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[en] A dual-mode thermochemical sorption energy storage system using working pair of expanded graphite/SrCl2-NH3 was proposed for seasonal solar thermal energy storage. The proposed system has two working modes to produce useful heat with an expected temperature during the discharging phase according to the different ambient temperatures, including the direct heating supply and temperature-lift heating supply. Solar thermal energy is transformed into chemical bonds and stored in summer, and the stored energy is released in the form of chemical reaction heat in winter. The direct heating supply mode is adopted at a relatively high ambient temperature in winter. The effective energy storage density is higher than 700 kJ/kg and the corresponding system COP is 0.41 when the heat output temperature and ambient temperature are 35 °C and 15 °C, respectively. The specific heating power increases with the decrease of heat output temperature for a given ambient temperature. The temperature-lift heating supply mode is adopted to upgrade the heat output temperature at a low ambient temperature below 0 °C in winter. It can produce heat with a temperature above 70 °C although the ambient temperature is as low as −15 °C. It is desirable to further improve the system performance using low mass ratio and high global conversion. Experimental results showed the advanced dual-mode thermochemical sorption energy storage technology is feasible and effective for seasonal solar thermal energy storage. - Graphical abstract: Working temperature range of dual-mode thermochemical sorption energy storage system during the discharging phase in winter. - Highlights: • A dual-mode seasonal solar thermochemical sorption energy storage system is developed. • The sorption working pair is strontium chloride/expanded graphite-ammonia. • Two working modes can be performed according to the different heat requirements in winter. • Energy density and COP of direct heating supply mode are 706 kJ/kg and 0.41 respectively. • Temperature-lift heating supply mode can meet heat demand at low ambient temperature.