Numerical Studies on Underground Thermal Energy Storages

The ever-increasing global energy consumption leads to a rise in demand for renewable energy sources and storage solutions. Nevertheless, in several cases energy is not sufficiently stored and dissipates unused (e.g., waste heat from air conditioning systems). This results in increasing the air temperature, especially in urban areas. Underground thermal energy storage (UTES) systems can be used to utilize underground soil to store unused energy for use when needed (e.g. district heating). The objective of this paper is to investigate the implementation of a UTES system in the 2D finite element software PLAXIS. Furthermore, the modelling approach used for thermally insulating the storage tank by using improved diaphragm walls was evaluated. In case of a UTES system, it is necessary to model a heating phase in which the storage stores energy for later use. For this purpose, different modelling approaches were investigated. Each analysis aimed to heat the storage reservoir to 90 °C. Afterwards, the difference in temperature distribution in the soil was investigated. Moreover, the influence of the thermal properties of the diaphragm walls on the temperature distribution was reviewed. Concrete with a density of 2400 kg/m3 was compared to a lightweight concrete developed at the Institute of Structural Concrete at Graz University of Technology. The thermal properties of the lightweight concrete were determined based on laboratory tests. The lightweight concrete had a thermal conductivity of 1.026 W/m K, which led to a significant reduction in heat losses from the storage.