Large thermal heat storages in rock caverns – numerical simulation of heat losses

In the future large energy storage facilities will play a key role in district heating systems that transport heat energy through tube systems with water as transport media. Energy storages enable storage of renewable energy and industrial waste heat through flexible buffer heat storage and allow a reduction in installed capacities of heat supply stations. In this article, the application of water-filled rock caverns for the use of large thermal energy storages is analysed. A key issue is the energy loss over the month/year. Therefore, this study focuses on the quantification of energy losses from water-filled rock caverns by means of numerical analysis. Three different rock cavern geometries are analysed, varying rock conductivity parameter and varying temperature profiles of the water storage. By simulations with the software COMSOL it could be shown that (i) the energy losses of underground caverns are only 25 % compared to the energy losses from the currently used insulated steel tanks located above ground, (ii) the energy losses can be further decreased by the application of a thermal insulation layer and (iii) the energy losses decrease over the lifetime due to the reducing temperature gradients in the surrounding rock. Since cavern reservoirs can be operated for more than 100 years, these findings are of great relevance and shall be further investigated with respect to economical assessment.