Experimental investigation of major rocks in Hong Kong as potential sensible thermal energy storage medium

Energy storage is considered a viable solution for managing renewable energies, and rock is recognized as an economically feasible and environmentally friendly medium for sensible heat storage. Following the principle of utilizing local resources, fifteen major rock types from Hong Kong—covering igneous, sedimentary, and metamorphic classifications—were collected and processed to required sizes for several characterization techniques, considering their heterogeneity and anisotropy. Thermophysical (thermal diffusivity/conductivity, heat capacity, and thermal expansion coefficient) and mechanical properties of the selected rocks were analyzed from room temperature to 1000 °C, along with their chemical and structural compositions. Through multidimensional evaluation, the suitability (optimal, average, poor) of these rocks from Hong Kong to serve as thermal energy storage media was assessed. The results obtained indicated that Hong Kong basalt is the optimal candidate for high-temperature thermal energy storage material, with 850 °C identified as the suitable maximum working temperature. Other igneous rocks from Hong Kong can be utilized for mid-to-low temperature range (100–500 °C) thermal energy storage engineering. However, sedimentary and metamorphic rocks from Hong Kong appear unsuitable for local thermal energy storage engineering.