Carbon dioxide vapor compression cycle improvements with focus on scroll expanders

Carbon Dioxide (CO₂) is under serious consideration worldwide as a replacement for conventional refrigerants, especially in automotive air-conditioning. The objective of this dissertation is to develop a CO₂ cycle that has comparable or better performance when compared to a cycle with conventional refrigerants.; Two air-conditioning systems were designed, built and tested, one operating with the state of the art refrigerant R134a, and the other with CO₂. Their performance was compared against each other, confirming the theoretical modeling that the CO₂ cycle falls short in performance with the components available at that time. Both systems were improved with a suction line heat exchanger, which shows larger benefits for CO₂; Potential improvements of components and advanced cycles with additional components were analyzed theoretically. An expander (a work recovering expansion machine) presents the best option. In the ideal case, it improves the COP of the CO₂ system by 40 to 65% over the range of operating temperatures, whereas the improvement potential for R134a is only about a third of these values. Various expander concepts were investigated. The scroll design was chosen for implementation, mainly because no valves are required. A scroll compressor was reduced in displacement volume, and this scroll expander prototype was operated for the first time for CO₂. The design was improved in several steps and has proven in principle the operability of a scroll expander in the transcritical and two-phase regions. However, the internal leak rates were still too high, and both an optimized design and improved machining accuracy are required to fabricate and operate a CO₂ expander at acceptable performance levels. Extensive modeling was conducted to optimize the CO ₂ scroll expander. The study demonstrates the influence of various geometric design parameters on the performance and size of the expander. The required machining accuracy for a scroll expander with acceptable performance is also calculated.; Different cycle integration concepts and the losses associated with them are discussed.