Bär, K. ; Rühaak, W. ; Welsch, B. ; Schulte, D. O. ; Homuth, S. ; Sass, I. (2016)
Seasonal high temperature heat storage with medium deep borehole heat exchangers – a conceptual case study.
European Geothermal Congress. Strasbourg (20.09.2016-22.09.2016)
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Kurzbeschreibung (Abstract)

More than 50 % of the overall energy demand in Germany is due to heating and cooling purposes. Therefore, ground-breaking techniques are needed to save energy and reduce greenhouse gas emissions especially in this low exergy sector. The combination of different renewable energy sources – solar thermal, industrial or waste heat and geothermal – with already existing district heating systems fed by combined heat and power stations (CHP) is a promising new approach. In summer, excess solar thermal energy and industrial/waste heat is available, while in winter when thermal energy is needed for heating systems its quantity is usually not sufficient. There are different options to cope with the seasonal offset of thermal energy supply and demand. Besides conventional storage tanks at the surface or thermal storage in shallow aquifers and shallow borehole thermal energy storages (BTES), geothermal heat storage in moderate depths is an innovative and yet barely tested concept. In difference to shallow heat storage systems, the proposed approach upgrades the naturally available geothermal energy in the subsurface by means of external heat input. This is done in summer when no space heating is required or at times when surplus energy from nearby sources is available. In winter, when other sources of energy are not sufficiently and cheaply available, the thermal energy from the geothermal storage is used for heating purposes. The focus of the presented study is the environment friendly and energy efficient redesigning of a more than 50 years old office and laboratory building. By storing excess heat from solar panels or thermal power stations of up to 110 °C in summer, a medium deep BTES can be operated on temperature levels above 45 °C. Storage depths of 500 m to 1,000 m below surface avoid conflicts with shallow groundwater use and additionally utilizes the geothermal site potential. Groundwater flow is decreasing with depth, making conduction the dominant heat transport process. To reach medium depths the water powered down the hole (DTH) hammer drilling method can be utilized with smaller drill rigs and corresponding small site requirements which results in lower compatible price ranges for medium deep drilling operations compared to conventional rotary mud drilling methods. Feasibility and design criteria of a coupled geothermal-solar thermal case study in crystalline bedrock for the office building are presented and discussed. A BTES system as well as an energy efficient building design will help to use sustainable energy sources for the next period of the building's lifetime.

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