Installation manual

74
3.2
3.2 Ground Heat Collector
The energy stored in the ground comes almost exclusively from
the surface of the earth. Precipitation and solar radiation are the
main sources of this energy. Thus, nothing should be built on top
of collectors nor should the surface be sealed in. The inflow of
heat from the interior of the earth is less than 0.1 W/m
2
and can,
therefore, be disregarded.
NOTE
The maximum annually extracted energy is 50 to 70 kWh/m
2
, which is
very difficult to realise in practice due to high costs.
3.2.1 Installation depth
The ground temperature at a depth of 1m can drop to freezing
point even without heat being extracted. The minimum
temperature at a depth of 2m is approx. 5 °C. This temperature
rises with increasing depth, although the heat flow from the
earth's surface decreases. Therefore, it cannot be guaranteed
that any ice will thaw in the spring. Thus, the installation depth
should be approx. 0.2 to 0.3m beneath the maximum frost line.
For most regions, this is between 1.0 and 1.5m.
ATTENTION!
For installing ground collectors in trenches, a surface depth of 1.25m
should not be exceeded in order to ensure lateral protection.
3.2.2 Installation clearance
When determining the clearance d
a
between pipe runs, ensure
that the ice radii which form around the pipe coils have melted
sufficiently to allow precipitation to drain, thus preventing
waterlogging.
The recommended installation clearance is between 0.5 and 0.8
m, depending upon ground conditions and pipe diameter.
 The longer the maximum duration of the frost period, the
greater the installation clearance should be.
 In the case of poor ground thermal conduction (i.e. sand),
the installation clearance should be reduced (with a
comparable horizontal surface), thereby increasing the total
pipe lengths.
NOTE
For climate conditions in Germany, an installation clearance of 0.8m has
proven successful for damp, cohesive ground (see Chap. 3.2.6 on p. 76.
3.2.3 Collector surface and pipe length
The area required for a horizontally-installed ground collector
depends upon the following factors:
 Cooling capacity of the heat pump
 Operating hours of the heat pump during heating period
 Ground type and ground humidity content
 Maximum duration of the frost period
NOTE
Chap. 3.2.6 on p. 76 indicates standard values for dimensioning of ground
heat collectors.
1. Step: Heat consumption of the heat pump as calculated
in the design (z.B. B0/W35)
2. Step: Calculation of the cooling capacity by deducting the
electric power consumption in the design of the
heat output.
3. Step: Calculate operating hours of the heat pump per
year
In Germany, approx.1,800 operating hours for heating and
domestic hot water preparation with monovalent heat pump
systems is the norm. . In
bivalent and mono energy systems, the operating hours increase
to approx. 2,400 (depending on the position of the bivalence
point).
4. Step: Select specific abstraction capacity depending
upon the ground type and expected annual
operating hours according to VDI 4640
Table 3.2: Specific abstraction capacities according to VDI 4640. with an
installation clearance of 0.8m
5. Step: The collector surface is obtained from the cooling
capacity and the specific abstraction capacity
NOTE
The calculated minimum pipe length is, in practice, rounded up to a full
100m circuit.
0
=
HP
- P
el
Example: SI 14TE
HP
= Heat output of the heat pump
14.5 kW
P
el
=
Electr. power consumption of the heat
pump as calculated in the design
3.22 kW
0
=
Refrigerating or abstraction capacity
of the heat pump from the ground as
calculated in the design
11.28 kW
4 4
4
4
Ground type Specific abstraction capacity
for 1,800 h for 2,400 h
Dry, non-cohesive ground
(sand)
approx. 10 W/m
2
approx. 8 W/m
2
Cohesive ground, damp
approx. 25 W/m
2
approx. 20 W/m
2
Saturated ground (sand,
gravel)
approx. 40 W/m
2
approx. 32 W/m
2
0
= Cooling capacity of the heat pump
11.28 kW
=
Specific abstraction capacity of the
ground
25 W/m
2
A = Collector surface
451m
2
Minimum pipe length for an
installation clearance of 0.8m
564 m
Number of brine circuits á 100m
6
PE pipe is available in standard 100m lengths. Thus, a
564m minimum pipe length results in 6 circuits á 100 and
a horizontal surface of 480m².
4
T