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Low Temperature Geothermal Resources: Ground Source Heat Pumps 191

Shown in Table 10.2 is the amount of heat per cubic meter that can be obtained from each mate-
rial, for a 1° temperature change at near surface conditions. Also shown in the table is the total
number of cubic meters of material that would be needed to supply the heat needed to account
for the heat loss. As discussed below, boreholes in many ground source heat pump applications
are drilled to depths of about 90 m. Thus, a single borehole could easily supply the needed heat,
in principle.
However, whether or not the heat can be supplied at the needed rate depends on the thermal
conductivity of the material in the vicinity of the closed loop. To establish the rate at which this will
occur requires solving the following equation and comparing the results to the demand:
Δ Q/Δt = [C × (T − T)/(t − t)] × V, (10.3)
i
f
V
i
f
where ΔQ is the amount of heat that is required to be added or removed from the system (in J), Δt
is the time over which the heat is added or removed (in seconds), C is the constant volume heat
V
capacity (in J/m -K), (T − T) is the temperature change between the initial (i) and final (f) states,
3
f
i
(t − t) is the time duration (in seconds), and V is the volume of interest (in m ). For a borehole
3
f
i
system operating under the conditions described above, and using a value of 2225.2 kJ/m -K for the
3
C for kaolinite, and assuming a volume of 1 meter
V
Δ Q/Δt = 61.81 J/s.
In Table 10.1, it is evident that kaolinite can only provide about 0.2 J/s per meter length of bore-
hole under optimal conditions. As a result, a borefield in such material would require substantially
more boreholes to provide the rate of heat transfer needed, even though the available heat in one
borehole is adequate. Comparing the thermal conductivity of kaolinite to that of other materials
in Table 10.2, it is evident that most other geological materials are much better suited to support a
ground source heat pump system than is a site composed solely of dry kaolinite. Indeed, for most
materials, the thermal conductivity is sufficiently high such that closed loop systems usually have
the capacity to accommodate loads at the rate of approximately 3520 watts per 180 meters, which is
approximately equivalent to one ton of cooling capacity per 90m (295 foot) borehole.

desIGn consIderaTIons For closed-loop sysTems

heaTinG and coolinG loads
Despite these myriad considerations, designing a ground loop is a relatively straightforward process.
The principle challenge is establishing the length of underground piping needed to satisfy a cooling
and/or heating load for a given building. The important parameters that must be quantitatively taken
into account are the heating and cooling load of the building or space to be controlled, the available
heating and cooling capacity of the local thermal reservoir, and the rate and efficiency with which
heat must be transferred from or to the reservoir.

calculaTinG loop lenGTh
Calculating building energy demands (heating and cooling loads) will not be treated in this book.
Excellent software packages are available to complete such calculations using state-of-the-art data
from a variety of sources, including those listed at the end of this chapter in Further Information
Sources. In the following discussion, it will be assumed that the required heating (C ) and cooling
H
(C ) capacities needed for the building loads are about 13,400 W and 11,750 W, respectively.
C
There are several approaches used for calculating the length of piping needed for a given system.
The primary variables such calculations must consider are the load, the efficiency of the heat pump,