Identification of natural rocks as storage materials in thermal energy storage (TES) system of concentrated solar power (CSP) plants - A review

Thermal energy storage (TES) system is a decisive technology for handling intermittent problems, and ensuring the dispatchability of electrical energy from concentrated solar power (CSP) plants. Indeed, the integration of a packed-bed TES system in these plants is a promising solution; however, it is also a challenge depending on the choice of storage materials. This system is a dual-media thermocline storage system composed of two phases: solid (storage material) and heat transfer fluid (HTF). At present, the selection process of suitable materials for this TES system remains a very critical issue, as it has to satisfy several criteria related to the storage materials stability and their performances inside the TES component. Within this scope, there have been many reviews of potential sensitive storage media, but there is no available review in the literature concerning natural rocks as potential storage materials at high temperatures, to the best of our knowledge. Therefore, the present paper reviews the available data about the previously investigated rocks. This comprehensive effort is divided into four main sections whose the first presents the operating principle of a packed-bed system. Then, the second one concerns the classification of rock types. The third one is entirely devoted to the methodology of rocks selection, as well as to the used equipment and methods for rocks characterization. In addition, the durability and compatibility of rocks with different HTFs are also examined in this section. Finally, the main conclusions, findings, and current research priorities are drawn and summarized in the last section.