Thermal stability of granite for high temperature thermal energy storage in concentrating solar power plants

Natural rock has been considered the promising material for thermal energy storage in concentrating solar power plants at high temperatures. Rocks need to keep stable when serving as storage materials and exchanging heat with working fluid many times. To investigate the feasibility of granite in this application, this paper evaluated the thermal stability of granite, including physical and mechanical properties, subjected to different thermal cycles with temperature ranging from 20 °C to 650 °C. Mass and volume of granites were measured after each desired cycle, as well as the P-wave velocity and porosity. Thermal cracks of granite samples in different thermal cycles were analyzed by adopting X-ray computed tomography technique. The results demonstrated that, most of the thermal damage in physical properties such as mass loss, porosity increase and P-wave velocity decrease of granites occurred within 5 thermal cycles, especially in the first cycle, before reaching almost the constant value after the 20 thermal cycles. Uniaxial compression tests were carried out on granites in different thermal cycles, revealing that the peak strength and elastic modulus of granite were also dramatically decreased after the first thermal cycle. Afterwards, the thermal damage in mechanical behavior of granites barely grown and granites kept stable when thermally cycled the granite from 20 to 100 times.