Looking for information on direct exchange geo

Discussion in 'Geothermal Loops' started by gte, Jul 31, 2011.

  1. gte

    gte New Member

    To start, I believe I can get away with copper because they use copper to bring city water to the house and it'll last a good 30 + years doing that in Maryland. I can also always add a cathodic low voltage counter action if the ph is too much. Scouring the web, I found this site

    http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx

    and it stated that soil ph in my area ranged from 4.5 to 7.3. I have 2.25 acres but would prefer to go in a conical pattern at a vertical angle < 45 degrees at a point that is roughly 25 feet away from the current heat pump location. Using the epa btu calculator, my home requires 58000 btus and 4.9 tons. A little more info is that the house is 3000 square feet, built in the late 50's, has wall insulation and 2 layers of attic insulation, half of the first floor is underground, poured concrete wall.

    Has anyone done direct exchange? Can you tell me about your setup?



    Also, I may get chastised for this last question (and I know I'm not the first person to ponder this), but I'd like to at least satisfy my curiosity as to why this is probably a bad idea ... why hasn't anyone tried a dx conversion/retrofit where the coils in a traditional heat pump are replaced with an in ground dx loop? Why would that not improve the efficiency of a heat pump in theory, all else being equal? I do understand that to maximize gains, a purpose built system is best.

    Thanks for reading.
     
  2. gte

    gte New Member

  3. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Heat pump conversion

    I'm sure a few folks have tried heat pump conversions. Most of who you are going to be talking with on these forums are qualified designers/installers that make a living putting geo in. For obvious reasons we can't do the ad hoc approach most talented DIYrs can do in their own home.
     
  4. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    and besides

    the cost and liability for design makes it something very few of us would take on. I would however take on another spring project with a sign off on, "What if it does not work".

    It is all about moving heat. If you know how to do that, you can make anything work for a price.

    Mark
     
  5. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    Most grouts fall in the .4 to 1.2 Btu/(hr·ft⋅F) range.

    Are in a heating or cooling climate?
     
  6. gte

    gte New Member

    Hi Mark,

    Thanks for the reply .. care to visit our nations capital in the spring for a project? Free room and board? I'm well versed in heat transfer from turbocharger system design, so there is familiarity there.

    I'll be glad to sign off on the "what if it doesn't work" clause. I do want to have in and out temperature monitors on each loop for performance monitoring as well.







     
    Last edited: Aug 1, 2011
  7. gte

    gte New Member

    Thanks for the reply ... so far superior to air, but far inferior to copper. Also moderately (up to 3 times) better than HPDE.

    So to put things into perspective


    • air is: 0.025 W/(m*K) or 0.173 Btu/(hr·ft⋅F)
    • HPDE is: 0.48 W/(m*K) or 3.33 Btu/(hr·ft⋅F)
    • grout is up to: 0.173 W/(m*K) or 1.2 Btu/(hr·ft⋅F)
    • soil is: 1.5 W/(m*K) or 10.4 Btu/(hr·ft⋅F) (type and water density of soil unknown)
    • concrete is: 1.7 W/(m*K) or 11.787 Btu/(hr·ft⋅F)
    • copper is: 401 W/(m*K) or 2780.322 Btu/(hr·ft⋅F)

    Looks like the theoretical winning combination there would be copper and concrete?


    As for whether I am in a heating or cooling climate, I'm not 100% sure what you are asking. I'd like to use this for both heating and cooling, and I am physically located in Maryland.


     
  8. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Careful

    You're getting drawn in to the wrong direction. The rate-limiting step is the soil. That is where the heat comes from. You can make % improvements with other materials but borehole efficiencies are primarily dependent on soil types.

    P.S. Don't you dare use a heat loss divided by w/m to calculate loop field:) I throw that in there as conventional mechanical engineers sometimes fake a design like that after sourcing all the thermal conductivities.
     
    Last edited: Aug 2, 2011
  9. gte

    gte New Member

    Thanks for the heads up on loop field and I agree that the soil is my limiting factor, which is why I chose 2 materials that were better than it in thermal conductivity. Maybe I'm wrong here, but I can't see any advantage to choosing a material that is less thermal conducting then my soil?

    Concrete is cheap and readily available, and I believe I'm correct in saying it has a higher thermal conductivity then grout. Coupled with copper or stainless or aluminum the weakest link would sit with the soil. Aluminum (unless it was thick wall) is probably a bad choice, although copper is pretty soft too, and so I guess any of those 3 materials that were readily available and the least expensive, would make a good choice? Do you agree?




     
  10. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    According to your numbers, HDPE is about 3 times better than the best bentonite grout.

    Most soils fall between 1 and 2 Btu/(hr·ft⋅F). The job we are on now is .65. Only supersaturated sand would come close 10.4.
     
  11. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Road trip

    to DC is possible, as our daughter lives there.

    Mark
     
    Last edited: Aug 3, 2011
  12. gte

    gte New Member

    Hi, thanks for the heads up about the soil conductivity, I had a feeling that number was a theoretical best.

    Is there a simplistic backyard way to do so, maybe with a coil/temp sensor inserted into the ground and then a certain distance away, another temp sensor. Then measuring the rate of change over an hours time?



     
  13. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    Your best bet is to use a table
     

    Attached Files:

  14. gte

    gte New Member

    Thanks!

    Are you sure those numbers aren't W/(m*K) ? Instead of Btu/(hr·ft⋅F)?

    The two listed that are "not possible" with the occurence, are 1.4 and 2.0 and saturated sand is 1.44 . Since none of them even remotely approach 10.4, it seems they are in W/(m*K) ?

    Looks like .75 to .9 is reasonably common? Which would be 5.2 to 6.24 Btu/(hr·ft⋅F) ... both higher than the HPDE.

    What do you think?


    • HPDE is: 0.48 W/(m*K) or 3.33 Btu/(hr·ft⋅F)
    • grout is up to: 0.173 W/(m*K) or 1.2 Btu/(hr·ft⋅F)
    • soil is: 1.5 W/(m*K) or 10.4 Btu/(hr·ft⋅F) (type and water density of soil unknown)
    [​IMG]
     
  15. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    The numbers are definitely Btu/(hr·ft⋅F).

    I don't know where you got the number of 10.4 Btu/(hr·ft⋅F), but it is entirely unrealistic .

    The list is for horizontal trenches, hence solid rock is not possible.
     
  16. gte

    gte New Member

    Ok, well thanks for clearing that up ... that's a downer.

    I got the
    soil is: 1.5 W/(m*K) or 10.4 Btu/(hr·ft⋅F) (type and water density of soil unknown)
    from Wikipedia, which is usually pretty accurate, but maybe not so much this time.


    Thermal conductivity - Wikipedia, the free encyclopedia



     
  17. Looby

    Looby Member Forum Leader

    The table in Wikipedia says: soil TC = 1.5 W/(m · K)

    Wikipedia also says: 1.0 BTU/(hr · ft · F) = 1.7 W/(m · K)

    Thus, soil TC = 1.5 / 1.73 = 0.87 BTU/(hr · ft · F)

    You seem to be off by a factor of exactly 12x, so I'll guess
    that your 10.4 TC is actually in units of BTU·in/(hr·sqft·F).

    The fault, dear gte, is not in our Wikipedia, but in ourselves.


    There are three kinds of people in the world:
    those who can count, and those who can't.
     
    Last edited: Aug 4, 2011
  18. gte

    gte New Member

    Hi Looby,

    1 W/(m · K) = 6.9334713 BTUin/(hr · ft · F)

    or

    1 BTUin/(hr · ft · F) = 0.1442278992342551 W/(m · K)

    Since wiki says the thermal conductivity of soil is 1.5 [W/(m·K)]

    1.5 [W/(m·K)] * 6.9334713 = 10.40020695 BTUin/(hr · ft · F)



     
  19. gte

    gte New Member

    Hi Lobby,

    I see you edited your post after I quoted it and started replying, yes the conversion calc I used was in BTU in /(hr ft2 F), I should have cut and pasted instead of typoing out the "in" part, which lead to all of the confusion.
     
  20. gte

    gte New Member

    Ok,

    recalc time

    • HPDE is 0.48 W/(m*K) or 0.2775 Btu/(hr·ft⋅F)
    • low end grout is 0.7788306558649778 W/(m*K) or 0.45 Btu/(hr·ft⋅F)
    • high end grout is up to: 1.730734790811062 W/(m*K) or 1 Btu/(hr·ft⋅F)
    • concrete is 1.7 W/(m*K) or 0.9833333333333333 Btu/(hr·ft⋅F)
    • soil is 1.5 W/(m*K) or 0.8667 Btu/(hr·ft⋅F) (type and water density of soil unknown)
    • copper is 401 W/(m*K) or 231.667 Btu/(hr·ft⋅F)
    So on average, soil is 4 times the thermal conductivity of hpde? High end grout is slightly better than soil and so is concrete, so the question then is ... which is cheaper to use? Copper blows everything out of the water and this also makes sense as to why it is claimed to be more efficient with a DX system, the limitation being the soil at that point. I doubt it is 4x as efficient, it'd be awesome to set up a real world comparison test though.




     

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