Environmental Engineering Reference
In-Depth Information
Size of Geothermal Heat Collector
The length l and the area A of a ground collector is calculated from the required
refrigerating capacity Q source of the low-temperature heat source and the extraction
capacity q
per metre of pipe and the pipe gap d A :
Q
source
l
=
and
Ald
=
.
A
q
If the heat capacity desired, for example, Q heating
= 10
kW
, and the COP
ε
= 4, a
refrigerating capacity of Q source
is required. With dry sandy soil the extrac-
tion capacity is around q = 00. kWm
= 7.
kW
; with dry clay soil, 0.02 kW/m. As a result,
on the basis of this example, the length of pipe needed for clay soil is calculated as
75
002
.
kW
kW m
l =
=
375
m
.
And with a pipe gap of d A = 0.8 m the collector area is
A =
375
mm m
0 8
.
=
300
2
.
If any doubt exists, it is recommended that the values be rounded off generously.
As individual pipe lines should not exceed lengths of 100 m, the recommendation
is that four circles of pipe, each 100 m in length, be used.
If suffi cient space is not available in a garden or there is no desire to dig up a whole
plot of land, ground probes can be used to reach the ground heat. Vertical boreholes
can reach depths of 100 m. Constant temperatures of around +10 °C or more exist
at these depths all year round. U-shaped pipe probes are inserted into the boreholes
through which the brine of the heat pump will later fl ow. The depth of the drilling
and the number of probes depend on the heat requirement and the composition of
the ground below. Geologists and specialized drilling fi rms can help with the speci-
fi cations. Depending on the composition of the ground at the bottom, the potential
extraction capacity is between 20 and 100 watts per metre. A rough estimate could
be calculated at about 55 watts per metre. Therefore, about 5.5 kilowatts of refrig-
erating capacity could be extracted from a 100 m-deep probe. Several parallel probes
with a gap of at least 5 to 6 m between them are necessary to achieve higher capaci-
ties (Figure 11.6 ).
In addition to the ground heat, heat from groundwater can be used. This requires a
production well and a reinjection well. The reinjection well transfers the cooled
groundwater back into the ground. It should be sited at least 10 to 15 m behind the
production well, in the direction of the groundwater fl ow, so that the cooled water
does not fl ow back to the supply drilling.
In many countries approval must be obtained from the responsible water authority
before groundwater can be extracted. This approval is usually given under certain
 
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