Geothermal Heat Exchanger 2

[aspearin1]

I'm researching Geothermal Heating energy and I am curious as to why hundreds to thousands of feet of coiled Polyethylene piping is preferred over perhaps a buried aluminum heat exchanger. Aluminum has great corrosion resistance (unlike copper) and thermal conductivity. In my opinion, using a well designed aluminum exchanger drastically reduces the size of the excavation needed to bury so much pipe (in a horizontally laid system), which should offset any material price increase by using a metal over a polymer. Your comments are appreciated.

Also, I'm wondering if anyone is familiar with a more passive geothermal heat exchange, using the temperature gradient in the pipeline to drive fluid motion (slowly) rather than constantly pumping the fluid. I'm sure it works somewhat in theory, but I haven't seen practical use.

Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee

 

[btrueblood]

"Aluminum has great corrosion resistance (unlike copper)"...

How much buried aluminum pipe have you seen? How many houses have aluminum water pipe, even as components?

Aluminum's corrosion resistance is not good enough for either situation. Plus, it is likely to dent and crack when backfilled, unlike a flexible polymer.

Although I have not run the numbers, I doubt the difference in thermal conductivity would give much benefit, the bigger issue is getting a large enough heat sink to balance the required heating/cooling load, i.e. getting enough cubic yards of dirt involved in the conduction process.

 

[Peckett]

Aluminimum doesn't handle geothermal fluid very well at all. It corrodes very quickly, and it kept out of geothermal plants.

My university project (many years ago) was on geothermal downhole heat exchangers, where we used the heat gradient to drive the fluid flow. You get less heat out than by pumping, and there is a balance between the two which determines which is the better system.

 

[aspearin1]

Peckett, By geothermal fluid, I assume you mean a system using some sort of anti-freeze or oil based transfer fluid, and not solely water? I also understand that there is an EROSION problem with aluminum that would weather a flowing system rather quickly.

By "corrosion resistance" I was considering only open-to air oxidation when comparing to copper... not considering contact with other fluids or interactions with earth minerals. My mistake.

Thanks for the info on your university project as well.

Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee

 

[carnage1]

One note is aluminum is a rigid pipe, requiring fitting, PEX is flexible and can be thrown in a ditch and buried easily.

Luck is a difficult thing to verify and therefore should be tested often. - Me

 

[mbt22]

The controlling resistance to heat transfer is unlikely to be the piping, whether polythene or copper. Heat transfer through the earth to the pipe is probably much, much larger. Remember (slightly simplified):

1/U = 1/h_in + 1/h_wall + 1/h_out

Increasing h_wall won't do much if h_out is tiny.

 

[DUMechEng]

"The controlling resistance to heat transfer is unlikely to be the piping, whether polythene or copper"

Precisely...the primary source of resistance is the ground itself.