Your soil and ground conditions refer to the type of soil
available in your area as well as the mean earth temperature in your area. Heat transfer principles apply here and
will tell you how long or deep your ground loop must be. There are some areas where GeoThermal
may not even be applicable.
In order to properly design a GeoThermal system there are
several items that you must be aware of right underneath your feet!
Soil temperature
Ground heat capacity
Ground thermal conductivity
Soil Temperature
The variation in soil temperature is a lot greater towards
the surface than below the surface.
As you move further down, the temperature of the Earth remains fairly
constant. The figure below shows
that at about 28 feet below the surface, the Earth’s temperature remains
constant.
The temperature of the Earth at this depth corresponds
roughly to the water temperature of groundwater wells. This is sometimes referred to as the
“mean earth temperature.” A map of
the “mean earth temperature” is shown below:
Ground Heat Capacity
Heat capacity which is also known as specific heat tells us
about the ability of a substance to store heat energy. The greater the heat capacity, the more
heat it can gain per unit rise in temperature, conversely, the more heat it can
lose per unit fall in temperature.
Heat capacity of soil is dependent on how dry or wet the
soil is. Dry soil has a heat
capacity of approximately 0.20 BTU per pound per degree F while moist or
saturated soils have a heat capacity of approximately 0.25 BTU per pound per
degree F. Dry soils will have
greater seasonal temperature swings because their lower heat capacity causes
their temperature to rise or fall more than wet soils for a given amount of
heat energy gained in the spring or lost in the fall.
Ground Thermal Conductivity
Thermal conductivity tells us about the rate at which heat
will be transferred between the ground loop and the surrounding soil for a
given temperature gradient. The
thermal conductivity of the soil and rock determines the length of pipe that
will be required for a GeoThermal system.