Just a quick thought... if the below ground temp is always ~55*, could one not produce electricity with a properly sized thermo-siphon system setup the same way as a geo-heat pump?
Dig a hole 2 feet across, 20 feet down, and 10 feet wide, make a loop of 18 inch dia. pipe with a generator in circuit with it and a burp valve at the highest point, fill it with active propylene glycol, then bury half of it in the hole, and leave half exposed... temperature gets below 45* air temp, the fluid starts to circulate, driving the generator, which (in theory) should then create electricity, right?
Has this been done?
How much electricity could one generate using the specs I've laid out? Would it even be worth it?
Found this:
http://www.patentstorm.us/patents/4546608/claims.html