Hybrid expander cycle with intercooling and turbo-generator

1. A gas turbine engine system comprising:

a gas turbine engine comprising:

a compressor section comprising:

a low pressure compressor configured to compress an inlet air flow to produce a first compressed air flow;

a high pressure compressor fluidly coupled to the low pressure compressor and configured to compress the first compressed air flow to produce a second compressed air flow;

a combustor fluidly connected to an outlet of the high pressure compressor and configured to combust a mixture of the second compressed air flow and a gaseous fuel, the gaseous fuel being at a first pressure, to produce a combustion gas flow;

a turbine fluidly coupled to the combustor and configured to extract energy from expansion of the combustion gas flow to produce an exhaust gas flow; and

a heat exchange system configured to transfer thermal energy from the first compressed air flow and the exhaust gas flow to a fuel to produce the gaseous fuel at a second pressure greater than the first pressure; and

a fuel turbine system comprising:

a multi-stage fuel turbine fluidly coupled to the heat exchange system and the combustor and configured to extract energy from expansion of the gaseous fuel at the second pressure, wherein the multi-stage fuel turbine comprises an interstage discharge outlet configured to extract fuel, before a final turbine stage of the multi-stage fuel turbine, at a higher pressure than a fuel pressure at a final outlet of the multi-stage fuel turbine, wherein the fuel turbine system is configured to mix fuel from the interstage discharge outlet and the final outlet to produce the gaseous fuel at the first pressure and supply the gaseous fuel at the first pressure to the combustor; and

a fuel pump fluidly coupled to the heat exchange system and configured to be driven by the multi-stage fuel turbine.

2. The gas turbine engine system of claim 1 , wherein the heat exchange system comprises an intercooler disposed in a flow path of the first compressed air flow between the low pressure compressor and the high pressure compressor and wherein the intercooler is fluidly coupled to a component selected from the group consisting of the fuel pump and an intermediate intercooler heat exchanger.

3. The gas turbine engine system of claim 2 , wherein the intercooler is fluidly coupled to the fuel pump.

4. The gas turbine engine system of claim 2 , wherein the intercooler is fluidly coupled to the intermediate intercooler heat exchanger, wherein the intermediate intercooler heat exchanger is disposed external to the compressor section and fluidly coupled to the fuel pump and wherein the intermediate intercooler heat exchanger is configured to place the fuel and a first working fluid in a thermal communication and wherein the intercooler is configured to receive the first working fluid from the intermediate intercooler heat exchanger and place the first working fluid in thermal communication with the first compressed air flow, such that thermal energy is transferred indirectly from the first compressed air flow to the fuel.

5. The gas turbine engine system of claim 2 , wherein the heat exchange system further comprises an exhaust heat exchange system configured to transfer thermal energy from the exhaust gas flow to the fuel, wherein the exhaust heat exchange system is fluidly coupled to one of the heat exchangers selected from the group consisting of the intercooler and the intermediate intercooler heat exchanger.

6. The gas turbine engine system of claim 5 , wherein the exhaust heat exchange system comprises an exhaust heat exchanger fluidly coupled to the intercooler.

7. The gas turbine engine system of claim 5 , wherein the exhaust heat exchange system comprises:

an exhaust heat exchanger; and

an intermediate exhaust heat exchanger fluidly coupled to the intermediate intercooler and

the exhaust heat exchanger, wherein the intermediate exhaust heat exchanger is configured to place the fuel in thermal communication with a second working fluid, and wherein the exhaust heat exchanger is configured to receive the second working fluid from the intermediate exhaust heat exchanger and place the second working fluid in thermal communication with the exhaust gas, such that thermal energy is transferred indirectly from the exhaust gas to the fuel.

8. The gas turbine engine system of claim 1 , wherein the fuel turbine system further comprises a combination motor/generator mechanically coupled to a rotor shaft of the multi-stage fuel turbine and configured to be driven by the multi-stage fuel turbine.

9. The gas turbine engine system of claim 1 , wherein the fuel pump is in fluid communication with a cryogenic fuel.

10. A method of operating a gas turbine engine system, the gas turbine engine system comprising a gas turbine engine, the gas turbine engine comprising a compressor section, the compressor section comprising a low pressure compressor and a high pressure compressor, the low pressure compressor configured to compress an inlet air flow to produce a first compressed air flow, the high pressure compressor fluidly coupled to the low pressure compressor and configured to compress the first compressed air flow to produce a second compressed air flow, the gas turbine engine further comprising a combustor fluidly connected to an outlet of the high pressure compressor and configured to combust a mixture of the second compressed air flow and a gaseous fuel, the gaseous fuel being at a first pressure, to produce a combustion gas flow, the gas turbine engine further comprising a turbine fluidly coupled to the combustor and configured to extract energy from expansion of the combustion gas flow to produce an exhaust gas flow, the gas turbine engine further comprising an engine heat exchange system configured to transfer thermal energy from the first compressed air flow and the exhaust gas flow to a fuel to produce the gaseous fuel at a second pressure greater than the first pressure, the gas turbine engine system further comprising a fuel turbine system, the fuel system comprising a multi-stage fuel turbine fluidly coupled to the engine heat exchange system and the combustor and configured to extract energy from expansion of the gaseous fuel at the second pressure, wherein the multi-stage fuel turbine comprises an interstage discharge outlet configured to extract fuel, before a final turbine stage of the multi-stage fuel turbine, at a higher pressure than a fuel pressure at a final outlet of the multi-stage fuel turbine, wherein the fuel turbine system is configured to mix fuel from the interstage discharge outlet and the final outlet to produce the gaseous fuel at the first pressure and supply the gaseous fuel at the first pressure to the combustor, the fuel turbine system further comprising a fuel pump fluidly coupled to the engine heat exchange system and configured to be driven by the multi-stage fuel turbine, the method comprising:

compressing, with the low pressure compressor, the inlet air flow to produce the first compressed air flow;

cooling the first compressed air flow in an intercooler heat exchange system of the engine heat exchange system to produce a cooled compressed air flow, wherein cooling the first compressed air flow comprises transferring thermal energy from the first compressed air flow to a liquid fuel to produce the gaseous fuel;

compressing, with the high pressure compressor, the cooled compressed air flow to produce the second compressed air flow;

extracting energy from expansion of the gaseous fuel through the multi-stage fuel turbine, wherein expansion of the gaseous fuel decreases a pressure of the gaseous fuel to a pressure greater than a pressure of the second compressed air flow;

extracting the gaseous fuel from the interstage discharge outlet upstream of a final turbine stage of the multi-stage fuel turbine;

combusting the mixture of the gaseous fuel, the gaseous fuel being at the first pressure, and the second compressed air flow in the combustor; and

pumping the liquid fuel to the intercooler heat exchange system via the pump, the pump being driven by the multi-stage fuel turbine.

11. The method of claim 10 , further comprising providing the gaseous fuel from the intercooler heat exchange system to an exhaust heat exchange system of the engine heat exchange system, and transferring thermal energy to the gaseous fuel in the exhaust heat exchange system, wherein the liquid fuel is cryogenic.

12. The method of claim 11 , wherein transferring thermal energy to the gaseous fuel comprises transferring thermal energy from an exhaust gas of the gas turbine engine to the gaseous fuel.

13. The method of claim 12 , wherein transferring thermal energy from the first compressed air flow to the liquid fuel to produce the gaseous fuel comprises pumping the liquid fuel through an intercooler of the intercooler heat exchange system, wherein the intercooler is disposed in a flow path of the first compressed air flow.

14. The method of claim 12 , wherein transferring thermal energy from the first compressed air flow to the liquid fuel to produce the gaseous fuel comprises:

pumping the liquid fuel through an intermediate intercooler heat exchanger of the intercooler heat exchange system;

pumping a first working fluid through an intercooler of the intercooler heat exchange system and the intermediate intercooler heat exchanger;

transferring thermal energy from the first compressed air flow to the first working fluid; and

transferring thermal energy from the first working fluid to the liquid fuel to produce the gaseous fuel.

15. The method of claim 14 , further comprising:

pumping the gaseous fuel from the intermediate intercooler heat exchanger through an intermediate exhaust heat exchanger of the exhaust heat exchange system;

pumping a second working fluid through each of a first exhaust heat exchanger of the exhaust heat exchange system and the intermediate exhaust heat exchanger;

transferring thermal energy from the exhaust gas to the second working fluid; and

transferring thermal energy from the second working fluid to the gaseous fuel received from the intermediate intercooler heat exchanger.

16. The method of claim 10 , further comprising extracting power from the multi-stage fuel turbine via a combined motor/generator.

17. A gas turbine engine system comprising:

a gas turbine engine, the gas turbine engine comprising a combustor configured to combust a mixture of air and a gaseous fuel; and

a fuel system configured to provide the gaseous fuel to the combustor, the fuel system comprising:

a heat exchange system configured to transfer heat to a liquid fuel to produce the gaseous fuel at a second pressure; and

a fuel turbine system, comprising:

a multi-stage fuel turbine in fluid communication with the heat exchange system and configured to extract energy from expansion of the gaseous fuel at the second pressure, wherein the multi-stage fuel turbine comprises an interstage discharge outlet configured to extract fuel, before a final turbine stage of the multi-stage fuel turbine, at a higher pressure than a fuel pressure at a final outlet of the multi-stage fuel turbine; and

a fuel pump in fluid communication with the heat exchange system and configured to deliver the liquid fuel to the heat exchange system, wherein the fuel pump is mechanically coupled to the multi-stage fuel turbine and configured to be driven by the multi-stage fuel turbine;

wherein the fuel system is configured to:

mix fuel from the interstage discharge outlet and the final outlet to produce the gaseous fuel at the first pressure and at a first temperature, and

provide the gaseous fuel at the first pressure and at the first temperature to the combustor.

18. The gas turbine engine system of claim 17 , wherein the heat exchange system comprises an intercooler of a compressor section of the gas turbine engine, the intercooler configured to cool a compressed air flow.

19. The gas turbine engine system of claim 8 , wherein the combined motor/generator is configured to drive the fuel pump when the multi-stage turbine is not in operation.