This paper investigates the dynamic characteristics of the heat-of-fusion type heat storage unit operated by the heat pipe having the evaporation part embedded in the storage material and the condensation end inserted in the solar collector or solar loop. The explicit finite-difference solutions are obtained for the transient response of the storage systems using a digital computer. The merits and drawbacks of using salt hydrates and eutectic fluoride mixtures as the storage media in the storage vessel of cylindrical and spherical configuration are discussed. A suggestion is made for the practical application of the heat-pipe operated thermal storage device.