Concentrating solar tower power systems use numerous large, flat, sun-tracking mirrors (heliostats) for focusing sunlight onto a receiver, based at the top of a tower. Thus a heat transfer fluid (HTF) is heated in the receiver, which is used to generate steam and therefore electricity. Due to the high conversion temperature, air used as a HTF has a particularly promising potential for a high solar-to-electric efficiency. One challenge for solar tower power plants is the reduced energy output, when the sun sets or is blocked by clouds, as well as the problem, that there is no sunlight and therefore no energy production during night. To face this issues thermal energy storage (TES) are used. A suitable heat storage technology is the regenerator storage based on directly heated solid media. It has a simple setup, is applicable to highest temperatures ( > 1000°C) and has best prospects for a deployment in large installations. Here slag from steel industry as an inventory material offers cost reduction potentials (Fig. 1). This is due to the fact that slag is treated as waste and can be turned into valuable low cost feedstock for TES. However, for a successful market introduction of this technology, efficient and up-scalable solutions for the heat storage are a prerequisite. Regenerator storages are well suited to the needs, but need further investigations to clarify open questions concerning the implementation in large installations in combination with slag-pebbles as inventory. The EU project RESLAG is dedicated to this.