Maker Pro
Maker Pro

Geothermal driveway heating

D

Dean Carpenter

Jan 1, 1970
0
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.
 
A

Anthony Matonak

Jan 1, 1970
0
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
 
Dean Carpenter said:
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
 
D

daestrom

Jan 1, 1970
0
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
 
W

William P.N. Smith

Jan 1, 1970
0
Dean Carpenter said:
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...
 
R

Richard

Jan 1, 1970
0
Dean Carpenter said:
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.
 
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: <[email protected]>
Cc: <[email protected]>
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.)
 
M

Me

Jan 1, 1970
0
http://bama.ua.edu/~geocool/Survival Kit _FAQ12.htm

From: "nick pine"
To: <[email protected]>
Cc: <[email protected]>
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 said:
...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
 
Astro said:
"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
 
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
 
A

Astro

Jan 1, 1970
0
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!
:)
 
E

Eric Tonks

Jan 1, 1970
0
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.
 

biGPete

Oct 7, 2010
1
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
 
Top