Numerical modeling and optimization of an insulation system for underground thermal energy storage

• Numerical analysis and design optimization of thermal insulation system for UTES.
• Temperature-dependent properties of insulation and other materials were applied.
• Component arrangement affected the performance of the insulation system.
• Optimal system consisted of inner & outer mineral wools, a steel liner and a lining.
• Regular emptying of the caverns slowed the temperature increase in the rock mass.

Underground thermal energy storage (TES) systems require an insulation system to control the heat flux from the TES system into the surrounding rock mass to minimize the adverse effects of the high-temperature storage media on the underground environment. In this study, numerical simulations were performed to investigate the temperature distribution and heat transfer in the insulation system and to determine the optimal design for the TES system. The performance of the mineral wool chosen as an insulation material is gradually degraded as its temperature is increased; thus, the temperature-dependent properties of component materials were applied to prevent the overestimation of the insulation performance. We demonstrated that the component arrangement could affect the performance of the insulation system under transient heat transfer conditions; thus, we compared three types of insulation systems according to the location of the mineral wool layer. An insulation system composed of an inner mineral wool layer, a steel liner, an outer mineral wool layer, and a concrete lining was proposed as the optimal system under the conditions given in this study. We showed that regular emptying of the storage caverns for a long time interval in periods of low energy demand would slow down the temperature increase in the surrounding rock mass, and we proposed that the operation schedule including idle time could also be an alternative to reduce the thickness of the mineral wool layers.