Alternate fresh air compressor intake for environmental control system

1. An environmental control system of an aircraft comprising:

a compression device including a compressor having a compressor inlet and a compressor outlet, and a turbine operably coupled to the compressor by a shaft, wherein energy extracted from a first medium within the turbine is used to drive the compressor;

a primary inlet supplying a second medium to the compressor inlet; and

a secondary inlet supplying a third medium to the compressor inlet, wherein the third medium is cabin discharge air, wherein the third medium is provided to the compressor inlet separately from the second medium when a pressure of the second medium is below a threshold.

2. The environmental control system of claim 1 , wherein a source of the second medium and a source of the third medium are the same.

3. The environmental control system of claim 1 , wherein the second medium includes fresh, outside air.

4. The environmental control system of claim 1 , wherein a source of the second medium is different than a source of the third medium.

5. The environmental control system of claim 4 , wherein the compressor receives energy derived from a flow of the third medium expanded across the turbine.

6. The environmental control system of claim 5 , wherein the source of the third medium is arranged upstream from the turbine.

7. The environmental control system of claim 5 , wherein the source of the third medium is arranged at or downstream from an outlet of the turbine.

8. The environmental control system of claim 1 , wherein the primary inlet is fluidly coupled to the compressor inlet by a conduit, and the secondary inlet is fluidly coupled to the conduit.

9. The environmental control system of claim 8 , further comprising a valve mounted within the conduit to control a supply of at least one of the second medium and the third medium to the compressor inlet.

10. The environmental control system of claim 9 , wherein the valve is a check valve operable to allow the third medium to flow to the compressor inlet when the pressure of the second medium is below the threshold.

11. The environmental control system of claim 9 , further comprising a controller operable to move the valve between a plurality of positions to control the supply of at least one of the second medium and the third medium to the compressor inlet.

12. The environmental control system of claim 11 , further comprising a sensor for measuring a parameter of a flow of the second medium, wherein the controller is coupled to the sensor and is configured to adjust a position of the valve in response to the measured parameter.

13. The environmental control system of claim 12 , wherein the measured parameter is flow rate.

14. The environmental control system of claim 12 , wherein the measured parameter is pressure.

15. A method of operating a compression device comprising:

expanding a medium across a turbine to operate a compressor coupled to the turbine;

providing a second medium from a primary inlet to an inlet of the compressor; and

providing a third medium from a secondary inlet to the inlet of the compressor, wherein a pressure of the second medium is below a threshold, wherein the third medium is cabin discharge air.

16. The method of claim 15 , wherein providing the third medium from the secondary inlet to the inlet of the compressor further comprises operating a valve to fluidly couple the secondary inlet to the inlet of the compressor.

17. The method of claim 16 , wherein providing the third medium from the secondary inlet to the inlet of the compressor further comprises operating a valve to fluidly disconnect the primary inlet from the inlet of the compressor.

18. The method of claim 16 , further comprising sensing a parameter associated with the second medium, wherein providing the third medium from the secondary inlet to the inlet of the compressor occurs in response to sensing the parameter associated with the second medium.