In Germany, the reduction of greenhouse gas emissions has been stagnating for a couple of years now. As a consequence, the climate targets for 2020 are at a risk of being missed. The energy transformation has strongly focused on the electricity sector while mostly disregarding the heating sector. Solar thermal energy and industrial waste heat offer a considerable potential for the replacement of fossil fuels in the heating sector. However, their utilization is hampered by the asynchronous seasonal fluctuation of heat demand and heat supply. Thermal energy storage technologies are required, which are able to absorb large amounts of heat in summer, store it for several months and release it during winter with minimal losses.
Borehole thermal energy storage (BTES) is such a technology for seasonally storing heat on a district scale. A dense array of multiple borehole heat exchangers (BHE) exploits the natural subsurface as a heat storage medium. Conventional BTES systems usually do not exceed a depth of 200 m. Consequently, their operation implies a large thermal impact on shallow geologic formations. This, in combination with comparatively strict groundwater regulations in Germany, impedes the construction of such shallow systems.
The unprecedented, still unrealized concept of medium deep borehole thermal energy storage (MD-BTES) is expected to remedy these shortcomings. MD-BTES systems consist of much less, but appreciably deeper BHEs (up to 1000 m). Consequently, they require significantly less ground surface. Therefore, they are particularly advantageous in densely populated urban areas, which are characterized by large heat demands and scarcely available space. More importantly, a large portion of the thermal energy is stored into deeper geologic formations, reducing the thermal impact on shallow aquifer systems. However, the magnitude of this reduction has not been quantified yet. Furthermore, the general applicability of MD-BTES systems, as well as their economic and environmental implications remain unclear.
As part of this thesis, a large number of numerical simulations was analyzed in a parameter study to investigate the influence of various design and operation variables on the performance of MD-BTES systems. In total, 200 different MD-BTES geometries were compared. Moreover, the influence of subsurface conditions, operating temperatures and the interconnection scheme of BHEs was studied. The results demonstrate the excellent suitability of MD-BTES systems for large scale seasonal heat storage. With a proper dimensioning and in convenient geological and hydrogeological framework conditions, these systems can reach storage efficiencies of 80% or more, while maintaining relatively high supply temperatures of 30 °C. Further numerical simulations provide evidence for a significant mitigation of the thermal impact on shallow groundwater resources by the application of MD-BTES systems instead of their shallow counterparts.
In order to resolve the economic and environmental questions connected to MD-BTES, a MATLAB based assessment tool was developed. It is used for a comprehensive economic and environmental life cycle assessment study on the integration of MD-BTES into district heating concepts. The results reveal the dependency of the economic and environmental impacts on the assumed financial and economic boundary conditions. However, they also demonstrate the high economic competitiveness of MD-BTES in combination with solar thermal collector fields, when supposing a likely increase of energy prices in the future. Furthermore, the combination of a solar thermal collector field, an MD-BTES system and a small combined heat and power plant undercuts the emissions of system combinations without any seasonal storage by 32% and more, when assuming a probable decrease in the emission factor of the electricity grid mix
