Downhole heat exchanger
A borehole heat exchanger (EWS ) is a closed tube filled with an a circulating heat transfer fluid U-shaped tube system. It is usually fitted in a vertical or rarely obliquely arranged borehole into the ground. With the borehole heat exchanger, heat is extracted from the earth, which mostly to the heat exchanger (evaporator) is passed a geothermal heat pump. With the help of the heat pump, the economic utilization of geothermal energy for heating buildings is possible. The EWS is a possibility to use of shallow geothermal energy. Besides the classic EWS further geothermal probe forms are used, for example, coaxial or carbon dioxide probes.
Species
The normal and widespread geothermal probe is made of polyethylene plastic tubes which are connected at the respective lower end with a U-shaped foot part. Therefore it is also called U- probes or double U - probes when two pipe pairs are used per well. Are also possible coaxial probes in which forward and reverse run or done ascent and descent of the heat transfer fluid in the inner tube and in the annular space between the inner and outer tube of the coaxial probe. The tubes, flows through the closed circuit of a brine, a mixture of water and antifreeze. Sole Stuffed geothermal probes are often not allowed in water economically sensitive areas.
Another variant probes that use carbon dioxide as a heat transfer medium on the principle of two-phase thermosyphon. These probes are made of stainless steel.
Installation and performance
In the double -U probe the most widely used construction process runs wiefolgt from, other probe types can vary slightly: With a mobile drilling rig is a hole in the rinse or dry boring, depending on the rock with or without piping, drilled. When using the usual double U - probes of the drill diameter is about 140 to 180 mm. After the sinking of the drilling to the target depth, the probe beam ( U- probes and Verpressrohr, all together, if necessary, weighed down with a draw weight on the probe base ) is introduced into the well. The remaining cavity in the well bore is pressed with a filler material (bentonite -cement slurry, or grout ) as possible with a good thermal conductivity through the tremie entrained with the probe beam Verpressrohr from bottom to top. A possibly built during the well casing is pulled back during compression. By -setting grout a good heat transfer from the surrounding mountains are achieved at the probe tubes and serves as a backup between groundwater storeys for the protection of groundwater. Simultaneously therewith, to an exit of the heat carrier liquid (brine) is prevented in the ground water.
After the erection of the remaining probes of the field and the final work (such as the pressure test of the individual probes ), the Sondenvor and recoiling moved by frost-proof, horizontal connecting lines with the heat pump connected and filled the system with the heat transfer fluid and air bled. Except manholes no more installations are visible above ground level after completion.
In operation, the heat transfer fluid in a closed circuit is pumped through the borehole heat exchanger and heated on its way to the deepest point and back through the geothermal heat across the wall using a circulating pump. Thus, the geothermal probe forms a large heat exchanger. The large surface area is also achieved through bundling of pipes (the principle of the tube bundle heat ), and in most cases two pairs of tubes are used per well in practice.
This heat can be transferred, the receiving heat transfer fluid must be cooler than the ground temperature. This need is previously provided by a heat pump. The heat transfer fluid is heated in the probe, but can not be warmer than the bottom.
The heated heat transfer fluid flows into a heat exchanger of the heat pump to extract by evaporation cooling the heat contained. Downstream the heat pump is used to raise the temperature level required for heating. The larger the temperature difference between the temperature of the earth, and the desired Heizmedientemperatur, the more mechanical pumping energy is required. Therefore, low-temperature heating systems such as underfloor heating advantageous.
Dimensioning
Plans for geothermal probes require substantial computation involving geological and technical heating parameters. A guidance from a geologist with experience in the sizing of geothermal probes is strongly recommended. The heat demand of the building to be determined ( = heat sink ) is compared with the fertility of the ground ( = heat source). To prevent damage to the probe circuit, for example, be avoided by freezing of the ground close to the probe, and other undesirable effects during operation of the plant, should the local meteorological (eg mean annual temperature ), geological ( rock parameters, including thermal conductivity), hydrogeological (eg presence of ground water through the probe length) and heating technology (including heat demand and supply temperature of the building to be heated ) mandatory parameters in the calculations.
To size or power calculation downloadable simulation models can be used. With such models, comparison observations are available for geothermal heat collector in a simple manner. These calculations can give a general overview. More precise calculations can be calculated only in terms of knowledge of the geological subsurface. In large systems (> 30 kW) more accurate geothermal heat output of the subsurface can be determined also by special investigations such as the Thermal Response Test (TRT ). To this end, a first probe hole is provisionally drilled as test drilling and expanded to test probe; based on the result of this probe, the TRT design planning to the rest of the probes or probe array is performed.
Geothermal unfavorable ground (eg dry sand ) require more meters of drilling ( = borehole heat meters), which consequently lead to higher investment for tapping the heat source. In addition, possible heating of the building in the summer is possible and possibly resulting in a lower number of meters drilled, as the ground is regenerated termisch in the summer months. In various sources mentioned values for extraction capacity of the subsoil are to be treated with caution, as each site ( geological) is different. Therefore, about theoretical calculation of the necessary geothermal probe lengths in the present geology in a feasibility study should be determined in each case, in the same time the efficiency of a geothermal probe system is considered.
Drilling depths
At a depth of about 10 meters, the temperature throughout the year is almost unchanged and is in the range of low mountains at 11 ° C. The temperature increases every 30 feet of water by 1 K in Central Europe on average. Therefore, the geothermal probe has a higher efficiency compared to the ground collector. The depth of a hole varies according to the geological structure of the subsurface and will be for normal residential buildings between 50 and 300 meters. Depending on local conditions and performance requirements, they can also be up to 400 meters and more. Mostly there are experimental drilling depths greater than 400 meters ( = scientific or industrial deep geothermal projects ), in which case the cost exceeds the benefits normally.
In private housing (single-family ) in Germany rich geothermal probes rarely deeper than 100 m. In other countries, greater depths are common. So is drilled in Switzerland regularly until about 300 meters depth. In addition to the high cost of the drill ( drilling ) a permit (eg water protection regulations ) must be obtained and at greater depths than 100 meters and the mining law are observed.
If larger heat transfer surfaces are necessary, several holes are usually placed at a distance of a few meters next to each other. As is drilled into the deep, the space requirement in comparison with the ground collector is low. According to VDI 4640, a minimum distance of 6 meters and the plot of 3 m is recommended between adjacent probe holes to avoid negative influences of the probes themselves. In the LAWA recommendation for water management requirements for geothermal probes and geothermal collectors a probe distance to each other is recommended by 10 m and 5 m from the property boundary.
Application
Mainly used geothermal probes the extraction of ambient heat via heat pumps. But the variant for cooling can be implemented via borehole heat exchangers. This heat is transferred from buildings on the heat pump into the ground. So the soil is used for cooling the heat transfer fluid. This can not be cooler than the ground temperature. If lower temperatures necessary, a downstream chiller is required.
Depth of borehole heat exchangers are used exclusively for heating. Setpoint, the cooling case are covered, the drilling depths can be reduced due to the storage application.
Large Equipment
Should be explored first area, the use of geothermal energy in the geothermal park in Neuweiler in the northern Black Forest, a construction area, where only geothermal energy is used for purposes of building heating and cooling. Here is to be first implemented as part of a pilot project and the heating or cooling of the existing roads.
Since 10 November 1994, the geothermal heat pipe Prenzlau is at a depth of 2790 m and a duration of heat output with heat pump of 520 kW at a rock temperature of 108 ° C in operation. The heat output without heat pump is 150 kW. The depth probe is characterized by a virtually trouble-free operation over the years, with rare interruptions of several hours.
The RWTH Aachen has achieved in the context of the construction of the building in November 2004 Super C with a geothermal probe a depth of 2500 m. The rock temperatures reach 70 to 100 degrees Celsius. The geothermal probe should provide a power of approximately 450 kW. This would have approximately 300 tonnes of CO2 in home heating saved annually. Unfortunately, the thermal performance fell far short of expectations.
Legal situation
Germany
According to the German Federal Water Act ( WHG) are drilling that may impact on groundwater, subject to notification ( § 49 WHG). The introduction of borehole heat exchangers in ground water aquifers is a use event within the meaning of § 9 WHG who makes occasional water rights permit or license under the Water Resources Act and the respective state water laws of the several states required. Likewise, the use of substances hazardous to water (eg cooling brine with WGK 1 ) represent a water-fast union event in underground plant parts.
Often performing drilling is restricted or prohibited in the protection zone of designated water protection areas or alternative source protection areas.
When drilling over 100 m depth the provisions of § 127 paragraph 1 is to be considered Federal Mining Act. Then the mining law applies with certain provisos.