## 1 Introduction  
A concentrated solar power (CSP) tower converts the sun's light energy into heat and electricity. Sun tracing mirrors are used to focus the sunlight onto a receiver located on the top of a tower. There, the solar energy is used to heat up a heat transfer fluid (HTF). This fluid serves to generate high-temperature steam in a steam boiler, to drive a turbine which is connected to an electrical power generator. The power generation is related to the availability of the sun. To increase the independence of CSP systems, thermal energy storage (TES) systems are used. Those systems are able to mitigate short load fluctuations during transient weather conditions, can shift the generation period to peak hours of demand and can extend the energy supply when the sun is not available. Different suitable TES technologies exist. The most common technology is the sensible heat storage. One representative is the two-tank molten salt thermal energy storage, which is extensively deployed and currently the state of the art [1]. The storage medium--molten salt--itself comprises commonly of sodium nitrate and potassium nitrate. Due to its specific properties, the operating temperature range is limited. Freezing and decomposition of the salt must be prevented. For the commonly used \"solar salt\", this means that its temperature must not fall below the melting point of 220 \({}^{\circ}\)C and must not exceed the maximum temperature of about 585 \({}^{\circ}\)C. Extensive research has been carried out and is still ongoing to lower the minimum temperature limit and increase the upper temperature limit. This is due to the fact that higher operating temperatures increase the overall solar-to-electricity efficiency, and lower melting points will reduce the additional heating power. In addition, the TES volume and levelized cost of electricity could be reduced [2; 3]. One promising TES option without the mentioned drawbacks is the regenerator type TES system with a solid inventory and air as HTF. Its prospects are the potential for higher efficiency and lower costs due to the free available, non-toxic heat transfer fluid (air) and the single-tank setup [4; 5]. However, this system results, due to the thermal properties of air, lower volumetric heat capacity and thermal conductivity compared to molten salt, in higher pressure drops and a larger storage tank size [6; 7]. Despite those drawbacks, the regenerator type storage is especially suitable for solar receivers with gaseous working fluids. Many studies on TES for CSP with air as HTF are already available from literature [2; 7; 8; 9; 10; 11]. However, most of the research work in this area is on the development of numerical models and only a few focus on experimental investigations and pilot plants [6; 12; 13; 14]. Due to the high operating temperatures of up to 1300 \({}^{\circ}\)C, one key point of many research projects is the inventory material of regenerator-type storages. Commonly used materials are bricks or packed beds of particles made of oxide ceramic [15]. To reduce the inventory costs, recent works have focused on the investigation of new inventory materials like low-temperature fired clay bricks and magmatic natural stones [16; 17; 18; 19]. Due to its classification as waste, slag from electric arc furnace (EAF) has a high potential for further cost reduction. The thermo-chemical stability and thermo-physical properties of EAF slag were investigated by [PERSON] et al. [20]. The European project REslag investigates the use of a slag pebble inventory in regenerator-based TES systems further [21]. Within this project, [PERSON] et al. [22; 23] already presented the basic work of lead design identification and the investigation of material and thermomechanical aspects. First performance studies as well as experimental results are given by [PERSON] et al. in [24]. However, there are still open questions regarding the thermal performance and thermo-structural stability of the slag pebble inventory under cyclic temperature load.