Subject: Norman Saunders on Earth Coupling, part 1

From: [email-masked] (Nick Pine)
Date: 9 Mar 1995 09:53:44 -0500

94N16 Earth Coupled Heat Storage and Loss. D93D01 94N15

   Copyright Norman B. Saunders, P. E. 15 Ellis Road, M3, Weston MA 02193
	     may be freely quoted if in context with appropriate credit.

A crawl space is thermally coupled to the earth. Water moving laterally
through earth within ten feet around or below the foundation washes the heat
away. See 94N15 Ground Coupling for Tempering Houses. Heat from non-flowing
water warmer than the space above moves up by vapor rising through the
intersticies of the soil. While heat or lack of it is carried downward 
by rain or surface water percolating down.

Consider however dry sand with conductivity of 0.355 W/m^2K and heat
storage of 355 Wh/m^3K which thereby has a time delay of 1,000 hr/m^2,
or basalt with 2, 2,000 and 1,000. Most rock has similar time delay
with conductivity and storage falling between. Thus in a half year a
temperature change has propagated down about two meters. The sand
under a 1,076 ft^2 floor (100 m^2) for a 10 kelvin change has stored
perhaps 300 kWh, possibly a day's worth of heat. Basalt then would
have stored nearly a week's worth. If the crawl space were filled with
400 barrels of water with space for moving air to circulate, heat
storage is increased by some 500 kWh.

My 93609 Earth Tube Drive and Expected Efficiency 2p+? gives for tubes
at the bottom of a sandy trench 1 m wide 2 m deep and 50 m long 800 kWh
or so of annual storage. This is a bit more than double that calculated
above for the sand under the crawl space.

It thus appears that the annual tempering of an earth tube that does
not also exchange heat with the outgoing ventilation is little better
than that of the uninsulated crawl space that is already there.

Where the store in the crawl space is a cool store, i. e. a place to
reject heat theat is well above earth temperature, ground water flow
is helpful and where it exists, the water store is pointless. However
a vapor barrier is in order.

On the other hand a warm store extending down to the base of the crawl
space needs insulation under it.

At Experimental Manor the crawl space has sometimes been used in summer
to cool the house. This is seldom done because the air though cooled picks
up moisture which decreases comfort more than the cooling increases comfort
[Norman adds a handwritten note: "Put a dehumidifier next to basement drain."]

Some references:

91N28 Ventilation, Earth Tubes, Review
92311 Capturing and Storing Solar Energy
93428 Temperatures and Heat Flow in the Ground
93609 Earth Tube Drive and Expected Efficiency 2p+?
93614 Heat Stored in Ground Around Pipe (equations only)
93615 Heat Flow Between Concentric Cylinders C93428 C93615
      (Programs to perform the calculations)
93D19 Voids in Soil, and Thermal Coupling to the Soil

It will be a while before I have time for continuing quantitative
analysis. Looks as if a minimum for three dimensional heat flow is
temperature, heat capacity, and conductivity for 10 x 10 x 10 points
at half-foot spacing stepped day by day through several years...

Norman

The rest of Norman's earth-coupling notes (he's written about 38
papers on earth-coupling) will appear only in alt.solar.thermal,
coming soon to a site near you, one hopes...

Nick