## 2 State of the Art of TES in CSP Plants  
In CSP plants, molten salt technologies have been extensively deployed in CSP applications for the storage of thermal energy prior to steam or electricity generation. An example among many is the Solana power plant with 280 MWe and a storage system able to supply full power for six hours. [2]  
The technology of regenerator-type storage is less developed, but has the potential for higher efficiency and lower costs. In particular, the application of regenerators in CSP power plants with an open volumetric receiver seems to be a promising approach, as shown in Figure 1.  
The Julich solar power tower in Germany, seen in Figure 2, is currently the only plant in the world to be based on this technology. This experimental central receiver plant was inaugurated in 2009 to facilitate the further development of this technology. The erection and operation of the plant have been accompanied by a research program, whose objective is to cover the remaining uncertainties around design and operation and to further promote the development of the technology towards a commercial deployment. As a part of this work program, heat storage operation and design are also addressed [8].  
As mentioned before, the plant uses an open volumetric receiver technology developed at DLR (German Aerospace Centre). In its primary cycle, air at atmospheric pressure is heated up to temperatures of about 700 \({}^{\circ}\)C. This solar heat then powers a steam generator, producing steam at 100 bars and 500 \({}^{\circ}\)C and driving a 1.5 MWel turbine-generator set. In parallel to the steam generator and receiver, thermal energy storage is integrated into the power cycle, as seen in Figure 1. It is implemented as an air-cooled regenerator storage system, an installation that is still unique in this application. The state of the art is set with the recently completed HOTSPOT project [9].  
Generally, with regenerator-type storage, temperatures of up to 1000 \({}^{\circ}\)C and even more can be realized by using solid storage material such as commercially available bricks or beds of smaller particles made of oxide ceramic material [10]. Recent work [11; 12; 13; 14] is investigating low-cost alternative inventory materials such as low-temperature-fired clay bricks and magmatic natural stones.  
Figure 1: Flowsheet of slag reuse as a thermal energy storage (TES) inventory for a concentrated solar power (CSP) plant with an open volumetric receiver.  
An alternative to this is slag from an electric arc furnace (EAF). Since this type of slag has not been considered as an inventory material for this purpose, it is important to clarify fundamental questions about the material and TES design. These questions are addressed by the REslag project. The main objective of the project is to make an effective valorization of the steel slag and reuse it as a feedstock for four innovative applications, one of which is thermal energy storage systems in CSP applications [15; 16].  
The degradation effects of the fluid by direct contact with the slag, which are reported, for example, by [PERSON] et al. [17] in another type of slag, are, in contrast to molten salt, not of importance in the air or can be equalized by correspondingly high air exchange rates. The issue of safety in the context of molten salt is also to be assessed much more critically than with air. The safety regulations for slag are also manageable with this system, as it is a closed circuit and the slag used is classified by the manufacturer as a non-hazardous substance.  
In this paper, design studies of slag-based thermal energy storage are carried out. Different design concepts are developed, calculated and evaluated. Finally, the lead concepts for further investigation in the project are defined.