Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 400

dimensioning and a corresponding excessive heat withdrawal. This leads to lower
temperatures of the heat carrier and thus a reduction of the COP of the heat pump.
In contrast to the horizontal heat exchanger installed at a depth of 1.0 to 1.5 m, the
deeper layers cannot recover entirely during the summer. Artificial heating would
have to be provided for (e.g. by solar collectors or from industrial waste heat).

S ection
A-A

25-32

Floor heating system

mm

50-70 mm

Hot water
storage

70-80 mm

U-probe Double-U-probe

Heat pump

Ground probe

A A

~ 46 mm ~ 70 mm

Coaxial probe Complex coaxial probe

Fig. 9.10 Types of common ground probes (see /9-4/)

Table 9.4 Specific heat withdrawal capacities of ground probes in smaller systems at dif-
ferent utilisation hours at full load (analogous to VDI 4640, sheet 2)

1,800 h/a 2,400 h/a

General guidelines

Bad subsoil (dry lose rocks) 25 W/m 20 W/m

Solid rock subsoil, water-saturated lose rock 60 W/m 50 W/m

Solid rock with high heat conductivity 84 W/m 70 W/m

Individual soils

Gravel, sand, dry < 25 W/m < 20 W/m
Gravel, sand, carrying water 65 - 80 W/m 55 - 65 W/m
Gravel, sand, strong groundwater flow, for small systems. 80 - 100 W/m 80 - 100 W/m
Clay, loam, moist 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 magmatites (e.g. granite) 65 - 85 W/m 55 - 70 W/m
Alkaline magmatites (e.g. basalt) 40 - 65 W/m 35 - 55 W/m
Gneiss 70 - 85 W/m 60 - 70 W/m
The requirement for using the table: only heat withdrawal (heating incl. hot water) takes place; length
of the individual ground probes between 40 and 100 m; smallest space between two ground probes
would be a minimum of 5 m for ground probe lengths of 40 to 50 m or at least 6 m for ground probes
with lengths of over 50 to 100 m. Suitable ground probes are double-U probes with an individual tube
diameter of 25 or 32 mm or coaxial probes with a diameter of at least 60 mm. The values given above
can fluctuate considerably, depending on rock formations such as crevasses, foliation and weathering.