42, Love the idea of using earth tubes. Have looked into them in years past and always wondered why no one is doing this. I was looking for inexpensive cooling to supplement a solar (evacuated tube) hydronic infloor radiant heat system. As you can tell this is an older article as the prices today are probably a little higher. After reading about problems you mentioned with direct air exchange I gave up on the idea. Though I still think decoupling it through an outside exchanger would work great. Should keep the outdoor unit right in it's sweet spot year round and save alot. Look over the link.
My thought is to use this setup and add one more set of 90 degree elbows to make almost a complete rectangle. Build both ends of the tubes into an insulated box. Place the outdoor unit inside in a fashion so it's fan would suck from one side (return) and blow out the box (supply) completing the loop in a push pull fashion. Not sure of the cfm of the outdoor unit though this is probably still overkill built in.
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