Geothermal Potential - Specific Conductivity and Specific Extraction Capacity 2017
Based on the borehole database of the Working Group Geological Survey of the Senate Department for the Environment, Transport and Climate Protection, the approximately 250,000 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 (Tab. 1).
In order to determine the specific extraction capacity of the ten classes of rock, a model approach using the Earth Energy Designer (EED, 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 capacity1. 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: 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
- groundwater flow: N/A
- subsoil temperature: constant at mean Berlin temperature (9° C)
- 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.
The resulting specific extraction capacity for heating without a hot water supply (1,800 h/a) and with a hot water supply (2,400 h/a) for each class of rock, both in the saturated and the unsaturated areas, is shown in Tab. 2.
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 1,500 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).