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groundsource heat-dump solar trickle-charger?!?

Royboy Member Posts: 223

have had a rather extended absence from this site but thinking about this question reminds me what a great bunch of minds & opinions are to be found here ...

OK here's the deal: my home, built in 1985-6. frost-protected floating-slab-on-grade 2 story. 24'x28' radiant slab on first floor (not too common back in 85!). polybutylene tubing (no PEX then) about a third of the way up in the slab. heat from freestanding wood-stove and slab. 9000 DD climate (N WI).

I'll spare the details of decision making but here's the slab insulation plan: 1' sand lift under slab above insulation/water barrier. 4" of foam around edge of slab and sand lift, 4' skirt of 2" foam outward to protect from frost heave, and underslab (undersand, actually) foam 5' in from perimeter (3" at outside edge tapering to 1" near center). this leaves a 14'x18' area in the center of the first floor where there is sand under the slab but no insulation. soil is heavy clay and, in retrospect, my sense is that this insulation strategy was not the best design choice, the ground-coupling would probably keep my house from freezing for a good long time even if unheated, but in normal conditions it makes for a real big heat suck, especially as I get the slab going again each fall.

til recently, slab was heated with wood boiler and off-peak electric. now the wood boiler is out (rusted out perhaps due to lack of O2 barrier in polybutylene??) and I have a new somewhat oversized solar DHW system (96 sf flat panels for a household of 1) in addition to the off-peak electric.

the solar system was intentionally oversized with the idea of trying to put some of the excess summer production into "preheating" the sand/ground under the slab. Bob Ramlow (and others) advocate high-mass solar charged systems which use an approach like this and take advantage of some of our late-summer abundant energy to get a running start into the heating season. the rub with my setup is I have no tubing down in the sand bed - its all up in the slab - and my guess is that I'd be doing a lot more overheating of my living space than heating of that big lump of cool clay under the house. I've thought about putting insulated carpet down on the center part of the house in the summer to resist overheating but that seems a bit clunky and still a challenge to get heat into clay rather than the house.

my latest Goldbergesque idea is the one I'm looking for feedback on. I'm imagining a few tube-in-tube "heat exchanger wands" stuck down through the slab, through the sand & into the clay (an interesting installation challenge, but doable, I think). I have a nice roughly 4'x6' hidden area under my stairs at the center of the house that could serve as the project area. I'm imagining ¾" copper tubes, capped on the bottom, maybe 4' (or more?) long, each with a internal ½" copper feed extending almost to the bottom (this concept inspired by the "butler solar wand" heat exchanger, the name of which I just now remembered.)

a small circ pump sends solar heat down to the bottom via the inner ½" pipe and on its way back up through the outer ¾" pipe would give up heat to the clay. over the course of the summer I am thinking a heat bubble will slowly expand out through the clay and result in a significant reduction of my slab-heating load, especially earlier in the season.

I invite comment on this idea. I'm not laboring under the delusion that this is going to "heat my house" as I'm sure most of the heat will end up flowing away from the house never to be felt again by me. but I am thinking that it could really cut into the annual fall & winter heat suck down into the 45°F clay abyss and thus be a worthwhile use of my extra summer solar BTUs.

whaddya think?



  • zacmobile
    zacmobile Member Posts: 211

    This subject has interested me a lot lately too. These guys seem to have a solid system figured out: http://www.icax.co.uk/interseasonal_heat_transfer.html
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Drake Landing Good

    None of the other interseasonal schemes have worked very well.

    Drake Landing was big, expensive, and complicated, however.

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  • hot_rod
    hot_rod Member Posts: 22,271
    give it a try

    I'll bet Barry would sell you some of that spiral copper, I think it comes from Wolverine.  That way you could bore the hole and screw the tube into the sand bed.  Much like those ground anchors for solar installs.  UPS will ship 9 foot long pipe or tube.  Screw or drive it into the bed ubtil it stops, cut it off and install the inner tube.  Like driving bridge pylons :)

    The key will be how much HX area you can get into the bed.  You can easily calculate how much solar you harvest above an beyond the DHW to calculate the excess to drive into the ground.

    Can you bore multiple holes without getting into the PB in the slab?  Maybe just remove that section of slab and install a big copper coil HX?

    If you do drill bore holes drill them at a 45 or so angle to spread the HX dimension as wide as possible.

    I watched a DX ground source system get installed that way.  A Bobcat with a drill attachment dug a 4 foot deep pit then bored holes around the pit at a angle to install a bunch of 100 foot long copper loops into the ground.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • zacmobile
    zacmobile Member Posts: 211
    edited June 2011

    It obviously doesn't have to be as expensive or complicated as Drake landing, the main reason for the expense of that system was the scale; they're basically running a whole village off of it. I think the key for maximizing your ground solar storage effectiveness is tubing density, if you look at any of the geo-solar products/systems on the market around the world the tubing density is very high compared to say a geoexchange ground loop. Contrary to popular belief  heat doesn't move around much once you put it in the ground, granted you wouldn't want to pump heat from your fossil fuel boiler but for renewables like solar I think interseasonal ground storage is the next big thing.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    I'm not sure I follow

    how does heat "not move around much" when you put it in the ground?

    ground does conduct... that's why drake landing is insulated. right?

    once you introduce a temperature differential the heat wants to equalize.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Karl_Northwind
    Karl_Northwind Member Posts: 139

    Hey Roy,

    it's been a while.

    That sounds like a fun project.  If you're going to use solar fluid for the heat transfer I would probably use type K copper pipe with a spun closure end (think the end of the cheaper copper pex manifolds) to allow driving.  some sort of grease or something might help too. possibly a Hammer drill with a hammer only function would allow driving without screwing up the tube. 

    if you're using solar fluid only, I would use all copper.  if you are using heating water or sometihing else I would use steel if you can.  Just thinking about being able to drive it without destroying it.  I know what the soil is like in your area.  not fun.

    I think most of the time storing heat in the ground is a marginal idea, but when the heat is free and available, use it.  I wish I had laid a couple of loops in the groundwater laden gravel below the insulation in my basement floor for summer heat dump or cooling. and wish I had laid a couple of long loops in the 400' long 5' wide by 6' deep trench I had to dig to run water and power in.  could have used them for cooling or GSHP down the road. 

    oh well.

    let us know how it goes.  we'll be up over the 4th.  you gonna be around?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    A boring job...

    Roy, I did the summer heat dump thing for my array. I did it somewhat different, in that I used 1-1/2" copper outer with 3/4" copper dip tube, and it worked fantastic in keeping the array from stagnating. It didn't, however, build up a "heat bubble" due to the proximity of ground water in my local soil structure. Too much wicking away of thermal energy, but my soil structure is different than yours and it may very well be beneficial. My "heat exchanger" was only 10' long, and I grouted it into place using a really wet sand/grout mixture. It was dumping the heat from 84 square feet of 45 degree angled collectors during peak summer insolation.

    I drilled my vertical bore holes with a self built device made of unistrut, that incorporated a HydroBore drilling tool, and a HoleHawg drill motor for a power source. You can use a water vacuum to suck up the drill cuttings. I will try and get a picture of the "rig" later today.

    I will be using this rig at Hydronicahh to drill some horizontal loops to explore the same potential, except in a horizontal mode as opposed to a vertical mode some time in the future. Stay tuned.

    The biggest problem you will run into is getting through the sand. You may have to "case" the first few feet of your drilling operation, otherwise you will wash out a lot of the intentionally placed sand. Again, not something that is not insurmountable, but something to consider.

    If necessity is the mother of invention, then I am the father :-)


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  • Royboy
    Royboy Member Posts: 223
    thanks ...

    for thoughts here from all. was just going to reply to Karl (hi Karl! see you this weekend at the energy fair, no doubt, and yes I''ll be around over the 4th. hope your place is coming along ...) and say that I am imagining drilling rather than driving the heat exchanger(s) into the ground.

    and then I see your post, Mark, which looks significantly like you've tried pretty much the same concept I have in mind. I have a drainback system, so stagnation & heat dump not the issue. and my soil is pretty much pure clay. it can be wet, but not really much, if any water flows through it due to its monolithic density. I'd love to get a better sense of how heat would behave in this - would the rate of conduction just pull it all away and pretty much equalize, as Rob suggests, or is it possible that some sort of thermal bubble would build up? some sort of temp gradient would result, I'm sure, but no idea how steep it would be. and I have to say that all the various college math courses that would help me think about this appear to be long gone from my head (like heat into clay??).

    there's a guy, Robert Aram, who has presented at solar conferences on soil conductivity and subslab insulation strategies. I'm going to try to chase him down and see if he has a gut sense of how he'd expect this to behave. (he'll be at the MREA Energy Fair this weekend, I hear)

    so Mark - did you do any monitoring of soil temps to conclude there was no "heat bubble" or was that from just watching supply/return temps on the fluid going into the heat exchanger? and I'm guessing you weren't injecting the heat under a structure, or were you?

    back to the drilling. what you did, Mark, sounds real close to what I am imagining. I was going to drill a hole in the slab (once I find an IR camera to help pinpoint tube locations). to get through my sand lift I was going to oversize and case the hole with PVC (a technique I've used before using a shop vac wand to suck out sand ahead of the PVC, drive PVC a little further, suck, drive, suck, etc). then I was thinking of either power drilling or using a hand auger to bore a slightly oversized hole the rest of the way. I was not thinking to go as deep as you did - maybe 5' overall - which would be 1' of sand and 4' of clay. what did you use for "grout"? - I was thinking about just a clay slurry, but would be better to have something that's not going to dry/shrink and pull away from the HX tube.

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    You are welcome!

    You asked;so Mark - did you do any monitoring of soil temps to conclude there was no "heat bubble" or was that from just watching supply/return temps on the fluid going into the heat exchanger? and I'm guessing you weren't injecting the heat under a structure, or were you?

    I drilled another hole 5 feet away from the HX hole, and had sensors looking at the soil temperatures through the walls of 1/2" copper every 2'. I could build the bubble during the day, but it would dissipate at night. And you are correct, it was NOT under a structure. Just out in my back yard.

    what did you use for "grout"? I literally went out and picked up a grout mix from the local HD, mixed it REAL wet so it would flow easily, dumped it down hole, and used a 1/2" piece of conduit to work it into the hole around the pipe annulus.

    I used 1.5" because of the increase of surface area, and it also made getting the gozinta and the gozouta easily into the 1.5" cap with silver soldering. You may want to consider the same thing, because in order for it (the HXer) to work, you need to introduce the incoming fluid in the bottom of the HXer, requiring a dip tube. If you drill a 2-1/2" hole, and sleeve it with 2" copper, you can slide the 1-1/2" HXer through it no problemo... (Famous last words.)

    I used my large Rigid shop vac to get the cuttings that were too heavy for the water. Boy does THAT machine suck :-)

    Here is a link to the drilling tool.


    Best of luck with your project.


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  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    edited June 2011
    Rig photos...

    I originally had the ability, through a come along, to exert downward pressure on the bit. In my case, it was not necessary. Gravity worked just fine.

    I cut the 3/4" drill pipe into 5 foot sections. My original intent was to go to 20' deep, but due to running into water bearing strata (pea gravel) I was only able to go to 10' deep. It was my intent/attempt to build a bubble of heat, that could then be extracted via a GSHP system, thereby increasing the efficiency of the GSHP system. Obviously, under the right soil conditions, it is doable, but my soil conditions are not conducive to long term energy storage.

    I built the "rig" using typical UniStrut components, including their trolley fittings to accommodate the need for raising and lowering the drilling assembly. The intent was to be able to easily and portably move the rig into a dwelling, and either drill vertically (7' tall rig) or horizontally, avoiding contact with close aquifers. Be prepared for a muddy mess. You might want to schedule it for when the wife is out of town :-) It is also helpful to have an assistant on hand, because when you get rolling, you'd best not stop. The assistant can be handling the waste products while you concentrate on making hole.

    I also developed a method of removing the solids from the water stream, and a means of being able to recycle the water back into the hole for washing the cuttings out, but as I said, pea gravel is not conducive to being washed out of the hole, and I had to resort to vacuum removal, and there are certain limitations to that.

    I am thinking that the whole process could be "enhanced" with the use of compressed air, except that I don't own a compressor large enough (CFM wise) to handle the load.



    EDIT: I turned the photos 90 degrees on my PC, but they came through sideways. Sorry for any kinked neck muscles :-)


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  • Royboy
    Royboy Member Posts: 223
    one more ...

    any suggestion (Mark or anyone) on reliable, cost-effective ground temp monitoring gear?

    I put the temp sensors for 3 of these:


    down CPVC tubes into a sand bed once. as far as I know they are still working after several years, but they seemed a little lightweight in construction. @ about $20/sensor the price seemed about as good as I was likely to do.

    anyone fond of another device?
  • Bob Gagnon plumbing and heating
    Excess Heat

    Why not a bigger tank, do you have room? I built a 1500 gallon tank and get 100% of my domestic hot water year round, it seems like a better way to use it.

    Thanks, Bob Gagnon
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Royboy
    Royboy Member Posts: 223
    not a lot of room

    beyond what I already have, which is a 120 gallon tank which feeds a 50 gallon electric water heater with the elements on switches. 50 gallon tank is priority for the solar energy, when its satisfied the 120 gallon tank gets heated. with only 96 sf of collectors it seems like this would be enough storage.

    thanks for the thought, though, Bob.
  • SunnySideofLIfe
    SunnySideofLIfe Member Posts: 3
    Solar Heat Articial


    I didn't have a chance to read this whole thread, but thought this articial could be of interest.  It was published in the mag Sun, Wind & Energy.

    Happy Reading:)
This discussion has been closed.