Ground Heat Source Or Heat Sink For Closed-Loop Ground Heat Exchangers - Bosch Geothermal Heat Pumps Applications Manual

Applications Manual
8 Ground Heat Source or Heat
Sink for Closed-Loop Ground
Heat Exchangers
Bosch geothermal heat pump systems utilize the sub-
surface of the ground as the heat transfer medium for
closed-loop ground heat exchangers since it typically
remains relatively constant. Typically, ground temperatures
are fairly constant from 5 to 30 feet below the surface;
below 100 feet, temperatures rise about 1°F for every 100
feet of further depth. Sub-surface temperatures uctuate
more widely near the surface. Soil temperatures can be
determined by direct measurement, by design graphs of
sub-surface temperature variation and by equations.
An important sizing consideration is that the performance
of the Bosch geothermal heat pump depends on the
performance of the ground heat exchanger, or "ground
loop", and vice versa. It is therefore essential to design and
size them together.
The length of ground loop piping required depends on
the structure load, soil conditions, loop con guration,
local climate and landscaping. Sizing of the ground loop is
critical. The more piping used (over-sizing), the greater the
output of the system, but since the costs associated with
the ground loop typically represent 30% to 50% of the
total system costs, over-sizing is uneconomical.
Conversely, under-sizing the ground loop can lead to it
running colder and could result in ground temperatures
not being able to recover during heat transfer. The ground
loop must be sized to meet the peak thermal power
but also to deliver energy at no greater rate than the
surrounding earth can collect energy over a twelve month
period. If a system provides heating and cooling, energy
transferred to the ground in summer can be stored and
used in winter.
Assuming that other conditions remain constant, the
heat that a ground loop can extract will be dependent on
the temperature difference between the circulating uid
and the "far eld" ground temperature (away from the
in uence of the heat exchange with the ground loop).
Bosch Thermotechnology Corp.
Bosch Geothermal Heat Pumps | 47
Data subject to change