My conclusion: I want to explore the concept of using an earth tube or tubes to precondition our fresh air intake. If I can build a tube system that will achieve a 50% reduction in moisture load on incoming air in the dog days of summer, then tubes are the way to go. My math is not up to the task of evaluating the design. I’ve been trying to connect with a mechanical engineer who could give advice on the psychrometrics. If I can’t find someone to help me geek out the system variables for our site (tube diameter/length, depth, rate of air flow, pressure) then we will have to go with a purely mechanical system to be on the safe side. And then I will always wonder if we missed a grand opportunity.
With enough surface area for incoming air to duct contact, you can eventually get the outdoor air almost to the same point (dry bulb) as the ground temperature. This means that in September, if your duct was 13 feet below ground, you could get the air to about 60 degrees even if it is 90 F outside. Conversely, you could get the incoming air as high as 45 degrees (13 foot duct) in February, even if it is 0 degrees outside. Unfortunately, to change the temperature that much, you need longer duct runs. Assuming you need about 55 cfm of ventilation (2500 sf x 0.01 cfm/sf + 4 people x 7.5 cfm/person), the graph below shows what percentage of the temperature difference (between earth temperature and outside air temperature) you could make up with different diameter ducts and different lengths (I assumed square / rectangle ducts). This says that 20 meters of 0.1 x 0.1 duct would make up 90% of the difference between outdoor air and ground temperature. For example, when it was 0 degrees outside and 50 degrees in the ground in January, you could make up 45 degrees just from going through the duct.