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Geothermal Potential - Specific Conductivity and Specific Extraction Capacity 2017

Introduction

In recent years, geothermal energy is been widely used in Berlin. Since 2004, the number of facilities using near-surface geothermal energy has risen from 132 to approximately 3,500, as of 2018 (Sept. 30th). This trend is continuing, and is an important factor in the energy mix for the future use of renewable energy sources.

Unlike most other renewable energy sources, such as wind, hydropower or solar power, geothermal energy is an energy form which is independent of the weather, of the time of day and of the season of the year; it is virtually always available.

The use of near-surface geothermal energy, which in Berlin means up to a maximum depth of 100 m, can draw on a wide spectrum of technological possibilities. All these procedures require heat pumps capable of increasing the relatively low temperatures of the subsoil or the groundwater at these depths, of between 8 and 11 °C, to a higher temperature level suitable for heating purposes, with the aid of mechanical or electrical energy.

While there are many other possible procedures, 93 % of the geothermal energy in Berlin is today obtained by means of closed-loop ground heat exchangers (see Figure 1). These are closed-loop plastic pipe systems installed in boreholes in which a water-brine mixture is circulated, which draws the heat from the surrounding groundwater-filled rocky soil. The depth of the heat exchanger is generally between 40 and 100 m, depending on the geological conditions and the technical specifications of the system.

Fig 1: Closed-loop ground heat exchanger for a single-family home
Fig 1: Closed-loop ground heat exchanger for a single-family home
Image: Bundesverband Wärmepumpe (BWP) e. V.

In order to increase planning security of these borehole systems, we are here providing maps of the potentials of specific conductivity (Maps 02.18.1 – 02.18.4) and, specifically for single-family homes, specific extraction capacities (Maps 02.18.5 – 02.18.12). The key geological and hydrological conditions necessary for this are provided herein.

Since the installation of geothermal portable facilities in the subsoil involves the potential risk of endangering the groundwater, strict legal hydrological requirements for the protection of the groundwater have been stipulated for the drilling procedure, the sealing of the borehole, the pressure tests, the documentation, etc., when installing such facilities. New research results, cases of damage, and the greatly increased numbers of ground heat exchangers have repeatedly underscored this danger.

Since Berlin gets almost 100 % of its drinking water from the groundwater, and almost all of that from within its own boundaries, especially strict requirements for the protection of the groundwater are stipulated in the permit procedure of the Water Authority, which is required for the installation of a closed-loop ground heat exchanger.

For more information please see Geothermal Guideline (in German only):
www.berlin.de/sen/uvk/_assets/umwelt/wasser-und-geologie/publikationen-und-merkblaetter/leitfaden_geothermie.pdf

Specific conductivity

The specific conductivity is the capacity of sub-surface rock to conduct heat. It is one of the most important quanta for the correct dimensioning of closed-loop ground heat exchangers. It is a measure for the speed with which the extracted heat can be replaced via the sub-surface rock and the groundwater.

The unit for measuring specific conductivity is watts per metre * Kelvin [W/mK]. The conductivity is a property specific to the type of rock, and depends on its mineral content, its porosity and its pore filling. Air is a poor conductor, so that dry sediments above the water table have low conductivity. Since water, on the other hand, has higher conductivity then air, the conductivity value of the water-saturated rock is considerably improved. For this reason, groundwater conditions are taken into account on the conductivity map.

Specific extraction capacity

The specific extraction is capacity, by contrast to specific conductivity, a quantum which depends on a large number of specific marginal conditions, especially the specific capacity of the rock in the subsoil to transport heat, but even more on the technical data of the ground heat exchanger, such as its number of operating hours, the interactive effect of neighbouring systems, the size of the borehole, the conductivity of the material pressed into it, etc.

The unit for measuring specific extraction capacity is watts per metre [W/m]. For heating systems with no hot water supply, 1800 operating hours per year [h/a] are estimated; for systems which do provide hot water, the figure is 2400 h/a (see below).

Groundwater flow

For ground heat exchangers, a high heat transport capacity of the subsoil is desirable, to ensure that the heat withdrawn from the subsoil is replaced from the surrounding soil as rapidly as possible. This heat replacement is provided both by the conductivity of the rock and by the very minimal groundwater flow. Since groundwater flow is difficult to ascertain, especially at great depths, and since this very low speed of groundwater flow moreover varies greatly, groundwater flow is disregarded in this calculation. Thus, the data on specific yield have been conservatively estimated, and can be viewed as a safety buffer for the sizing of the system. Moreover, the statements on specific extraction capacity apply only for the marginal conditions stated (see below).