Storage of Mechanical Energy and Extraction of Heat Via Artificial Fractures In Low Permeable Rock

The storage of surplus power generated by wind turbines or solar panels under favourable weather conditions is of significant importance for a successful transition of current energy systems. Surplus power can be used for hydrogen production or for charging batteries. Later, by demand, it can be retransformed and fed into the power grid again. However, there is also an option to store surplus power as mechanical energy in the underground and to combine this storage option with the extraction of geothermal heat. At the geothermal research well “Horstberg” in Germany experiments have been performed to generate heat out of deep and low permeable sedimentary rock. Here, a large and highly conductive artificial fracture was created in the Buntsandstone formation at approximately 3800 m depth. For the hydraulic stimulation a calculated amount of ca. 200 MWh electricity was necessary. In succeeding production tests, the water was produced back at a high pressure above 100 bar and a significant portion of the injected hydraulic energy would have been retrievable with a turbine in the water stream. Further cyclic injection and production tests designed for heat extraction revealed as a side effect that approximately half of the electric energy necessary for injection could have been recovered while producing. Obviously, a large portion of the hydraulic pump energy was stored in the underground as mechanical energy due to ballooning of the fracture and due to elastic compression of water and rock surrounding the fracture. The efficiency of energy storage can be improved significantly by implementing a horizontal well design with multiple fractures. This is shown based on model calculations. If water is injected in parallel artificial fractures the static pressure level between the fractures increases, water losses into the far field decrease and the back-production is improved. Furthermore, overpressurised reservoirs and low permeable rock are favourable. Thereby the injected water remains in the closed surrounding of the fractures and the complete artesian back production at high pressure is ensured. Overpressurised formations seem to be widespread in the deep underground of sedimentary basins as in the North German Basin. Mechanical energy storage in the deep underground should be combined with geothermal heat extraction. At the test site Horstberg thermal water at a temperature of more than 100°C was produced in cyclic tests. Numeric modelling results suggest that a thermal power of appr. 1 MW can be extracted by cyclic production in the long term via the large fracture in Horstberg. For the realisation of the storage concept several challenges have to be met. Besides the creation of good underground conditions, the handling of the produced saline water and its reinjection without scaling or corrosion are serious issues. On the other hand the storage of surplus power as mechanical energy in the underground and its retransformation to power can be more efficient than a conversion into hydrogen and less expensive than battery storage. The reuse of abundant deep wells for energy storage could be a very cost-efficient starting point for this concept.