Air

We’ve talked in other posts about how water will condense out of air when it is cooled past the dewpoint temperature.  So I got to wondering how much it would cost to produce water that way.  In this post we’ll look at a very rough estimate of the minimum cost.

This figure shows the process of condensing water out of the air on a psychrometric chart:

Assuming that our outside air was starting at the green dot, the air first has to be cooled to the dewpoint (the horizontal line going to 100% relative humidity).  After that, continued cooling will follow the 100% relative humidity line downward, and liquid water will be condensed out of the air.  Now, the cost of cooling will depend heavily on where you start because a significant percentage of the cooling is required to get to 100% relative humidity.  However, if we are after liquid water rather than cool air, we could use the cool, saturated air at the red dot to “pre-cool” our air—possibly most or all of the way to the 100% relative humidity line.

A system that looked like this, for example, might do it:

On a psychrometric chart, the processes would look like this:

Now, assuming that we could do all of the sensible cooling (horizontal line) with the heat exchanger, the only net cooling to produce liquid water happens along the 100% relative humidity line, and it is easy to determine the amount of heat that needs to be removed to get liquid water along that line.

The remaining variables for a calculation of the cost of that water are the cost of electricity, and the efficiency of the chiller (that is, how much electricity it requires to remove a given amount of heat).  The chiller efficiency is expressed as COP which stands for Coefficient of Performance.  Typical values for commercial equipment might be in the range of 2.5-4.

This figure shows the cost of liquid water as a function of the cost of electricity and the COP:

 The range of costs, about 6 cents to about 18 cents per gallon, may not sound like too much, but compared to the average U.S. consumer cost of tap water of about 0.15 cents per gallon, it is roughly 40-120 times as much.  Note also, that this calculation only includes the electricity costs of condensing the water—we’ve ignored capital costs of the equipment and collection and storage infrastructure.  So, overall it is probably best to continue to use the water utility, but it’s nice to know a ballpark figure for another way.