Quote:
Originally Posted by charlesfl
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 |
I think that this paper will get more attention when conclusion number six is stated:
Quote:
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Conclusion 6: The studies show also that up to 40% more heat from the ground can be extracted by using an insulated pipe in a CBHE instead of a non-insulated pipe.
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There has been work on co-axial borehole pipe before, but I don't recall seeing any study that is this thorough.
Previous suggestions regarding coaxial boreholes have been largely dismissed because the cost of coaxial borehole pipe was high relative to the 'U' shaped pipe, and by putting in more of the cheaper pipe, the same heat could be extracted for cheaper.
However, a 40% improvement is not to be ignored. On first analysis, a 40% improvement seems very attractive. But this does not mean that 40% more heat can be harvested from a given piece of ground over the long term. Thermal depletion would surely result, unless the boreholes were spaced farther apart.
What it does suggest however, is that 40% fewer boreholes might be need to be deployed. Since the earthwork is the biggest expense of a GSHP system, this is significant.
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Taking this all back to a DIY perspective, the additional difficulty of home-fabricating insulated, coaxial borehole pipe might be a deterrent to potential DIY readers. They should know that the 'U' shaped borehole pipe has been widely used successfully, in many parts of the world, and there is plenty of engineering data available to correctly size a U-pipe loop field for specific locations and soil types.
Best,
-AC