By Roy Emrick and Russell Lowes
An earlier version of this article appeared in the April-June 2010 Sierra Club Rincon Group Newsletter.
Which cooling system is best for energy use? Which is best for water use? Which is best for reducing CO2 output of electrical plants?
For several years, a business columnist at the Arizona Daily Star regularly berated evaporative coolers as water wasters and outmoded technology. He said refrigeration was the way to go in the modern world. Many readers disagreed with him but they gave only qualitative arguments. We decided to see if we could find some quantitative data to compare the two systems. We put together our data on our own rooftop systems. One of us (Roy) has had only evaporative coolers since he came to Tucson in 1960. The second author (Russell) has a combined evap/air conditioner/heat pump unit.
Although evaporative coolers used to be the standard cooling device for Tucson homes, they are less common today, so a brief description of how they work is in order. You’ve probably noticed that even on a very hot summer day, when you come out of swimming pool you find yourself shivering. This is because it takes energy to evaporate water (or any liquid for that matter). This energy, called the latent heat of evaporation, comes from your body and cools it. The evap cooler uses the same principle. It is a box with a tank of water, pads of aspen fiber, corrugated paper, or composite (MasterCool), a pump to distribute water to wet the pads, and a blower fan to pull air in through the pads and force it into your house. The air is cooled as it flows through the pads by the evaporating water. On a hot, dry summer day, this method of cooling is very effective; however, because less water evaporates when the air is more humid, these coolers are admittedly not as effective during the humid monsoon season.
Also, as you probably know, Tucson’s water contains lots of dissolved minerals. These minerals precipitate out on the cooler pads eventually making them useless. To combat this problem, the more modern coolers have pumps that empty out the water tank every eight or twelve hours of operation, thereby purging the salty water. This is good for cooler pad life but uses more water. Because this latter type of cooler is more common today, we included the use of this pump in our experiment.
Refrigeration or “air conditioning” systems are based on the Joule-Thomson effect: a gas cools when it expands. For example, when you let air out of a tire, it is cool. Here a mechanical pump compresses a gas (usually Freon), which warms it. It then goes through a copper coil where air cools it until it condenses. The resulting liquid then flows through a small opening and expands, causing it to cool, and chill your house.