Ground Heat Exchanger Types - Bosch Geothermal Heat Pumps Applications Manual

Applications Manual

5 Ground Heat Exchanger Types

The two most prevalent types of residential geothermal
heat pump systems today include "Open-Loop" and
"Closed-Loop" ground heat exchangers. They will be
referred to as open-loop and closed-loop in this manual
from this point forward.
Open-loop geothermal heat pump systems using
groundwater were the most widely used type for many
years, and are still used today in different locations.
An open-loop geothermal heat pump system is a loop
established between a water source and a discharge
area, in which the water is collected and pumped to a
geothermal heat pump unit inside the structure, then
discharged to its original source or to another acceptable
location. Unlike an air-source heat pump, where one heat
exchanger (and frequently the compressor) is located
outside, the entire geothermal heat pump unit, or units, is/
are located inside the structure.
Open-loop geothermal heat pump systems are
characterized by the fact that the main heat carrier,
groundwater, ows freely in the underground, and acts as
both a heat source/sink and as a medium to exchange heat
with the solid earth. The main part of this type of system
is groundwater wells, to extract or inject water from/
to water bearing layers in the underground ("aquifers").
In most cases, two wells are required, one to extract the
groundwater, and one to re-inject groundwater into the
same aquifer from which it was drawn, but discharge is
Fig. 19 Extreme Frost Penetration (In Inches) Based on State
Averages.
Courtesy of http://www.dpwater.com
Bosch Thermotechnology Corp.
Bosch Geothermal Heat Pumps | 35
also possible to the surface where allowed. Ponds and/or
lakes are often used for this type of system as well.
Typically, closed-loop geothermal heat pump systems
have two parts dedicated to the uid ow: a circuit of
underground piping (ground loop) outside the structure,
and a geothermal heat pump unit inside the structure.
Closed-loop geothermal heat pump systems include a
ground loop, or "ground heat exchanger" (often called
"ground-coupled"), typically made of a vertical or
horizontal high-density polyethylene piping array placed
in the ground. Using coils of piping placed in a large body
of water (pond/lake application) approximately eight feet
below the surface is also another acceptable closed-loop
system. With pond/lake applications, the water is the
source of heat extraction and/or discharge instead of the
ground.
A closed-loop geothermal heat pump system (the most
common today) works on a simple premise: the earth
below the "Frost Line" (usually about six inches to
four feet down depending on location) is a constant
temperature of normally 40 to 70 degrees year round
depending on location (Fig. 19). During winter, heat is
removed from the earth through a liquid, such as water
or an antifreeze and water solution, upgraded by the
geothermal heat pump, and transferred to indoor air.
During summer months, the process is reversed: heat is
extracted from indoor air and transferred to the earth
through the water or antifreeze and water solution.
Data subject to change