A method for operating a thermal energy storage plant comprising a charging circuit where a first working fluid is circulated, the charging circuit including: - a first fluid transporting machine for generating a flow of the first working fluid in the charging circuit, - a heating device for transferring heat to the first working fluid, - a heat accumulator for storing the thermal energy of the first working fluid, the heat accumulator including a hot end for receiving the first working fluid at a first temperature and a cold end for letting the first working fluid exit the heat accumulator at a second temperature lower than the first temperature, the heat accumulator comprising a plurality of heat storage units (135a, 135b, 135c) connected in series between the hot end and the cold end, wherein the heat accumulator is oriented in such a way that the first working fluid is circulated from the hot end to the cold end along a horizontal direction, and at least one valve (137a, 137b) interposed between two heat storage units (135a, 135b, 135c) of said plurality of heat storage units (135a, 135b, 135c), wherein the thermal energy storage plant further comprises a discharging circuit where a second working fluid is circulated, the discharging circuit including: - the heat accumulator, - a second fluid transporting machine for generating a flow the second working fluid in the discharging circuit, the flow being oriented from the cold end to the hot end of the heat accumulator, - a heat exchanger included in a thermal cycle for transferring the thermal energy from the second working fluid to the working fluid of the thermal cycle, the method comprising the steps of: - heating the first working fluid in the heating device, - generating a flow of the first working fluid in the charging circuit with the first fluid transporting machine, for charging the heat storage units (135a, 135b, 135c) in series from the hot end to the cold end, such that the temperature front moves from the first heat storage unit to the last heat storage unit, characterized in that the method further comprises the steps of: - stopping the heating and the flow of the first working fluid after the last heat storage unit has been charged, namely when the temperature front reaches the outlet of the last heat storage unit and the second temperature at the cold end rises, - isolating the charged heat storage unit from the other heat storage units (135a, 135b) by means of at least one valve (137a, 137b). - a first fluid transporting machine for generating a flow of the first working fluid in the charging circuit, - a heating device for transferring heat to the first working fluid, - a heat accumulator for storing the thermal energy of the first working fluid, the heat accumulator including a hot end for receiving the first working fluid at a first temperature and a cold end for letting the first working fluid exit the heat accumulator at a second temperature lower than the first temperature, - the heat accumulator, - a second fluid transporting machine for generating a flow the second working fluid in the discharging circuit, the flow being oriented from the cold end to the hot end of the heat accumulator, - a heat exchanger included in a thermal cycle for transferring the thermal energy from the second working fluid to the working fluid of the thermal cycle, - heating the first working fluid in the heating device, - generating a flow of the first working fluid in the charging circuit with the first fluid transporting machine, for charging the heat storage units (135a, 135b, 135c) in series from the hot end to the cold end, such that the temperature front moves from the first heat storage unit to the last heat storage unit, - stopping the heating and the flow of the first working fluid after the last heat storage unit has been charged, namely when the temperature front reaches the outlet of the last heat storage unit and the second temperature at the cold end rises, - isolating the charged heat storage unit from the other heat storage units (135a, 135b) by means of at least one valve (137a, 137b).