Geothermal driveway heating

Discussion in 'Home Power and Microgeneration' started by Dean Carpenter, Oct 9, 2004.

  1. A friend is in construction of his house right now, and plans to install a
    home-grown geothermal heating system in the driveway to keep snow off. I'm
    not convinced it will work, but since we're planning to build next year, if
    it does work, it sounds very cool, and I'm going to want to do it too :)

    Basically, he's planning on putting down pex (I think ?) tubing before the
    asphalt is laid down, so it's embedded in the asphalt. That will lead to
    several loops dug down below the frost line for heat transfer.

    Hrm - not sure if he's going vertical or wide horizontal loops. It's a
    closed loop system, with a small pump to move the glycol solution through.
    He hasn't done any calculations at all, just "it should work" :) It sounds
    like it will, but I don't know how to calculate if it will or not.

    I thought of using a manifold system, with two main pipes along the sides
    of the driveway and ladder-like rungs of pipe between them. Each run would
    have several loops, to ensure sufficient time for heat transfer.

    He's against that - worried about the T-joints leaking. He wants to run
    one or two looong loops. I'm thinking that at the end of the loop, all the
    heat will have already been removed from the fluid, and it won't be
    effective. So you would get one end nice and clear of snow, the other end
    as if the system was off.

    So, questions are ... Is this even feasible ? Would there be enough heat
    transferred to keep the driveway above freezing point ? I would think so,
    sort of, since below-ground is a fairly static 50degF or so, isn't it ?
    And what would the best layout be ? Long loop, or manifolds ?

    Anything else ?

    D.
     
    Dean Carpenter, Oct 9, 2004
    #1
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  2. Dean Carpenter wrote:
    ....
    > Basically, he's planning on putting down pex (I think ?) tubing before the
    > asphalt is laid down, so it's embedded in the asphalt. That will lead to
    > several loops dug down below the frost line for heat transfer.

    ....
    > So, questions are ... Is this even feasible ? Would there be enough heat
    > transferred to keep the driveway above freezing point ? I would think so,
    > sort of, since below-ground is a fairly static 50degF or so, isn't it ?
    > And what would the best layout be ? Long loop, or manifolds ?


    Sure it's feasible, takes a lot of heat (of course) but there are plenty
    of people selling systems just like this.

    A quick google search found me these links...
    http://www.warmzone.com/SnowMelting.asp
    http://www.slantfin.com/heating_guide/main.html
    http://www.pmmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2379,4510,00.html
    http://www.hpac.com/member/feature/1998/9812/9812hayden.htm

    Anthony
     
    Anthony Matonak, Oct 9, 2004
    #2
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  3. Dean Carpenter

    Guest

    Dean Carpenter <> wrote:

    >A friend is in construction of his house right now, and plans to install a
    >home-grown geothermal heating system in the driveway to keep snow off...
    >Basically, he's planning on putting down pex (I think ?) tubing before the
    >asphalt is laid down, so it's embedded in the asphalt. That will lead to
    >several loops dug down below the frost line for heat transfer... It's a
    >closed loop system, with a small pump to move the glycol solution through.
    >He hasn't done any calculations at all, just "it should work" :) It sounds
    >like it will, but I don't know how to calculate if it will or not.


    Sounds like this could work in principle, but it might require
    a LOT of water pumping and tubing. You can find some clues here:

    http://www.geothermie.de/egec-geothernet/ci_prof/america/usa/pavement_snow_melting.htm

    They describe a Japanese system in which 60 F water circulates through
    a heat exchanger under a sidewalk, melts off snow, cools to 45 F, then
    gets sprinkled onto the road next to the sidewalk.

    And a $3 million Swiss system that melts snow off a 14K ft^2 bridge by
    storing about 20% of the summer's heat (512K million Btu) from tubing
    below the asphalt surface in about 2 million ft^3 of sandstone via 91
    200' boreholes with heat exchangers :) The store loses 35% of the heat
    by wintertime, when the rest is recovered to melt snow off the bridge.

    It looks like most snow melting systems aim at 100 Btu/h-ft^2 min, enough
    to melt about 1" of snow per hour. You might sprinkle the driveway and
    collect water it in a trench along each side. A few leaks might help,
    since the main mechanism for upwards heatflow in soil is evaporation
    from lower soil layers, upward vapor migration through pores, and
    condensation in layers above.

    Nick
     
    , Oct 9, 2004
    #3
  4. Dean Carpenter

    daestrom Guest

    <> wrote in message
    news:ck8qbn$...
    > Dean Carpenter <> wrote:
    >
    >>A friend is in construction of his house right now, and plans to install a
    >>home-grown geothermal heating system in the driveway to keep snow off...
    >>Basically, he's planning on putting down pex (I think ?) tubing before the
    >>asphalt is laid down, so it's embedded in the asphalt. That will lead to
    >>several loops dug down below the frost line for heat transfer... It's a
    >>closed loop system, with a small pump to move the glycol solution through.
    >>He hasn't done any calculations at all, just "it should work" :) It
    >>sounds
    >>like it will, but I don't know how to calculate if it will or not.

    >
    > Sounds like this could work in principle, but it might require
    > a LOT of water pumping and tubing. You can find some clues here:
    >
    > http://www.geothermie.de/egec-geothernet/ci_prof/america/usa/pavement_snow_melting.htm
    >
    > They describe a Japanese system in which 60 F water circulates through
    > a heat exchanger under a sidewalk, melts off snow, cools to 45 F, then
    > gets sprinkled onto the road next to the sidewalk.
    >
    > And a $3 million Swiss system that melts snow off a 14K ft^2 bridge by
    > storing about 20% of the summer's heat (512K million Btu) from tubing
    > below the asphalt surface in about 2 million ft^3 of sandstone via 91
    > 200' boreholes with heat exchangers :) The store loses 35% of the heat
    > by wintertime, when the rest is recovered to melt snow off the bridge.
    >
    > It looks like most snow melting systems aim at 100 Btu/h-ft^2 min, enough
    > to melt about 1" of snow per hour.


    Darn!!! So when it snows over 6"/hour here (a near weekly occurance in
    Jan/Feb), I'm out of luck :-/

    Oh, well. Time to PM the snowblower anyway as it's already October.

    daestrom
     
    daestrom, Oct 9, 2004
    #4
  5. Dean Carpenter <> wrote:
    >Basically, he's planning on putting down pex (I think ?) tubing before the
    >asphalt is laid down, so it's embedded in the asphalt. That will lead to
    >several loops dug down below the frost line for heat transfer.


    I'd think he'd just freeze the ground around the loops "below the
    frost line". There's nothing special about the "frost line", it's
    just where the ground doesn't freeze because it's insulated from the
    surface...
     
    William P.N. Smith, Oct 10, 2004
    #5
  6. Dean Carpenter

    Richard Guest

    "Dean Carpenter" <> wrote in message
    news:N_E9d.25915$...
    > A friend is in construction of his house right now, and plans to install a
    > home-grown geothermal heating system in the driveway to keep snow off.


    I think this is more correctly called a Ground Source Heat Pump. Geothermal would indicate you have
    some sort of naturally occuring hotspot or hotspring. Boy to live in Iceland. :)

    Home Power Magazine issue #98 (Dec 2003/Jan 2004) has several pages on using residual ground heat,
    bodies of water, etc as a heating medium and gives diagrams on how to work it out. Also includes
    some tables on costs, etc. The end of the article suggests these URL's for additional information:

    Geo Thermal Heat Pump Consortium = http://www.geoexchange.org/
    Earth Energy Society of Canada = http://www.earthenergy.ca/
    IEA Heat Pump Center = http://www.heatpumpcentre.org/
    European Heat Pump Association = http://www.ehpa.org/
    International Ground Source Heat Pump Association = http://www.igshpa.okstate.edu/

    If you want a copy of that Home Power Magazine, I have it in .PDF format (Adobe Reader) and can
    e-mail it to you if you wish. Set up a temporary e-mail address at Yahoo or Hotmail.

    Hope this helps! I know of a housing development in western Canada that uses ground heat to heat
    homes and works extremely well.
     
    Richard, Oct 10, 2004
    #6
  7. Dean Carpenter

    Guest

    <> wrote:

    >Check out this link to see why it's not even feasible with a full
    >geothermal hot water system:


    http://bama.ua.edu/~geocool/Survival Kit _FAQ12.htm

    From: "nick pine"
    To: <>
    Cc: <>
    Subject: FAQ #12
    Date: Tue, 12 Oct 2004 07:22:51 -0400

    Gentlemen,

    Your FAQ #12 portrays snow melting as difficult and expensive,
    even when using compressors. Are you familiar with the Japanese
    system with 60 F water entering heat exchangers under sidewalks
    and emerging at 45 F for sprinkling over the street?

    The water might come from a well, with a stone-filled trench
    alongside a driveway with a layer of poly film under groundcloth
    to collect and reheat most of it. Sprinkling it over the driveway
    would eliminate the cost of the driveway heat exchanger and
    the thermal lag you mention.

    If it's damp from a few trench leaks, the soil below the trench
    may have a substantial thermal capacitance (50 Btu/F-ft^3) and
    a high effective conductance for upward heatflow (perhaps 20
    Btu-in/h-F-ft^2), with water evaporating from lower soil layers
    and condensing in upper layers and keeping a non-porous driveway
    surface warmer all winter.

    As Steve Strong says "What exists must be feasible" :)
    Why don't we do this in the US?

    Nick

    (No response yet.)
     
    , Oct 11, 2004
    #7
  8. Dean Carpenter

    Me Guest

    In article <ckdtv2$>,
    wrote:

    > <> wrote:
    >
    > >Check out this link to see why it's not even feasible with a full
    > >geothermal hot water system:

    >
    > http://bama.ua.edu/~geocool/Survival Kit _FAQ12.htm
    >
    > From: "nick pine"
    > To: <>
    > Cc: <>
    > Subject: FAQ #12
    > Date: Tue, 12 Oct 2004 07:22:51 -0400
    >
    > Gentlemen,
    >
    > Your FAQ #12 portrays snow melting as difficult and expensive,
    > even when using compressors. Are you familiar with the Japanese
    > system with 60 F water entering heat exchangers under sidewalks
    > and emerging at 45 F for sprinkling over the street?
    >
    > The water might come from a well, with a stone-filled trench
    > alongside a driveway with a layer of poly film under groundcloth
    > to collect and reheat most of it. Sprinkling it over the driveway
    > would eliminate the cost of the driveway heat exchanger and
    > the thermal lag you mention.
    >
    > If it's damp from a few trench leaks, the soil below the trench
    > may have a substantial thermal capacitance (50 Btu/F-ft^3) and
    > a high effective conductance for upward heatflow (perhaps 20
    > Btu-in/h-F-ft^2), with water evaporating from lower soil layers
    > and condensing in upper layers and keeping a non-porous driveway
    > surface warmer all winter.
    >
    > As Steve Strong says "What exists must be feasible" :)
    > Why don't we do this in the US?
    >
    > Nick
    >
    > (No response yet.)
    >


    That is all well and good, but if the outside Air temp is lower than
    32F then sprinkling the water on the roads will leave a sheet of ICE,
    that vehicles would find very hard to navigate. Your assumption that
    your melting snow in an above freezing air temp, is a bit misguided.

    In the college I went to, all the sidewalks were also Utility vaults for
    the Building Steam lines and stayed snow and Ice free even at -30F.

    Me
     
    Me, Oct 11, 2004
    #8
  9. Dean Carpenter

    Guest

    Me <> wrote:

    >...if the outside Air temp is lower than 32F then sprinkling the water
    >on the roads will leave a sheet of ICE...


    A thin film that evaporates or sublimates, on a carefully sloped roadway.

    >Your assumption that your melting snow in an above freezing air temp,
    >is a bit misguided.


    We might call that "your assumption" :)

    Nick
     
    , Oct 11, 2004
    #9
  10. Dean Carpenter

    Guest

    Astro <> wrote:

    >"Latent heat of sublimation (Ls): Refers to the heat lost or gained by
    >the air when ice changes to vapor or vice versa. Ls=2833 Joules per
    >gram (J/g) of water or 680 calories per gram (cal/g) of water. "


    Sounds like you own a reference book.

    >So this will cool down the driveway even more...


    I don't think that's a big deal, since evaporation will happen very slowly
    during snow melting at 32 F to cold dry air, and the final layer of water
    or ice (much less likely, on 50 F pavement) will be very thin. Perhaps you
    can tell us how many Btu/h a square foot of 32 F water would lose on an
    average Jan day in Phila, with w = 0.0025.

    >...we're talking about lots of energy required to keep the driveway ice free.


    That Japanese groundwater system has no heat pump.

    Nick
     
    , Oct 12, 2004
    #10
  11. Dean Carpenter

    Guest

    daestrom <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:

    >> It looks like most snow melting systems aim at 100 Btu/h-ft^2 min, enough
    >> to melt about 1" of snow per hour.


    >Darn!!! So when it snows over 6"/hour here (a near weekly occurance in
    >Jan/Feb), I'm out of luck :-/


    http://www.geothermie.de/egec-geothernet/ci_prof/america/usa/pavement_snow_melting.htm

    ....describes a Japanese system in which 60 F water circulates through
    a heat exchanger under a sidewalk, melts off snow, cools to 45 F, then
    gets sprinkled onto the road next to the sidewalk.

    The batch simulation below seems to indicate that a groundwater system with
    a 2'x2' trench on one side of a driveway might keep up with 10"/hour of snow,
    not counting the useful energy in the original trenchful of water. It seems
    to do surprisingly well, with a 3.3 hour layer time constant and 0.1 hour
    timesteps. The 10" of snow over a 10' wide x 1' long strip of driveway is
    like 53 pounds of ice, which requires 7680 Btu to melt, with no heat loss
    to the outdoors (in this first-order model.) The first 4" layer of soil below
    the trench has a heat capacity of 1361 Btu/F, so it (alone) can supply the
    snow melting energy with a 7680/1361 = 5.6 F temperature drop, which is close
    to the final temp drop in the simulation.

    We might keep the soil under the trench damp in wintertime by measuring
    its lengthwise conductance and automatically adding water as needed with
    a solenoid valve when the soil conductance becomes too low.

    Nick

    20 SNOWDEPTH=10'(inches)
    30 SNOWDENSITY=6.4'(lb/ft^3)
    40 DRIVEWIDTH=10'feet
    50 MELTLOAD=144*SNOWDEPTH/12*SNOWDENSITY*DRIVEWIDTH'(Btu)
    60 TG=55'deep ground temp (F)
    70 GC=20'damp soil conductivity (Btu-in/h-F-ft^2)
    80 CG=50'damp soil heat capacity (Btu/F-ft^3)
    90 RI=24'trench radius (inches)
    100 THICKNESS=4'layer thickness (inches)
    110 FOR LAYER = 0 TO 10'(10 is deepest)
    120 RLAYER=RI+LAYER*THICKNESS/2'mean layer radius (inches)
    130 SLAYER=3.14159*RLAYER'mean layer surface (ft^2)
    140 R(LAYER)=THICKNESS/GC/SLAYER'layer resistance (h-F/Btu)
    150 VLAYER=SLAYER*THICKNESS/12'layer volume (ft^3)
    160 C(LAYER)=VLAYER*CG'layer capacitance (Btu/F)
    170 TEMP(LAYER)=TG'initialize layer temps (F)
    180 NEXT LAYER
    190 DT=.1'timestep (h)
    200 TEMP(0)=32'trench temp (F)
    210 FOR LAYER = 0 TO 9
    220 IF LAYER=0 THEN Q=0:GOTO 240
    230 Q=-(TEMP(LAYER)-TEMP(LAYER-1))/R(LAYER-1)*DT'heatflow out of layer (Btu)
    240 Q(LAYER)=Q+(TEMP(LAYER+1)-TEMP(LAYER))/R(LAYER)*DT'flow into layer (Btu)
    250 TEMP(LAYER)=TEMP(LAYER)+Q(LAYER)/C(LAYER)'new layer temp (F)
    260 NEXT LAYER
    270 T=T+DT'elapsed time (h)
    280 ICEMELT=ICEMELT+Q(0)'total ice melting energy (Btu)
    290 IF ICEMELT<MELTLOAD GOTO 200'melt more ice...
    300 PRINT SNOWDEPTH,T
    310 TEMP(0)=32'trench temp (F)
    320 FOR LAYER=0 TO 10'final temp distribution in layers
    330 PRINT LAYER,TEMP(LAYER)
    340 NEXT LAYER

    snow depth melting time
    (inches) (hours)

    10 1

    layer # layer temp (F)

    0 32
    1 50.12458 <--This barely uses the first layer's energy...
    2 54.31883
    3 54.92732
    4 54.99355
    5 54.9995
    6 54.99996
    7 55
    8 55
    9 55
    10 55
     
    , Oct 12, 2004
    #11
  12. Dean Carpenter

    Astro Guest

    just saw your reference to the paper on snow melting. Missed that the
    first time through.

    So based on your reference, it's actually 78000 BTU/hour needed for a
    system in New York based on the sample driveway I gave (4x15 meters).
    That's a HUGE amount of energy.

    Now, reading the article about the system in Japan, it says that they take
    60F water and run it through a heat exchanger under the road/sidewalk.
    They then sprinkle the cooled water on the adjacent roadway (as you noted).

    The applicability of this to the original case in question, I believe, is
    nil. They use a continuous supply of fresh 60F water to melt the snow. The
    article notes that the coole water is 45F. That's after a single
    runthrough.

    Now imagine if that 45F water were going back to the guy's pipes in his
    under-yard energy store. Maybe it will pick up much of the ground
    temperature, but it's got to drop it some. Let's just say that it drops it
    brings it back to 0.1F of the ground temperature. Now you've got 59.9F
    water after a single circulation. You circulate the water every minute,
    for example. So the temperature of your ground is dropping 6F per hour. In
    5 hours, the ground will be freezing. This is all totally hypothetical. I
    don't know what the actual chilling factor will be. But the fact is, it
    will be chilling and eventually the ground will cool to the point that the
    system is in a state of near equilibrium and no water will be freezing.

    Maybe, just maybe, he'll get lucky and the solar gain will pump the
    temperature back up during the day enough to hold slow down the cooling
    factor.

    Ultimately, for this guy's system, only time will tell. Right now, this is
    all just mental masturbation. Nonproductive but fun!
    :)
     
    Astro, Oct 13, 2004
    #12
  13. Dean Carpenter

    Eric Tonks Guest

    Pex buried in hot asphalt may be a problem. The asphalt is over 400 degrees
    F, and may be too hot and damage if not destroy the plastic pipe. Check out
    the maximum heat it can tollerate, or if it is softened, the compaction
    rollers may flatten it. You may have better results with rigid foam covered
    with sand with the pipe covered with more sand then asphalt over this -- not
    sure if it will work, just a guess.

    "Dean Carpenter" <> wrote in message
    news:N_E9d.25915$...
    > A friend is in construction of his house right now, and plans to install a
    > home-grown geothermal heating system in the driveway to keep snow off.

    I'm
    > not convinced it will work, but since we're planning to build next year,

    if
    > it does work, it sounds very cool, and I'm going to want to do it too :)
    >
    > Basically, he's planning on putting down pex (I think ?) tubing before the
    > asphalt is laid down, so it's embedded in the asphalt. That will lead to
    > several loops dug down below the frost line for heat transfer.
    >
    > Hrm - not sure if he's going vertical or wide horizontal loops. It's a
    > closed loop system, with a small pump to move the glycol solution through.
    > He hasn't done any calculations at all, just "it should work" :) It

    sounds
    > like it will, but I don't know how to calculate if it will or not.
    >
    > I thought of using a manifold system, with two main pipes along the sides
    > of the driveway and ladder-like rungs of pipe between them. Each run

    would
    > have several loops, to ensure sufficient time for heat transfer.
    >
    > He's against that - worried about the T-joints leaking. He wants to run
    > one or two looong loops. I'm thinking that at the end of the loop, all

    the
    > heat will have already been removed from the fluid, and it won't be
    > effective. So you would get one end nice and clear of snow, the other end
    > as if the system was off.
    >
    > So, questions are ... Is this even feasible ? Would there be enough heat
    > transferred to keep the driveway above freezing point ? I would think so,
    > sort of, since below-ground is a fairly static 50degF or so, isn't it ?
    > And what would the best layout be ? Long loop, or manifolds ?
    >
    > Anything else ?
    >
    > D.
     
    Eric Tonks, Oct 13, 2004
    #13
  14. Dean Carpenter

    biGPete

    Joined:
    Oct 7, 2010
    Messages:
    1
    This system is wonderful, I have snow melting in my driveway and it really works wonders. No more backbreaking shoveling jobs all I have to do is use the sensor and the snow melts right away.
    http://www.usheatingsystem.com
     
    biGPete, Oct 7, 2010
    #14
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