Ultimate solar fan

The plastic shroud that the fan is mounted in would require very little modification to put it into a duct.  This fan is so powerful that if I turn it on it makes it hard to open the door if all the windows are closed.  You can feel all the air rushing out of the house.

I plan to install it using a standard cold air return type duct for an A/C unit (filter on ceiling type) and have the air vent into the attic.  The air should be able to escape through the roof venting.  That will solve two problems it will draw cool into the house and it will cool off the attic.  A double whammy for the watts

Thats my crazy plan anyway.

Total cost thus far.  15 dollars (Junk Yard Fan)

Sounds great!  I just assumed that radiator fans were belt-driven.  I had no idea they used a 12v motor.  I may look into that for my whole house fan.  In my setup, I'll be pulling in fresh cool air through earth tubes.

Earth tubes may provide some benefit in Florida, but not as much as in the north.  The principal works primarily on passive annual heat (or coolth) storage, similar to ground source heat pumps.  Without a great deal of seasonal variation in Florida, the ground temperature won't be much lower than the average outside air temperature.  In Pennsylvania, our ground temperature is around 50 degrees because we go below zero in the winter.  In Florida, it's in the 70s.  Still, that's not too shabby on a 100 degree day, so it may still be worth it. 

Similarly, on those rare 30 degree days (like when I went to Orlando in December), a 70 degree intake air temp might be quite nice too. 

Also, another benefit is if the ground temp is below the dew point, the incoming air will be much drier than outdoor air because water will precipitate out underground in the earth tubes just like it does with a traditional air conditioner.  That alone is probably well worth the effort when you live in a swamp.  And speaking of swamps -- you could then run that dry air through a "swamp cooler", i.e. adding moisture back in, which cools the air in the process.  That gives you the choice of cool dry air or colder, wetter air.

I haven't built my earth tube system yet, because it is going to be incorporated into a larger earth works project when the first stage of my house is complete.  My plan is to build a reflecting pool directly in front of my passive solar windows to increase winter insolation, and to run my earth tubes through the bottom of that pool.  The purpose of that is to maximize the conduction from the earth into the tubes, which is accomplished much better by water than by soil.  But if your water table is high, as it is in most of Florida, you may have water at a few feet depth anyway, and so direct burial would achieve good results.  Another trick is to redirect your downspouts to let out over your earth tubes to keep the soil moist and conductive.

Yeah, the decision on material is a tough one. 

The reason you're better off with a direct water connection rather than sand is because of contact surface area for conduction -- you want your earth tube to always be the temperature of the surrounding soil, not the air passing through it.  Sand or other fill has air gaps that act as an insulator, so your tube could more likely adjust to the temperature of the air passing through it instead of the ground temperature.  Water not only moves heat through conduction, but also convection.  During the summer, hot air will pass through the tube, heating up the water around it, and the water will rise to the surface of the pond to be replaced with new, colder water around the tube. 

I think that using water as the medium instead of soil will improve performance and reduce the length and depth required.  This part will improve the economics of the project, though the earth-moving aspect will not.  But since the pond will serve multiple roles -- solar reflector, passive evaporative cooler, landscaping, extra water storage, fire mitigation reservoir, etc -- in addition to intake air moderation, it is cost-effective overall and isn't too far removed from the cost of trenching alone.

But that does make choosing a water-proof material important.  Not that it won't get wet inside from condensation anyway, but as long as a drainage path is designed in, that shouldn't be too bad.  I wouldn't want water to be constantly seeping in though, as with a porous material. 

A perfectly water-proof material does imply plastic or other oil-based material.  I guess certain metals could work, but would most likely be prohibitively expensive, particularly for a perfect seal.  I don't know what 4-6" copper, galvanized, or stainless would cost, but probably way too much.  Copper does have anti-microbial properties, which would be a plus, but not enough to offset the cost.  Dropping a few scraps of copper wire or pipe in the bottom of a pipe of another material might be enough to achieve the same effect anyway... as these scraps corrode from condensation, they'll leave a blue-green trail of copper ions as they flow toward the drain, inhibiting mold & bacteria in the wettest part of the tube.

With clay I would be worried about the long-term integrity being exposed to water, and also I'm not sure how to make water-tight seals with clay.

PVC seems like the easy way.  I would use a thin-walled variety for best conduction, but the R-value of PVC isn't too high anyway.  I am a bit concerned about off-gassing.  I know that PVC does get used for air ducts under slabs, and has for a few decades, so perhaps the off-gassing concern isn't too big of a problem.  That's most likely the path I will take, gluing the joints for a perfect air-tight seal (to keep both water and radon out). 

I read about an earth tube system in India that used a 4" metal tube, about 150' long, and 9' deep buried in sand.  It both cooled in summer and heated in winter the intake air by about 25 degrees Fahrenheit.  But it accomplished almost all of the conditioning in only half the length, so really 75' would have been effective.  I figure by using water as the medium, I can achieve similar results at about half again that length, or about 40', and only 5' depth with 4" PVC.  Even if it didn't quite perform as well, even a 10-15 degree change would still be a huge success for a relatively low cost.