For future parabolic trough plants direct steam generation in the receiver pipes is a promising option for reducing the costs of solar thermal energy . These new solar thermal power plants require innovative storage concepts, where the two phase heat transfer fluid poses a major challenge . For the regions where the heat transfer fluid is in a single phase (water or steam), sensible heat storage using molten salt  or concrete  as storage material can be applied. However, efficient energy storage in the two-phase evaporation/condensation region requires heat storage operation within a narrow temperature range.
For this two phase region, a high performance PCM storage technology was developed and demonstrated by DLR. A test module using 14 tons of PCM with 700 kWh capacity was built in 2009 and commissioned in 2010 in a direct steam test loop, set up at the power plant Litoral of Endesa in Carboneras, Spain . The PCM-storage uses Sodium nitrate as phase change material with a melting temperature of 305 °C. Cycle testing has started end of 2010. Cycling tests have proven the expected discharge capacity of approx. 700 kWh for the PCM-storage module. System operation in constant pressure mode and sliding pressure mode has been conducted for the PCM-storage. While in the constant pressure mode a peak performance of the storage of more than 700 kW could be demonstrated, in the sliding pressure mode a constant power output over almost the whole charge and discharge period could be provided.
The paper discusses the test results and evaluation for different operation modes for the phase change storage for discharge operation. Charging of the phase change storage is always in a once-through mode. However, for discharge, the steam can be generated either in forced or natural circulation mode or in once-through mode, leading to very different effects for the two-phase flow and filling level inside the heat exchanger pipes in the storage. The effects for forced and natural circulation discharge will be analysed and described in the paper.