Gas turbine bleed energy recovery via counter rotating generator

1. A gas turbine engine comprising:

a main compressor section for compressing air;

a main combustor section positioned downstream of the main compressor section;

a main turbine section positioned downstream of the main combustor section;

a spool extending from the main compressor section to the main turbine section;

a second turbine fluidically connected to the main compressor section by a bleed passage; and

a counter-rotating electrical generator having a first rotor mechanically driven by the spool and a second rotor mechanically driven by the second turbine.

2. The gas turbine engine of claim 1 , and further comprising:

a locking mechanism for limiting rotation of the second rotor.

3. The gas turbine engine of claim 2 , wherein the second rotor and the second turbine are substantially stationary with respect to the bleed passage when the locking mechanism is locked.

4. The gas turbine engine of claim 1 , and further comprising:

a heat exchanger positioned along the bleed passage.

5. The gas turbine engine of claim 1 , wherein one of the first and second rotors includes a magnetic field producer and the other of the first and second rotors includes an armature.

6. The gas turbine engine of claim 1 , wherein the first rotor of the counter-rotating electrical generator rotates in a first direction when air flows through the main turbine and the second rotor rotates in a second direction when air from the bleed passage flows through the second turbine.

7. The gas turbine engine of claim 1 , wherein the spool is a high pressure spool.

8. The gas turbine engine of claim 1 , and further comprising:

an accessory gearbox having gearing for connecting the spool to the first rotor.

9. The gas turbine engine of claim 1 , and further comprising:

a bypass fluidically connected to the bleed passage both upstream and downstream of the second turbine; and

a bypass valve for selectively directing air flow to either the bypass or the second turbine.

10. A method for operating a gas turbine engine on an aircraft, the method comprising:

supplying air from a main compressor through a main flow path to drive a main turbine and through a bleed passage to drive a second turbine;

generating electrical power with a generator mechanically driven by both the main turbine and the second turbine; and

supplying air from the second turbine to a cabin of the aircraft.

11. The method of claim 10 , wherein the generator is simultaneously driven by both the main turbine and the second turbine under a first set of operating conditions and wherein the generator is driven by only the main turbine under a second set of operating conditions.

12. The method of claim 11 , wherein the first set of operating conditions includes pressure upstream of the second turbine having a value above a threshold and the second set of operating conditions includes pressure upstream of the second turbine having a value below the threshold.

13. The method of claim 10 , and further comprising:

supplying air from the second turbine to a wing de-icing system.

14. The method of claim 10 , and further comprising:

removing heat from the air in the bleed passage with a heat exchanger fluidically positioned along the bleed passage between the main compressor and the second turbine.

15. The method of claim 10 , and further comprising:

rotating a magnetic field producer of the generator in a first direction with respect to the aircraft; and

rotating an armature of the generator in a second direction with respect to the aircraft.