Environmental Engineering Reference
In-Depth Information
connections has to be provided. Heat transfer tubes can only be added to the pre-
fabricated pile length. Hollow piles, where heat transfer tubes can be added to the
hollow in the pile at a later stage, allow the utilisation of the entire pile length.
However, they reduce the available diameter for the tubes.
Apart from foundation piles, other concrete components can also be used as
heat exchanger in the earth (e.g. claddings of foundation trenches made of slot-die
or pile walls), as these fixtures are generally no longer required for static purposes
after the building is finished. Supporting walls, cellar walls or foundation plates
can also be used as heat exchangers. In these cases, a good insulation against the
interior is equally necessary, as with the manifold of energy pile systems which
are laid under the floor plate; this enables the actual withdrawal of heat from the
subsoil and prevents for example, the cellar from becoming cold and damp.
Open systems.
Open systems for near-surface ground energy utilisation are
groundwater wells. They are discussed in the following.
Because of its relatively constant temperature between 9 and 10 °C, groundwa-
ter is very suitable as a heat source for heat pumps. Limitations are the lack of
availability of the heat source. Rich enough and not too deep groundwater layers
(aquifers) with a suitable water quality are not available everywhere. Further limi-
tations might be caused by regional water legislation.
The heat source system for groundwater utilisation consists of a production
well that provides the groundwater, and an injection well that is used to recharge
the groundwater layers with the cooled down water (doublet). The extraction and
the injection well have to be at a reasonable distance in order to avoid a thermo-
hydraulic shortcut. The extraction well should also not be within the cold zone of
the injection well as this reduces the efficiency of the heat pump system.
The well capacity needs to ensure a continuous extraction for the nominal flow
of the connected heat pump, this corresponds to approximately 0.2 up to 0.3 m
3
/h
for each kW of evaporator capacity. The capacity of the well depends on the local
geological conditions. The temperature change of the groundwater that is re-
charged to the injection well or wells should also not exceed ± 6 K. The extracted
amount and the minimum recharging temperature should be in line with the re-
spective regulations.
Fig. 9.12 shows the typical construction of a heat pump system for groundwa-
ter utilisation. Common well depths are 4 to 10 m (e.g. /9-1/, /9-3/), which can be
deeper in larger systems (the transition to hydrothermal energy utilisation is fluid
in this case, see Chapter 10). The clay barrage above the gravel fill holds air and
gravitational water back. The gravel filling between the well borehole and the fil-
ter tube should be between 50 to 70 mm in thickness. The suction tube, the intake
of the tailwater pump in the extraction well and the downspout in the injection
well always have to end below the water surface in each operating state.
Before designing the well, hydro-geological analyses should clarify the chemi-
cal structure of the groundwater, the aquiferous and non-water permeable layers,
plus the groundwater level and the permeability of the aquiferous layers. For these
Search WWH ::
Custom Search