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Berlin Environmental Atlas

02.18 Geothermal potential: Specific conductivity and specific extraction capacity (Edition 2015)

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Methodology

Based on the borehole database of the Geology and Groundwater Management Working Group of the Senate Administration for Health, the Environment and Consumer Protection, the approximately 227,000 strata obtained in the bore holes and the interpretation profiles were compiled and assigned to ten classes of rock with associated specific conductivities and specific heat capacities (Table 1).

Tab. 1: Classes of rock with associated specific conductivities and specific heat capacities
Rock Number of strata present Conductivity
λ [W / (m · K)]
unsaturated
Conductivity
λ [W / (m · K)]
saturated
Heat capacity 2
c [MJ/m³, K]
unsaturated
Heat capacity 2
c [MJ/m³, K]
saturated
Landfill / anthropogenic 4,023 0.4 2.7 1.6 2.5
Soil 3,658 2.1 2.8 2.25 2.25
Gyttja 2,111 0.4 2 0.4 2 0.8 0.8
Peat / peat gyttja 835 0.4 2 0.4 2 0.8 0.8
Sand 120,032 0.4 3 2.7 3 1.6 2.5
Gravel 9,629 0.4 2 1.8 2 1.45 2.4
Boulder clay 5,183 2.9 1,3 2.9 3 2 2
Glacial till 48,769 2.9 1,3 2.9 3 2 2
Clay / silt 29,375 0.5 3 1.5 3 1.55 2.4
Brown coal 3,921 0.4 2 0.4 2 0.8 0.8
1 Moist soil; 2 From VDI 4640 (2010); 3 From potential study, Module 1
Tab. 1: Classes of rock with associated specific conductivities and specific heat capacities

Excel
[Table is also available as Excel-File (MS-Excel is required).]

In order to determine the specific extraction capacity of the ten classes of rock, a model approach using the Earth Energy Designer (EEG, Version 3.16) was used. The case calculated was that of the energy load of a single-family home, close to real conditions, with identical marginal conditions for each class of rock. Only the rock-specific parameters, conductivity and thermal capacity, were adapted. In that way, maximum heating yield for each class of rock, and from that, the specific extraction capacity, could be determined.

Marginal conditions for the ascertainment of specific extraction capacity


1. Marginal conditions of the site/heating requirement:
mean temperature of the earth's surface: 9 °C
arrangement of ground heat exchangers: 2 exchangers, 100 m in length each, 6 m apart
borehole diameter: 180 mm
volume flow per exchanger: 0.5 l/min (lower level of turbulence in fluid
ground heat exchanger: Doppel-U, PE DN 32 PN 10
distance to middle: 0,07 m
conductivity of the backfill: 1,5 W/(m*K)
coolant: mono-ethylene glycol, 25%
borehole resistance: corresponds to above design
simulation period: 25 years
annual coefficient of performance: 4.3 (subsidy guideline of the Federal Office of Economics and Export Control/ BAFA)
minimum temperature marginal condition of fluid: 1,5 °C

2. Marginal conditions of the groundwater/minimum soil temperature:
groundwater flow: N/A
subsoil temperature: constant at mean Berlin temperature (9 °C)

3. Validity:
Applicable only for small ground heat exchangers (two exchangers)
For larger systems with more than two exchangers (even < 30 kW), corresponding reductions must be taken into account, since the interactive effect of the exchangers increases with their number.

Annual operating hours

The calculation of the extraction capacity refers to heating operations without hot water supply, with 1,800 operating hours per year for the heat pump (Maps 02.18.5-8); and for heating operations with hot water supply, with 2,400 operating hours per year for the heat pump (Maps 02.18.9-12).

The variations in annual heating load distribution are shown in Figures 2 and 3, for 1,800 and 2,400 operating hours, respectively. The share of hot water supply is constant, while the heating supply varies over the course of the year.

Figure 2
Fig. 2: Annual heating load distribution for 1,800 operating hours

Figure 3
Fig. 3: Annual heating load distribution for 2,400 operating hours

Excel
[Statistical base of Figure x is also available as Excel-File (MS-Excel is required).]

The resulting specific extraction capacity for heating without a hot water supply (1,800 h/a) and with a hot water supply (2400 h/a) for each class of rock, both in the saturated and the unsaturated areas, is shown in Table 2.

Tab. 2: Specific extraction capacity of particular classes of rock
Rock unsaturated saturated
  P 1,800 h/a
[W/m]
P 2,400 h/a
[W/m]
P 1,800 h/a
[W/m]
P 2,400 h/a
[W/m]
Landfill / anthropogenic 9.94 8.02 45.56 37.30
Soil 37.86 30.88 46.41 38.02
Gyttja 9.20 7.43 9.20 7.43
Peat / peat gyttja 9.20 7.43 9.20 7.43
Sand 9.94 8.02 45.56 37.30
Gravel 9.82 7.94 34.01 27.75
Boulder clay 47.16 38.82 47.16 38.82
Glacial till 47.16 38.82 47.16 38.82
Clay / silt 11.91 9.63 29.84 24.22
Brown coal 9.20 7.43 9.20 7.43
Tab. 2: Specific extraction capacity of particular classes of rock

Excel
[Table is also available as Excel-File (MS-Excel is required).]

The mean specific conductivity and the mean specific extraction capacity for the entire borehole system has been calculated for selected depth segments (0 - 40 m, 0 - 60 m, 0 - 80 m and 0 - 100 m), by weighted averaging of the conduction and extraction capacity for each stratum.

Since especially for the drafting of the map for the depth of 100 m, there were only 1,300 bore holes available, greater density was achieved using virtual drillings based on geological cuts at intervals of 500 m. The assignment of conductivities and of associated extraction capacities for these points was carried out by means of average values for the petrographic properties of the surrounding rock. A total of approx. 1,900 additional virtual boreholes were thus used.

In order to draft the maps, the values for the specific conductivity and the specific extraction capacity calculated for all boreholes and virtual points were then interpolated by means of inverse distance weighting (IDW).

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