Thermal energy storage plant

1. A thermal energy storage plant comprising a charging circuit and a discharging circuit, the charging circuit including:

a first fluid transporting machine for generating a flow of a working fluid in the charging circuit,

a heating device for transferring heat to the working fluid,

a main heat accumulator for storing the thermal energy of the working fluid;

the discharging circuit including:

the main heat accumulator,

a heat exchanger included in a thermal cycle for transforming the thermal energy stored in the main heat accumulator into mechanical power,

wherein thermal energy storage plant further comprises a secondary heat accumulator including:

a first end connected to the charging circuit, downstream the heating device,

a second end connected to the discharging circuit, upstream the heat exchanger, and

wherein, when the temperature at a hot end of the main heat accumulator is below a predetermined value, the first end of the secondary heat accumulator is connected to the charging circuit and the second end of the secondary heat accumulator is connected to the discharging circuit.

2. The thermal energy storage plant according to claim 1 , wherein the secondary heat accumulator has a lower thermal capacity than the main heat accumulator.

3. The thermal energy storage plant according to claim 1 , further comprising at least a valve between the secondary heat accumulator and the charging circuit.

4. The thermal energy storage plant according to claim 1 , further comprising at least a first and a second discharging valves between the secondary heat accumulator and, respectively, the main heat accumulator and the heat exchanger.

5. The thermal energy storage plant according to claim 1 , further comprising a second fluid transporting machine upstream or downstream the secondary heat accumulator.

6. The thermal energy storage plant according to claim 1 , wherein the heating device is powered from a renewable energy source.

7. The thermal energy storage plant according to claim 1 , wherein the secondary heat accumulator has 10% to 40% of the thermal capacity of the main heat accumulator.

8. A method for operating the thermal energy storage plant according to claim 1 , the method comprising the steps of:

heating the working fluid,

generating a flow of the working fluid in the charging circuit, for charging the main heat accumulator and the secondary heat accumulator.

9. The method according to claim 8 , wherein the main heat accumulator and the secondary heat accumulator are charged in parallel by connecting a hot end of the main heat accumulator and the first end of the secondary heat accumulator downstream the heating device and by connecting the second end of the secondary heat accumulator to the hot end of the main heat accumulator.

10. The method according to claim 8 , wherein the main heat accumulator and the secondary heat accumulator are charged in series by disconnecting a hot end of the main heat accumulator from the heating device, by connecting the first end of the secondary heat accumulator downstream the heating device and by connecting the second end of the secondary heat accumulator to the hot end of the main heat accumulator.

11. The method according to claim 8 , further comprising the step of stopping the heating and the flow of the working fluid after a constant temperature profile is created in the secondary heat accumulator, from a first end to a second end of the secondary heat accumulator.