Solar thermal systems using absorber evaporating steam directly require isothermal energy storage. The application of latent heat storage systems is an option to fulfill this demand. This concept has been demonstrated mainly for low temperature heating and refrigeration applications, the experience for the power level and temperature range characteristic of solar process heat and solar thermal power plants is limited. Cost effective implementation of the latent heat storage concept demands low cost phase change materials (PCMs). These PCMs usually show low thermal conductivity limiting the power density during the charging/discharging process. This paper describes various approaches, which have been investigated to overcome these limitations. Based on fundamental PCM-research and laboratory-scale experiments, the sandwich concept has been identified to show the highest potential. The sandwich concept has been demonstrated successfully for three different storage units ranging from 2 kW to 100 kW at melting temperatures of $145°C$ and $225°C$.

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