## 1 Introduction  
The depletion of fossil fuels and the increasing number of problems caused by global warming have increased interest in renewable energy sources in recent years [1]. At present, 46% of the electricity generated in Germany is obtained from renewable energy sources. As a result of the increasing share of sustainable power generation, the load on the power grid has increased because such power generation has significant dependencies on meteorological influences. In order to enable a further expansion of renewable energy sources, it is necessary to relieve and stabilise the power supply through the use of energy storage devices. Energy storage is divided into electrical, chemical, mechanical and thermal energy storage according to its operating principle [2]. Electrical storage uses electrical and magnetic fields to store energy (for a short time). Chemical storage is the storage of energy with the help of material energy carriers [3]. In electrochemical storage systems, the stored energy is in chemical form in the electrodes. These also act as energy stores and converters. They have a fast response time, but have a short lifespan (<10 years) and show a constant loss of capacity. Mechanical storage uses the forced change in position (potential), speed (kinematics) or the thermodynamic state (pressure) of a material to store energy [4]. Thermal energy storage is discussed as the last operating principle, and can be divided into three types of thermal storage: latent, thermochemical, and sensitive. It is expected that thermal storage will gain increasing importance in the future because the heating sector is the largest energy sector in Europe, accounting for approx. 50% of energy consumption [1]. The most important type of energy storage for this work is that of sensible heat storage. In this type, heat is stored based on temperature difference. More precisely, a medium is heated during charging in order to absorb (excess) heat, and cooled during discharging, or the thermal energy is removed again. In sensible heat storage, the heat capacity of the storage medium, the mass, and the temperature difference are important. Furthermore, effective thermal insulation is essential, especially for long-term storage. Because of its low price and high level of development, it is currently the most common form of heat storage. Thermal salt storage systems are currently used in a large number of solar power plants to ensure a constant power supply even though the solar radiation fluctuates in daylight and disappears during the night. The research and development of these storage systems are currently largely limited to steady-state process simulation models [5]. These do not require any control structures and are mathematically based on mass, momentum, and energy balances, which simplifies the initial system description. However, the transient and continuous changes in operating conditions can influence the efficiency of the investigated power plant. A recent literature review shows that there are few studies regarding the optimisation of the parabolic trough power plant employing dynamic and steady-state models. A high temperature storage model of a solar tower power plant can be explained with open volumetric receiver technology using air as the heat transfer fluid (HTF) [6]. The model of the storage system was designed in the MATLAB/Simulink. The type of storage system investigated in this study is a bed-packed thermal energy storage system that has regeneration properties. It was used to develop various models of thermal storage systems using the lumped parameter method. First, the implemented models were stimulated to achieve high accuracy for the obtained results. Second, the dynamic properties of the thermal storage systems were extensively evaluated by applying turbulence of 15% to the mass flow. The results indicate that the charge and discharge properties of the three developed storage models have the same behaviour [7].