Simulation and Optimization of Medium Deep Borehole Thermal Energy Storage Systems

In the heating and cooling sector, borehole heat exchangers (BHE) have become increasingly popular for supplying renewable energy. When grouped in compact arrays, BHEs represent suitable thermal energy storage systems for fluctuating heat sources such as solar energy or district heating grids. Tapping into greater depth allows for storage operation on a higher temperature level. This so called medium deep borehole thermal energy storage (BTES) requires negligible groundwater flow in the reservoir rock and the thermal insulation of the upper part of the boreholes to meet legal requirements and to improve the BHEs’ performance. Medium deep BTES is characterized by a slow thermal response and a large storage capacity, which makes it particularly suitable for seasonal heat storage applications. However, the economic feasibility of these systems is compromised by high investment costs, especially by the expensive drilling of the boreholes. Therefore, a priori numerical simulations of the storage operation 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 and are difficult to handle in mathematical optimization applications. This thesis presents a versatile tool for the simulation and optimization of medium deep BTES systems. The Borehole Heat Exchanger Array Simulation and Optimization tool (BASIMO) includes models for the three most common BHE types: U-pipe, double U-pipe and coaxial pipe BHEs. In a dual-continuum approach, the simulator couples a numerical subsurface model with an analytical solution for the BHEs, which allows for the efficient, but detailed consideration of the relevant thermo physical and operational parameters. With the presented tool, many aspects of BTES systems can be simulated and optimized. The concept of medium deep BTES has not been put into practice so far. However, simulations yield promising results and show that large scale medium deep BTES can achieve more than 80 % storage efficiency. The performance is sensitive to many geological, material and operational parameters, but also to the interaction between the BHE array and the downstream heating system. Therefore, future research should focus on coupled simulations including the above ground facilities and, more importantly, on the realization of field experiments including first and foremost a pilot plant, which could help to push this promising technology to economic viability