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re: geothermal driveway heating
9 oct 2004
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




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