技術情報とプレゼンテーション

Small-scale short-term thermal energy storage (TES) systems and large-scale seasonal TES systems have been the focus of many research projects. However, large-scale short-term TES units have not been covered in great depth, even though they play an important role by decoupling the heat demand from the heat generation, allowing for more efficient district heating networks (DHN) and a reduction of carbon dioxide emissions. Results of studies on small-scale storage units or long-term TES systems cannot be transferred easily, as the thermo- and hydrodynamic properties of the tank vary with its size. Therefore, the corresponding processes of a short-term TES unit with a size of 4300 m³ are investigated using the COMSOL Multiphysics^® software. Losses occur during charging, standby, and discharging of the TES unit for various reasons. Two principal mechanisms are the heat flux through the wall and the destratification due to mixing. These and other losses are observed and their magnitude is estimated for a storage unit of the given dimensions.

A 3D simulation is performed using the COMSOL Multiphysics^® software, as this leads to more accurate results than a 2D-axisymmetric simulation. The storage vessel including the wall, insulation, and internal structures is simulated in a reference environment. A Turbulent Conjugate Heat Transfer multiphysics selection in COMSOL Multiphysics^® includes the Heat Transfer modules, Heat Transfer in Solids and Fluids interface along with the Turbulent Flow interface and a Nonisothermal Flow Multiphysics coupling. For charging and discharging, the timeframe of the simulation encompasses several hours, while the standby phase is simulated for up to 24 hours. This, in combination with the large dimensions of the storage unit, requires significant computational resources. Therefore, the simulation is performed using the Lichtenberg high performance computer of the TU Darmstadt and managed by the COMSOL Multiphysics^® software’s Cluster Computing Functionality.

For a TES unit of this size, the external losses via the wall are expected to be larger than all other losses, though these should still be relevant. The turbulent mixing during charging and discharging is expected to affect the development of thermal stratification inside the vessel significantly. Some mixing should also occur near the wall due to the heat losses to the environment. While a disturbance of the thermal stratification does not lead to a loss of energy, it reduces the exergy stored in the tank. Therefore, both energetic and exergetic analyses are performed. In the future, the created model will be validated using measurement data from a real-world TES unit in the city of Darmstadt, which is currently under construction. Additionally, a simplified one-dimensional model for a TES unit of this size will be developed to aid the analysis and design of DHN.