The apparatus for storing and retrieving thermal energy by employing a phase change material (PCM) comprises thermally conductive units, energy sources for providing thermal energy to PCM, an inert gas system, insulation layers and outer enclosure member. The conductive unit has trench(es) for holding the PCM. The trench has opposing significantly curved longitudinal walls and side walls meeting at a base thereby having a greater internal pressure withstanding capacity. In some embodiments, the conductive units are configured to maximise the volume ratio of the PCM to conductive material. In one embodiment, the trench is partially elliptical in a planar cross section that is perpendicular to a major axis or a minor axis of the trench. The apparatus is in thermal contact with thermal energy conversion system for transferring retrieved thermal energy to thermal energy conversion system that converts retrieved thermal energy into electrical energy.