Scroll Expander for Carbon Dioxide Cycle

A design has been developed and refined for a scroll energy recovery expander for use in a CO2 airconditioning circuit for an 18,000 Btu/hr military environmental control unit (ECU). The design is based on integration of the expander with an established-design two-stage rotary CO2 compressor. Key technical issues have been addressed through analysis, including bearing loads, friction, leakage, and material stresses and deflections. Testing in a static leakage fixture has been done to verify that refrigerant leakage through the expander, the largest anticipated loss mechanism, is less than initially projected. A proof-ofconcept prototype with an internal geometry identical to the expander’s design has been fabricated initially for testing in a shaft-output configuration. Initial testing of the prototype has been carried out at off-design conditions. Further testing to quantify the expander performance is ongoing.