Installation manual
78
3.3
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 20m to 100m in the ground. When double U
pipes are used, there is an estimated average heat source output
of approx. 50 W per drilling metre of loop. However, exact
dimensioning depends on the respective geological and hydro-
geological conditions, which are generally unknown by the
heating technician. Dimensioning should, therefore, be left to a
drilling company accredited with the seal of approval from the
International 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. 3.6 on p. 78 ).
NOTE
The extraction of heat causes the temperatures in the pipe to drop.
Dimensioning must ensure that no permanent brine outlet temperatures
fall below 0 °C.
Fig. 3.6: 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 Dimensioning of borehole heat exchangers
For single systems with a heat pump heat output of up to 30kW
(that are installed for heating and DHW preparation),
dimensioning can be carried out based on specific abstraction
capacities according toTable 3.4 on p. 78, which is based on the
following assumptions:
Length of the individual borehole heat exchangers between
40 and 100 m
At least 6m clearance between two borehole heat
exchangers
The borehole heat exchangers used are either double U
pipes with an individual pipe diameter of DN 32 or DN 40.
These abstraction capacities are permissible for borehole heat
exchangers for standard installation with low output. In the case
of longer runtimes, both the specific abstraction capacity already
mentioned and the annual abstraction factor should also be
taken into consideration. This should be between 100 and
150 kWh per drill meter and per year.
For heat pump systems that
consist of several single systems
require more than 2,400 operating hours annually
are installed for both heating and cooling
are over 30kW of heat pump total heat output,
the system dimensioning must be approved using calculations
from a geothermal planning professional.
Long-term, calculated simulation of low gears allows
identification of the long-term effects and, therefore, take these
into consideration during dimensioning.
Table 3.4: Typical specific abstraction capacities for borehole heat exchangers (double U pipes) (according to VDI 4640 Part 2)
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Foundations Specific abstraction capacity
for 1,800 h for 2,400 h
Standard values:
Poor subsoil (dry sediment) (λ < 1.5 W/(m * K)) 25 W/m 20 W/m
Normal bedrock and saturated
sediment (λ = 1.5 - 3.0 W/(m * K)) 60 W/m 50 W/m
Bedrock with a high thermal conductivity (λ > 3.0 W/m * K)) 84 W/m 70 W/m
Types of stone:
Gravel, sand, dry < 25 W/m < 20 W/m
Gravel, sand, aquiferous 65 – 80 W/m 55 - 65 W/m
Gravel and sand with strong ground water current, for single systems 80-100 W/m 80-100 W/m
Clay, loam, damp 35 – 50 W/m 30 - 40 W/m
Limestone (solid) 55 – 70 W/m 45 - 60 W/m
Sandstone 65 – 80 W/m 55 - 65 W/m
Acidic igneous rock (e.g. granite) 65 – 85 W/m 55 - 70 W/m
Alkaline igneous rock (e.g. basalt) 40 – 65 W/m 35 - 55 W/m
Gneiss 70 – 85 W/m 60 - 70 W/m