Evaporative Coolers Don't Guzzle Water After All

A man installing an evaporative cooler on a roof.

Dagny Gromer / Flickr / CC BY-NC-ND 2.0

After seeing the new AMAX evaporative cooler at Greenbuild, I wrote:

The unit is able to deliver 3.5 tons of cooling with only 456 watts of electricity, easily within reach of a solar panel. The problem is the water; the unit sucks up almost 2.5 gallons per ton of cooling per hour, which could add up quickly in a place like Phoenix. It is just vapourized and lost into the atmosphere. There really is a tradeoff between electricity and water, and right now both are a problem.

2.5 gallons per ton per hour sounded like a lot to me. For a three-ton unit it would be like flushing a toilet every twelve minutes. But it turns out that it is less than the amount of water that would have been used to make the electricity that has been saved.

Si Hyland of AMAX pointed me to a study titled "Consumptive Water Use for U.S. Power Production" by P. Torcellini, N. Long, and R. Judkoff of the National Renewable Energy Laboratory. It lists the amount of water used to make power, and where it goes:

In a coal or nuclear plant, a lot of cooling water is used. Often cooling towers evaporate water, where there is an obvious direct loss. But even if it is cooled by a river, putting the water back at a higher temperature increased the rate of evaporation. In thermoelectric plants, the average across the country is estimated to be .47 gallons per kWh of electricity used by the consumer.

But the real shocker was hydroelectric power; when rivers are dammed and reservoirs created, there is a huge increase in surface area and evaporation compared to a free-running river. So much so that they estimate that it averages 18 gallons of freshwater evaporated for every kWh of water used by the consumer.

Overall, the national average is two gallons per kWh of electricity consumed.

The evaporative cooler runs on 450 watts; a three ton conventional unit uses ten times that, consuming running at 4500 watts, or about 4 kWh every hour more if running constantly. That production consumes 8 gallons of water on average.

So in fact, having an evaporative cooler uses a tenth the amount of electricity and a little bit less water than the conventional unit. There is no trade-off to be made.

Of course, it would be preferable to have a system that didn't use much electricity and didn't use water; some of the solar-powered absorption units we have talked about are like that, but they have not come to market in North America yet. But one can't fault the AMAX evaporative cooler for using more water than a conventional air conditioner.