With regard to the construction of the seasonal TES in Eggenstein several boundary conditions had to be considered. As the ground water level is only 7.5 m below top ground surface, the store had to be constructed in such a way that even in the case of a hundred year flood, the thermal insulation is protected from penetration of ground water. Hence, an external HDPE liner for ground water protection was installed.

A further limiting boundary condition is that the store is located in the area of a schoolyard. Unrestricted accessibility for the pupils and also trafficability was demanded by the customer. Obviously, a 100% safe construction — even in the case of a total failure of the liner — was required. Therefore, a concept with a gravel-water store was favoured over a hot water store.

The geometry of the store consists of two truncated cones, see Fig. 5. 2/3 of the volume of the store is located below ground surface. It is filled with 16-32 mm gravel to a height of 2.5 m. In the remaining volume the excaveted gravel/sand is refilled in order to reduce construction costs. The upper 1/3 of the store is formed as an truncated cone with washed 16-32 mm gravel. Charging and discharging of the gravel-water TES is realized by two vertical wells. One is embedded in the bottom gravel layer, the other in the top gravel laye

An inverted truncated cone with a height of 7 m and a diameter of 35 m was excavated. The

The internal liner consists of of a HDPE membrane with vapor barrier. The aluminum layer prevents water vapour diffusion and thus protects the thermal insulation from getting wet during the entire period of operation of more than 30 years. On the present market high temperature liner materials are not available. Hence, standard geomembranes known from landfill constructions had to be used for the lining of the store. Reliable information about service lifetime of polymer liners under operation conditions of a seasonal TES is not available. Therefore, the maximum operation temperature of the store is limited to 80 °C.

Due to the situation that the ground water level is only some centimeters below the store, special attention was drawn to the design of the store with respect to insulation type and thickness. Simulations conducted by ITW and Solites yielded that an increased insulation thickness at the bottom is required. Hence, on bottom and on side walls below top ground surface 50 cm expanded glass granules was suggested. Above top ground surface 90 cm foam glass gravel form the insulation on side walls and at the top of the store. Both, foam glass gravel and expanded glass granules are pressure resistant in the required range. Foam glass gravel was favoured over expanded glass granules for the upper part as it is shapeable due to its relatively high friction angle. The resulting dimensions of the insulation corresponds to the economic optimum with regard to the material and installation costs within the limits of the available budget.

insulation

covered with HDPE liner

Fig. 6: Primary and secondary chambers, installation of the insulation by blowing from silo-truck into
chambers or loose; lower left: installation of the foam glass gravel by pouring from big bags. It is delivered
by trucks and filled on-site into the big bag by a front loader, lower right: filled and evacuated chamber.

In order to protect the insulation from getting wet an external liner for ground water protection was installed. The external liner and the internal barrier liner are welded together such that they form chambers, which are filled with the bulk insulation material. In total 30 primary and secondary chambers were built, see Fig. 6.

Based on a method tested in Steinfurt-Borghorst [6] a leakage detection system was established using these chambers. Preliminary tests were carried out in the framework of the R&D project “Further development of pit heat stores” at ITW [4]. As the tests showed good results the evacuation system was established. After filling the 30 chambers with the insulation they were hermetically sealed by hot air and extrusion welding technique and evacuated to roughly 0.5 bar. This procedure enables leakage detection during construction and if desired also during operation.

For future projects with gravel-water store such an evacuation system is recommended. However, it is recommended that state-of-the-art vacuum equipment is applied and that a warning system signals in case of increasing pressure.