If you dig down five feet in the middle of a desert, the ground will be cool. You can use this inherent coolness to keep the temperature down in your home as well.
One of the things you notice when you visit a cave is the cool temperature. The outside air can be hot enough to boil an egg on the sidewalk, but once you step down into the cave, you encounter cool and dehumidified air. This is because the temperature below ground level tends to remain at a constant level. This temperature can be calculated roughly by knowing the average temperature of the above ground air. For example, the city of Chicago has an average temperature in the area of 72 degrees. This average comes from the fact that it drops to 10 degrees in the winter, and swelters at over 100 degrees in the summer. The 72 degree day is a rare, if welcome, break in the extremes.
It is the average, however, and the layers of earth tend to insulate very well. It is reasonable to assume that after centuries of this average temperature, any air trapped underground would settle out at this average temperature and maintain it. This has led to the idea that this constant temperature air underground could be tapped for both heating and passive air conditioning. If your home happened to be built on top of a large cave, you could run a pipe from your home to the cave. Using fans to cause circulation, the cool air could be pulled up from the cave, and the ambient air from the home could be pushed down into the cave.
Since many homes do not happen to be located on top of caves, the idea has been conceived of digging artificial caves under homes and laying pipes to connect them to the homes. A system like this would require only the power it takes to run the fans that would facilitate the proper circulation between the “cave” and the home. It would eliminate the use of compressors which draw the majority of power used in air conditioning.
Another version of this plan would involve the laying of long pipes underground. The idea here is that the “cave” is not necessary. Simply running the air underground through the pipes would allow the air to be cooled. An underground pipe system would be a bit expensive and awkward to install in an existing building. It would be a little less difficult in new construction as the pipes could be laid during the excavation that would take place in laying the foundation of the building.
Regardless of any initial expense, a passive underground system would need little or no maintenance, and beyond circulating fans, little or no energy consumption. Although passive pipe air conditioning, like solar powered air conditioning, is new and mostly experimental today, the combination of global warming and increased population is making an energy solution to air conditioning critical.
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