Environmental Engineering Reference
In-Depth Information
Due to the lack of separation from the natural subsoil (a plastic liner does not
seal off completely), antifreeze cannot be used in coaxial wells. The heat pump
has thus been run in a way that prevents freezing - in the same way as using
groundwater. A maximum annual number of operation hours is generally fixed in
advance for that reason. Furthermore, long seeping paths, large amounts of water
in the borehole ring section and an increased temperature at the bottom of the
borehole have been perceived as useful. Therefore coaxial wells are normally be-
tween 100 and 250 m deep.
Measured specific heat withdrawal capacities of coaxial wells under normal
operation are between 36 and 44 W/m and under short-term operation at full load
at around 90 W/m /9-11/. Hence they have similar dimensions as those of the
ground probes. The average heat source temperatures are, however, a little higher
compared to ground probes. This achieves a better COP of the heat pump.
Cavity and tunnel water.
Artificial hollows in the subsoil can serve as collectors
of groundwater or groundwater reservoirs. They are mainly mines (no longer or
still operational) or tunnels, where the hollows had not primarily been built for a
thermal utilisation. This special creation of hollow spaces is normally ruled out
due to high costs (with the exclusion of thermal subsoil storage). At times we
move away from the field of shallow geothermal energy when dealing with pits
and tunnels. To give an example, the water for thermal use from a coal mine in the
Eastern Ruhr area in Germany would be obtained from depths significantly below
1,000 m and from the interior of an Alpine tunnel e.g. in Switzerland sometimes at
a depth of over 2,000 m.
Water from mines can be obtained e.g. through drillings from above ground.
Above all, the depth of the water level in the pit determines the heat withdrawal
method. It may lead to high pumping heads and correspondingly high energy in-
put to operate the pumps. In general, after cooling down, the water has to be trans-
ferred back through another borehole into the pit. The flow between the with-
drawal and the intake borehole should be as long as possible (achieved e.g. by
drilling at different levels). Mines in the low mountain range areas that ascend via
drifts from valleys, water flowing naturally from these drifts can also be used as a
heat source.
Water from large tunnel constructions normally flows to the portals and can be
utilised as a heat source there. In some Alpine tunnels this water has temperatures
which are significantly above the annual mean temperature.
Preheating/precooling of air.
Utilisation of air preheating in the subsoil (without
heat pump) already existed in the eighties in the farming sector. Intake air for the
pig pens was sucked in through tubes in the ground. Winter and summer tempera-
ture peaks were broken. As a further development, in order to extend the opera-
tion time of heat pumps utilising the heat source air during the winter, some sys-
tems were operated that transferred air through tubes in the ground, pre-heated it
there and then transported it to the heat pump evaporator /9-1/, /9-3/ (Table 9.5).
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