I

— Undisturbed groundwater level Affected groundwater level Extraction tube

" Immersed pump

Filtering gravel layer

Fig. 9.12 Elementary diagram of a groundwater-heat pump installation (see /9-10/)

One particular problem is the sedimentation of iron ochre in the injection wells. It occurs very often in oxygen-free groundwater with a low redox potential. Such groundwater should not get in contact with ambient air. Therefore the entire system needs to be closed and be kept under excess pressure all the time, otherwise water treatment by deferrisation and de-manganesing would be required. Lime precipitation, however, does not play a role at temperature fluctuations of a maximum of ± 6 K.

Under certain conditions groundwater heat pumps are possible that exclusively consist of one or several production wells. Such concepts rule potential problems with injection wells out. Technically, this requires that the aquifer has enough newly produced groundwater and that the water can be channelled appropriately or sunk again. In Germany, such systems are generally not authorised.

Other systems. Other systems are the utilisation of groundwater with a coaxial well, the utilisation of pit and/or tunnel water and air preheating or cooling in near-surface soil.

Coaxial wells. Coaxial wells ("Standing Column Wells") /9-11/ are positioned between ground probes and groundwater wells. An ascending tube with a filter at the bottom end and surrounded by a stack of gravel is built into a borehole. Towards the rock, the stack of gravel can be separated with a plastic liner. Water is pumped from the ascending tube with a submersible pump - in a similar way as in a groundwater well. It is then cooled down in a heat pump (or heated up) and then seeps out again through the stack of gravel in the ring section. During the sinking process the water absorbs heat from the surrounding subsoil or discharges heat into the subsoil.

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 between 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 input to operate the pumps. In general, after cooling down, the water has to be transferred back through another borehole into the pit. The flow between the withdrawal 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 temperature peaks were broken. As a further development, in order to extend the operation time of heat pumps utilising the heat source air during the winter, some systems 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).

Such heat sources are called concrete collectors, air wells or air registers. As air has a very low heat capacity, comparatively large amounts of air have to be moved. Lately, preheating and pre-cooling of intake air in tubes in the ground (without heat pumps) have gained significance for the ventilation of buildings with a low-energy and passive-energy standard.

Table 9.5 Designs and configurations of tubes for preheating of air in the ground Types of design

Concrete tubes (can absorb humidity), PVC-tubes (low pressure decrease)

Tubes free in the ground, tubes insulated at the top, tubes flat underneath the foundation

Single tubes or tube registers Types of operation

Fresh air is always transported through the tubes

Fresh air is only transported through the tubes if the outflow temperature is above the ambient air

Fresh air is transported through the tubes every time the outlet temperature is below the ambient temperature, for evaporators heat sources with a higher temperature are _always used (additional charging of the ground)_

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