This article presents the latest experimental results of the European DISS (DIrect Solar Steam) project. The experiments are subdivided into steady state and transient tests. The goal of the steady state tests is the investigation of the thermohydraulic phenomena of the occurring two phase flow, whereas the transient tests are needed for the controller design. The experimental results are compared to simulation studies. Implications for the plant operation will be discussed. INTRODUCTIONThe direct steam generation (DSG) in parabolic trough collectors is a promising option for the improvement of the reliable parabolic trough technology for solar thermal electricity generation. Several open questions concerning the DSG process have to be answered before a commercial plant can be designed. These are among others the:• operation of single and parallel rows under steady-

The direct steam generation (DSG) is an attractive option regarding the economic improvement of parabolic trough technology for solar thermal electricity generation in the multi megawatt range. According to Price, H., Lu¨pfert, E., Kearney, D., Zarza, E., Cohen, G., Gee, R. Mahoney, R., 2002, “Advances in Parabolic Trough Solar Power Technology,” J. Sol. Energy Eng., 124 and Zarza, E., 2002, DISS Phase II-Final Project Report, EU Project No. JOR3-CT 980277 a 10% reduction of the LEC is expected compared to conventional SEGS like parabolic trough power plants. The European DISS project has proven the feasibility of the DSG process under real solar conditions at pressures up to 100 bar and temperatures up to 400°C in more than 4000 operation hours (Eck, M., Zarza, E., Eickhoff, M., Rheinla¨nder, J., Valenzuela, L., 2003, “Applied Research Concerning the Direct Steam Generation in Parabolic Troughs,” Solar Energy 74, pp. 341–351). In a next step the detailed engineering for a precommercial DSG solar thermal power plant will be performed. This detailed engineering of the collector field requires the consideration of the occurring thermohydraulic phenomena and their influence on the stability of the absorber tubes.

This article presents the latest experimental results of the European DISS (DIrect Solar Steam) project. The experiments are subdivided into steady state and transient tests. The goal of the steady state tests is the investigation of the thermohydraulic phenomena of the occurring two phase flow, whereas the transient tests are needed for the controller design. The experimental results are compared to simulation studies. Implications for the plant operation will be discussed.

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Increasing energy prices and shortage of fossil fuels lead to a growing interest in alternative energy sources. In combination with energy storage systems the generation of solar process heat can be provided independent from the weather leading for example to a cost efficient stabilization of power output. For this application latent heat storage units with phase change materials (PCMs) can be designed to store solar process heat within a narrow temperature interval utilizing the high storage density of the different PCMs. This is achieved using the latent heat of melting in the melting / solidification process, or the latent heat of re-crystallization in a solid / solid phase transition. However, this advantage can only be used in technical applications if the heat transfer in the PCM is sufficiently high. As most pure PCMs exhibit a low thermal conductivity (about 1 W/(m•K) or less), methods to improve heat transfer in PCMs have been under investigation for decades. The heat transfer in a PCM can be increased by addition of highly thermal conductive materials. Due to its superior properties - high thermal conductivity, good processability, and chemical inertness - graphite has distinct advantages for this purpose. Depending on the requirements of the respective application, various routes to combine PCM and graphite are used. For example, besides the fabrication of PCM/graphite composite materials, the increase of heat exchanger surface by highly thermal conductive graphite plates is a favorable method for large scale applications, in particular. Effective thermal conductivities up to 30 W/(m•K) have been realized. This paper gives an overview of actual and potential applications of PCM/graphite heat storage systems focusing on storage of solar heat for high temperature applications such as process heat generation and solar thermal power plants.