I found a masters thesis on Coaxial Borehole Heat Exchanger which comes up with interesting conclusions about earth coupled heat exchangers comparing the two pipe system we use. It is very detailed with lots of testing. It can be found at
http://www.kth.se/polopoly_fs/1.2039...0Guillaume.pdf
Analysis of a Novel Pipe in Pipe Coaxial Borehole
Heat Exchanger
Hope this helps . Charlesfl