The time-dependent behavior of a thermal storage device incorporating an integrated thermosyphon is analyzed numerically. Energy is added from a constant temperature heat source situated within one portion of the thermosyphon loop. Comparison is made between the stratified thermosyphon-based system and a fully mixed storage volume with constant temperature heat addition by an immersed heat exchanger. In both systems, a fundamental limitation is the decay of the heating power as energy accumulates within the storage volume. The results show however, that the energy transfer rate to storage during charging is significantly higher for the thermosyphon-based system. In addition, the undesirable consequence of energy accumulation can be strongly offset by targeting the dominant frictional losses to a specific flow regime.