A novel GAHP-TSU (gravity-assisted heat-pipe thermal storage unit) is presented, in which the composite granular solid liquid PCM (phase-change material) is piled up as the porous-medium layer. The composite PCM, which melts over a temperature range of 200-250 degrees C, is composed of a mixed metal salt and inorganic bentonite. Naphthalene is presented as the phase-change heat-transfer medium of the GAHP-TSU. The mechanism of charging/discharging modes is similar to that of the gravity-assisted heat pipe, which is superior to the traditional direct-contact or indirect-contact heat storage unit. Heat transfer between the PCM and the heat resource or cold resource in this device occurs in the form of a cyclic phase change of the heat-transfer medium, which directly occurs on the surface of the PCM and has an extremely high heat-transfer coefficient. In this paper, a series of experiments are carried out first to investigate the properties of the new heat storage unit. Then, a series of theoretical analyses, such as heating time, power distribution, temperature difference between the inside and outside of the granular PCM, are presented. The results show that the whole system has an excellent heat-storage/heat-release performance. (C) 2015 Elsevier Ltd. All rights reserved.