Tuesday, July 17, 2007

Evaporative Cooling

Each summer we depend on our air conditioner to keep our home's temperature at a livable level – otherwise it would be 100°+ in the house, and darned uncomfortable. We have a completely conventional air conditioner (for the U.S.): a standard electric unit that depends on the evaporation and condensation of a synthetic refrigerant. It works great, and keeps our house at a very comfortable temperature – but it costs a small fortune to run. Surely there must be a better way!

And there is.

In any dry part of the country (and we certainly qualify for that!), evaporative cooling is a technology that costs much less for any given cooling capacity. This technology (which comes in several forms) leverages a simple fact about water: it absorbs a lot of heat (2272 Joules) for each gram of water evaporated. Our air conditioner is a “five ton” unit (60,000 BTU/hour). To provide an equivalent amount of cooling, we'd need to evaporate water at the rate of about 7.4 gallons per hour.

Two forms of evaporative cooling are common in homes in certain areas of the U.S.: misting systems and “swamp coolers”. Misting systems force water at high pressure through tiny holes, creating extremely fine droplets that evaporate almost immediately, cooling down the air around them. Many people in Arizona, New Mexico, and other places use misting systems to cool their patios. There are a few of these in the San Diego area, but they are not common. Swamp coolers work by blowing air through a porous pad that is kept wet, usually by rolling the pad continuously through a tub of water. The air is then routed into the home, directly cooling the house. These systems are common
in some areas (again, in Arizona and New Mexico), but they have one big drawback: they raise the humidity in the house to undesirably high levels.

The third form of evaporative cooling technology isn't found (to my knowledge) in homes at all, though it is quite common in industrial buildings. This is the “forced-draft cooling tower”, which works by forcing air over droplets of water to cool the water. A conventional heat exchanger then runs a separate loop (of refrigerant or chilled water) to heat exchangers in the building being cooled. These cooling tower system are slightly less efficient than swamp coolers (as there are some thermal losses in the heat exchanger loop), but they have the great advantage of not humidifying the air in the building being cooled. The only reason I can think of that these are not used for homes is that they are relatively complex pieces of machinery. A small capacity cooling tower is just as complex as a large one, and would probably be much more expensive to build than a small capacity refrigerant-based air conditioner. But much cheaper to run!

To cool our home on a typical summer day, I estimate that we'd have to evaporate about 40 gallons of water. I'm exploring the notion of building my own cooling tower to replace our conventional system. The basic engineering challenge is to create enough droplets, with air blowing over them, to evaporate water at the required rate of 7.4 gallons/hour; I've not yet found any reference that would help me design this. The rest of it is very straightforward, basically just plumbing.

Any “evaporation engineers” out there?

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