Experimental characterization of siliceous rocks to be used as filler materials for air-rock packed beds thermal energy storage systems in concentrated solar power plants

Due to the intermittency of solar radiation, the integration of a thermal energy storage system is a key solution to ensure the dispatchability and enhance the cost-effectiveness of concentrated solar power plants (CSP). Recently, the use of rocks as sensible heat storage materials in packed bed systems has been the main object of several research studies. Nevertheless, limited knowledge is available on the thermal behavior of rocks at high temperatures and their suitability is yet to be deeply studied. The present paper aims to investigate the potential of quartzite and flint rocks to be used as filler materials in a packed bed storage system with air as the heat transfer fluid (HTF). These siliceous rocks have been subjected to an experimental characterization in terms of the properties required in a sensible heat storage material. In fact, the geochemistry and structure of these rocks were identified and some mechanical characteristics were measured. The influence of temperature variation on thermal capacity, thermal diffusivity and thermal conductivity was also analyzed. Besides, thermogravimetric analysis was performed up to 1000 °C, in order to check the thermal stability of rocks. In addition, 110 thermal cycles were achieved in a furnace up to 550 °C. Characterization experiments demonstrated the potential of silica rocks as storage materials. However, the preliminary results of accelerated thermal cycling tests indicate that performing a thermal treatment of these rocks could be of great interest, especially when used at temperatures higher than 250 °C, in order to enhance their lifetime.