Start-up and control of liquid salt energy storage combined cycle systems

1. A method of operating a combined cycle power plant,

the combined cycle electric power plant comprising:

a combustion turbine generator that combusts fuel to generate electricity and produce hot exhaust gases;

a second heat source, different from the combustion turbine;

a thermal energy storage system that stores heat from the second heat source;

a steam turbine generator that expands superheated steam across a steam turbine to generate electricity;

a feedwater reservoir that stores feedwater condensed from steam exhausted from the steam turbine;

a feedwater preheater configured to heat feedwater from the feedwater reservoir with heat exclusively from the combustion turbine exhaust gases;

a boiler configured to boil feedwater from the feedwater preheater with heat exclusively from the thermal energy storage system to generate steam;

a superheater configured to heat steam from the boiler exclusively with heat from the combustion turbine exhaust gases to generate the superheated steam;

a third heat source, different from the combustion turbine; and

a startup feedwater heater configured to use heat from the third heat source to heat feedwater from the feedwater reservoir to a temperature greater than a freezing point of a heat transfer fluid used to transfer the heat from the thermal energy storage system to the boiler;

the method comprising heating feedwater from the feedwater reservoir to a temperature greater than the freezing point of the heat transfer fluid using the startup feedwater heater.

2. The method of claim 1 , comprising recirculating the feedwater heated by the startup feedwater heater to the feedwater reservoir to heat feedwater in the feedwater reservoir to a temperature greater than the freezing point of the heat transfer fluid.

3. The method of claim 1 , comprising heating the boiler to a temperature greater than the freezing point of the heat transfer fluid using feedwater heated by the startup feedwater heater.

4. The method of claim 3 comprising, after heating the boiler to a temperature greater than the freezing point of the heat transfer fluid, circulating heat transfer fluid from the thermal energy storage system through the boiler to generate steam from feedwater using heat transferred by the heat transfer fluid from the thermal energy storage system.

5. The method of claim 4 , comprising directing steam generated in the boiler to the feedwater reservoir to heat feedwater in the feedwater reservoir.

6. The method of claim 5 , wherein steam flow to the feedwater reservoir is regulated to heat feedwater in the feedwater reservoir to a temperature greater than the freezing point of the heat transfer fluid.

7. The method of claim 5 , wherein as the temperature of the feedwater in the feedwater reservoir increases, a flow rate of feedwater through the startup feedwater heater is increased.

8. The method of claim 5 , wherein as the temperature of the feedwater in the feedwater reservoir increases, heat input to the startup feedwater heater from the second heat source is decreased.

9. The method of claim 1 , comprising:

bypassing the superheater to provide steam from the boiler to a startup superheater connected in parallel with the superheater between the boiler and the steam turbine and configured to heat steam from the boiler using heat exclusively from the thermal energy storage system;

superheating the steam in the startup superheater; and

expanding the superheated steam across the steam turbine.

10. The method of claim 1 , wherein the wherein the startup feedwater heater is arranged in parallel with the feedwater preheater between the feedwater reservoir and the boiler.