## 1 Introduction  
The application of thermal energy storage (TES) combined with conventional power plants and power plants driven by renewable energy sources offer considerable advantages. For conventional power plants, the integration of TES potentially improves the operating flexibility and reduces minimum load, thereby a decrease in operational costs and an increase in power plant system efficiency are prospected. For power plants based on renewable energy, e.g., concentrated solar power (CSP) plants, TES opens up the opportunity to continue operation beyond sunshine hours and to compensate for alternating solar radiation income during daytime. Additionally, technologies like adiabatic compressed air energy storages (ACAES) are based on the application of TES as well. Further applications can be found in steel and glass manufacturing or other industrial processes with large amounts of waste heat, where the heat can be stored in a TES for later recirculation into the process. For applications with gaseous heat transfer media, regenerator-type heat storages offer cost-effective and straightforward design solutions. In such arrangements, the gaseous heat transfer media, e.g., flue gas or air, flows through a solid storage material, and sensible heat is transferred via direct contact from the heat transfer media to the storage material (charging) or vice versa (discharging). The following paragraph gives an overview of existing regenerator-type heat storage technologies and materials. Conventionally used solid media energy storage in the steel manufacturing industries consists of fixed bed installations of shaped ceramic bricks or clinkers, which are stacked inside of a container. Depending on specific requirements of the storage application, a variety of different brick sizes and shapes can be utilized. A selection of possible inventory options is shown in Figure 1. Another important part of the arrangement is the isolation, which is typically installed between the storage material and the inner container wall. There are multiple options of viable isolation materials, e.g., refractory or fire clay bricks. The regenerator is operated by channeling a gaseous heat transfer fluid (HTF) through the storage material. Thermal charging of the storage material is achieved by a hot HTF streaming through the storage material. The flow direction usually is from top to bottom of the arrangement in order to ensure lower temperature at the load-bearing lower part of the container. In search of cost-efficient inventory material with adequate thermal performance, packed beds of ceramic pebbles or simply of natural stone were identified as promising options. While offering low cost potential (especially in the case of natural stone packed beds), the heat transfer rates are still comparable to shaped bricks installations. While smaller-sized particles increase the thermal efficiency of the packed bed compared to larger particle sizes, they also increase the pressure loss during operation of regenerator-type heat storages. This tradeoff leads to typical particle sizes between 10 mm and 100 mm, depending on the specific application. However, when used as heat storage material, packed beds lead to a high technical risk for the inventory itself and the encircling containment isolation. This issue is a result of the punctiform inter-particle and particle-isolation contacts, further amplified by cyclic thermal swelling and shrinking of the particles during the thermal charging and discharging process. As a consequence, mechanical and thermo-mechanical loads are induced. Considering thermal cycling on a daily basis and large bed heights, those induced loads might have a severe negative impact on the life expectancy of the storage. Therefore, proper adjustments in container and packed bed design have to be made. For that reason, knowledge of the occurring stresses and loads is essential, which, again, requires appropriate test rigs and simulation tools in order to calculate those loads.