• Energy Research

In the heating sector, borehole heat exchangers have become popular for supplying renewable energy. They tap into the subsurface to extract geothermal energy for heating purposes. For advanced applications, borehole heat exchangers require insulation in the upper part of the borehole either to meet legal requirements or to improve their performance. A priori numerical heat transport models of the subsurface are imperative for the systems’ planning and design. Only fully discretized models can account for depth-dependent borehole properties like insulated sections, but the model setup is cumbersome and the simulations come at high computational cost. Hence, these models are often not suitable for the simulation of larger installations. This study presents an analytical solution for the simulation of the thermal interactions of partly insulated borehole heat exchangers. A benchmark with a fully discretized OpenGeoSys model confirms sufficient accuracy of the analytical solution. In an application example, the functionality of the tool is demonstrated by finding the ideal length of a borehole insulation using mathematical optimization and by quantifying the effect of the insulation on the borehole heat exchanger performance. The presented method allows for accommodation of future advancements in borehole heat exchangers in numerical simulations at comparatively low computational cost.

Abstract An interface is developed to couple the finite element program FEFLOW with the MATLAB-SIMULINK software. The TCP/IP based data exchange routine allows for a co-simulation of borehole heat exchangers and of HVAC components, so that each subsystem can be modeled in its specialized simulation environment. Hereby, the interaction of the subsystems is taken into account, which leads to a more precise representation of the systems’ dynamic behavior. Furthermore, the concept supports the application of mathematical optimization algorithms that can be utilized to automatically determine several design parameters for an overall improved system performance.

AbstractInnovative applications and novel modifications of borehole heat exchangers (BHE) require new simulation tools. Currently, features like inclined or partly insulated boreholes necessitate fully discretized models. However, those models come at high computational cost. We present a tool, which uses an analytical solution for the BHE coupled with a numerical solution for the subsurface heat transport. A tetrahedron mesh bypasses the limitations of structured grids for borehole path geometries, while BHE properties changing with depth are considered. The tool benefits from the fast analytical solution of the BHEs while still allowing for a detailed consideration of the BHE properties.

AbstractThe modeling of freezing and thawing of water in porous media is an area of increasing interest. Examples include the modeling of permafrost degradation due to climate change, geotechnical applications in tunneling, and borehole heat exchanger performance in cold regions. Different code implementations have been developed and an interest has arisen in benchmarking different codes with analytical solutions, experiments, and numerical results. The name for this benchmark consortium is INTERFROST. Benchmark results are shown for a 1D analytical solution and two different 2D set-ups. All compared codes exhibit a similar behavior.

AbstractArrays of medium‐deep borehole heat exchangers are characterized by their slow thermal response and large storage capacity. They represent suitable thermal energy storage systems for seasonally fluctuating heat sources such as solar energy or district heating grids. However, the economic feasibility of these systems is compromised by high investment costs, especially by the expensive drilling of the boreholes. This study presents an approach for the simulation and optimization of borehole thermal energy storage systems. To exemplify the concept, a software tool is used to optimize the number and length of borehole heat exchangers with regard to a specific annual heat demand. The tool successfully determines the ideal size of the thermal energy storage. Furthermore, the prediction of the system’s performance also indicates that borehole thermal energy storage systems only operate efficiently in large‐scale applications. With the presented tool, many aspects of borehole thermal energy storage systems can be simulated and optimized.

Geothermal energy has the potential to significantly contribute to the cooling of buildings. A shallow aquifer system in north east Jordan was proven as a geothermal resource for its efficiency for cooling utilization. A numerical 3D model was developed in order to predict the future performance of the geothermal cooling reservoir. Different possible geothermal installations were studied, using various approaches. The study shows that a geothermal utilization of the respective basaltic reservoir is feasible. It features sufficient hydraulic and thermal properties to be utilized for cooling purposes. The developed model has proven to be robust and flexible. It can be easily extended for analyzing other sites.

AbstractSubsurface temperature is one of the key parameters in geothermal exploration. The estimation of the reservoir temperature is of high importance and usually done either by interpolation of temperature data or numerical modeling. However, temperature measurements of depths larger than a few hundred meters are generally very sparse. A pure interpolation of such sparse data always involves big uncertainties and usually neglects knowledge of the reservoir geometry or reservoir properties.Kriging with trend does allow including secondary data to improve the interpolation of the primary one. Using this approach temperature measurements of depths larger than 1,000 m of the federal state of Hessen/Germany have been interpolated in 3D. A conductive numerical 3D temperature model was used as secondary information. This way the interpolation result reflects also the geological structure. As a result the quality of the estimation improves considerably.

AbstractHeating of buildings requires more than 25% of the total end energy consumption in Germany. By storing excess heat from solar panels or thermal power stations of more than 110°C in summer, a medium deep borehole thermal energy storage (MD-BTES) can be operated on temperature levels above 45°C. Storage depths of 500 m to 1,500 m below surface avoid conflicts with groundwater use. Groundwater flow is decreasing with depth, making conduction the dominant heat transport process. Feasibility and design criteria of a coupled geothermal-solarthermal case study in crystalline bedrock for an office building are presented and discussed.