Storage in a Field of Downhole Heat Exchangers

For the storage of thermal energy in a field with downhole heat exchangers (DHE), the underground strata also serve as the storage medium. Here, the reservoir is tapped by a number of boreholes of 20 to 100 m depth, usually sym­metrically arrayed, and these are outfitted with probes.

A geothermal field with DHE is an underground closed system in which there is no hydrological connection be­tween the storage medium (earth) and the heat transport medium. The heat transport for charging and discharging takes place through the walls of the probes. The charging and discharging thermal power is thus proportional to the

surface area of the probes and to the temperature difference between the thermal transport medium and the storage medium. The cross-sectional area of the probes is therefore dimensioned in such a way that the probe surface area is as great as possible.

Figure 5 shows four types of probes in cross section. The U – and double U-tube probes consist of two or four tubes, respectively, which are connected together at the bottom of the borehole. In coaxial probes, the medium flows downwards in the outer space and upwards in the in­ner tube. A water-glycol (antifreeze) mixture is used as heat – transport medium, so that the liquid does not solidify at low temperatures.

Owing to the temperature difference between the heat – transport medium and the storage material, which is re­quired for heat transfer, the temperature in the storage reser­voir is always higher (when heat is being extracted from the reservoir) or lower (during charging) than the temperature of the heat-transport medium. These losses appear in the storage balance as exergy annihilation. The heat-transport medium in a DHE field storage system has a smaller tem­perature range in comparison to that in an aquifer storage system. Therefore, in energy-management systems with DHE field storage, heat pumps which raise the heat to a us­able temperature level are usually employed.

The heat-transport medium flows through the heat-ex­changer tubes and, during winter operation, takes on heat from the surrounding earth. In summer operation, it gives up heat to the earth and thus recharges the storage reser­voir. In contrast to aquifer storage systems, the heat-trans­port medium always flows through the DHE in the same di­rection. The workable temperature variations and the stor­age capacity of the borehole field are dependent on the composition of the subsurface earth and on the heat-trans­fer properties of the tubes and their thermal contact with the earth. The reservoir cannot be isolated from its sur­
roundings below ground. In order that the stored thermal energy not be lost, such reservoirs are usually located at sites where the ground water has only a very low – or zero – flow velocity.