Solutions for seasonal energy storage systems are an essential component for the reliable use of fluctuating renewable energy sources and to bridge the gap between abundant heat availability from renewable sources in summer and an increased heat demand in winter. As a part of the research project ’solar crystalline borehole thermal energy storage system‘ – ‘SKEWS’, a field-scale demonstrator for a medium deep borehole thermal energy storage (BTES) system with a maximum depth of 750 m is to be built at Campus Lichtwiese of the Technische Universität Darmstadt, Germany, to demonstrate this innovative technology. In this first demonstration phase, the storage array consists of four coaxial BHEs (Borehole Heat Exchangers), which tap into a crystalline reservoir rock underneath a thin sedimentary cover. Prior to project launch a numerical model of the storage system was to be built to investigate the storages behavior under the local geological and hydrogeological conditions. In the first stage, the geological context in the surroundings of the project location was investigated using archive drilling data and groundwater measurements. The data obtained facilitated the development of a geological model concept. It suggests the assumption that the uppermost part of the intended storage domain is crosscut by a normal fault, which displaces the Permian rocks east of Darmstadt against granodioritic rocks of the Odenwald crystalline complex. The simplified geological model was implemented in a 3D-finite-element numerical model to simulate the thermal effect of the storage system operation on the surrounding subsurface. The model for the four planned BHEs did not show the formation of any significant heat plumes by groundwater flow with only a minor increase in groundwater temperature.
Additionally, the numerical model was used to estimate the effect of the potentially highly permeable fault zone on the planned storage site. For this purpose, a storage operation over a time span of 30 years was simulated for different parametrizations of the fault zone and the storage system. The simulations reveal a limited but visible removal of heat from the storage region with increasing groundwater flow in the fault zone. However, since the section of the BTES system affected by the fault is very small in comparison to the system’s total depth, only a minor impairment of the storage efficiency could be observed in the worst case.
Lukas Seib, Bastian Welsch, Matthis Frey, Claire Bossennec, Kristian Bär, Ingo Sass
Technische Universität Darmstadt, Germany