Technical data
www.dimplex.de 01.2012 | Project planning manual for heat pumps for heating and domestic hot water preparation | 43
Brine-to-water heat pump 3.3.2
3.3 Borehole heat exchangers
When implementing a borehole heat exchanger system, a heat
exchanger system is constructed in boreholes, usually with a
depth of between 20 m to 100 m in the ground. When double U
pipes are used, there is an estimated average heat source output
of approx. 50 W per metre of heat exchanger length. However,
exact dimensioning depends on the respective geological and
hydro-geological conditions, which are generally unknown to the
heating technician. The design should, therefore, be left to a drill-
ing company accredited with the seal of approval from the Inter-
national Heat Pump Association or authorised according to
DVGW W120. In Germany, the information in the Association of
German Engineers' (VDI) publication VDI -4640 Parts 1 and 2
should be taken into consideration.
Ground temperatures
The ground temperature below a depth of approx. 15 m is around
10 °C all year round (see Fig. 3.11 on page 43).
NOTE
The extraction of heat causes the temperatures in the pipe to drop. The
design must ensure that no permanent brine outlet temperatures fall
below 0 °C.
Fig. 3.11: Illustration of the temperature curve at different depths underground
in relation to the seasonal, mean temperature values on the Earth's
surface.
3.3.1 Design of borehole heat exchangers
Borehole heat exchangers must always be designed for the geo-
thermal planning office. An approximate estimation, even in
small output ranges, is not permissible for ground heat exchang-
ers. This is necessary, as the abstraction capacity is based on
the composition of the ground and the water-bearing layers.
These factors can only be determined on-site by the company
carrying out the work.
NOTE
When planning and designing ground heat exchangers, the legal require-
ments in the individual countries must be taken into account.
Long-term, calculated simulation of low gears makes it possible
to identify the long-term effects and thus take these into consid-
eration during dimensioning.
NOTE
When designing borehole heat exchanger fields as a heat source, it is im-
portant to ensure that the size of the borehole heat exchanger field is se-
lected based on the annual heat consumption in the building. This is es-
pecially important with bivalent systems. Normally, the abstraction
capacity of the borehole heat exchanger field is dimensioned for an an-
nual heat pump runtime of 1,800 to 2,400 hours. As the heat pump runtime
is higher with bivalent systems, the borehole heat exchanger fields for
such systems must be of a larger scale.
3.3.2 Preparation of boreholes
The clearance between the heat exchangers should be at least
6 m so that reciprocal interference is kept to a minimum and re-
generation is guaranteed in the summer. If several heat ex-
changers are required, these should not be laid out parallel to the
direction of ground water flow, but rather perpendicular to it (see
Fig. 3.12 on page 43).
NOTE
The same rules as for ground heat collector systems apply to the brine
concentration, the materials used, the layout of the manifold shaft and the
installation of the pump and expansion vessel.
Fig. 3.12: Layout and minimum clearances between borehole heat exchang-
ers in relation to the direction of the ground water current
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