Assessment of Geothermal Heat Provision from Deep Sedimentary Aquifers in Berlin/Germany: A Case Study

Regions of active volcanism offer high-enthalpic production rates at comparably low development costs, but most of the developed and highly energy-consuming countries do not possess such geothermal resources. Even here, the crust's heat potential is enormous and may be mined for direct heating or energy conversion purposes, but the potential is largely undeveloped for different reasons: Limited geological data induce development risks, technological challenges, high investment and limited public reception. This situation holds true for the low-enthalpic hydrothermal heat reservoirs of the Northeastern German Basin, and specifically for the city of Berlin, capital of Germany. This contribution aims at illustrating the thermal power of hydrothermal heat plants at urban scale for the Berlin region. For this purpose, we selected a specific plant design and evaluated the heat production capacity of PostPermian reservoir units (Middle Buntsandstein). This was based on a regional-scale structural model and statistically derived geohydraulic model parameter of the Northeast German basin; see Kastner et al. (2014) for the details of the underlaying model. In this contribution, we discuss the model results for the campus of the Technical University of Berlin, which may serve as a showcase. The primary energy consumption of the German economy amounts to 13,908 peta joule in 2013, cf. AG Energiebilanzen e.V. (2014). This number is regarding the total energy supply from any imported, home mined and renewable sources, excluding exports and bunkering, but prior to any conversion processes take place. The primary energy consumption has slightly increased 2012-2013 by 2.5%, while the total electric energy production remained stagnant during this time. A closer examination traces the increase of the primary energy back to enhanced needs for space heating during the particularly cold winter season of 2013. Heating in Germany is almost entirely based on fossil fuels, primarily natural gas and mineral oil. The cold winter in 2013 induced increased shares, by 2.2% for mineral oil and by 6.4% for natural gas. As a result the carbon emissions kept stagnant at a high level in 2013, despite a significant share of renewable electric power in Germany, thus thwarting the overall governance goal of carbon emission reduction. Therefore, to meet this goal the heat supply market has to develop a significant share from renewable resources in the future. Geothermal systems have the potential to contribute in principle, however, this potential is largely undeveloped for different reasons: Limited geological data inducing development risks, technological challenges, high investment costs and limited public reception. This situation is true especially for densely populated urban regions, comprising high demand and complex, heterogeneous supply structures. The city of Berlin/Germany may serve as a representative example of this. An enormous potential (heat in place) of the deep sedimentary aquifers in Berlin's underground has been demonstrated (Kastner et al. (2013), Sippel et al. (2013)). Geologically, Berlin is situated in the Northeast German Basin, Fig. 1. The thermal gradient in this region is moderate, but promising hydrothermal reservoirs are expected in porous rock formations predicted at elevated depths between 1,000 and 5,000 meters. Typical reservoir formations in the region consist of sandstone units of variable thicknesses. The pore space of these units contains aqueous fluids characterized by depth-related temperature and salinity; this water body may be developed to utilize the heat potential by hydrothermal plant systems: The fluid is pumped to the surface through production wells and after heat extraction pumped back through corresponding injection wells to stabilize the formation pressure. A preliminary assessment of the thermal capacities of such heat plants in Berlin are investigated in the follow-up study Kastner et al. (2014) (submitted). In the framework of this extended conference paper we refer to this work. The model relies on an existing structural model of the Berlin/Brandenburg region (Fig. 1a), which resolves the relevant reservoir formations at major unit scale. The required transport parameters are derived statistically from available field data of the Northeast German Basin within specific intervals of confidence. The resulting geological model, therefore, represents the present state of public knowledge for this region. The hydrothermal heat plant model was elaborated under the fundamental assumption that this database is sufficiently accurate to address the main tendencies on average. Under the constraint imposed by the geological uncertainties, an idealized heat plant design is considered, which deliberately ignores a range of inferior details: temperature and pressure losses along the piping, impact of the fluid chemistry, technical restrictions resulting from pump efficiencies, surface heat networks and buildings, as well as possible improvements due to different well design, development strategies and plant operations. To rationalize this complexity, we select a specific scenario and evaluate the geological model. This scenario is characterized by the assumption of a specific plant design and re-injection temperature, standard confined aquifer conditions and idealized fluid properties along the pipe system. The operation point of the heat plant is selected so as to prevail absorbing conditions in the injection well. This model is employed to assess the heating powers of virtual doublet systems fed from the Middle Buntsandstein. In this contribution, we specify the results for a selected site in Berlin, the location of the campus of the Technical University of Berlin. The campus is located at the city's decumanus in the western part of the city (N52.511332°, E13.326912°), indicated by the white triangle “TUB” in the contour map of Fig. 2B.