After the demonstration of the feasibility of direct steam generation in parabolic troughs [4] one focus of further research activities of this technology lies in the development of a suitable storage technology. The development of a cost effective DSG-storage concept (Fig. 7) is the aim of the recently launched DISTOR project funded by the European Community within the 6th Framework Programme on Research, Technological Development and Demonstration.

Regarding efficiency, a fundamental demand for thermal storage systems in power plants is the minimization of temperature differences between working fluid and storage medium. This requires isothermal storage systems for the DSG-process. An obvious solution is the application of latent heat storage materials. Fig. 8 shows the process in the T-s diagram: during the charging period, heat from the condensating steam is transferred to the melting storage material (Fig. 8, 2-3). During the discharge process, heat from the solidifying storage material is used to generate steam (Fig. 8, 6-7).

The selection of the latent heat storage material depends strongly on the saturation temperature resulting from the pressure in the steam cycle; the DSG-process with an operation range of 30-100 bar requires melting temperatures between 250°C and 300°C. Considering also economic aspects, candidate materials for latent heat storage systems are salts. Although this approach has often been suggested, only limited experience is available in this temperature range. Most problems result from the low thermal conductivity of salts, particularly in the solid phase.

Basically, there are two methods to overcome the problems resulting from the low thermal conductivity:

о reduce the specific resistance for heat conduction in the latent heat storage material

о reduce the average distance for heat conduction within the storage material Solutions based on both methods are investigated within the DISTOR project. The specific resistance for heat conduction can be reduced by embedding the storage material in a matrix made of a material with a high thermal conductivity, such as expanded graphite. This approach has been tested for low temperature applications and will be extended to the operation range of the DSG-process. This development aims at a composite material with an effective thermal conductivity in the range of 5-10W/(mK).

Reducing the average distance for heat transport in the storage material means an increase of the ratio of surface area to mass of storage material. By introducing an intermediate heat transfer medium between storage material and steam pipes the surface of the storage material can be extended while the mass of piping remains constant.

Within the DISTOR-project three basic storage concepts will be tested in laboratory scale. Based on the experiences gained with 10kW lab-units, one concept will be selected for the design of a 100kW storage unit that will be connected to the DISS test facility to assess the storage system under realistic operating conditions.