Medium Deep Borehole Thermal Energy Storage Pilot Plant

Abstract

Borehole thermal energy storage (BTES) is eminently suitable for seasonal storage of large amounts of excess heat. However, water authorities regard the associated thermal impact on shallow aquifers as critical. Medium deep borehole heat exchangers (BHE) tap into larger depth and allow for the use of thermal insulation in the topmost section of the boreholes. As a result, the thermal load on topmost aquifers is significantly reduced and shifted to the rock volume below. Numerical simulations have shown, that these medium deep BTES systems can store heat on a large scale and effectively protect critical drinking water reservoirs. However, the existence of modern district heating grids and the large system size required for efficient operation raises the capital cost. As a consequence, medium deep BTES systems have not been put into practice so far. We present the concept of the first pilot plant for a medium deep BTES system built on the campus of Technische Universität Darmstadt. The array consists of four coaxial BHEs, which tap into a crystalline reservoir rock underneath a sedimentary cover of approximately 50 m thickness. All BHEs are equipped with flowmeters and fiber optic cables for depth-dependent temperature measurements. In close vicinity, wells with multi-parameter probes complete the extensive monitoring setup. Experiments run using mobile heating and cooling units, which simulate charging and discharging cycles and allow for full control of the storage operation. Together with geophysical exploration data from the construction phase we expect the pilot plant to provide comprehensive data for the validation of our numerical experiments and to proof the concept of medium deep BTES.