For the development of solid media storage material the thermo-physical properties of the materials, such as density p, specific heat capacity cp, thermal conductivity Л, coefficient of thermal expansion (CTE) and cycling stability as well as availability, costs and production methods are of great relevance. A high heat capacity (p*cp) reduces the storage volume and a high thermal conductivity Л increases the dynamic in the system. The CTE of the storage material should fit to the CTE of the material of the embedded metallic heat exchanger. A high cycling stability is important for a long lifetime of the storage.

Two different storage materials have been developed in parallel [2], as an innovative storage material a castable ceramic and alternatively, a high temperature concrete. Both developed materials are principally composed of a binder system, aggregates and a small amount of auxiliary materials.

The castable ceramic is based on a binder including Al2O3. The binder is set chemically under ambient conditions and forms a solid, stable matrix, which encloses the aggregates. As main aggregate iron oxides, accumulated as waste material in strip steel production, is used. Auxiliary materials are needed to improve the handling of the ready mixed material, for example as accelerator or for reduction of viscosity.

For the high temperature concrete blast furnace cement is used as binder, again iron oxides are used as main aggregate, as well as flue ash and again a small amount of auxiliary materials.

The material properties have been analyzed at DLR. The results are shown in table 1. Shear stress analysis has proven that the contact between tubes and material is very good at ambient temperature as well as at 350°C.

In an overall view high temperature concrete seems to be the more favorable material. Reasons are the lower costs, higher strength of the material and easier handling of the ready mixed material. However, the further development of cracks in the test modules needs to be investigated, when cycling at operation temperature has been demonstrated. On the other side castable ceramics has a 20% higher storage capacity and 35% higher thermal conductivity and still some potential for cost reduction.