Effect of groundcover on loop performance

Discussion in 'Geothermal Loops' started by mtrentw, Nov 1, 2013.

  1. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    Here begin the musings of an inquisitive mechanical engineer.

    There is lots of discussion and information that loop performance improves in year 2 due to compaction of loops field and corresponding decrease in thermal contact resistance and increased soil conductivity.

    Does anyone have data or anecdotal evidence of how much improvement or effect is realized by improved/increased ground cover versus the bare ground. (this would be a great study for those mechanical engineering students who were on the other day)

    During my first year, the ground was pretty bare and still a lot of settling. Now, in year 2, I have 50% of my loop field which is covered by pretty thick wild grasses, &c. Another half of my loop field is a 50' x 100' plot which was my garden during summer and is now fairly bare after harvest. I have planted a crop of winter wheat, but still a lot of exposed dirt.

    We know that our winter heat recovery is up-welling heat from deep earth to surface. The question becomes what are the R-Values of certain types of ground cover and could changes in land management lead to increased winter time performance. How much of that up-welling heat goes right past the loops to the surface and the sky and how much would a heavy ground cover improve that.

    It seems it would be an easy theory to check by someone who has:
    1. An interior manifolded loop system where you could check temps of individual return lines. (before/after or season 1/season 2)
    2. A loop field design which would allow for one side to be close cropped with the mower and another let to grow dense grasses and foliage.
    3. A wife/significant other that don't mind an area of lawn that is left to go to pot.

    At present I am thinking there would definitely be some increase in performance and am currently considering taking my 5 acres worth of collected leaves and spreading them over my loop field for the winter.

    Appreciate any thoughts.

    best and later,
    Trent
     
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    You could probably do an academic search through the Agricultural side of things as they like to look at heat flux through evaportranspiration, ground cover and so forth. You would look at deep (to them) seasonal ground temperature changes and their delays. I think snow would be best.
     
  3. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    1 word,
    compost. If properly composted material generates heat, it would be a biological heat blanket during the winter. Limiting the effects of convection? from cold winter air on the ground above your loop field. Other than the cost factor, I have wondered why more attention is not paid to the backfill of horizontal loops. Even mixing a slurry of water and natural material would ensure better loop contact than waiting for mother nature to fill and settle more completely.
    Eric
     
  4. Calladrilling

    Calladrilling Member Industry Professional Forum Leader

    Would it be cost effective enough to completely flood the trenches with our water truck to help with settling faster?
    We could send our 3000 gallon water truck out while backfilling.
     
  5. ChrisJ

    ChrisJ Active Member Forum Leader

    The looper that did my horizontal loops put PVC pipes, with holes drilled every 2 ft., in the trenches 2 ft above the black pipes. I sent water down into those pipes this summer adding moisture to the ground around my loops. Dropped the EWT 1.5- 2 degrees. I am curious to see if it will show any difference this winter.

    Looper had me pay for materials and I did most of the drilling of the holes.

    Chris
     
    Last edited: Nov 9, 2013
  6. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Several thoughts

    1) It's hard to get excited by a 1-2*F delta-T for EWT...is that worth any more than a few hundredths delta COP?
    2) Heat from decaying compost would seem to tend to move up, not down toward loop tubes. Snow would insulate soil from bitter cold nights, but also from solar gain on sunny winter days. White color reflects solar heat.
    3) Organic material is absolutely to be avoided beneath any structure depending on soil bearing for support. Organic material decays and shrinks, causing voids, settling, loss of bearing. I would think that organic material deliberately introduced near a horizontal loop field's tubing could cause voids as it shrinks during decay. Any heating related to decay would be minimal owing to lack of oxygen underground.
    4) I like the idea of burying a horizontal loop field in lifts, and watering the heck out of the first couple lifts to cave in voids and promote rapid consolidation.

    Disclaimer: All of the above is based on zero actual experience with horizontal loop fields.
     
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Filling in layers, compacting or watering in appears to be counter-acting fast and efficient loop installs. I rather add a larger loop, like one additional circuit, I would imagine it benefits the homeowner more in the long run. I must admit we have dense clay with a high moisture content, so our horizontals are performing like a charm.
     
  8. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    Someone earlier suggested looking for agricultural research data. I found the attached which demonstrates significant variance between asphalt/concrete, bare ground, and vegetated ground, it appears there is no notable difference between types of ground cover, be it crop, prairie or lawn. This only covered summer season.
     

    Attached Files:

  9. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Do you have the link to the data. It appears to be modeling, not actual measures.
     
  10. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    Good catch Doc. I reviewed and the title of the article is:
    Ground surface temperature simulation for different
    land covers

    If I were a true believer in simulations, I'd be more worried about raising my house on jacks to avoid the 10 meter seal level rise. I'll go look for some real data.
    http://static.msi.umn.edu/rreports/2008/319.pdf
     
  11. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

  12. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    Okay, Maybe not the exact holy grail I was seeking, but a very interesting find on insulating layers. This study was done by some guys at University of Buffalo, so maybe DocJ can go beat on them for more data.

    Summary I get from a read of highly technical 19 pages was: a layer of insulating material (tire derived aggregate(tire chips)) {see picture} can improve heat transfer to loop pipe by 17% in cold climates and 5-6% in moderate climates. They had loop pipe at 1.8 meters depth and varied the top of the insulating layer from 0.1 to 1.2 meters below ground surface. Optimum depth of top of insulation ranged from 0.3 meters to 0.7 meters depending on temperature swings outside.

    I'd expect a backfill plan that involved 2 feet of dirt, 2 feet of tire chips with another 2 feet of dirt would never be economical when the option of digging loops a foot deeper probably has a better payback.

    Full study here which has some good references for more data sources.

    https://ia601207.us.archive.org/4/i...rmanceWithTireDerivedAggregate/Heatmass3p.pdf
     

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  13. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    So the University of Buffalo guys did a study with data from Turkey. Now comes the key question: Did they meassure the effect of tire chips, or did they model it?
    Buffalo guys actually put a loopfield in a couple years ago, I actually bid on it, and lost the bid since I realized how much work it takes to pay attention to detail for the sensor placement, calibrations checks etc. My driller actually won the bid, who never put in a horizontal loopfield before, we will see how it goes….
     
  14. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    The economics would depend on the availability of the tire chips. 8 years ago DE had a huge scrap tire problem. A state funded program leased a huge chipper that created mountains of tire chips, that were given to the septic guys to be used as drain field material. We do not have a tire problem anymore.
    Eric
     
  15. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Been busy, but I have been watching this thread.

    I feel since we did not dig deep enough, in Ternt's yard, to be using the heat from the Earth's core, his loop system is really solar, at least in heating mode. Insulation might be a hindrance.

    Mark
     
  16. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    C'mon Mark,

    You know I'm not looking for lava. I am looking for the warmth of a constant layer 30-40 feet down and 50-60 degrees that is pretty stable as heat flux moves up. My use of the word deep is in comparison to my 6 foot depth and not on geologic scale. The further down, the more influence from this more constant/stable layer. While I'd love the nuclear fusion and subduction zone heating, I still have some sense of realism. I am hopeful that I can stop some heat from running to sky with my dark mulchy leaf piles.

    Since my WEL server consists of me reading gauges and typing, I'll try and start logging a little better and see how low it goes in 2013-2014 winter. I think I got down as low as 29 degrees EWT in the first winter.

    Best and later,

    Trent
     
  17. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Trent:

    I am pulling your leg.

    I am not saying insulation is not a plus, I just "saw" you tending a garden covered with 2' of shredded tires.

    I will read the attached files soon. Off to play Nancy nurse to my best friend.

    Mark
     
  18. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader


    Just to verify, the aggregated tire chip data was modeled modeled in the study!
     
  19. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    There I go again, biting on the modeling data again.
     
  20. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Yes, it is confusing. They used loopdata from Turkey to confirm their model, although they are up to 50% off, and then they plug the tire aggregate into their model.
     

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