Ground Characteristics; Soil Analysis - Bosch Geothermal Heat Pumps Applications Manual

Applications Manual

9.2 Ground Characteristics

The ground moves heat slowly and has high heat
storage capability, so its temperature changes very
slowly depending on the depth in the ground where a
measurement is taken. As a consequence of this low
thermal conductivity, the soil can transfer some heat from
the cooling season to the heating season. Heat absorbed
by the ground during the summer gets used in the winter.
This yearly, continuous cycle between the air and the soil
temperature results in a thermal energy potential that can
be utilized to heat or cool a structure very ef ciently with
a Bosch geothermal heat pump system (Fig. 35).
Fig. 35 Ground temperature characteristics
Courtesy of Geo4VA, "Seasonal Temperature Cycles", Virginia Tech, http://
www.geo4va.vt.edu/A1/A1.htm, accessed September 2012
Soil type can also in uence the performance of geothermal
heat pump system installations. Moist soils such as clay
and loam are best (for closed-loop systems). Dry, sandy
soils, in contrast, contain millions of tiny air pockets which
insulate against the heat-transfer process. In these cases,
the installer will need either to extend the piping loop (up
to 30 percent) or back ll the bottoms of the trenches with
grout or a better soil.
Another ground (thermal) characteristic is that a few feet
of surface soil insulates the ground and groundwater
below, minimizing the variation in soil temperature in
comparison with the temperature in the air above the
ground (Fig. 36). This thermal resistivity uctuation helps
shift the heating or cooling load of a structure to the
season where it is needed. Thus, the ground is warmer
than the ambient air in the winter and cooler than the
ambient air in the summer.
Bosch Thermotechnology Corp.
Fig. 36 Ground temperature annual variation
Courtesy of American Society of Heating, Air-conditioning and Refrigeration
Engineers Inc., www.ashrae.org

9.3 Soil Analysis

The thermal conductivity of the soil must be analyzed or
estimated from local data for any particular geothermal
heat pump system installation ground heat exchanger.
Resistance to heat transfer in the soil around a buried
pipe is a function of the thermal conductivity of the soil,
the number, diameter and con guration of the pipes in
the trench or bore, and the distance between trenches or
boreholes.
Thermal conductivity testing is typically a good approach
in the soil analysis if possible. Cost often prohibits this
type of test. In any event, composition and soil and rock
properties must be considered when designing the ground
.
loop. Soil with good heat transfer properties requires less
piping to absorb heat than soil with poor heat transfer
abilities. Soil amount available also contributes to system
design. Hard rock or soil too shallow to trench will often
require vertical ground loops. Ground or surface water
availability is also a major deciding factor for what type of
ground loop to use. Bodies of surface water can be used
as a repository for coils of piping for a closed-loop system.
Thermal conductivity of soil depends on the type of soil.
This means the percentage of sand, silt and clay, the
density of the soil, and the water content of the soil.
Density does not typically change signi cantly for a given
soil type, but the water content can change over time,
depending on the soil type and precipitation for that
particular locale.
Sandy soils do not hold water well and tend to exhibit
large swings in water content and thermal conductivity
depending on the season and precipitation.
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Data subject to change