A latent heat thermal energy storage (LHTES) system is an efficient thermal battery using a phase change material (PCM) for key applications of intermittent renewable energy. In this study, a flexible elliptical-shaped capsule is investigated and subsequently proposed as a container of the PCM used for LHTES. The novel elliptical PCM capsule is made of polyethylene and designed to achieve maximum enhancement of heat transfer. The thermal flow inside the capsule is analyzed via the combined methods of particle image velocimetry and laser induced fluorescence. In the present study, the upward and downward motion of a solid PCM lump in an elliptical capsule are reported for the first time. This is shown to increase the heat transfer rate by more than twice that of a spherical capsule. A prototype heat storage tank with 1000 L of water and 500 kg of the PCM is fabricated for application as a large-scale thermal battery. Consequently, the developed PCM capsule exhibits a decrease of 50% and 35% in the charging and discharging process time, respectively.