Environmental Engineering Reference
In-Depth Information
Ground probes are vertically sunken into, boreholes up to, and over, 250 m
deep. Their principal layout variations are shown in Fig. 9.9. A good heat transfer
between the soil and the probe has to be ensured. This can either be done by in-
jecting a bentonite-cement suspension or by additional filling with quartz sand.
Drilled
probe
Pile-driven probe
Fig. 9.9
Different layouts of vertically arranged drilled and pile-driven ground-coupled
heat exchanger (see /9-4/)
By using pile-driven steel probes and drillings with small machinery (up to a
depth of approximately 30 m) the layout presented in Fig. 9.9 on the right can be
achieved. The pile-drive and the drill are installed in one spot enabling them to
rotate. They can sink the ground probes without further relocation. For pile-
driving, mostly metal-coaxial probes are used. If stainless steel is not used, a ca-
thodian protection from corrosion has to be applied. Other processes have been
developed to sink U-type slings of plastic tubes directly into soft soil, using the
appropriate auxiliary mechanisms.
The layout of the most common ground probes is shown in cross-section in
Fig. 9.10. Single or double-U tube probes consist of two or four tubes that are
linked at the bottom end to enable the heat carrier to flow down in one tube and
up in another. In the coaxial basic form the withdrawal of heat from the soil only
takes place on one flow section (depending on the system either upwards or
downwards).
The material mainly used for ground probes is High-Density-Polyethylene
(HDPE) (e.g. PE 80 or PE 100 according to DIN 8074 or DIN 8075). Typical tube
dimensioning is 25 x 2.3 mm at a probe design length of 60 m and 32 x 2.9 mm at
100 m. For coaxial ground probes, plastic-coated high-grade steel or copper tubes
can also be used - at high costs. In general, the danger of leakage due to corrosion
of the ground probe has to be kept as low as possible by choosing the right mate-
rial.
Like for horizontally ground-coupled heat exchangers, there is also a danger
for ground probes that the soil is cooled down too much due to under-
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