Twin pipe probe

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As Twin-pipe probes are geothermal probes designated, for the recovery of geothermal energy in the rinse process in the soil are incorporated. In contrast to normal geothermal probes or depth probes, the twin-pipe probes are inserted a maximum of 20 meters deep into the ground. Twin-pipe probes can be installed in the uppermost groundwater level up to about 15-20 meters using the flushing lance method, provided that the subsoil is a well flushable sandy soil . Twin-pipe probes in the uppermost water-bearing layer have a high level of heat extraction when they are in the groundwater. The "flushing probe" is less complex than the flushing drilling method, which is used for geothermal probes up to 100 or in individual cases even up to 200 meters.

In theory, it is possible to flush until the first layer of marl is reached. In practice, probes of 12 or 14 meters are the rule, as these lengths can be made from rod tubing and can be used on any construction site. The mostly clayey marl layer under the uppermost aquifer has a high thermal conductivity, which also supplies the probe field well with energy from below. The individual probes are placed at a distance of two to three meters, connected to one another below the frost line and led to a distribution shaft.

Rinsing probes that are completely in the groundwater flow have very stable source temperatures even under high loads, as they are constantly supplied with new energy, similar to a water-water heat pump. If the dimensions are correct, icing can be ruled out. In summer the probe field is completely regenerated by the groundwater.

The probes are filled or slurried with the stone-free items to be washed in order to prevent subsequent sagging. Grouting with bentonite is usually not necessary because the operating pressures are low and because the flushing lance method makes it impossible to penetrate several water-bearing layers.

Extract from VDI 4640 / Sheet 2

In the case of permeable, loose rock (pore aquifer) and boreholes up to 50 meters deep, the aforementioned type of backfill (betonite) can be deviated from if the geothermal probe is only located in the uppermost aquifer over its entire length. In these cases fine gravel or, if fine-grained enough, the cuttings can be slurried .

In most cases, the water authorities agree to the stone-free wash ware being backfilled, provided the following requirements are met:

1. The entire length of the twin pipe probes lies in the uppermost aquifer up to 20 meters; the backfill material is fine-grained enough
2. The twin pipe probes have already been subjected to a pressure test by the manufacturer
3. The heat pump system has a brine circuit monitoring system, which switches off the system in the event of a brine pressure loss.

The operation is then generally classified as safer than with deeper geothermal probes.

Connection of the twin pipe probes

The individual probes are connected to one another in a pipe trench below the frost line using the plastic welding process. Depending on the pressure loss, three to five probes are always connected in series and routed together to the manifold. Here, too, it is important that the lengths and thus the total pressure loss of the circles are the same. In the case of very elongated probe fields, it is possible to combine five probes close to the distributor and 4 or 5 at points further away.

Variants are also offered in which spacers are used to create a distance of about 20 cm between the two pipes. Since an oval hole can be rinsed with a double lance and a corresponding amount of water, these probes can be inserted with spacers. This creates a greater distance between the probe tubes. This increases the heat yield.

Between 1000 and 2000 liters of water are required for each twin pipe probe for a flushing process. A petrol-powered flushing pump should deliver 80 m³ / h at a pressure of eight bar.

Design: For heat extraction in the groundwater, the following services are assumed for a probe distance of 2.5 meters:

Rinsing probe: 30–35 W / m (1800h / a)

This means that twin-pipe probes are usually designed more generously than depth probes. The risk of structural damage , artesians or uplift cracks such as B. in Staufen im Breisgau 2007 does not exist with twin-pipe probes, since no groundwater storeys can be penetrated in the flushing process. Twin-pipe probes are therefore considered to be a particularly safe method for extracting near-surface geothermal energy .

literature

Thorsten Schröder: Thermal use of the subsoil - Earth-coupled heat pump systems In: Heat sources for heat pumps , June 2013, ISBN 978-3-9812130-7-2 , VDI 4640 / sheet 2.