Geothermal heat exchanger
Geothermal heat exchanger (EWT), geothermal heat exchanger and geothermal heat exchangers are in the soil laid heat exchanger , which are flowed through by a fluid.
In connection with low-energy and passive houses , it is often common to only refer to systems through which air flows as geothermal heat exchangers, which are used to preheat the fresh air of a controlled living space ventilation system (KWL). Corresponding water-carrying systems are called brine geothermal heat exchangers.
Water-bearing heat exchangers are referred to as either geothermal collectors or geothermal probes , depending on whether the pipes are predominantly horizontal or vertical.
construction
Geothermal heat exchangers usually consist of a pipe system through which air, water or a mixture of water and antifreeze ( brine ) flows. Air-to-ground heat exchangers can also do without pipes, as the air flows through underground cavities such as cellars, channels , caves, crevices or gravel fillings. These heat exchangers are also known as air wells .
Pipes or hoses e.g. B. plastic or concrete are laid in the ground. The beginning of the pipe system is above ground, the end, for example, at the ventilation system. Air is sucked in by a fan and conveyed through the system. The diameter can be a few centimeters to many meters, the total length up to a hundred meters. The DIN 1946-6 requires under point 9.2.5.8 (3) that the air-carrying lines of ground-air heat exchangers are laid with a gradient of at least 1% in order to discharge condensate that occurs when the dew point is not reached into the next shaft, where it is in the Usually seeps away. If the slope does not run in the direction of flow, it should be increased if necessary.
Another structure manages with less overall length and conveys the air with direct "earth contact". To do this, a trench or pit is filled with gravel or other coarse material. A pipe begins in the material and ends at a fan. Here, too, air is sucked in and transported through the spaces between the coarse material. This system was used in ancient times .
Hypo exchanger in the winter garden
In winter gardens , a so-called "hypo exchanger" system is sometimes used, with which overheating and drafts can be avoided. In the warm air in the winter garden, water (irrigation water or fountain water) evaporates and thus extracts heat from the air; The moist air that has risen is extracted mechanically at the highest point in the winter garden and passed through hypocaust pipes that are laid in the floor or the walls of the building. The water vapor condenses in the hypocaust and the released heat of condensation is transferred to the structure. The dehumidified air is then fed back into the winter garden and enables further cooling through evaporation.
Working method
The temperature of the transported air approaches the temperature of the earth as it flows through the heat exchanger. In well-dimensioned systems, the air temperature can change by up to around 10 degrees. The principle is basically the same as that of the geothermal collector , which works with liquid media.
Procedure
The temperature of the upper ground follows the annual course of the ambient temperature. The closer to the surface the geothermal heat exchanger is installed, the closer the air temperature is to the daily average. Lower-lying heat exchangers dampen the temperature profile of the seasons, so that the temperatures change out of phase. From a depth of around six meters, the temperature is constant throughout the year.
This temperature difference is used:
- If the outside air is colder than the ground, the air is heated by the ground before it is directed into the building.
- Conversely, in very warm weather, the incoming outside air is cooled by the ground.
Energy is exchanged in both working directions: heat is either extracted from the ground or added to it.
Use for heat generation
The deeper the pipes of the geothermal heat exchanger are laid, the less the temperature level is determined by the season or the ambient temperature (outside air temperature), but the less the sun is able to rewarm the ground that has cooled down in winter in summer. If the groundwater is so deep that it cannot compensate for the energy losses, a system with deep pipes works particularly effectively if the heat extracted in winter can be returned to the ground in summer. This can be done by passing air through the system in summer. On hot days, the air cooled in this way can also be used to cool the building. In the summer, surpluses from solar collectors can be fed into liquid-carrying heat exchangers , which also prevents overheating and stagnation of the solar system.
Air preheating before heat recovery or air heat pumps
If the fresh air in a ventilation system is heated by heat recovery using a heat exchanger, the condensate that forms inside the heat recovery device can freeze at air temperatures below about 0 ° C. To avoid this, geothermal heat exchangers can be used to first preheat the outside air supplied. This also increases the heat recovery rate of the ventilation system.
The geothermal heat exchanger can also be installed in front of an air heat pump .
Air quality
In the summer, condensate forms on the cold surfaces of the pipe system, which should be drained off via slopes and seepage points in order to avoid health risks from standing water in the geothermal heat exchanger. If infiltration is not possible due to impermeable layers or high groundwater levels, the condensation water from collecting shafts must be pumped out regularly. Alternatively, air dryers can be connected upstream.
Initial studies have established that the number of bacteria and fungal spores in the supply air fed through the air / geothermal heat exchanger is reduced compared to the outside air. This result is partly attributed to the upstream air filter.
Another study suggests the reason for the deposits of fungal spores and bacteria on the wall of the pipe. Towards the end of the pipeline, higher concentrations were found in the deposited dust than at the beginning of the pipe. An influence of the material from which the pipeline was made could not be determined. To avoid or remove deposits, it is advisable to either install a (coarse) filter upstream of the pipe or to provide enough shafts in the course of the pipe (e.g. at every change of direction of more than 30 degrees) through which the pipe can be flushed.
A radon exposure of transported air due to a permeable or leaky pipe system, direct contact with the soil or rock at radonbelastem gravel bed should be avoided for health reasons.
See also
literature
- Air fountain. In: Johann Georg Krünitz : Economic Encyclopedia . Volume 81, 1801, p. 372 f. ( kruenitz1.uni-trier.de ).
Web links
- The solar furnace laboratory building as a low-energy house. EWT planning guide. In: dlr.de
Individual evidence
- ↑ Planning of living space ventilation according to DIN1946 part 6 - Pluggit. (PDF; 2.5 MB), p. 35, In: Pluggit.com, accessed June 2019.
- ^ Rainer Wagner, Stefan Beisel, Astrid Spieler, Klaus Vajen; Philipps-Universität Marburg, Department of Physics: Measurement, Modeling and Simulation of an Earth-to-Air Heat Exchanger in Marburg (Germany). 4th ISES Europe Solar Congress, Copenhagen, Denmark, 2000 ( citeseerx.ist.psu.edu [PDF; 1.1 MB]).
- ↑ Rabindra Nath Bhattarai, Shailendra Kumar Mishra, Pawan Basnyat: Use of earth air tunnel HVAC system in minimizing indoor air pollution . In: Air Quality Monitoring and Management, Proceedings of Better Air Quality . 2004.
- ↑ B. Flückiger, C. Monn: Microbial investigations and general measurements in ground-coupled Earth-to-Air Heat Exchangers. (PDF; 585 kB) Institute for Hygiene and Applied Physiology, Environmental Hygiene Section, ETH Zurich. 20th AIVC and Indoor Air 99 Conference “Ventilation and indoor air quality in buildings”. Edinburgh, Scotland, 9-13 August 1999.
- ^ Earth Tube Concerns. In: HomeInTheEarth.com, accessed September 2019.