Soil & Groundwater Temperature - Bosch Geothermal Heat Pumps Applications Manual

5 4 | Bosch Geothermal Heat Pumps
9.5 Soil & Groundwater Temperature
The average soil/groundwater temperature of the upper
100 to 200 feet in most places in the continental U.S.
is equivalent to the average ambient air temperature of
approximately 60°F to 74°F. The map shown (Fig. 40)
indicates the approximate groundwater temperatures
in the U.S. There are seasonal variations to the soil/
groundwater temperature of approximately 20°F
approaching a constant temperature at a depth of
approximately 30 feet where it is equivalent to the
earth's normal thermal gradient. The undisturbed ground
temperature will typically remain constant throughout the
year below 30 ft. Above 30 ft. the ground temperature will
typically change with the season.
It is this constant soil/groundwater temperature which
Bosch geothermal heat pump systems utilize for heating
47°F
52°F
52°F
57°F
62°
F
Fig. 40 Approx. ground water temperatures
Courtesy of epa.gov., accessed September 2012
Data subject to change
42°F
42°F
42°F
52°F
67°
F
and cooling. Even if soil temperatures rise to 90°F during
the summer or even 50°F during the winter, these systems
can still utilize the 90°F soil temperature to provide
adequate cooling during summer. During winter, the
system operates when the air temperature is less than the
soil/groundwater temperature.
The temperature difference between the ground and the
uid in the ground heat exchanger drives the heat transfer,
so it is important to determine the ground temperature.
At depths of less than approximately 6-½ feet the ground
temperature will show marked seasonal variation above
and below the annual average air temperature. As the
depth increases the seasonal swing in temperature is
reduced and the maximum and minimum soil temperatures
begin to lag the temperature at the surface. At a depth of
about 5 feet the time-lag is approximately one month.
Figure 62
72°F
Applications Manual
42°
47°
F
52°F
57°F
62°F
67°F
72°F
77°
F
Bosch Thermotechnology Corp.