Combined Storage System Developments for Direct Steam Generation in Solar Thermal Power Plants

Abstract

For future parabolic trough plants direct steam generation in the absorber 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. A three-part storage system is proposed for the two phase fluid water/steam. Concrete storage is used for the process steps involving transfer of sensible heat – i.e. preheating of water and superheating of steam - while for the two-phase evaporation a phase change material (PCM) storage will be deployed. This technology is currently being tested by DLR and Ed. Zublin AG within the project ITES, funded partly by the German Ministry for the Environment, Nature Conservation and Nuclear Safety. A combined storage solution with a 22 m³ concrete storage test module for superheating of steam and a 8.5 m³ PCM-storage for evaporation of water was build in 2009 in a direct steam test loop, set up at the power plant Litoral of Endesa in Carboneras, Spain. This high temperature storage system has a total capacity of ca. 1000 kWh and is the first demonstration of such a combined storage system for the two phase heat transfer fluid water/steam, details were reported in [Laing et al., 2009]. Cycle testing has started end of 2010. First, cycling tests for each storage unit separately have been performed successfully, showing the expected discharge capacities of ca. 250 kWh for the concrete unit and ca. 720 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 the peak power 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. Combined system operation was also proven in constant pressure and sliding pressure mode, showing good system operability. The paper will describe the storage system and present the test results and evaluation of the single module testing as well as on the combined storage system testing in different operation modes.