Open-Loop Geothermal Heat Pump Systems - Bosch Geothermal Heat Pumps Applications Manual

3 6 | Bosch Geothermal Heat Pumps
6 Open-Loop Geothermal Heat
Pump Systems
Since groundwater is a relatively constant temperature
year-round, it is an excellent heat source and heat sink.
The average soil/groundwater temperature of the upper
100 to 200 feet in most of the U.S. is equivalent to the
average ambient air temperature of the location. There are
seasonal variations to the soil/groundwater temperature of
approximately 10 to 20 F degrees, depending on incident
solar radiation, rainfall, seasonal swings in overlying air
temperature, local vegetation cover, type of soil, and
depth in the earth, approaching a constant temperature
at a depth of approximately 30 feet (where it is equivalent
to the earth's normal thermal gradient). The average
soil/groundwater temperature is typically in the range of
normally acceptable comfort temperature levels for people
by being higher than the air temperature in the winter and
lower during the summer.
Open-loop geothermal heat pump systems do not con ne
uid to a ground heat exchanger, or "ground loop".
Rather, they extract groundwater (only) directly from a
groundwater well or pond and run it through a water-
to-refrigerant coaxial heat exchanger within the Bosch
geothermal heat pump. Although surface water could
often be used, most open-loop geothermal heat pump
systems rely on groundwater.
After the transfer of heat between the groundwater and
the Bosch geothermal heat pump system occurs, the
water is discharged back into a secondary well, into a
pond or other body of water, or into a drainage ditch or
other acceptable location, depending on local codes (Fig.
20). Local environmental of cials should be consulted
whenever an open-loop geothermal heat pump system is
being considered.
This open-loop geothermal heat pump system method
is used less frequently today than a couple of decades
ago, but may be employed cost-effectively if groundwater
is plentiful and water quality is appropriate. Open-loop
geothermal heat pump systems are the simplest to install
and have been used successfully in areas where local
codes permit. Because open-loop geothermal heat pump
systems utilize water on a "once through" basis, they
are often referred to as "pump and dump" systems. The
performance of the open-loop geothermal heat pump
system may degrade over time if water quality issues are
present (high mineral or dissolved solids content, etc.) or
if the water supply diminishes for any reason.
The Bosch geothermal heat pump is normally provided
with a copper water-to-refrigerant coaxial heat exchanger,
but often a cupro-nickel water-to-refrigerant coaxial heat
exchanger is recommended for open-loop geothermal heat
pump systems. This optional water-to-refrigerant coaxial
heat exchanger can be ordered prior to unit assembly
Data subject to change
and helps offset the effects of heavy scale formation or
brackish water.
Fig. 20 Open-loop overview
With open-loop geothermal heat pump systems, if the
possibility of groundwater use where scaling could be
heavy or where biological growth such as iron bacteria
could be present, an open-loop geothermal heat
pump system is not recommended. If mineral deposits
accumulate in the coaxial heat exchanger its heat transfer
capabilities can deteriorate over time resulting in loss
of capacity and ef ciency. Water-to-air coaxial heat
exchangers must only be serviced by a quali ed contractor
or technician, as typically acid and special pumping
equipment is required if clogged. If the Bosch geothermal
heat pump is equipped with the optional Heat Recovery
Package ("desuperheater") its coil can become scaled and
possibly plugged as well.
Applications Manual
Bosch Thermotechnology Corp.