Abstract: Thermal energy storage systems and methods are provided including a bed, a blend of aggregates packed in the bed, and a high-density heat transfer fluid flowing through the blend of aggregates. The blend of aggregates includes rock materials and may also include non-rock materials. The heat transfer fluid flows through the blend of aggregates such that heat is transferred between the heat transfer fluid and the blend of aggregates. The porosity of the aggregates increases heat transfer and the high density of the heat transfer fluid reduces the pressure gradient of the heat transfer fluid. In exemplary embodiments, the heat transfer fluid is a liquid comprised of carbon-based molecules. Methods of safely storing and releasing energy are provided in which axial thermal conductivity of the bed is minimized and inadvertent pressure release failures are mitigated.