Thermal Storage Concept for Solar Thermal Power Plants with Direct Steam Generation

Abstract

One possibility to increase the efficiency and thus economic viability of solar thermal power plants is to increase their operating temperature. This approach demands the substitution of the state-of-the-art heat transfer fluid (HTF) that limits the operating temperature to roughly 400°C. Promising heat transfer fluids for future applications are molten salts or water/steam. If water/steam is used as HTF, the feed-water from the power block is fed to the solar field (SF) and directly evaporated and superheated. This process is called direct steam generation (DSG). A recent study [1] has pointed out that the economic potential of the DSG process is utilized only, if the SF design is simplified and a competitive thermal storage is available. Thus, an R&D project was launched in Germany to develop a complete storage system covering the energy of the evaporation as well as of the pre- and superheating section. It consists of a phase change material (PCM) storage for evaporation and a molten salt storage for pre- and superheating. One specific feature of superheated steam is its changing specific heat capacity with temperature. Using molten salt as storage medium with a nearly constant specific heat and the application of an obvious simple heat exchange would lead to an inefficient process. A significantly reduced live steam temperature and thus power block efficiency during discharge would be the results. Furthermore, the specific storage density of the molten salt system would be reduced too. In this paper this effect will be discussed in more detail. The consequences for the storage system will be discussed and solutions of the developed processes for the integration of such storage into a DSG power plant will be presented that reduce or overcome the mentioned restrictions.