## 6 Summary and Conclusions  
The use of metallurgical slags from EAF as inventory material for thermal energy storage (TES) has not yet been thoroughly investigated. Since the thermo-physical properties of the slags under consideration are very similar to those of conventional refractory materials and ceramics, this material offers the economic advantage of being a very cost-effective alternative to conventional inventory materials for solid media TES. However, for a successful market introduction of this technology, efficient and up-scalable solutions for the TES are a prerequisite. In this work, thermal performance and thermo-structural stability of slag-based TES are evaluated. For this purpose, two different test rigs are used. The thermo-structural stability is investigated with a uniaxial compression test rig, while the thermal performance is tested by using the high-temperature test facility HOTREG located at the German Aerospace Center in Stuttgart. In addition, a TES simulation model was set up and validated to calculate the thermal performance of scale up and full-scale TES plants. Results of the test campaign with the pilot TES plant confirm the thermal suitability of EAF slag as an inventory material for concentrated solar power tower plants. By comparing the experimental thermal results with the simulation results, a good agreement can be seen. A maximum deviation of 10% could be achieved. The detected deviations are a result of simplified model assumptions. Additionally, the prototype scale of the test rig leads to higher impact of losses, which results from the higher surface-to-volume ratio of the pilot compared to the full-scale plant. The relatively high losses of the prototype can also be seen in the downtime behavior, where an energy loss of 35% after two days occurs. The thermo-mechanical tests with the slag and different insulation options confirm, on the one hand, the suitability of raw EAF slag as an inventory material for a full-scale TES plant. However, after the UCT's, the sintered slag pebbles show damages of around 10 wt. %. This results in improvement needs regarding the manufacturing process of the pebbles. On the other hand, the uniaxial compression tests indicate that a protection layer is required for the perlite brick insulation. By investigating three different protection options, a promising one could be identified. The best option here is to use super-duty firebricks as a protection layer in front of the perlite brick insulation. In summary, it can be stated that CSP plants can benefit from slag-based TES. However, there are still some uncertainties that need further investigation such as the long-term stability and dust generation of the slag as well as the exact thermomechanical behavior. Figure 16: UCT of perlite brick insulation protected with super-duty firebricks. **Author Contributions:** The paper was written by [PERSON] under the guidance of [PERSON] and [PERSON]. The experimental investigation of the slag and the insulation options was carried out by [PERSON] and [PERSON]. The simulation model was set up by [PERSON] All authors have read and agreed to the published version of the manuscript. **Funding:** This project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement No. 642067. **Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript or in the decision to publish the results. **Nomenclature**